Ultralarge hyperpolarizability twisted π-electron system electro-optic chromophores: Synthesis, solid-state and solution-phase structural characteristics, electronic structures, linear and nonlinear optical properties, and computational studies

Published on Web 02/20/2007

Ultralarge Hyperpolarizability Twisted π-Electron System

Electro-Optic Chromophores: Synthesis, Solid-State and

Solution-Phase Structural Characteristics, Electronic

Structures, Linear and Nonlinear Optical Properties, and

Computational Studies

Hu Kang,^†Antonio Facchetti,^†Hua Jiang,^†Elena Cariati,^‡Stefania Righetto,^‡

Renato Ugo,^‡Cristiano Zuccaccia,^§Alceo Macchioni,^§Charlotte L. Stern,^†Zhifu Liu,^|

Seng-Tiong Ho,^|Eric C. Brown,^†Mark A. Ratner,^†and Tobin J. Marks*^,†

Contribution from the Department of Chemistry and the Materials Research Center and

Department of Electrical and Computer Engineering, Northwestern UniVersity, 2145 Sheridan

Road, EVanston, Illinois 60208-3113, Dipartimento di Chimica Inorganica Metallorganica e

Analitica and Centro di Eccellenza CIMAINA dell’UniVersita` di Milano and Unita` di Ricerca

dell’INSTM di Milano, Via Venezian 21, I-20133 Milano, Italy, and Dipartimento di Chimica,

UniVersita` di Perugia,Via Elce di Sotto 8, I-06123 Perugia, Italy

Received October 24, 2006; E-mail: t-marks@northwestern.edu

Abstract: This contribution details the synthesis and chemical/physical characterization of a series of

unconventional twisted π-electron system electro-optic (EO) chromophores. Crystallographic analysis of

these chromophores reveals large ring-ring dihedral twist angles (80-89°) and a highly charge-separated

zwitterionic structure dominating the ground state. NOE NMR measurements of the twist angle in solution

confirm that the solid-state twisting persists essentially unchanged in solution. Optical, IR, and NMR

spectroscopic studies in both the solution phase and solid state further substantiate that the solid-state

structural characteristics persist in solution. The aggregation of these highly polar zwitterions is investigated

using several experimental techniques, including concentration-dependent optical and fluorescence

spectroscopy and pulsed field gradient spin-echo (PGSE) NMR spectroscopy in combination with solid-

state data. These studies reveal clear evidence of the formation of centrosymmetric aggregates in

concentrated solutions and in the solid state and provide quantitative information on the extent of aggregation.

Solution-phase DC electric-field-induced second-harmonic generation (EFISH) measurements reveal

unprecedented hyperpolarizabilities (nonresonant µâ as high as -488 000 × 10^-48esu at 1907 nm).

Incorporation of these chromophores into guest-host poled polyvinylphenol films provides very large electro-

optic coefficients (r₃₃) of ∼330 pm/V at 1310 nm. The aggregation and structure-property effects on the

observed linear/nonlinear optical properties are discussed. High-level computations based on state-averaged

complete active space self-consistent field (SA-CASSCF) methods provide a new rationale for these

exceptional hyperpolarizabilities and demonstrate significant solvation effects on hyperpolarizabilities, in

good agreement with experiment. As such, this work suggests new paradigms for molecular hyperpolar-

izabilities and electro-optics.

Introduction

The essential requirement for large bulk EO response is that

the active component chromophores have a large microscopic

molecular first hyperpolarizability tensor (â), and the quest for

such chromophores has been a very active research field.^1-3

To date, the vast majority of effective EO chromophores have

been devised according to very similar design principles: planar

The development of high-performance molecule-based elec-

tro-optic (EO) materials has been the focus of much current

research. Such materials are of great scientific and technological

interest not only for applications as diverse as optical telecom-

munications, signal processing, data storage, image reconstruc-

tion, logic technologies, and optical computing, but also for the

fundamental understanding of how matter interacts with light.^1-3

(1) (a) Dalton, L. R. Pure Appl.Chem. 2004, 76, 1421. (b) Kajzar, F.; Lee,

K.-S. A.; Jen, K.-Y. AdV. Polym. Sci. 2003, 161, 1. (c) Dalton, L. R. AdV.

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^†Department of Chemistry and the Materials Research Center, North-

western University.

^‡Universita` di Milano.

^§Universita` di Perugia.

^|Department of Electrical and Computer Engineering, Northwestern

University.

9

J. AM. CHEM. SOC. 2007, 129, 3267-3286

3267

A R T I C L E S

Kang et al.

conjugated π-electron systems end capped with electron-donor

and -acceptor (D, A) moieties. This design algorithm gives rise

to a dominant intramolecular charge-transfer (ICT) transition

from the ground state to the first excited state and produces

effective polarization along the π-conjugated axis. Considerable

efforts⁴have been directed toward the molecular engineering

of such chromophore structures, and a variety of strategies has

emerged within the framework of the classical “two-state model”

for molecular hyperpolarizability â.⁵This simple model invokes

a polar ground state and a charge-separated first excited state,

where â is determined by the energy gap between the two states

(∆E_ge), the transition dipole moment (µ_ge) between the two

states, and the difference in dipole moment between the two

states (∆µ_ge) µ_ee- µ_gg) (eq 1).

and/or multiple (including fused) thiophene ring-containing

bridges (e.g., CLD and FTC), with the chromophore figures-

of-merit, µâ (µ ) the molecular dipole moment), as high as

35 000 × 10^-48esu being achieved.^3d

â

∆µ_ge(µ_ge)²/(∆E_ge)²

(1)

One approach to enhancing â proposed by Marder and co-

workers involves tuning “bond length alternation” (BLA; the

difference between average single and double bond lengths in

the conjugated chromophore core).⁶They argued that BLA,

hence â, can be optimized by controlling the relative neutral

and charge-separated contributions to the ground state via

modifying D/A substituent strength, the polarity of the solvent,

or the strength of an applied electric field. Another model,

“auxiliary donors and acceptors”,⁷correlates molecular hyper-

polarizability with the electron density of the π conjugation,

arguing that electron-excessive/deficient heterocycle bridges act

as auxiliary donors/acceptors and lead to substantial increases

in â values. Directed by these strategies, the largest hyperpo-

larizabilities have, to date, been observed with protected polyene

Note that such strategies focus primarily on extensive planar

π conjugation and that such molecules are inherently structurally

complex, complicating synthetic access and introducing potential

chemical, thermal, and photochemical frailties.⁸Furthermore,

extended conjugated systems typically introduce bathochromic

shifts in optical excitation, thus eroding transparency at the near-

IR working wavelengths of many photonic applications. Other

â enhancement strategies have emerged recently, including

multidimensional charge-transfer chromophores (e.g., HPEB

and X-CHR)⁹and a class of “right-hand-side” zwitterionic

chromophores (e.g., PCTCN and RHSC).^4d,10These chro-

mophores exhibit improved transparency and stability but not

significant enhancement in hyperpolarizability. Interestingly,

Kuzyk has argued that the â responses of all known organic

EO chromophores fall far short of the fundamental quantum

limits by a factor of ∼10^-3/2for reasons that are presently not

entirely clear but the understanding of which may afford

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3268 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

Chart 1

Chart 2

completely new insights.¹¹Alternative paradigms for very large

â chromophores would clearly be desirable, and there is growing

evidence that simple two-state systems are inadequate.¹²

could thereby exhibit far larger âs than conventional planar π

chromophores and, in principle, be as simple as two connected

arene rings, hence less susceptible to thermal/oxidative/

photochemical degradation. Notably, the mechanism of the

response to the light field here is distinctly different from the

current BLA/two-state models, suggesting a promising new

strategy for ultrahigh response EO chromophores.

Twisted intramolecular charge-transfer (TICT) molecules

have received significant attention in the quest to understand

their nonlinear optical response.¹³In the TICT response mech-

anism, rotation about a bond connecting conjugated D/A

substituents can reduce the overlap between the orbitals of the

D/A groups, providing a means by which nearly complete

electron transfer can occur upon optical excitation,¹⁴strongly

enhancing CT interactions and leading to large hyperpolariz-

abilities. Recent theoretical work in this laboratory¹⁵suggests

that molecules with twisted π-electron systems bridging D and

A substituents (TICTOID; Chart 1) might exhibit unprecedent-

edly large hyperpolarizabilities. Such chromophores could have

relatively simple zwitterionic biaryl structures in which â_zzzand

the linear optical response are sterically tunable via R¹, R²

modification of the interplanar dihedral angle (θ). Maximum â

magnitudes are predicted at θ ≈ 70-85°,¹⁵with twist-induced

reduction in D-π-A conjugation leading to aromatic stabiliza-

tion and formal charge-separated zwitterionic ground states,

relatively low-energy optical excitations, and large dipole

moment changes from the ground to first excited state. For

example, the maximum nonresonant µâ estimated for a tetra-

tert-butyl-substituted 4-quinopran (TBQP) with a full AM1-

optimized interplanar dihedral angle of 104° is ∼70 000 × 10^-48

esu at 0.1 eV.¹⁵Molecules with small numbers of π electrons

In recent communications¹⁶we briefly described the first

synthetic realization of twisted π-electron system chromophores

(tictoids). Those preliminary reports described initial synthetic

approaches, aggregation tendencies in solution, and evidence

of unprecedented molecular hyperpolarizabilities, in turn raising

a number of intriguing questions. In the present contribution,

we disclose full details and discussion of synthetic approaches,

structural characteristics, aggregation properties, and the ex-

ceptional NLO/EO properties of this unconventional tictoid

chromophore family (TM and TMC; Chart 2), investigated

using a full battery of experimental techniques, including

condensed state X-ray diffraction, solution-phase NOE NMR,

and optical/IR/¹³C NMR spectroscopic studies in both the

solution phase and the solid state, electrochemistry, concentra-

tion-dependent optical and fluorescence spectroscopy, pulsed

field gradient spin-echo (PGSE) NMR spectroscopy, solution-

phase DC electric-field-induced second-harmonic generation

(EFISH) spectroscopy, and Teng-Man electro-optic measure-

ments, combined with high-level state-averaged complete active

space self-consistent field (SA-CASSCF) computational ap-

proaches. The results include molecular hyperpolarizabilities as

large as 15× greater than ever previously reported (µâ as high

as -488 000 × 10^-48esu at 1907 nm) and poled polymers with

EO responses 3-5× greater than reported heretofore in the open

literature³(r₃₃as high as 330 pm/V at 1310 nm). Thus, we

suggest new paradigms for molecular hyperpolarizability and

organic electro-optics.

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9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3269

A R T I C L E S

Kang et al.

Under the initial rate approximation, the NOE build-up is only

Experimental Section

proportional to the cross relaxation rate constant σ_ISaccording to eq 2

Materials and Methods. All reagents were purchased from Aldrich

Chemical Co. and used as received unless otherwise indicated. THF

was distilled from sodium/benzophenone and methylene chloride from

CaCl₂. Chloroform was dried and distilled from anhydrous K₂CO₃.

Toluene was dried by passing through two packed columns of activated

alumina and Q5 under N₂pressure and regularly tested with benzophe-

none ketyl in ether solution. The reagent 1-iodo-4-vinyl-benzene was

purchased from Karl Industries Inc. and 2-propyl-1-heptanol from

Narchem Co. The reagents 4-bromo-3,5-dimethyl-pyridine 1-oxide (1)¹⁷

and ligand dicyclohexyl-(2-phenanthren-9-yl-phenyl)-phosphine (DCP-

PP) for catalytic Suzuki coupling were synthesized according to

literature procedures.¹⁸Solution NMR spectra were recorded on a Varian

Mercury 400 MHz or Varian INOVA 500 MHz spectrometer except

for NOE and PGSE measurements (see below). Solid-state ¹³C CPMAS

NMR spectra were recorded on a Varian VXR-500 MHz spectrometer

at room temperature with a spinning rate of 10 kHz using a 3.2 mm

zirconia rotor with Aurum caps. The cross-polarization contact time

was 4.0 ms, and the repetition time was 5.0 s. Mass spectra were

recorded on a Micromass Quattro II Triple Quadrupole HPLC/MS/

MS mass spectrometer. Elemental analyses were performed by Midwest

Microlabs. Optical spectra were recorded on a Cary 5000 spectropho-

tometer. Emission spectra were recorded on a PTI QM2 Fluorescence

Instrument. Thermal analysis was performed with a TA Instruments

SDT 2960 simultaneous DTA-TGA instrument under N₂at 1.0 atm.

