X-shaped electro-optic chromophore with remarkably blue-shifted optical absorption. Synthesis, characterization, linear/nonlinear optical properties, self-assembly, and thin film microstructural characteristics

Article abstract of DOI:10.1021/ja060185v

A novel type of "X-shaped" two-dimensional electro-optic (EO) chromophore with extended conjugation has been synthesized and characterized. This chromophore is found to exhibit a remarkably blue-shifted optical maximum (357 nm in CH₂Cl₂) while maintaining a very large first hyperpolarizability (β). Hyper-Rayleigh Scattering (HRS) measurements at 800 nm provide a β_zzz value of 1840 × 10^-30 esu. Self-assembled thin films of this chromophore were fabricated via a layer-by-layer chemisorptive siloxane-based approach. The chromophoric multilayers have been characterized by transmission optical spectroscopy, advancing contact angle measurements, synchrotron X-ray reflectivity, atomic force microscopy, and angle-dependent polarized second harmonic generation spectroscopy. The self-assembled chromophoric films exhibit a dramatically blue-shifted optical maximum (325 nm) while maintaining a large EO response (χ⁽²⁾333 ～ 232 pm/V at 1064 nm; r₃₃ ～ 45 pm/V at 1310 nm). This work demonstrates an attractive approach to developing EO materials offering improved nonlinearity-transparency trade-offs.

Full text of DOI:10.1021/ja060185v

Published on Web 04/07/2006
X-Shaped Electro-optic Chromophore with Remarkably
Blue-Shifted Optical Absorption. Synthesis, Characterization,
Linear/Nonlinear Optical Properties, Self-Assembly, and Thin
Film Microstructural Characteristics
Hu Kang,† Guennadi Evmenenko,§ Pulak Dutta,§ Koen Clays,‡ Kai Song,‡ and
Tobin J. Marks*,†
Contribution from the Department of Chemistry and the Materials Research Center, and
Department of Physics and Astronomy and the Materials Research Center, Northwestern
UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113, and Department of Chemistry,
UniVersity of LeuVen, Celestijnenlaan 200D, B-3001 LeuVen, Belgium
Received January 10, 2006; E-mail: t-marks@northwestern.edu
Abstract: A novel type of “X-shaped” two-dimensional electro-optic (EO) chromophore with extended
conjugation has been synthesized and characterized. This chromophore is found to exhibit a remarkably
blue-shifted optical maximum (357 nm in CH2Cl2) while maintaining a very large first hyperpolarizability
(â). Hyper-Rayleigh Scattering (HRS) measurements at 800 nm provide a âzzz value of 1840 × 10-30 esu.
Self-assembled thin films of this chromophore were fabricated via a layer-by-layer chemisorptive siloxane-
based approach. The chromophoric multilayers have been characterized by transmission optical spec-
troscopy, advancing contact angle measurements, synchrotron X-ray reflectivity, atomic force microscopy,
and angle-dependent polarized second harmonic generation spectroscopy. The self-assembled chro-
mophoric films exhibit a dramatically blue-shifted optical maximum (325 nm) while maintaining a large EO
response (ø(2)333 ∼ 232 pm/V at 1064 nm; r33 ∼ 45 pm/V at 1310 nm). This work demonstrates an attractive
approach to developing EO materials offering improved nonlinearity-transparency trade-offs.
Introduction
polar arrays of high-â chromophores is particularly attractive
in that it yields self-assembled superlattices (SASs) that can be
grown on silicon or a variety of related substrates with dense
Realization of large-response molecule-based electro-optic
(EO) materials and development of “all-organic” photonic
devices are predicted to greatly enhance optical network speed,
capacity, and bandwidth for high-speed optical communications,
integrated optics, and optical data processing/storage. For these
reasons, the quest for high-performance EO materials has been
a very active research field.1-3 Crucial synthetic challenges for
large bulk EO response in these materials are that individual
chromophore components have large molecular hyperpolariz-
abilities (â) and that they be arranged in a noncentrosymmetric
architecture.1-3 Among the various approaches to preparing
suitable EO materials,4-10 layer-by-layer chemisorptive siloxane-
based self-assembly (SA) for templated formation of intrinsically
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† Department of Chemistry, Northwestern University.
§ Department of Physics and Astronomy, Northwestern University.
‡ University of Leuven.
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10.1021/ja060185v CCC: $33.50 © 2006 American Chemical Society