The temperature ramp rate was 1.5 °C/min. IR spectra were obtained

on a Bio-Rad FTS-40 FTIR spectrometer. Cyclic voltammetry was

performed with a BAS 100 electrochemical analyzer using a three-

electrode cell (carbon working electrode, Ag wire pseudo-reference

electrode, and Pt wire counter electrode) with 0.1 M Bu₄NPF₆in

anhydrous CH₃CN as the electrolyte. All potentials are quoted vs the

ferrocene/ferrocenium (Fc/Fc⁺) couple internal standard. Synthetic

procedures and characterization are reported in the Supporting Informa-

tion.

d(NOE_I{S})

|

_{τ ) 0}) 2σ_IS

(2)

dτ

σ_IScan be expressed in terms of internuclear distance r_ISand correlation

time τ_c

according to eq 3

2

µ₀²p²γ_I²γ_S

6τ_C

1 + (ω_I+ ω_S) τ_C

σ_IS

)

-

2

(_4π)

(

10

τ_C

1 + (ω_I- ω_S)²τ_C

-6

r_IS(3)

2

)

τ_Cvaries with temperature according to the following eq 4²¹

τ_C) τ⁰‚e^{E /kT}

(4)

R

where E_Ris the activation energy for rotational reorientation, τ⁰is a

constant, and k is the Boltzmann constant. For a homonuclear

experiment, combining eqs 3 and 4 affords eq 5, where the temperature

dependence of σ_ISis explicit

2

µ₀²p²γ_I²γ_S

6

-6

σ_IS

)

‚τ⁰‚e^{E /kT}

- 1 r_IS

R

2

0

E_R/kT 2

(_4π)

(

)

10

1 + (2ω) (τ ‚e

)

(5)

Equation 5 was used to fit the experimental σ_ISvalues (Table S2). The

fitting results provide the corresponding r_IS, E_R, and τ⁰.²²Despite the

goodness of the fits, the error on the derived r_ISvalues was estimated

to be ca. 5%, corresponding to an uncertainty of (0.1-0.2 Å.²²The

experimentally determined r₂₃distance was incremented by 10% to

account for the known overestimation of short distances when confor-

mational averaging, aryl liberation in this case, is present (see ref 20,

pp 171-173). Comparison between the obtained r₂₃and the computed

Single-Crystal X-ray Diffraction. All diffraction measurements

were carried out on a Bruker SMART CCD diffractometer with

graphite-monochromated MoKR (0.71073 Å) radiation. Data were

collected using the Bruker SMART detector, processed using the

SAINT-NT package from Bruker, and corrected for Lorentz and

polarization effects. The structures were solved by direct methods

(SHELXTL-90) and expanded using Fourier techniques (SHELXTL-

97). The non-hydrogen atoms were refined anisotropically. Hydrogen

atoms on water molecules of solvation were refined with group isotropic

displacement parameters. The remaining hydrogen atoms were included

in idealized positions but not refined. All calculations were performed

using the Bruker SHELXTL9 crystallographic software package.

Crystallographic data are summarized in Table S1.

r₂₃distance for seven static conformations, having a dihedral angle of

89.6°, 84.4°, 80.0°, 75.2°, 69.6°, 64.4°, and 59.6° allows the average

dihedral twist angle between the two aryl rings in solution to be

estimated.

Pulsed Field Gradient Spin-Echo (PGSE) NMR Spectroscopy.

¹H PGSE NMR measurements were performed using the standard

stimulated echo pulse sequence²³on a Bruker AVANCE DRX 400

spectrometer equipped with a GREAT 1/10 gradient unit and a QNP

probe with a Z-gradient coil at 295.7 K without spinning. The shape

of the gradients was rectangular, their duration (δ) was 4 ms, and their

strength (G) was varied during the experiments. All spectra were

acquired using 32K points and a spectral width of 5000 Hz and

processed with a line broadening of 1.0 Hz. The semilogarithmic plots

of ln(I/I₀) vs G²were fit using a standard linear regression algorithm;

the R factor was always greater than 0.99. Different values of “nt”

(number of transients) and number of different gradient strengths (G)

were used for different samples, depending on solution viscosity and

solute concentration. The two TMC ortho methyl group resonances

NOE Measurements of Twist Angle in Solution. A sample of

TMC-2 in dry DMSO (dried and stored over activated molecular sieves)

was degassed via four freeze-pump-thaw cycles. NOE measurements

were performed on a Bruker Avance spectrometer operating at 400.13

MHz using the 1D-GOESY pulse sequence of Keeler and co-workers¹⁹

under the initial rate approximation.²⁰The singlet relative to H(3) was

inverted, and NOEs were measured on both H(4) and H(2) as a function

of temperature (294-342 K).

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2002, 124, 1162.

(19) Stonehouse, J.; Adell, P.; Keeler, J.; Shaka, A. J. J. Am. Chem. Soc. 1994,

116, 6037.

(20) The mixing time (τ_m) was set to 150 ms as a good compromise between

initial rate approximation (τ_m< 5 times the shorter T₁) and a sufficient

signal to noise ratio. Neuhaus, D.; Williamson M. The Nuclear OVerhauser

Effect in Structural and Conformational Analysis; VCH Publishers: New

York, 1989.

(21) (a) Doddrell, D. M.; Bendall, M. R.; O’Connor, A. J.; Pegg, D. T. Aust. J.

Chem. 1977, 30, 943. (b) Farrar, T. C.; Becker, E. D. Pulse and Fourier

Transform N.M.R.; Academic Press: New York, 1971.

(22) Zuccaccia, C.; Bellachioma, G.; Cardaci, G.; Macchioni, A. J. Am. Chem.

Soc. 2001, 123, 11020.

(23) Tanner, J. J. Chem. Phys. 1970, 52, 2523-2526.

9

3270 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

(one from each ring) were used for the analysis. The dependence of

the resonance intensity (I) on a constant diffusion time and on a varied

gradient strength (G) is described by eq 6

direction of µ, were performed by solution-phase DC electric-field-

induced second-harmonic (EFISH) generation methods,²⁹which provide

direct information on the intrinsic molecular NLO response via eq 8

I

I₀

δ

3

γ_EFISH) (µâ/5kT) + γ( - 2ω;ω,ω,0)

(8)

ln ) - (γδ)²D_t∆ - G²

(6)

(

)

where µâ/5kT is the dipolar orientational contribution and γ(-2ω; ω,

ω, 0), a third-order term at frequency ω of the incident light, is the

electronic contribution to γ_EFISH, which is negligible for molecules of

the type investigated here.³⁰

where I ) intensity of the observed spin echo, I₀) intensity of the

spin echo without gradients, D_t) diffusion coefficient, ∆ ) delay

between the midpoints of the gradients, δ ) length of the gradient

pulse, and γ ) magnetogyric ratio. For pure solvents, the diffusion

coefficient D_t, which is directly proportional to the slope of the

regression line obtained by plotting log(I/I₀) vs G², was estimated by

measuring the proportionality constant using an HDO sample (0.04%)

in D₂O (known diffusion coefficient in the range 274-318 K)²⁴under

the exact same conditions as the sample of interest [D_t(CD₂Cl₂) ) 33.2

× 10^-10m²s^-1, D_t(DMSO-d₆) ) 6.5 × 10^-10m²s^-1]. Residual solvent

signals were then used as internal standards to account for systematic

changes in solution viscosity (i.e., the reported D_tvalues refer to a

hypothetical experiment carried out in a solution having the nominal

reported concentration but the viscosity of the pure solvent at that

temperature)²⁵or random changes in the actual probe temperature as

well as gradient strength reproducibility.²⁶

EFISH measurements were carried out in CH₂Cl₂and DMF solutions

over a broad range of concentrations (10^-4-10^-6M) at a nonresonant

fundamental wavelength of 1907 nm using a Q-switched, mode-locked

Nd³⁺:YAG laser [pulse durations of 15 ns (90 ns) at a 10 Hz repetition

rate]. The 1064 nm initial wavelength was shifted to 1907 nm by a

Raman shifter with a high-pressure H₂cell. Solutions were prepared

under N₂. CH₂Cl₂was freshly distilled from CaCl₂. The organic base

1,4-diaza-bicyclo[2.2.2]octane (DABCO) (molar ratio DABCO/chro-

mophore equal to 0.5/1.0) was added to the solutions to increase

chromophore stability. The µâ values reported (Table S3) are the

averages of 16 successive measurements performed on each sample.

The standard deviation was never greater than 22%.

In Situ Poling and Direct Electrooptic Measurements on TMC-

Containing Guest-Host Polymer Thin Films. The effective electro-

optic coefficient, r₃₃, of poled TMC-based guest-host materials was

measured using the Teng-Man reflection technique.³¹Figure S1 shows

a schematic diagram of the in situ poling and direct Teng-Man EO

measurement setup. A 633 nm He-Ne laser is used for alignment of

optical components, and a CW diode laser at 1310 nm (TEC) is used

as the working laser source. The laser beam (1310 nm) travels through

a polarizer, where the orientation is set at 45° with respect to the laser

incident plane, then passes through the transparent conducting oxide

(TCO) top electrode, and then the EO-active thin film before being

reflected by the bottom gold electrode. The reflected laser beam travels

through a compensator and then an analyzer, which is cross-polarized

with respect to the polarizer. The signals are collected by a detector

and then monitored with an oscilloscope and lock-in amplifier.

Apertures are used for collimation purposes. The sample is mounted

under an N₂flow on a temperature-controlled heated stage with good

thermal contact. An AC modulation voltage, amplifying the input signal

from a signal generator, is applied to the sample with a frequency of

1 kHz, with the amplitude ranging from 1.0 to 10 V. A DC bias

generated by the high-voltage amplifier is applied to the sample, with

a microamp meter monitoring the poling current (∼10 µA). In the

present experiments, assuming r₃₃) 3r₁₃, r₃₃is derived from eq 9³²

PGSE measurements were carried out for TMC-2 and TMC-3 in

CD₂Cl₂and DMSO-d₆over a range of concentrations. Hydrodynamic

radii of the diffusing particles (r_H) were derived from the experimentally

determined D_tdata using the Stokes-Einstein equation

kT

D_t)

(7)

f

6πηr_H

(_f)

0

where k is the Boltzmann constant, T is the temperature, and η is the

solution viscosity. The f/f₀ratio is introduced to take into account the

ellipsoidal shapes of TMC-2 and TMC-3, where f is the frictional

coefficient of the ellipsoid and f₀is that of a sphere having an equal

volume.²⁷The f/f₀ratio depends on the shape (prolate or oblate) and

ratio of the major (a) to minor (b) axis of the ellipsoid. For prolate

ellipsoid-like TMC-2, a/b ≈ 2.9 and f/f₀≈ 1.1, while for TMC-3, a/b

≈ 4.1 and f/f₀≈ 1.2.²⁷From the hydrodynamic radii obtained, the

hydrodynamic volumes (V_H) of the diffusing chromophores were

calculated and then compared with the van der Waals volumes (V_vdW

)

obtained from crystallographic data or molecular modeling. The V_H/

V

_vdWratio represents the aggregation number (N), similar to that defined

by Pochapsky,²⁸and is useful in comparing trends over a range of

concentrations.

2

EFISH Measurements. Measurements of µâ, the products of the

chromophore dipole moment (µ) and the projection of â_VEC, the vector

part of the molecular first-order hyperpolarizability â tensor along the

V_AC

x

3

2 λ 1

π V_MV_DC

n - sin θ

r₃₃

)

(9)

n²sin²θ

4

where λ is the wavelength of the laser, V_DCis the working voltage,

V_ACis the modulated signal, V_Mis the modulation voltage applied to

the sample, θ is the incident angle of the laser beam (fixed at 45°),

and n is the refractive index of the thin film.

The EO measurement sample has the structure glass substrate/TCO/

EO thin film/Au (Figure S1). The TCO electrode (NIR transparent

In₂O₃), having a thickness around 160 nm and a conductivity of 100

S/cm, was grown on Eagle 2000 glass substrates at room temperature

by ion-assisted deposition (IAD).^31cTMC guest-host thin films (∼1

(24) Mills, R. J. Phys. Chem. 1973, 77, 685. Data at different temperatures were

estimated by interpolation of the data reported by Mills, giving D_HDO

)

1.780 × 10^-9m²s^-1at 295.7 K.

(25) (a) The viscosity of CD₂Cl₂was estimated to be 0.4256 cp at 295.7 K by

interpolation of the data reported for CH₂Cl₂(CRC Handbook of Chemistry

and Physics, 67th ed.; Weast, R. C., Ed.; Chemical Rubber: Cleveland,

1986) and corrected for the reduced mass as proposed by Holz et al. (Holz,

M.; Mao, X.; Seiferling, D.; Sacco, A. J. Chem. Phys. 1996, 104, 669). (b)

The viscosity of DMSO-d₆was estimated to be 2.138 cp at 295.7 K by

interpolation of the data reported for DMSO (Higashigaki, Y.; Christensen,

D. H.; Wang, C. H. J. Phys. Chem. 1981, 85, 2531) and corrected for the

reduced mass as proposed by Holz et al. (Holz, M.; Mao, X.; Seiferling,

D.; Sacco, A. J. Chem. Phys. 1996, 104, 669).

(26) (a) Zuccaccia, D.; Macchioni, A. Organometallics 2005, 24, 3476. (b)

Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M. C.; Roberts, J. A.;

Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448.

(30) (a) Kanis, D. R.; Lacroix, P. G.; Ratner, M. A.; Marks, T. J. J. Am. Chem.

Soc. 1994, 116, 10089. (b) Roberto, D.; Ugo, R.; Bruni, S.; Cariati, E.;

Cariati, F.; Fantucci, P.; Invernizzi, I.; Quici, S.; Ledoux, I.; Zyss, J.

Organometallics 2000, 19, 1775. (c) Lesley, M. J. G.; Woodward, A.;

Taylor, N. J.; Marder, T. B.; Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton,

A.; Bruce, D. W.; Kakkar, A. K. Chem. Mater. 1998, 10, 1355.

(31) (a) Teng, C. C.; Man, H. T. Appl. Phys. Lett. 1990, 56 (18), 1734. (b)

Schildkraut, J. S. Appl. Opt. 1990, 29, 2839. (c) Wang, L.; Yang, Y.; Marks,

T. J.; Liu, Z., Ho, S.-T. Appl. Phys. Lett. 2005, 87 (16), 161107/1.

(32) Shuto, Y.; Amano, M. J. Appl. Phys. 1995, 77, 4632.

(27) Perrin, F. J. Phys. Radium 1936, 7, 1.

(28) (a) Mo, H.; Pochapsky, C. T. J. Phys. Chem. B 1997, 101, 4485. (b)

Pochapsky, S. S.; Mo, H.; Pochapsky, C. T. J. Chem. Soc., Chem. Commun.

1995, 2513.

(29) (a) Levine, B. F.; Bethea, C. G. Appl. Phys. Lett. 1974, 24, 445. (b) Singer,

K. D.; Garito, A. F. J. Chem. Phys. 1981, 75, 3572. (c) Ledoux, I.; Zyss,

J. Chem. Phys. 1982, 73, 203.

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3271

A R T I C L E S

Kang et al.

µm) were prepared by spin-coating onto the TCO substrate a solution

of TMC/host polymer in DMF at 1000 rpm for 60 s and curing the

resulting film at 110 °C in a vacuum oven (60 mTorr) for 16 h. The

bottom Au electrode (∼150 nm) was then deposited on the TMC

guest-host thin films by e-beam evaporation techniques.

angles across the series and that these tictoid chromophores

possess highly charge-separated zwitterionic ground states in

the solid state. Molecular properties and structural characteristics

in solution are then studied by a combination of techniques,

including NOE NMR, optical absorption (UV-vis) spectros-

copy, fluorescence spectroscopy, cyclic voltammetry, and

infrared (IR) vibrational spectroscopy. It is further shown that

the dihedral twist angle and structural characteristics observed

in the solid state persist in the solution phase by NOE-derived

dihedral twist angle measurements in solution and comparative

solid-state vs solution ¹³C NMR, IR, and optical spectroscopic

analysis. To understand in depth how aggregation of these highly

dipolar zwitterions may affect chromophore measured linear/

nonlinear optical properties, the solution-phase molecularity is

fully investigated using several complementary experimental

techniques, including concentration-dependent optical absorp-

tion, fluorescence, and pulsed field gradient spin-echo (PGSE)

NMR spectroscopies, in comparison with the solid-state X-ray

diffraction data. These studies provide clear evidence for the

formation of centrosymmetric aggregates at high concentrations

in nonpolar solvents and in the condensed state and provide

quantitative information on the state of aggregation. Molecular

hyperpolarizabilities are then evaluated by the solution-phase

DC electric-field-induced second-harmonic generation (EFISH)

methods, and the electro-optic coefficients (r₃₃) of poled host-

guest polymers containing these chromophores are directly

measured by Teng-Man reflection techniques. Aggregation

effects on these measurements are also taken into account and

discussed. Finally, high-level computations provide a rationale

for the observed exceptional hyperpolarizabilities in these

twisted chromophores and demonstrate significant solvation

effects on hyperpolarizabilities, in good agreement with experi-

ment.

Chromophore Synthetic Approaches. The syntheses of the

present new families of tictoid chromophores are summarized

in Scheme 1. The highly encumbered asymmetric tetra-ortho-

methylbiaryl core 3 was constructed via Suzuki cross-coupling

of hindered 4-bromopyridine N-oxide 1 and phenyl boronic acid

2 using a Pd/dicyclohexyl-(2-phenanthren-9-yl-phenyl)-phos-

phane (DCPPP) catalyst.¹⁸Pyridine N-oxide 3 was next reduced

to pyridine 4 in a facile and quantitative fashion using

Pd-catalyzed hydrogenation with sodium hypophosphite as the

hydrogen source.³⁵Subsequent cleavage of the 4 methoxyl group

with pyridine + HCl affords pyridylphenol intermediate 6. This

product was then quaternized with methyl iodide and deproto-

nated with sodium methoxide to afford chromophore TM-1.

N-Methylpyridinium salts 5 and 7 were also prepared for X-ray

diffraction studies.

Computational Methodology. The model compound TM-1′ was

used as an analog of chromophore TM-1 to reduce the computational

demands. All electronic structure calculations were performed using

the MOLCAS quantum chemistry package³³with the 6-31G** basis

set.³⁴All geometries were optimized with respect to the ground-state

wave function at a particular geometry where the ring-ring dihedral

twist angle had been fixed. For twist angles between 50° and 90°,

excitation energies were obtained with a three-state state-average

complete active space (SA-CASSCF) level of theory with four π

electrons in three π molecular orbitals. At the 90° geometry, the lowest

three states converged to a ground-state Ψ_D, the first excited state Ψ_Z

(notationally D and Z represent diradical and zwitterion states,

respectively), and the second excited state, which can be thought of as

an n fπ * transition (Ψ_nfπ*). In order to account for the effects of

dynamical electron correlation, CASPT2 single-point energy calcula-

tions were performed at the CASSCF geometry. The CASPT2 and

CASSCF results differ by no more than 0.1 eV in all of these

calculations; therefore, the CASSCF active space is adequate for the

purpose of describing the differences in computed quantities.

Instead of explicitly including the four ortho-methyl groups of TM-1

in state-average calculations, a model methyl-repulsion potential was

derived by subtracting the energy of the ground state of TM-1′ as a

function of twist angle from the ground-state torsional potential of TM-

1′. The model methyl-repulsion potential was then shifted so that its

potential energy is zero at the 90° geometry, where the ortho-methyl

groups interact the least. This methyl-repulsion potential was added to

the ground- and excited-state energies from the three-state SA-CASSCF

calculations. Finally, the SA-CASSCF calculations were slightly shifted

by 0.3 eV so that the energy difference matched that of the higher-

quality single-state-CASSCF(14,13) calculations at the 90° geometry.

Considering that nonlinear response depends on the difference in state

energies, these electronic states were characterized by describing their

composition in terms of the frontier molecular orbitals at the highly

symmetric (C_2V) twisted geometry. Each state was classified after

inspecting its CI coefficients, molecular orbitals, and dipole moment.

Results

The synthesis of a new series of tictoid chromophores is first

reported. Of particular interest here are the enforced dihedral

twist angles and related structural characteristics that define the

architectures of these zwitterions. To this end, single-crystal

X-ray diffraction structure determinations are performed on a

number of the target chromophores as well as on their synthetic

precursors (including neutral, positively charged, and zwitteri-

onic molecules) to provide structural information in the solid

state. It will be seen that the present tetra-ortho-methylbiaryl

substitution pattern indeed enforces very large and uniform twist

The crucial synthetic biaryl iodide intermediate 11 was

prepared via a four-step phenol-to-aryl iodide conversion. Phenol

6 was first converted to triflate 8. Next, Pd-catalyzed coupling³⁶

of triflate 8 with benzophenone imine leads to diphenyl ketimine

9 in 98% yield. Subsequent quantitative hydrolysis of 9 to

primary aniline 10 was achieved using hydroxylamine hydro-

chloride. Biaryl iodide 11 was then obtained via diazotization

of 10 and iodination of the corresponding diazonium salt with

NaI. Chromophore TM-2 was synthesized via conventional

Heck cross-coupling of 11 and styrenic coupling partner 12,

(33) Karlstro¨m G.; Lindh R.; Malmqvist , P.-Å.; Roos, B. O.; Ryde, U.;

Veryazov, V.; Widmark, P.-O.; Cossi , M.; Schimmelpfennig, B.; Neogrady,

P.; Seijo, L. Comput. Mater. Sci. 2003, 28, 222.

(34) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,

2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.

(35) Balicki, R.; Kaczmarek, L. Gazz. Chim. Ital. 1994, 124 (9), 385.

(36) Wolfe, J. P.; Åhman, J.; Sadighi, J. P.; Singer, R. A.; Buchwald, S. L.

Tetrahedron Lett. 1997, 38, 6367.

9

3272 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

Scheme 1. Synthesis of Twisted π-Electron System Chromophrores^a

^a(i) Pd₂(dba)₃/DCPPP, K₃PO₄, toluene; (ii) NaH₂PO₂, Pd/C, AcOH; (iii) pyridine + HCl; (iv) MeI; (v) MeONa, MeOH; (vi)Tf₂O, pyridine; (vii) NHdCPh₂,

Pd(OAc)₂/BINAP, Cs₂CO₃, THF; (viii) NH₂OHHCl, NaOAc, MeOH; (ix) NO⁺BF₄^-, CH₃CN, NaI; (x) Pd(OAc)₂/PPh₃, Et₃N; (xi) n-C₈H₁₇I, CH₂Cl₂; (xii)

MeONa, MeOH; (xiii) NaCH(CN)₂, Pd(PPh₃)₄, DME; (xiv) ROTf, CH₂Cl₂; (xv) aq. NaOH (xvi) MeONa, MeOH; (xvii) Pd(OAc)₂/PPh₃, Et₃N, DMF; (xviii)

ROTf, CH₂Cl₂; (xix) MeONa, MeOH.

which was readily obtained via thermal decarboxylation of 3,5-

di-tert-butyl-4-hydroxycinnamic acid in DMF. The resulting

stilbene precursor 13 was quaternized with n-octyl iodide and

then deprotonated with sodium methoxide to afford chro-

mophore TM-2 in 87% yield.

mediates 3 and 6, N-methyl pyridinium salts 5 and 7, and tictoid

chromophores TM-1, TM-2, TMC-1, and TMC-2. Single

crystals were obtained via slow evaporation of saturated

solutions. Important crystallographic data for these compounds

are collected in Table S1. The ORTEP drawings of the

molecular structures, selected bond lengths, and inter-ring twist

dihedral angles are summarized in Table 1. All of these

molecules exhibit consistently large arene-arene dihedral twist

angles (80-89°). The (ring)C-C(ring) distances in these

molecules are slightly longer than in typical biaryls (∼1.487

Å)³⁸and close to that in bimesityl (1.505(2) Å),³⁹doubtless a

result of the pronounced steric hindrance about this region of

the molecules.

The (ring)C-C(ring) and (ring)C-O distances in the two

zwitterionic TM chromophores TM-1 and TM-2 are only

modestly shorter than in those of the corresponding neutral (3,

6) and positively charged (5, 7) species (Table 1), probably a

result of the very small contribution of quinoidal limiting forms

(Chart 1) to the ground state due to the twist-induced intra-

Pd-catalyzed coupling of 11 with sodium dicyanomethanide

affords 15 in high yield (96%), which is next regioselectively

N-quaternized³⁷with alkyl triflates and then deprotonated to

afford chromophores TMC-1 (80% yield) and TMC-2 (67%

yield). Styrene precursor 18 was synthesized via Pd-catalyzed

coupling of 1-iodo-4-vinyl-benzene with sodium dicya-

nomethanide in 72% yield. Subsequent Heck coupling of 11

with styrene 18 affords chromophore precursor 19 in 55% yield.

This intermediate was alkylated and then deprotonated to afford

chromophore TMC-3 in 66% yield. All new compounds have

been fully characterized via conventional analytical/spectro-

scopic techniques, including multinuclear NMR spectroscopy,

mass spectrometry, and elemental analysis.

Condensed-State Structural Chemistry. X-ray diffraction

structural analyses were performed on neutral synthetic inter-

(38) Database of average bond lengths in organic compounds: Allen, F. H.;

Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R.

J. Chem. Soc., Perkin Trans. 2 1987, S1.

(37) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A. J. Org. Chem.

1997, 62, 5755.

(39) Fro¨hlich, R.; Musso, H. Chem. Ber. 1985, 118, 4649.

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3273

A R T I C L E S

Kang et al.

Table 1. ORTEP Drawings of the Molecular Structure and Selected Metrical Parameters for Twisted π-Electron System Chromophores and

Several Synthetic Intermediates

^aDrawn with 50% probability ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity. The iodide counterions are included in the

drawings of 5 and 7. ^bAverage of four dihedral angles in the respective crystal structures. ^cThere are two independent molecules in the TMC-1 unit cell.

molecular charge transfer (see more below). Thus, there is a

pronounced reduction in inter-ring π conjugation, and a

dominant zwitterionic ground state prevails in all of the TM

chromophores, evidenced by the departure from quinoidal

structures where typically (ring)CdC(ring) ≈ 1.349 Å and CdO

≈ 1.222 Å.³⁸

cyclopentadienylidene-1,4-dihydropyridines.⁴²Taken together,

the solid-state metrical parameters confirm a highly charge-

separated zwitterionic TMC ground state (cf., Chart 2).

The solid-state packing diagrams of chromophores TM-1,

TM-2, TMC-1, and TMC-2 are shown in Figure 1. All of the

present zwitterions crystallize in antiparallel pairs, doubtless a

result of electrostatic interactions⁴³involving their large dipole

moments.⁴⁴The TM-1 molecules exist in the crystal as a

complex with NaI units where two molecules are linked head-

to-head by Na⁺ions, a result of the strong phenoxide affinity

for Na⁺(Figure 1A). A centrosymmetric dimer of two dimeric

complexes is linked between two neighboring zwitterions from

each complex, arrayed in an antiparallel fashion with an

intermolecular distance of 6.475 Å between the aromatic

backbones. This distance is considerably larger than the sums

of van der Waals radii between planar cofacial π-electron

systems (∼ 3.50 Å)⁴³and is likely the result of the difficulty in

closely face-to-face packing the aromatic rings in these sterically

encumbered twisted π-electron molecules. The dimers are

arranged within the (101) plane in a herringbone-like packing

with an angle of 30° between the backbones of adjacent dimers.

TM-2 molecules are arrayed along the a direction in loosely

interacting antiparallel pairs. These dimers have a larger

Similar structural characteristics are revealed in the TMC

chromophores. The observed (ring)C-C(ring) distances imply

strong reduction in inter-ring π conjugation. The phenylenedi-

cyanomethanide fragments display a markedly different bond

length pattern than in typical TCNQs.^38,40The (NC)₂C-bound

phenylene rings exhibit significantly less quinoidal character,

and the (dicyanomethanide)C-C(aryl) distances lack typical

TCNQ CdC(CN)₂exocyclic character (∼1.392 Å).³⁸This

indicates substantial negative charge localization within the

-C(CN)₂group, also evident from the observed C-CN bond

shortening (1.397(2)-1.412(2) Å in TMC-1 and 1.402(5)-

1.402(6) Å in TMC-2 vs 1.427 Å in typical TCNQs³⁸), and

CtN bond elongation (1.152(2)-1.158(2) Å in TMC-1,

1.160(5)-1.165(6) Å in TMC-2, vs 1.144 Å in typical

TCNQs³⁸), a result of charge resonant stabilization via the two

CN groups. Finally, there is significant pyridinium aromatic

character in the TMC chromophores with metrical parameters

paralleling N-methyl-p-phenylpyridinium salts⁴¹rather than

(42) Ammon, H. L.; Wheeler, G. L. J. Am. Chem. Soc. 1975, 97, 2326.

(43) Wu¨rthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. J. Am. Chem.

Soc. 2002, 124, 9431.

(44) DFT-derived ground-state dipole moments are 20.0, 37.3, 27.0, and 50.6

D for TM-1, TM-2, TMC-2, and TMC-3, respectively.

(40) Cole, J. C.; Cole, J. M.; Cross, G. H.; Farsari, M. J.; Howard, A. K.;

Szablewski, M. Acta Crystallogr. 1997, B53, 812.

(41) Das, A.; Jeffery, J. C.; Maher, J. P.; McCleverty, J. A.; Schatz, E.; Ward,

M. D.; Wollermann, G. Inorg. Chem. 1993, 32, 2145.

9

3274 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

Figure 2. (A) Nonlinear regression analysis of the cross-relaxation rate

constant σ_ISfor TMC-2 measured from NOE NMR in dry DMSO-d₆as a

function of temperature according to eq 5. The solid lines are the best fit

to the data. (B) Computed distance r₂₃for TMC-2 as a function of dihedral

twist angle. The cross point indicates the NOE-derived distance r₂₃and the

corresponding twist angle.

Table 2. Internuclear Distance r_IS(Å),^aActivation Energy for

Rotational Reorientation E_R(kJ/mol), τ⁰(10^-15s) Values

Estimated from the Best Nonlinear Least-Squares Fit of NOE Data

for TMC-2, and the Corresponding Correlation Time τ_C(10^-15s) at

300.1 K

Figure 1. Crystal packing diagrams of chromophores TM-1 (A), TM-2

(B), TMC-1 (C), and TMC-2 (D). A single dimeric or tetrameric unit is

indicated by the dashed lines and shown at the bottom. The two independent

molecules in the TMC-1 asymmetric unit are colored by symmetry

equivalence. Solvent molecules (A-C), tetra-o-methyl substituents (A, top),

and N-alkyl chains (D, top) are removed for clarity.

r_IS

E_R

τ⁰

τ_C

H4-H3

H2-H3

2.7 ( 0.1

3.5 ( 0.2^a

26.7 ( 0.8

27.4 ( 2.6

7.5 ( 2.3

3.9 ( 4.1

343

232

^ar₂₃average distance obtained from the nonlinear least-squares fit has

been incremented by 10% to account for the overestimation of short

distances due to conformational averaging.²⁰

separation than that in the aforementioned TM-1 dimer, 8.166

Å, clearly a result of steric hindrance from the bulky tert-butyl

groups and N-alkyl chains. The structures of TMC-1 and

TMC-2 exhibit similar but more complex packing features. A

tetramer-like unit is found in the TMC-1 unit cell (Figure 1C)

consisting of two independent molecules with a distance of 6.208

Å between two neighboring molecules oriented in an antiparallel

fashion and an average distance of 6.366 Å between two skew-

packed neighbors. The formation of tetramers is consistent with

strong dipolar interactions between TMC molecules, doubtless

a consequence of their large dipole moments.⁴⁴In the case of

TMC-2, which possesses a bulky N-alkyl chain at the pyri-

dinium fragment, the two molecules in the dimeric unit are

forced apart to accommodate the bulky chain with a larger

average intermolecular distance of 8.232 Å (Figure 1D). This

asymmetric dimer is related to the neighboring dimer via an

inversion center to form the centrosymmetric unit cell.

Twist Angle Measurement in Solution by NOE. Nonlinear

least-squares fitting of experimental σ_ISvalues (see Experimental

Section for details) as a function of temperature according to

eq 5 (Figure 2A) yields r_IS, E_R, and τ⁰parameters. The results

are summarized for TMC-2 in Table 2. The r₄₃distance is

independent of the dihedral twist angle and can test the validity

of this methodology. The obtained value from the NOE

measurements (r₄₃) 2.7 ( 0.1 Å) is in excellent agreement

with that calculated from the solid-state structure under r^-6

averaging (2.68 and 2.83 Å for the two limiting static conforma-

tions of the methyl group). The dependence of the distance r₂₃

on the dihedral angle (θ) was then computed in the 60-90°

range using the solid-state structure as described in the

Experimental Section, and the relationship is plotted in Figure

2B. From the plot, an average dihedral angle θ of 88° ( 10°,

corresponding to the derived r₂₃of 3.5 ( 0.2 Å (Table 2), can

be estimated in solution, in excellent agreement with the solid-

state structures (Table 1).

Optical Spectroscopy. All the tictoid chromophores exhibit

intense optical absorption in the UV-vis region with extinction

coefficients as large as 38 400 M^-1cm^-1. Optical maxima λ_max

,

extinction coefficients ꢀ and the lowest energy optical gaps E_g

(estimated from the onset of the absorption maxima at the low-

energy edges) are collected in Table 3. TM chromophore optical

absorption spectra in methanol are shown in Figure 3A. The

TM-1 optical spectrum consists of two maxima at 269 (ꢀ )

6900 M^-1cm^-1) and 400 nm (ꢀ ) 790 M^-1cm^-1), assignable

to phenyl subfragment intra-ring excitations and inter-subfrag-

ment charge-transfer (CT) excitation, respectively.¹⁵Pyridinium

intra-subfragment excitation is expected to be at higher energy¹⁵

and overlap with the solvent cutoff. Such a modest extinction

coefficient for the CT band indicates a pronounced reduction

in inter-ring π conjugation due to the twist. The TM-2 spectrum

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3275

A R T I C L E S

Kang et al.

Table 3. Optical Absorption (λ_max, nm, and Extinction Coefficient ꢀ, M^-1cm^-1) Data, Optical Gap E_g(eV), Redox Potentials E vs SCE (V),

Estimated Ground-State Dipole Moments µ_g(D), and the Maximum EFISH-Derived µâ Values (10^-48esu) for TM and TMC Chromophores

d

λ_max

(ꢀ)

E_g

E^e

µâ

compound

MeOH

CH₃CN

CH₂Cl₂

CH₃CN

E_ox

E_red

µ_g^f

CH₂Cl₂

DMF

TM-1

269^a(6900)

304^a

-1.54

2.04

2.32

2.24

2.16

2.61

0.30

20.0

400^b(790)

474^b

448^a

304^a

-1.65

-1.64

TM-2

326^a(20700)

462^a

1.25

37.3

27.0

50.6

-315000

-49000

495^b(2010)

296^a

TMC-1

TMC-2

TMC-3

314^a

440^b

297^a

462^b

304^a

556^b

314^a(27200)

0.39

0.20

-1.56

-1.50

-24000

-5620

451^b

397^a

472^b

416^a

569^b(1840)

433^a(38400)

-488000

-84000

540^c(2090)

^aAssigned to intra-subfragment excitation. Another high-energy subfragment excitation overlaps with the solvent and is not tabulated here. ^bAssigned to

low-energy inter-subfragment charge-transfer (CT) excitation. ^cDeconvoluted from the subfragment excitation band and assigned to inter-subfragment CT.

^dEstimated from the onset of the absorption maximum at the low-energy edge. ^eReferred to ferrocene internal reference E_1/2) 0.43V vs SCE in MeCN.

^fDFT-derived ground-state dipole moment.

Figure 3. (A) Optical absorption spectra of TM chromophores in methanol solution: solid line, TM-1; dotted line, TM-2. (B) Optical absorption spectra

of TMC chromophores in CH₂Cl₂solution: dashed line, TMC-1 (enlarged for clarity); solid line, TMC-2; dotted line, TMC-3. (C) Optical absorption

spectra of TMC-2 in different solvents. Only interfragment CTs band are shown here. (D) Optical absorption spectra of TMC-3 in solvents of varying

polarity. Inter- and intrafragment excitation bands are overlapped.

also features an intense band at 326 nm (ꢀ ) 20 700 M^-1cm^-1),

assigned to stilbenyl subfragment excitation and a relatively

weak CT band at 495 nm (ꢀ ) 2010 M^-1cm^-1) with a shoulder

at short wavelength. All of the TM chromophores have limited

solubilities in moderate- and low-polarity solvents, precluding

a full comparative optical spectroscopic study as a function of

solvent.

The TMC chromophore optical properties were studied in a

range of solvents having varying polarity. Their spectra in CH₂-

Cl₂are shown in Figure 3B. It can be seen that TMC-1 and

TMC-2 exhibit similar spectra with two fairly short wavelength

maxima, tentatively assigned to pyridinium and phenyl sub-

fragment high-energy intra-ring excitations, and one low-energy

inter-subfragment charge-transfer (CT) excitation (Figure 3B).¹⁵

The CT band has a smaller oscillator strength (λ_max) 569 nm,

ꢀ ) 1840 M^-1cm^-1for TMC-2 in CH₂Cl₂) than the intra-

subfragment excitation band (λ_max) 314 nm, ꢀ ) 27 200 M^-1

cm^-1for TMC-2 in CH₂Cl₂). The principal difference between

the TMC-1 and TMC-2 spectra is that the TMC-1 CT band is

slightly blue shifted vs that of TMC-2, suggesting that any

aggregation (reasonable considering the large dipole moments⁴⁴)

is greater for methyl-functionalized TMC-1 than for bulky alkyl-

functionalized TMC-2. Such CT band blue shifts due to

antiparallel centrosymmetric dimer formation have also been

9

3276 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

reported in merocyanine dye aggregation studies.⁴³Figure 3C

shows TMC-2 optical absorption spectra in solvents of different

polarity. It can be seen that the TMC-2 CT bands exhibit strong

negatiVe solvatochromic effectsslarge blue shifts with increas-

ing solvent polarity (the spectra were recorded in the concentra-

tion range where the aggregation is not significant). The TMC-2

solvatochromic shift from CHCl₃to MeOH is ∼153 nm toward

shorter wavelength, comparable to the largest solvatochromic

effects reported in heavily studied betaine⁴⁵and merocyanine

derivatives.^4d,37,46Within the conventional interpretation of

solvatochromic interactions,⁴⁷the negative solvatochromism

indicates that the magnitude of the dipole moment in the excited

electronic state is significantly smaller than in the ground state.

Although other factors can affect the sign of solvatochromism

(e.g., aggregation or different solvent polarizabilities),⁴⁸within

the reasonable hypothesis that the various solvents here do not

drastically affect the electronic nature of the ground and excited

states, the negative solvatochromism indicates that the TMC

ground state is best approximated by the zwitterionic limit

formula. The intra-subfragment excitation bands exhibit similar

but weaker negative solvatochromism. The TMC-3 optical

spectrum (Figure 3B) features an intense band centered at ∼433

nm in CH₂Cl₂(ꢀ ) 38 400 L mol^-1cm^-1) assigned to

predominant stilbenyl subfragment excitation, overlapping a

relatively weak CT band centered at 540 nm (ꢀ ) 2090 M^-1

cm^-1). Both subfragment and CT bands (a CT band of 578 nm

can be deconvoluted from the subfragment excitation band in

THF) exhibit negative solvatochromism (Figure 3D).

Figure 4. Optical absorption spectra of TMC-2 (A) and TMC-3 (B) in

the solid state (solid line) vs those in DMF solution (dashed line).

LUMO gaps of ∼1.84 and ∼1.95 eV for TM-1 and TMC-2,

respectively, in good agreement with the optical gaps (Table

3). In the case of TM-2 and TMC-3, there are considerable

discrepancies between the electrochemical gaps and the optical

gaps (Table 3), perhaps because the HOMO-LUMO optical

transitions are mixed with other transitions.

Solid-state optical studies were also performed on films of

TMC-2 and TMC-3 prepared by spin-coating chromophore

cyclopentanone solutions onto fused quartz slides followed by

drying in a vacuum oven. The optical spectra of these films

exhibit close correspondences to those acquired in DMF solution

(Figure 4). Taking into account the negative solvatochromic shift

of the CT excitations and aggregation effects (vide supra), the

spectral patterns exhibit no significant differences between the

solid state and solution phase.

Infrared Vibrational Spectroscopy. The IR spectra of

chromophores TM-1, TM-2, TMC-2, and TMC-3 are shown

in Figure 5. Both TM-1 and TM-2 exhibit several sharp, intense

infrared transitions in the 1000-1800 cm^-1region (Figure 5A),

which correspond to principally aromatic ring stretching modes.

The features at 1585 cm^-1in TM-1 and 1578 cm^-1in TM-2

correspond to the aromatic stretching modes typical of phenox-

ide rings.⁴⁹The CdO stretching mode ν(CdO), which is usually

the strongest band at 1640-1660 cm^-1in typical benzoquinone

Electrochemistry. Cyclic voltammetry (CV) was carried out

under N₂in a 0.1 M Bu₄NPF₆solution in anhydrous MeCN

with scanning rates between 60 and 150 mV/s. Voltammograms

of ∼10^-3M CH₃CN solutions of TM-1, TM-2, TMC-2, and

TMC-3 exhibit one chemically irreversible oxidation wave and

one irreversible reduction wave (two in TM-1), which can be

attributed to the redox chemistry of the phenoxide/phenyldi-

cynomethanide donor and the pyridinium acceptor moieties.

Data are summarized in Table 3. All the chromophores exhibit

comparable reduction potentials. The surprisingly high oxidation

potentials of TM-2 can be attributed to weak solvation of its

cation radical, possibly due to the steric effects of the two ortho

tert-butyl substituents at the phenoxide portion. HOMO-LUMO

(electrochemical) gaps can be estimated from the oxidation and

reduction potentials. We are aware that a precise E_gapdetermi-

nation requires knowledge of the standard potentials. However,

from the present oxidative/reductive data, we estimate HOMO-

structures,⁵⁰is very weak or nonexistent in TM-1 (∼1640 cm^-1

)

and TM-2 (∼1636 cm^-1), arguing for only very small quinoidal

limit contribution to the TM ground state.

In the TMC infrared spectra (Figure 5B and 5C), the CN

stretching vibration, ν(CtN), is observed for both TMC-2 and

TMC-3 as the most intense band at around 2164 cm^-1with a

low-energy side component (2126 cm^-1in TMC-2 and 2118

in TMC-3). This two-peak feature is also observed in some

dianionicTCNQsandzwitterionicdicynomethylenederivatives^4b,51

with the more intense band assigned to an in-phase ν(CtN)

normal mode and the second to the related out-of-phase

51

motion.^4b,

In contrast, compounds 15 and 19, the neutral

precursors to TMC-2 and TMC-3, respectively, exhibit only a

very weak transition at 2257 cm^-1. The ν(CtN) energy is

known to be highly sensitive to the electron density localization

(45) Reichardt, C. Chem. ReV. 1994, 94, 2319.

(46) Citterio, D.; Kawada, T.; Yagi, J.; Ishigaki, T.; Hisamoto, H.; Sasaki, S.;

Suzuki, K. Anal. Chim. Acta 2003, 482, 19.

(47) SolVents and SolVent Effects in Organic Chemistry 2; Reichardt, C., Ed.;

VCH: Weinheim, 1990.

(50) Yates, P.; Ardao, M. I.; Fieser, L. F. J. Am. Chem. Soc. 1950, 78, 650.

(51) Casado, J.; Pappenfus, T. M.; Mann, K. R.; Milia´n, B.; Ort´ı, E.; Viruela,

P. M.; Ruiz Delgado, M. C.; Herna´ndez, V.; Lo´pez Navarrete, J. T. J. Mol.

Struct. 2003, 651-653, 665.

(48) Chen, C.-T.; Marder, S. R. AdV. Mater. 1995, 7, 1030.

(49) Kotorlenko, L. A.; Aleksandrova, V. S.; Yankovich, V. N. J. Mol. Struct.

1984, 115, 501.

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3277

A R T I C L E S

Kang et al.

Figure 5. (A) IR spectra of chromophores TM-1 and TM-2 as KBr pellets.

(B) IR spectra of TMC-2 and its neutral precursor 15 in a KBr pellet and

CH₂Cl₂solution. (C) IR spectra of TMC-3 and its neutral precursor 19 in

a KBr pellet and CH₂Cl₂solution.

Figure 6. (A) ¹³C NMR spectra of chromophore TMC-2 in the solid state

(top) and DMSO-d₆solution (bottom). (B) ¹³C NMR spectra of chromophore

TMC-3 in the solid state (top) and DMSO-d₆solution (bottom).

onto the CtN fragment,⁵²e.g., converting phenylmalononitrile

into the corresponding carbanion is accompanied by a strong

decrease of the CN stretching frequency (for phenylmalononitrile

ν(CtN) ) 2254 cm^-1versus ν(CtN) ) 2163, 2117 cm^-1in

the carbanion), dramatic increases in the corresponding inte-

grated intensities (∼136-fold), and strong enhancement of the

oscillator-oscillator ν(CtN) vibrational coupling (ν(CtN)

splitting ) 46 cm^-1).^52aThus, the electron density localized on

the dicyanomethanide fragments of TMC-2 and TMC-3 is

qualitatively similar to that of the phenylmalononitrile carban-

ion,^52awhere the dicyanomethanide unit supports nearly one

negative charge. Furthermore, note that both the TMC-2 and

TMC-3 IR spectra recorded in the solid state (KBr pellet) and

solution (CH₂Cl₂) are virtually identical, indicating very similar

electron density distributions, hence similar molecular structural

characteristics, in the solid state and solution. One interesting

observation is that neutral precursors 15 and 19 exhibit similar

spectral features in the solid state (KBr pellet) which are very

similar to those of the corresponding zwitterions (Figure 5B

and 5C), implying a possible proton transfer between the strong

acidic dicyanomethylene group and pyridine group. It was not

possible to obtain diffraction-quality crystals of precursors 15

and 19.

Solid-State vs Solution NMR Spectroscopy. Solid-state

CPMAS ¹³C NMR and solution-phase ¹³C NMR spectra of

chromophores TMC-2 and TMC-3 are compared in Figure 6.

Comparison of solid-state NMR spectra with those in solution

provides additional insights into chromophore molecular struc-

ture and electronic charge distribution in the solid state vs

solution since any significant change in twist angle should

drastically shift the distribution between zwitterionic and

quinoidal electronic structures and hence the chemical shifts of

the nuclei proximate to the π-electron core. A change of one

electron in π-electron density for a π-conjugated system is

expected to result in a total change of ∼160 ppm in carbon

atom chemical shift.⁵³As can be seen in the NMR spectra, the

TMC-2 resonance positions (Figure 6A) are virtually identical

in the solid state and DMSO-d₆. The signals adjacent to the

positively charged pyridinium center in the solid-state NMR

spectra, δ (N-CH₂, 67.2 ppm) and δ (pyr, 160.3 ppm), are

displaced only 3.2 and 1.1 ppm upfield, respectively. Further-

more, TMC-3 ¹³C NMR spectra in the solid state and DMSO-

d₆are nearly identical (Figure 6B) with only a minor change of

0.3 ppm in δ (N-CH₂, 64.5 ppm) and 0.7 ppm in δ (pyr, 158.2

ppm). These results exclude any significant changes in molecular

twist angles on going from the condensed state to the solution

phase, consistent with the observations in the optical and IR

spectra discussed above.

(52) (a) Binev, Y. I.; Georgieva, M. K.; Novkova, S. I. Spectrochim. Acta. 2003,

59A, 3041. (b) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063.

(53) Bradamante, S.; Pagani, G. A. J. Chem. Soc., Perkin Trans. 2 1986, 1035.

9

3278 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

Figure 7. (A) Concentration-dependent TMC-2 optical spectra in CHCl₃solution. Arrows indicate changes in CT bands upon dilution from 5.5 × 10^-4to

4.5 × 10^-6M. (Inset) Concentration-dependent TMC-2 optical spectra in CH₂Cl₂solution (3.0 × 10^-3-2.0 × 10^-5M). The monomer (dashed line) and

dimer (dotted line) spectra were derived from data at two different concentrations and K_dimerizeaccording to eq 12. (B) A typical nonlinear regression analysis

of the TMC-2 apparent extinction coefficient ꢀ as a function of concentration c₀at 490 nm in CHCl₃solution according to eq 12. The solid line is the best

fit to the data. (C) Variable-concentration fluorescence spectra (λ_ex) 350 nm) of TMC-2 in CH₂Cl₂. (D) Variable-concentration fluorescence spectra

(λ_ex) 300-350 nm) of TMC-3 in CH₂Cl₂. Intensities are normalized for clarity.

Studies of TMC Chromophore Aggregation in Solution

by Optical Absorption and Fluorescence Spectroscopies. The

aggregation state of chromophore TMC-2 in solution was

studied by variable-concentration optical spectroscopic meth-

ods.⁴³Significant TMC-2 spectral changes in moderately polar

CHCl₃(ꢀ_r) 4.81) solutions can be observed upon variation of

dimer, respectively. Combining eqs 10 and 11 yields an

expression for the apparent extinction coefficient ꢀ as

8K c + 1 - 1

x

d

0

ꢀ )

(ꢀ_M- ꢀ_D) + ꢀ_D

(12)

4K_dc₀

the concentration over the range 5.5 × 10^-4-4.5 × 10^-6

M

Nonlinear regression analysis (Figure 7B) of ꢀ as a function of

solution concentration at a specific wavelength based on eq 12

yields K_d) 13 300 ( 1420 M^-1(average of the nonlinear

regression analysis data at six different wavelengths), and the

related Gibbs dimerization energy ∆G°_d) -23.5 ( 0.3 kJ

mol^-1can be derived from eq 13

(Figure 7A). The spectra at highest dilution can be ascribed to

the monomeric chromophore with a CT band centered around

621 nm. Upon increasing the concentration, the spectra exhibit

diminution of this CT band and concomitant appearance/growth

of a new transition at shorter wavelength, revealing the onset

of aggregation. A well-defined isosbestic point clearly is

consistent with equilibrium between monomer (M) and dimer

(D). Pragmatically employing the simplest dimerization model

for the moment (2M h D), the dimerization constant K_dcan

be defined as

∆G°_d) - RT ln K_d

(13)

The respective monomer and dimer spectra can be then

calculated from the derived K_dand the absorption data at two

different concentrations according to eq 12 (Figure 7A).

Concentration-dependent TMC-2 optical absorption studies in

more polar CH₂Cl₂(Figure 7A, inset) indicate similar aggrega-

tion effects but at higher concentration ranges (10^-4-10^-3M).

Nonlinear regression analysis yields smaller K_dand ∆G⁰_dvalues

of 246 ( 30 M^-1and -13.6 ( 0.3 kJ mol^-1, respectively.

The TMC-2 aggregation in CH₂Cl₂is also evidenced from

variable-concentration fluorescence spectra despite the low

emission intensity. Significant TMC-2 spectral changes can be

observed upon variation of the concentration over the range

1 × 10^-3-1 × 10^-5M (Figure 7C) with a transition from λ_em

) 462 to 522 nm, in good agreement with the concentration

variation range in optical absorption spectral data. TMC-3

c_D

1 - R

2R²c₀

K_d)

)

(10)

2

c_M

where c_Dand c_Mare the equilibrium concentrations of dimer

and monomer, respectively, c₀is the initial concentration, and

R is the fraction of the monomer in solution, defined as R )

c_M/c₀. The apparent extinction coefficient ꢀ can then be

expressed as

ꢀ ) ꢀ_MR + ꢀ_D(1 - R)

(11)

where ꢀ_Mand ꢀ_Dare extinction coefficients of monomer and

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3279

A R T I C L E S

Kang et al.

Table 4. Diffusion Coefficient (D_t, 10^-10m²s^-1), Hydrodynamic

Radius (r_H, Å), Hydrodynamic Volume (V_H, Å³), and Aggregation

Number (N ) V_H/V_VdW^a) for TMC-2 and TMC-3 as a Function of

Concentration c

entry

solvent

c (mM)

D_t

r_H

V_H

N

TMC-2

1

2

3

4

5

6

7

8

9

CD₂Cl₂

DMSO-d₆

0.004

0.07

0.5

10.0(6)

4.60

4.96

5.11

5.43

5.95

6.33

4.64

4.74

4.72

408

512

558

670

881

1064

417

448

440

1.0(4)

1.3(1)

1.4(3)

1.7(1)

2.2(5)

2.7(3)

1.1(3)

1.1(4)

1.0(7)

9.3(2)

9.0(5)

8.5(2)

7.7(7)

7.3(0)

1.9(9)

1.9(4)

1.9(5)

1.6

4.0

8.4^b

0.05

0.34

4.83

TMC-3

10

11

12

13

14

15

16

17

CD₂Cl₂

0.005

0.01

0.083

0.25

0.49^b

0.08

1.34

3.4

7.9(7)

7.8(7)

7.4(6)

7.0(7)

6.4(2)

1.6(8)

1.6(9)

1.6(6)

5.32

5.39

5.68

5.99

6.60

5.02

4.99

5.07

630

654

769

903

1205

529

522

547

1.2(9)

1.3(5)

1.5(8)

1.8(6)

2.4(8)

1.1(2)

1.0(7)

1.0(9)

CD₂Cl₂

DMSO-d₆

Figure 8. Comparative PGSE NMR-derived aggregation number (N, left

scale) data and EFISH-derived µâ data (right scale) as a function of

concentration for twisted chromophores TMC-2 (A) and TMC-3 (B). PGSE

NMR data are obtained from CD₂Cl₂(2) and DMSO-d₆(1) solutions.

EFISH measurements are performed in CH₂Cl₂(9) and DMF (b) solutions.

Lines are drawn as guides to the eye. c ) concentration.

^avan der Waals volumes: TMC-2, 389 Å³(from X-ray data); TMC-3,

486 Å,³computed from X-ray data of TMC-2 adding the volume of a styryl

moiety (97 Å³)]. ^bSaturated solution^.

aggregation is more difficult to observe in optical absorption

spectroscopy due to overlapped CT transitions (Figure 3B).

Alternatively, variable-concentration TMC-3 fluorescence spec-

tra in CH₂Cl₂(Figure 7D) reveal a clear transition from dimer

(515 nm) to monomer (485 nm) emission upon dilution in the

range of 5 × 10^-4-2.5 × 10^-6M. Although quantitative

aggregation information is difficult to estimate in fluorescence

spectra due to the quenching of fluorescence in aggregates, these

results provide further evidence of TMC aggregation.

10). For TMC-3, aggregates somewhat larger than dimers are

determined to be present at the highest concentrations (N )

2.4, Table 4, entry 14).

Hyperpolarizability Measured by EFISH Spectroscopy.

EFISH-derived µâ values for chromophores TM-2, TMC-2,

and TMC-3 in CH₂Cl₂and DMF are summarized in Table S3.

The variation of the µâ values for TMC-2 and TMC-3 as a

function of the concentration is shown in Figure 8 in comparison

with PGSE-derived aggregation numbers. In less polar CH₂Cl₂

the µâ values exhibit a pronounced concentration dependence.

The TMC-2 µâ rapidly increases as the concentration falls in

the range of 10^-3-10^-5M, with µâ saturating at a large value

of -24 000 ( 4320 × 10^-48esu at 5 × 10^-6M (Figure 8A).

Significant deaggregation of TMC-3 in CH₂Cl₂occurs at lower

concentrations, in the range 10^-5-10^-6M (Figure 8B), with

an unprecedented µâ value of -488 000 ( 48 800 × 10^-48esu

measured at 8 × 10^-7M. The TM-2 µâ exhibits similar

concentration dependence in CH₂Cl₂, with a µâ value of

PGSE NMR Measurements on TMC Chromophores.

PGSE (pulsed field gradient spin-echo) NMR techniques⁵⁴

represent an incisive tool to quantify molecular dimensions in

solution and, consequently, levels of aggregation.²⁶Experimen-

tally determined translational self-diffusion coefficients (D_t),

hydrodynamic radii (r_H), volumes (V_H), and the ratio between

V_Hand the van der Waals volume (V_vdW) afford aggregation

numbers N (N ) V_H/V_vdW) readily indicating aggregation levels

in solution (N ) 1.0, 1.5, or 2.0 indicates 100% monomer, 50%

monomer + 50% dimer, or 100% of dimer, respectively).^26a

Table 4 summarizes the results of the PGSE measurements

carried out on chromophores TMC-2 and TMC-3 in both CD₂-

Cl₂and DMSO-d₆over a broad range of concentrations. The

variation in the aggregation number N as a function of the

concentration is shown in Figure 8. The trends in N vs

concentration (Table 4, Figure 8) clearly show that monomer

predominates in very polar DMSO-d₆(ꢀ_r²⁵) 46.45) for both

TMC-2 and TMC-3 over the entire concentration range (Table

4, entries 7-9 and 15-17). In CD₂Cl₂(ꢀ_r²⁵) 8.93), TMC-2 is

exclusively monomeric only at the lowest concentrations

examined (4 × 10^-6M, Table 4, entry 1) while dimers (Table

4, entries 4-5) and even somewhat larger aggregates (N ) 2.7,

Table 4, entry 6) are present at the highest concentrations (Figure

8). TMC-3 exhibits a greater tendency for aggregation and is

not exclusively monomeric even at 5 × 10^-6M (Table 4, entry

-315 000 ( 56 700 × 10^-48esu measured at 1 × 10^-6

M

(Table S3). From the computed µ values⁴⁴we estimate â_0.65eV

≈ 8450, 890, and 9800 × 10^-30esu for TM-2, TMC-2, and

TMC-3, respectively.

TMC µâ values measured in highly polar DMF are consider-

ably less sensitive to concentration. The µâ of TMC-2 (Figure

8A) begins to saturate at ∼5 × 10^-5M, with µâ ) -5620 (

618 × 10^-48esu at 5 × 10^-6M, while the µâ values of TMC-3

(Figure 8B) still exhibit a gentle increase up to the highest

dilution, with µâ ) -84 000 ( 10 080 × 10^-48esu measured

at 1.1 × 10^-6M. Similarly, the TM-2 µâ in DMF exhibits

saturation in dilution, with µâ ) -49 000 ( 9800 × 10^-48esu

measured at 1 × 10^-5M.

Electro-Optic Measurements on TMC-Based Poled Guest-

Host Polymers. Poly(vinylphenol) films containing 10 wt %

TMC-2 and 5 wt % TMC-3, poled at 100 V/µm, give

nonresonant r₃₃values of 48 and 330 pm/V, respectively, as

(54) (a) Johnson, C. S., Jr. Prog. Nucl. Magn. Reson. Spectrosc. 1999, 34, 203.

(b) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1.

9

3280 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

electrons, a second excited state appears fairly close in energy

to the first excited state over a wide range of twist angles. The

molecular orbitals which comprise the active space are shown

in Figure 10. The SA3-CASSCF(4,3) potential-energy surfaces

of the three lowest states of TM-1′ are shown in Figure 11. It

can be seen how these states change upon application of a

dipolar field, which is intended to simulate the effects of a polar

solvent medium. At a twist angle of 90°, the nonlinear response

(â) was computed as the finite-second derivative of the dipole

moment with respect to an additional applied field.

Discussion

Tictoid Chromophore Synthetic Strategies and Stability

Characteristics. The formation of highly encumbered substitu-

tion patterns via the coupling of two arenes possessing bulky

ortho substituents, specifically the synthesis of the tetra-ortho-

methylbiaryl cores of the TM and TMC chromophore skeletons,

presents a nontrivial synthetic challenge. We chose Suzuki

methodology since it has previously demonstrated excellent

tolerance to steric constraints among the catalytic cross-coupling

reactions useful for unsymmetrical biaryl synthesis.^18,55Employ-

ing a highly active Pd(0)/DCPPP Suzuki catalyst,¹⁸4-bromo-

pyridine N-oxide 1 can be coupled with boronic acid 2 to afford

the key tetra-ortho-methylphenylpyridine core 3 in 53% yield

(Scheme 1). Use of the pyridine N-oxide precursor here is crucial

since the corresponding pyridine species evidence a pronounced

sluggishness in the present coupling process for reasons possibly

involving coordinative inhibition at the Pd center.¹⁶Interestingly,

the N-oxide functionality does not appear to induce detrimental

oxidation of the Pd(0) form of the catalyst here.

Figure 9. In situ poling and Teng-Man direct EO measurements on a

polyvinylphenol (PVP) film containing 5 wt % TMC-3, poled at 100 V/µm.

Another synthetic challenge involved in the TMC syntheses

is the synthesis of twisted biaryl iodide 11. This is crucial not

only for facilitating the Heck coupling process in the TM-2

synthesis, but also for enabling dicyanomethanide group intro-

duction into the TMC skeletons, a transformation which cannot

be efficiently achieved via nucleophilic triflate substitution⁵⁶at

8. Converting phenol 6 into aryl halides, which can subsequently

undergo cross-coupling with active methylene regents such as

malononitrile in the presence of Pd catalysts,⁵⁷is thereby crucial

to the TMC synthetic strategy. The only examples in the

literature for the effective direct conversion of phenols to aryl

halides, such as the thermolysis of a phenol-triphenylphosphine

dibromide adduct⁵⁸and displacement of triflate by iodide or

bromide,⁵⁹were unsuccessful in the present case. We therefore

devised an effective new strategy for converting phenol 6 to

iodide 11 by combining Pd-catalyzed aryl triflate amination⁶⁰

with arylamine-to-aryl halide conversion⁶¹(Scheme 1). The

Figure 10. Active space orbitals of model chromophore TM-1′. The

symmetry labels are shown for the C_2Vextrema (θ ) 0° and 90°) as well

as for the intermediate (C₂symmetry) twist angles.

determined by Teng-Man electro-optic measurements at 1310

nm. Figure 9 shows a typical profile for in situ poling and

Teng-Man measurement on 5 wt % TMC-3/PVP guest-host

films. Under the applied DC electric poling field, the effective

r₃₃undergoes a rapid increase when the temperature rises to

∼140 °C (the DSC-derived T_gof 5 wt % TMC-3/PVP is

∼148 °C) and reaches a saturation value of 330 pm/V when

temperature is held at 140 °C. Further temperature increases

result in a slow decrease of r₃₃, and when heating is removed,

the r₃₃quickly drops to the noise level. This poling behavior

presumably reflects the strong aggregation tendency of these

zwitterions, which is also observed in less polar matrices, e.g.,

amorphous polycarbonate (APC, ꢀ_r) 3 vs 4.5 for PVP), where

only very low EO responses are observed.

Computational Results. Calculations aimed at determining

the lowest energy states yield orbital and state correlation

diagrams for TM-1′ as a function of torsional coordinate (Figure

10). The states chosen here are the lowest three states, two of

which certainly correspond to the well-known inter-ring charge

transfer in the twisted chromophores. In addition to identifying

the lowest two states (Ψ_Dand Ψ_Z), the second excited state

was also investigated to determine whether higher energy states

could contribute strongly to the observed nonlinear response

properties. During the investigation of the vertical excitation

spectrum of TM-1′ it was found that when the CAS active space

is expanded from the SS-CASSCF(2,2) to a SA3-CASSCF(4,3)

active space, hence including the in-plane oxygen p AO and its

(55) (a) Suzuki, A. Proc. Jpn. Acad. Ser. B 2004, 80, 359. (b) Bellina, F.; Carpita,

A.; Rossi, R. Synthesis 2004, 15, 2419. (c) Kotha, S.; Lahiri, K.; Kashinath,

D. Tetrahedron 2002, 58, 9633. (d) Johnson, M. G.; Foglesong, R. J.

Tetrahedron Lett. 1997, 38, 7001. (e) Miyaura, N.; Suzuki, A. Chem. ReV.

1995, 95, 2457.

(56) (a) Atkinson, J. G.; Wasson, B. K.; Fuentes, J. J.; Girard, Y.; Rooney,

C. S.; Engelhardt, E. L. Tetrahedron Lett. 1979, 20, 2857. (b) Williams,

H. W. R.; Rooney, C. S.; Bicking, J. B.; Robb, C. M.; De Solms, S. J.;

Woltersdorf, O. W.; Cragoe, E. J. J. Org. Chem. 1979, 44, 4060.

(57) (a) Uno, M.; Seto, K.; Takahashi, S. J. Chem. Soc., Chem. Commun. 1984,

932. (b) Gao, C.; Tao, X.; Qian, Y.; Huang, J. Chem. Commun. 2003, 1444.

(58) Wiley, G. A.; Hershkowitz, R. L.; Rein, B. M.; Chung, B. C. J. Am. Chem.

Soc. 1964, 86, 964.

(59) Prugh, J. D.; Alberts, A. W.; Deana, A. A.; Gilfillian, J. L.; Huff, J. W.;

Smith, R. L.; Wiggins, J. M. J. Med. Chem. 1990, 33, 758.

(60) (a) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 1997, 62, 1264. (b) Louie,

J.; Driver, M. S.; Hamann, B. C.; Hartwig, J. F. 1997, 62, 1268.

(61) Kosynkin, D.; Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1997,

119, 4846.

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3281

A R T I C L E S

Kang et al.

Figure 11. Potential-energy surfaces (PESs) for the lowest three singlet states of TM-1′ as a function of angle and applied field (A-C). The field is applied

in the longitudinal direction. The black curve corresponds to a state which has a diradical wave function at θ ) 90°, the red curve corresponds to a state

which is a zwitterion at θ ) 90°, and the blue curve represents the PES for the state formed from an n f π* transition.

dimethylphenol is 10.2).⁶²Such a charge-localized structure may

also account for the low thermal stability observed in TM-1,

where the pyridinium N-methyl substituent undergoes alkylative

migration to the aryl O^-group to form neutral 4-(4-methoxy-

2,6-dimethyl-phenyl)-3,5-dimethyl-pyridine (4, Scheme 1) when

heated to 200 °C in vacuum, reflecting the strong nucleophilicity

of the aryl O^-group. TM-2 exhibits slightly enhanced stability

compared with TM-1, probably due to the steric protection of

the two bulky ortho tert-butyl substituents adjacent to the aryl

O^-group. In addition to moisture sensitivity, both TM

chromophores have only modest solubilities in most organic

solvents, even in the case of N-octyl-functionalized TM-2.

In marked contrast to the TM chromophores, the TMC

chromophores are air and moisture stable and readily purifiable

by conventional column chromatography. Thermogravimetric

analysis indicates that these materials have very high thermal

stability (T_d≈ 306 °C for TMC-2 and 330 °C for TMC-3,

1

Figure S2), while H NMR spectroscopy indicates stability in

DMSO-d₆solution at 150 °C for periods of hours under air.

This marked chemical and thermal robustness doubtless results

from introduction of the charge-stabilizing dicyanomethanide

Figure 12. ¹H NMR spectra (400 MHz, 25 °C) of precursor 7 (A), TM-1

(B) in dry DMSO-d₆, and TM-1 in DMSO-d₆solution exposed in air during

the period of 1 (C) and 5 (D) days.

overall yield of this four-step phenol-to-aryl iodide conversion

is 65% and therefore represents an efficient general route for

phenol-to-aryl halide conversion.

functionality, where negative charge is delocalized by two strong

electron-withdrawing cyano groups through resonant delocal-

ization (also observable in the crystallographic data, vide supra).

Here the dicyanomethanide portion is expected to be a weak

Lewis base (the pK_aof related phenylmalononitrile is 4.2).⁶³

For donor-acceptor EO chromophores, the TMC series is a

noteworthy example of introducing a stabilized anion into a

twisted π-electron system zwitterion, where significant charge

stabilization through a quinoidal limiting resonance structure

is unlikely.

The product TM and TMC chromophores exhibit very

different chemical and thermal stability characteristics. TM

chromophores exhibit considerable sensitivity to moisture. Dark

red TM-1 solid quickly bleaches when exposed to moisture,

implying that protonation may take place at the aryl O^-group,

evidenced by ¹H NMR spectroscopy in DMSO-d₆(Figure 12).

The signals from phenylene ring and methyl protons adjacent

to the aryl O^-group (H_aand H_b, Figure 12) are displaced

significantly downfield when the NMR solution is exposed to

moisture. This marked proton affinity is consistent with a twist-

induced reduction in inter-ring π conjugation, thereby resulting

in a dominant charge-separated zwitterionic ground state,

describable as a noncommunicating phenoxide anion adjacent

to a pyridinium cation. Here the phenoxide portion is expected

to be a strong Lewis base (the pK_aof the related 3,5-

Tictoid Chromophore Structural Characteristics. The

combination of single-crystal XRD data with optical, IR, and

NMR spectroscopies, both in the solid state and in solution,

provides fundamental architectural and electronic structural

(62) Handbook of Organic Chemistry; Dean, J., Ed.; McGraw-Hill: New York,

1987.

(63) Algrim, D.; Bares, J. E.; Branca, J. C.; Bordwell, F. G. J. Org. Chem.1978,

43, 5024.

9

3282 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

insight into these new tictoid chromophores. The most important

feature revealed from the crystallographic analyses of the target

chromophores as well as of their synthetic precursors is the

consistently large and relatively uniform arene-arene dihedral

twist angle (80-89°; Table 1), suggesting that the tetra-ortho-

methylbiaryl substitution pattern indeed provides sufficient steric

encumbrance to achieve the desired inter-ring staggering, an

anticipated¹⁵prerequisite for large molecular hyperpolarizabili-

ties in such twisted π-electron system chromophores. Further-

more, it can be seen that the magnitude of this twist is governed

primarily by sterics and essentially independent of chromophore

architecture and charge distribution. Indeed, neutral, positively

charged, and zwitterionic molecules all exhibit comparable twist

angles (Table 1). A number of earlier studies have shown that

the electronic structure of merocyanines can be described as

some hybrid of the quinoid and benzenoid limiting structures

and that any external perturbation, such as solvation or an

external electric field, can lead to stabilization of the charge-

separated benzenoid structure.^4dThe electronic structures of

these chromophores are therefore highly dependent on the state

of the materials and medium surrounding the molecules.

vibrational spectroscopic benzenoid phenoxide ring stretching

features and the strong reduction of the quinoid CdO stretching

frequency in the TM chromophores and the almost pure

phenyldicynomethnide anion-like CtN stretching frequency in

the TMC chromophores clearly support a dominant zwitterionic

structure in the ground state. Comparative solid-state ¹³C NMR,

optical, and IR spectroscopic studies provide additional structure-

property information regarding solution phase vs solid phase

TM and TMC properties. Close correspondence between solid-

state ¹³C NMR, optical, and IR spectra and those recorded in

solution suggest that the large inter-ring dihedral twist angle

and zwitterionic ground state observed in the solid state persist

essentially unchanged in the solution phase and that it is

reasonable to interpret TMC solution-phase properties (such

as linear/nonlinear optical response) in terms of solid-state

crystal structure metrical parameters (in particular the twist

angle).

Aggregation Properties. The aggregation of dipolar NLO

chromophores has been identified as an important issue in

understanding basic molecular EO response as well as in

applications.^43,64In the case of merocyanine dyes, the cen-

trosymmetric nature of the dye aggregation is clearly anticipated

by the nature of possible electrostatic dipole-dipole interactions

and was recently verified experimentally.⁴³

This type of aggregation is obviously detrimental to applica-

tions in electro-optics where a microstructurally polar chro-

mophore arrangement is essential. It is only by removing the

impediment of aggregation that the exceptional molecular

hyperpolarizability properties in the present tictoid chro-

mophores can be accessed and fully exploited. Thus, a full

investigation and understanding of the nature of tictoid chro-

mophore aggregation is important. As might be expected, the

new tictoid chromophores described here exhibit significant

aggregation tendencies in the solid state and concentrated

solutions, evidenced from X-ray diffraction data and concentra-

tion-dependent optical absorption, fluorescence, and PGSE NMR

spectroscopic studies. These results are entirely reasonable

considering the large computed dipole moments.⁴⁴

Analysis of the tictoid zwitterion packing in the crystalline

state clearly shows the formation of centrosymmetric antiparallel

dimers. Although the substituent-modulated intermolecular

distances between tictoid molecules in the dimeric units are

significantly larger than those observed in planar merocyanine

zwitterion dimers (∼ 3.50 Å),⁴³the very strong dipole moments

in the highly charge-separated tictoid molecules still result in

comparable or even larger electrostatic interactions (this can

be readily seen from estimated binding energies compared with

those of planar zwitterions, vide infra). These intermolecular

distances depend principally on steric details. It can be seen in

In the present tictoid chromophores, the steric encumbrance-

induced twist leads to a pronounced enforced reduction in inter-

ring π conjugation, and this in turn leads to aromatic stabilization

of the resultant pyridinium and phenoxide/phenyldicynomethanide

fragments, resulting in a chemically, thermally robust, dominant

charge-separated zwitterionic ground state. Molecular electronic

structure should be predominantly governed by the twist and

affected by external perturbations such as dielectric constant.

This solid-state zwitterionic structural assignment is supported

by a full complement of single-crystal metrical parameters, such

as the (ring)CsC(ring), (ring)CsO, and (dicyanomethanide)Cs

C(aryl) distances, and the bond length patterns within the

pyridinium and phenoxide/phenyldicynomethanide fragments.

Importantly, NOE NMR measurements of the TMC twist

angle in solution confirm that solid-state twist angle persists in

solution. Optical spectroscopies provide further evidence for a

zwitterionic tictoid ground state in the solution phase. The tictoid

spectra exhibit both inter-ring HOMO-LUMO charge-transfer

(CT) excitations as well as intra-subfragment transitions within

the pyridinium and phenyl (stilbenyl in TM-2 and TMC-3)

fragments. This implies that the tictoid ground state is better

described in terms of linked pyridinium acceptor and phenoxide/

phenyldicyanomethanide donor portions. The relatively small

oscillator strengths in the CT band appear to reflect the

pronounced reduction in inter-ring π conjugation and are

consistent with large inter-ring dihedral angles observed in the

X-ray diffraction studies. Furthermore, the strong negatiVe

solvatochromism effects observed in the TMC optical spectra

indicate that the ground-state dipole moment is substantially

larger than in the excited state, consistent with a dominant

zwitterionic ground-state description.

The twist chromophore IR vibrational spectroscopic features

provide structural information consistent with that obtained from

the XRD analyses and optical spectroscopic studies. The typical

(64) (a) Cross, G. H.; Hackman, N.-A.; Thomas, P. R.; Szablewski, M.; Palsson,

L.-O.; Bloor, D. Opt. Mater. 2002, 21, 29. (b) Wu¨thner, F.; Yao, S. Angew.

Chem., Int. Ed. 2000, 39, 1978. (c) Dalton, L. R.; Harper, A. W.; Robinson,

B. H. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 4842.

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3283

A R T I C L E S

Kang et al.

Figure 1 that the TMC-2 dimeric units have larger intermo-

lecular distances separating monomers than in TMC-1 dimeric

units, clearly the result of accommodating the bulky N-alkyl

chain. The relatively looser packing in TMC-2 vs TMC-1 can

also be seen in the experimental crystal densities (1.110 g/cm³

for TMC-2 vs 1.184 g/cm³for TMC-1), which is also consistent

with the reduced dimerization energy of TMC-2 in solution

compared with TMC-1 (vide supra).

aggregation levels than TMC-2. At the highest accessible

concentrations, TMC-2 and TMC-3 exhibit aggregation num-

bers of N ) 2.7 and 2.5, respectively, implying that more

complex aggregates than dimers are also present, consistent with

the tetramer-like aggregates observed in TMC crystal structure

data (Figure 1C). The aggregation numbers decrease upon

dilution, tending toward N ) 1.0 and revealing dissociation of

aggregates in dilute solutions in the 10^-3-10^-4M concentration

range for TMC-2, in good agreement with the optical spectro-

scopic data (Figure 7A, inset). TMC-3 exhibits greater ag-

gregation tendencies and is not exclusively monomeric even at

5 × 10^-6M, consistent with the results of the variable-

concentration TMC-3 fluorescence spectroscopy (Figure 7B).

The aggregation numbers for both TMC-2 and TMC-3 in more

polar DMSO-d₆(ꢀ_r²⁵) 46.45) are close to 1.0 and almost

independent of the concentration, clearly the result of the

aforementioned strong polar solvation effects on aggregation.

The strong aggregation tendency of TMC-2 is further

supported by concentration-dependent optical absorption spectra,

exhibiting a pronounced blue shift and decrease of the CT

excitation intensity upon increasing the concentration. Moreover,

an isosbestic point is observed, supporting well-defined ag-

gregation equilibria and indicating the formation of H-type

antiparallel centrosymmetric aggregates,⁴³in agreement with the

X-ray diffraction data. The binding energetics, estimated by

analyzing the TMC-2 extinction coefficient as a function of

concentration assuming a simplistic (but tractable) dimerization

model, not unexpectedly indicate that the aggregation is weaker

in more polar solvents, with derived binding constant K_d)

250 ( 30 M^-1and Gibbs free energy ∆G°_d) -13.6 ( 0.3 kJ

mol^-1in CH₂Cl₂(ꢀ_r²⁵) 8.93) vs K_d) 13300 ( 1420 M^-1and

Nonlinear Optical and Electro-Optic Properties. EFISH-

derived µâ values for chromophores TMC-2 and TMC-3 in

CH₂Cl₂and DMF exhibit negative signs, indicating that the

ground state is more polar than the excited state,^4d,10in

agreement with the observed negative solvatochromism (vide

supra). In less polar CH₂Cl₂(Figure 8), the µâ values exhibit a

pronounced concentration dependence due to the aforementioned

aggregation effects of a type previously reported for zwitterionic

TCNQ derivatives.^64aThe comparison between EFISH and

PGSE data over a wide range of concentrations shows good

agreement with the trends in aggregation (Figure 8). The TMC-2

µâ in CH₂Cl₂rapidly increases as concentration falls, implying

dissociation of (presumably centrosymmetric) aggregates. In-

deed, the µâ approaches a maximum at concentrations where

the PGSE measurements indicate the presence of greater than

80% monomer (Figure 8A). Again, TMC-3 deaggregation

trends in the EFISH data (Figure 8B) closely parallel those for

the PGSE data, with µâ rapidly increasing at concentration levels

where the monomer content is ∼80% (Figure 8B) and saturating

at an unprecedented µâ value of -488 000 ( 48 800 × 10^-48

esu at 1907 nm. The chromophore figure of merit, defined by

µâ/M_w, is as large as 945 × 10^-48esu, almost 20 times larger

than the highest value of 46 × 10^-48esu previously reported.^3d

The off-resonance â_0.65eVestimated from the computed µ

values⁴⁴is as large as 9800 × 10^-30esu. In recent elegant

work,¹¹Kuzyk estimated the fundamental limits of hyperpo-

larizability using quantum sum rules and found that the apparent

limit for all the experimentally achieved hyperpolarizabilities

in organic molecules falls far short of the fundamental limit by

a factor of 10^-3/2. For example, a chromophore having 22 π

electrons and an absorption maximum of 540 nm, as in tictoid

chromophore TMC-3, could possess a maximum nonresonant

â as large as 20 000 × 10^-30esu according to Kuzyk’s estimates.

However, all âs achieved previously fall below an apparent limit

of ∼600 × 10^-30esu. For the first time, the âs achieved in the

present tictoid chromophores approach the fundamental limit,

clearly suggesting a new paradigm for organic electro-optics.

The present results may provide insights into the reasons for

this universal gap between the experimental â results and the

fundamental limits.

∆G°_d) -23.5 ( 0.3 kJ mol^-1in less polar CHCl₃(ꢀ_r²⁵

)

4.81). The aggregation free energy is expected to be highly

dependent on solvent polarity when the electrostatic interactions

between two interacting monomer dipole moments dominate

the binding forces.⁴⁷These K_dand ∆G°_dvalues indicate stronger

aggregation in TMC-2 than in typical planar merocyanine

zwitterions in the same solvents (e.g., the dye HPOP has a

binding constant K_dof 520 M^-1and Gibbs free energy ∆G°_d

of -15.2 kJ mol^-1in CHCl₃),⁴³doubtless caused by the larger

TMC molecular dipole moments,⁴⁴clearly a consequence of

the highly charge-separated ground states. Variable-concentra-

tion TMC-2 and TMC-3 fluorescence spectra in CH₂Cl₂provide

further evidence of TMC aggregation. The unexpected blue shift

of the TMC-2 fluorescence spectra for the H-type aggregates

could conceivably be due to unusually slow radiationless

relaxation between exciton-split states, resulting in TMC-2

fluorescence from the higher energy state.⁶⁵An alternative

explanation for the inverse trend observed in the TMC-2

fluorescence spectra invokes strong reabsorption.⁶⁶In contrast,

the TMC-3 dimers exhibit more conventional red-shifted

florescence. It can be seen that significantly higher concentra-

tions are necessary to observe aggregation effects in TMC-2

than for TMC-3 in the same solvent, clearly a result of a much

greater dipole moment in TMC-3 than in TMC-2.⁴⁴

PGSE NMR spectroscopy provides additional, quantitative

information on the state of TMC aggregation. Both TMC-2

and TMC-3 exhibit pronounced aggregation tendencies in CD₂-

Cl₂(ꢀ_r²⁵) 8.93), with TMC-3 exhibiting substantially greater

The µâ values measured in highly polar DMF (ꢀ_r²⁵) 37.8)

(65) Park, J.-M.; Zhang, W.; Nakatsuji, Y.; Majima, T.; Ikeda, I. Chem. Lett.

2000, 29, 182.

(66) Principles of Instrumental Analysis Chemistry, 7th ed.; Skoog, D. A., Leary,

J. L., Eds.; Saunders College Publishing: Philadelphia, 1996.

exhibit a less abrupt increase as concentration falls, a sign of a

weaker aggregation in more polar DMF than in CH₂Cl₂(ꢀ_r²⁵

)

9

3284 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Twisted

π

-Electron System Electro-Optic Chromophores

A R T I C L E S

8.93), consistent with the PGSE data. That the magnitude of

the EFISH-derived µâ is lower in more polar solvents has also

been reported for other zwitterionic chromophores and theoreti-

cally investigated^4d,67and is in agreement with the present

theoretical study. It was found previously that the first hyper-

polarizabilities of typical zwitterionic molecules exhibit remark-

able solvation effects.^4dâ values are small and positive in the

gas phase but become negative with increasing solvent polarity.

They remain negative in all polar solvents and reach a maximum

at a moderate ꢀ_r≈ 6-8 and then again decrease slowly with

further increase of ꢀ_r. This behavior is ascribed principally to

the change in charge distribution and dipole moment. Once the

structure is dominated by the zwitterionic limit, â values

decrease with increasing solvent polarity.^4d,67

whereas the zwitterion and n f π* state term symbols are both

¹A₁. When the ground-state wave function is a diradical (in the

gas phase), and the first two excited states, which would be

expected to contribute most strongly to the ground state for

energetic reasons, are dipole-disallowed transitions; hence, â

should be very small. When a dipolar field is applied, the

zwitterionic state shifts to become the ground state. Since it

has an allowed dipole transition with the n f π* state, this

state interaction is expected to contribute significantly to the â

of the ground-state zwitterion. Further increasing the electric

field begins to lower the energy of the zwitterionic state relative

to the excited-state manifold; hence, â begins to decrease with

increasing field (also observed here experimentally).

In evaluating the present EFISH results, external electric field

effects on reducing the chromophore aggregation must also be

taken into account, as shown in previous studies.^43,69This effect

may be expected to be significant if the species in aggregation

equilibrium strongly differ in dipole moments, as observed for

the monomeric and dimeric species in the present zwitterionic

chromophores. The competition between internal dipolar inter-

actions leading to aggregation and the external electric fields

leading to dissociation could conceivably shift the aggregation

equilibrium to some degree. Taking a simple dimerization

model, the new dimerization constant under an applied electric

The SA3-CASSCF(4,3) computations in this work provide a

new rationale for these exceptional hyperpolarizabilities and the

observed solvation effects on hyperpolarizabilities. It was

recently found⁶⁸that SS-CASSCF(2,2) gas-phase calculations

afford a zwitterionic ground state (Ψ_Z) for the fully methylated

chromophore TM-1 at a 90° twist angle. In agreement with this

result, in this work it is also found that SS-CASSCF(4,3) and

SA2-CASSCF(2,2) calculations lead to this seemingly unphysi-

cal result. Increasing the active space and adding the second

excited state (SA3-CASSCF(4,3)) affords a dominant diradical

ground state (Ψ_D) in the gas phase in agreement with the much

larger SS-CASSCF(14,13) calculations.⁶⁸The first excited state

is easily characterized as being the zwitterionic state (Ψ_Z), and

the second excited state corresponds to the product (Ψ _nfπ*) of

a n f π* transition. Figure 11 reveals that the potential-energy

surfaces of the lowest three states are very dependent upon the

strength of an external applied dipolar field (to simulate polar

solvation). In the gas phase (F ) 0.0 au) at θ ) 90°, the CI

coefficients suggest that the diradical state is the ground state.

However, applying a dipolar field strength of F ) 0.007 au to

simulate polar solvation leads to an exclusive zwitterionic

ground state. At an intermediate field strength of F ) 0.006

au, the energy differences between the two states at θ ) 90° is

negligible. As suggested by the sum-over-states (SOS) equation,

this small energy difference should lead to a very large static

â. At 90° twist angles, the SA3-CASSCF-computed static

hyperpolarizabilities (â) in the gas phase at field strengths of

0.006 and 0.007 au are 1.1 × 10¹, 5.7 × 10⁴, and 1.7 × 10⁴

(10^-30esu), respectively, in agreement with a previous SA-

CASSCF investigation⁶⁸where the computed â was found to

be very small in the gas phase. At higher field strengths, the â

values in the present study are found to be 3 orders of magnitude

larger than those in the gas phase and negative, in accord with

experiment. Further increasing the dipolar field strength from

0.006 to 0.007 au yields a significant decrease in hyperpolar-

izability, consistent with the remarkable solvation effects

observed for the experimental hyperpolarizabilities.

E

field, K_d, is given by eq 14⁴³

E

K_d^E) K_dexp(-∆G_d/RT)

(14)

E

where ∆G_dis the electric field-induced contribution to the

Gibbs free energy of the dimerization, which can be estimated,

assuming a vanishing dimer dipole moment, by eq 15⁴³

N_AL²µ_gE²

2

E

∆G_d

)

(15)

3kT

where N_Ais Avogadro’s number, L a Lorentz local field factor

given by L ) (ꢀ_r+ 2)/3,⁷⁰µ_gthe dipole moment of the

chromophore, and E the electric field. For chromophore TMC-2

in CH₂Cl₂with a DFT-derived ground-state dipole moment of

27.0 D, a Lorentz local field factor of L ) 3.64, and an estimated

dimerization constant K_d) 246 M^-1and applying an EFISH

field of 7 × 10⁶V/m, the field-dependent dimerization constant

K_dis then estimated to be 222 vs 246 M^-1in the absence of

E

the field. As can be seen, the present electric fields in the EFISH

measurement are insufficient to greatly affect the aggregation

equilibria.

Teng-Man³¹experiments on poled TMC-2- and TMC-3-

based guest-host polyvinylphenol (PVP, ꢀ_r) 4.5) films reveal

very large EO coefficients (r₃₃) at 1310 nm, confirming the

exceptional hyperpolarizabilities in these chromophores. The

exact poling behavior of TMC-3/PVP guest-host films (Figure

9) presumably reflects the strong aggregation tendencies of these

zwitterions as discussed above, which is also observed in less

polar matrices, e.g., amorphous polycarbonate (APC, ꢀ_r) 3),

where only very weak EO responses are observed. Chromophore

dipolar aggregation is clearly detrimental to the performance

of EO materials. The aforementioned aggregation studies on

TMC chromophores demonstrate how the aggregation can be

characterized quantitatively in solution and how molecular

modification of the chromophores can be utilized to minimize

aggregation. While the dimerization constant determined in

With knowledge of the TMC potential energy surfaces, a

number of predictions can be made about the nonlinear response

(â) by employing the sum-over-states (SOS) formula. At θ )

90°, the diradical state wave function term symbol is ¹A₂,

(67) Ray, P. C. Chem. Phys. Lett. 2004, 395, 269.

(68) Isborn, C. M.; Davidson, E. R.; Robinson, B. H. J. Phys. Chem. A 2006,

22, 110.

(69) (a) Wortmann, R.; Ro¨sch, U.; Redi-Abshiro, M.; Wu¨rthner, F. Angew.

Chem., Int. Ed. 2003, 42, 2080. (b) Liptay, W.; Rehm, T.; Wehning, D.;

Schanne, L.; Baumann, W.; Lang, W. Z. Naturforsch. A 1982, 37, 1427.

(70) Wortmann, R.; Bishop, D. M. J. Chem. Phys. 1998, 108, 1001.

9

J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007 3285

A R T I C L E S

Kang et al.

solution may not directly correlate with the degree of the

aggregation in host polymers, it is reasonable to use it to estimate

the order of magnitude of the aggregation effects in these guest-

host materials. For example, the polarity of PVP (ꢀ_r) 4.5) is

close to that of chloroform (ꢀ_r) 4.8), and the TMC-2

dimerization constant K_dis estimated to be 13 300 M^-1in

chloroform (vide supra). We can then estimate that for TMC-2

K_d≈ 13 300 M^-1in PVP. A TMC-2 doping level of 10 wt %

in PVP corresponds to a concentration of c₀≈ 0.28 M, and it

follows from K_dthat monomer concentration c_M≈ 0.003 M,

so that about 99% of the TMC-2 chromophore molecules are

predicted to be present in the form of dimers in PVP at room

temperature. However, upon applying poling field (100 V/µm

in the present experiments), we can then derive the field-

and practically independent of chromophore architecture/charge

distribution. NOE NMR measurements confirm that solid-state

twist angle persists in solution. Optical spectroscopy of these

chromophores reveals a twist-induced reduction of inter-ring

charge transfer (CT) and strong negative solvatochromism,

evidence of charge-separated zwitterionic structure in solution.

The solid-state vs solution-phase ¹³C NMR, IR, and optical

spectroscopic studies on these chromophores further support that

zwitterionic ground-state structural characteristics observed in

solid-state persist virtually unchanged in the solution phase and

that these chromophores exhibit a strong tendency for cen-

trosymmetric aggregation in concentrated solution. Detailed

information on the state of this aggregation, such as aggregation

model, level, and binding energies, has been provided by a

combination of techniques, including concentration-dependent

optical spectroscopy, pulsed field gradient spin-echo (PGSE)

NMR measurements, and X-ray crystallography. Most impor-

tantly, exceptional molecular hyperpolarizabilities of these

unconventional chromophores have been achieved in this

work, with EFISH-derived nonresonant µâ values as high as

-488 000 × 10^-48esu at 1907 nm. Preliminary direct Teng-

Man reflection measurements on guest-host poled polymers

containing these chromophores reveal a very large electro-optic

coefficient (r₃₃) of ∼ 330 pm/V at 1310 nm. Aggregation effects

are also observed in these measurements and shown to be an

issue to be addressed in future EO applications. SA-CASSCF

computations provide a new rationale for these exceptional

hyperpolarizabilities and demonstrate significant solvation ef-

fects on hyperpolarizabilities in good agreement with experi-

mental results. In summary, this work shows twisted π-electron

system chromophores to be promising candidates for EO

applications and provides new insights into the design of

molecule-based EO materials.

dependent dimerization constant K_d) 7.14 M^-1at room

E

temperature according to eqs 14 and 15 (with a Lorentz local

field factor of L ) 2.17). This is a significant shift of the

dimerization equilibrium with the monomer concentration c_M

changing to 0.11 M, so that ∼40% of the TMC-2 chromophores

is present as the dissociated monomer in the host at room

temperature. Furthermore, since the present dimerization of the

chromophore is an exothermic reaction with an expected

negative dimerization enthalpy and entropy,⁴³an increase of

the temperature will result in the decrease of dimerization

constant K_d. Therefore, upon heating the polymer matrix during

the poling, a greater proportion of the chromophore will be

present as dissociated monomers. It can be seen that there is

still considerable room to realize the optimal values offered by

such exceptional hyperpolarizabilities. Judging from the afore-

mentioned crystallographic observations and spectroscopic

aggregation studies, it is clear that bulky skeletal substituents

should prevent close aggregation, and from recent successes

that steric inhibition has had in dramatically improving bulk

EO response,⁷¹there is reason to believe that the present

aggregation issues can be similarly addressed and that further

enhanced r₃₃values will be achieved.

Acknowledgment. We thank DARPA/ONR (SP01P7001R-

A1/N00014-00-C) and the NSF-Europe program (DMR 0353831)

for support of this research. We thank the Northwestern

University MRSEC for support of the characterization facilities

(DMR 0076097). We also thank MIUR (FIRB 2003: “Molec-

ular compounds and hybrid nanostructured materials with

resonant and non-resonant optical properties for photonic

devices”) for support of the EFISH measurements. A.M. and

C.Z. thank the Ministero dell’Istruzione dell’Universita` e della

Ricerca (MIUR, Rome, Italy) Programma di Rilevante Interesse

Nazionale, Cofinanziamento 2004-2005. We thank Profs. A.-

K. Jen and L. R. Dalton (University of Washington) and S. R.

Marder (Georgia Tech) for helpful discussions and Dr. L. Wang

for growth of the In₂O₃electrodes.

Conclusions

A series of theory-inspired, unconventional twisted π-electron

system EO chromophores (TM and TMC) have been designed

and synthesized. Efficient synthetic approaches were developed

to realize these sterically hindered zwitterionic biaryls. All of

these new compounds have been fully characterized via

conventional analytical/spectroscopic techniques. The TMC

chromophores exhibit excellent thermal and chemical stability.

Crystallographic analysis of these molecules reveals large and

nearly invariant ring-ring dihedral twist angles (80-89°) and

highly charge-separated zwitterionic ground states. This twist

is governed primarily by the o,o′,o′′,o′′′-substituted biaryl core

Supporting Information Available: Synthetic details and

characterization, crystallographic data (CIF), NOE data, EFISH

data, setup for in situ poling and Teng-Man EO measurements,

TGA plot for TMC-2 and TMC-3, PGSE NMR data, and CV

plots of TM and TMC chromophores. This material is available

free of charge via the Internet at http://pubs.acs.org.

(71) (a) Zhang, C.; Sun, S.-S.; Bonner, C. E.; Kim, S.; Fetterman, H. R.; Dalton,

L. R. SPIE Proc. 2005, 6020, 56. (b) Luo, J.; Ma, H.; Haller, M.; Jen,

A. K.-Y.; Barto, R. R. Chem. Commun. 2002, 888. (c) Ma, H.; Chen, B.

Q.; Sassa, T.; Dalton, L. R.; Jen, A. K. Y. J. Am. Chem. Soc. 2001, 123,

986. (d) Robinson, B. H.; Dalton, L. R. J. Phys. Chem. A 2000, 104 (20),

4785. (e) Harper, A. W.; Sun, S.; Dalton, L. R.; Garner, S. M.; Chen, A.;

Kalluri, S.; Steier, W. H.; Robinson, R. H. J. Opt. Soc. Am. B 1998, 15,

329.

JA0674690

9

3286 J. AM. CHEM. SOC. VOL. 129, NO. 11, 2007

Article Doi

Ultralarge hyperpolarizability twisted π-electron system electro-optic chromophores: Synthesis, solid-state and solution-phase structural characteristics, electronic structures, linear and nonlinear optical properties, and computational studies

DOI: 10.1021/ja0674690

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Article abstract of DOI:10.1021/ja0674690

Full text of DOI:10.1021/ja0674690

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