Blue-Shifted Absorption of X-Shaped Electro-Optic Chromophore
A R T I C L E S
Chart 1. High-â Chromophoric Building Blocks Used in Self-Assembled Superlattices
chromophore packing, covalently cross-linked microstructural
orientation, high thermal stability, and large EO coefficients.
These characteristics eliminate the need for postdeposition
processes, such as electric field poling, allow ready device
integration, and therefore significantly reduce device design
complexity.9,10 To date, the large-â chromophore building blocks
incorporated into such SAS materials,9,10 as well as those
employed in typical poled polymer4 and Langmuir-Blodgett-
based films,5 are devised according to common design motifs:
conjugated one-dimensional (1D) planar conjugated π systems
with end-capping electron donor and acceptor (D, A) substit-
uents. Chromophores of this type usually exhibit a single intense
low-lying longitudinal charge-transfer (CT) excitation and
possess optical nonlinearities which are essentially 1D in nature,
that is, dominated by a single â tensor component. Considerable,
elegant efforts have striven to increase â by optimizing D/A
strengths and/or the conjugation pathways of such 1D chro-
mophores.11 However, increases in â for such chromophores
are almost invariably accompanied by bathochromic shifts of
the optical maximum, eroding the optical transparency in the
NIR region essential for many EO applications. Furthermore,
molecules with extended π systems and low-lying excited states
are frequently subject to chemical and thermal instability.12
Alternative design strategies at the molecular level would
therefore be desirable, especially for the siloxane-based SAS
strategy building blocks where pyridinium and aminophenyl
fragments are exclusively utilized as D/A functionalities/covalent
linkers, and the key component to tune chromophore molecular
properties is the engineering of the conjugative pathway.9,10
Recent investigations have revealed that unconventional NLO
chromophores with multiple donor-acceptor substituents, hence
multiple intramolecular charge transfer, ranging from dipolar
“X-shaped”13 and “Λ-shaped”14 to octopolar molecules,15 can
afford 2D â tensor character where off-diagonal components
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A R T I C L E S
Chart 2
Kang et al.
become significant, offering potential advantages over 1D
chromophores, such as increased â responses without undesir-
able losses of transparency, and improved phase-matching via
the larger off-diagonal components.16 Moreover, off-diagonal
contributions to the â tensor have been shown to significantly
decrease NLO film anisotropy, opening new possibilities for
the development of polarization-independent materials that
would be suitable for optical telecommunications.17 Regarding
siloxane-based SAS fabrication strategies, dipolar X-shaped
molecules which possess two D/A pairs straddling the C2V
symmetry axis (also the molecular dipolar axis; see structure
below) might offer not only possible relaxation of nonlinearity-
transparency trade-offs but also enhanced microstructural
regularity and orientational stability via chelative, double
chemisorptive anchoring at both donor and acceptor ends of
the synthon. To date, however, no studies have reported
implementing 2D EO chromophore building blocks in SAS
strategies, and there have been only few experimental studies
incorporating dipolar 2D chromophores in LB or poled polymer
thin films.13d,e,15a,b In this contribution, we present a full
discussion of the synthesis, characterization, and chemisorptive
self-assembly into intrinsically acentric EO-active thin films of
the novel type of X-shaped 2D chromophore, X-CHR, contain-
ing a central aromatic core fused to extended conjugated D/A
aminophenyl/pyridinium groups. Here the optical transition
dipoles are arranged to couple in such a manner as to drastically
blue-shift λmax while maintaining a very large â response.
AM1/ZINDO computations predict âω)0 ) 407 × 10-30 esu
with a HOMO-LUMO CT excitation λmax ) 369 nm for this
chromophore versus âω)0 ) 312 × 10-30 esu with λmax ) 551
nm for the analogous 1D chromophore APVPVP.18 Even
considering a slightly lower value of the computed molecular
mass normalized NLO response (â/Mw) of 0.56 × 10-30 esu
for X-CHR versus 0.77 × 10-30 esu for its 1D analogue,
APVPVP, this minor attenuation in hyperpolarizability still
represents a small compromise considering the great enhance-
ment of the optical transparency predicted. Some preliminary
experimental results on this new π chromophore have been
previously communicated.19 We present in this contribution a
detailed discussion of chromophore synthetic approaches, solu-
tion structural characterization, linear optical (UV-vis) and
NLO (Hyper-Rayleigh Scattering) spectroscopic observations,
and efficient self-assembly into siloxane-based superlattices of
this new X chromophore. We report detailed characterization
of these self-assembled chromophoric films using a combination
of techniques, including transmission optical spectroscopy,
advancing contact angle measurements, synchrotron X-ray
reflectivity, atomic force microscopy, and angle-dependent
polarized SHG spectroscopy. It will be seen that X-chro-
mophore-derived SAS films exhibit a remarkably blue-shifted
optical maximum while maintaining a very large EO response,
as well as high structural regularity.
Experimental Section
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
calcium hydride. Toluene and pentane were dried/deoxygenated by
passing through two packed columns of activated alumina and Q5 under
nitrogen pressure and were tested using benzophenone ketyl in ether
solution. The reagent 4,5-dibromo-o-xylene was purchased from Alfa
Aesar. The reagents 1,2-dibromo-4,5-bis(bromomethyl)benzene (1),20
[(4,5-dibromo-1,2-phenylene)bis(methylene)]bisphosphonic acid tetra-
ethyl ester (2),21 and 4-N,N-bis(2-tert-butyldimethylsiloxyethyl)ami-
nobenzaldehyde (3)22 were synthesized according to the literature
procedures. The coupling agent dichloroiodo-(4-iodomethylphenyl)-
silane was prepared following procedures previously described.9c,10a,b
Single-crystal silicon(111) substrates were purchased from Semiconduc-
tor Processing Company, Inc. NMR spectra were recorded on a Varian
Mercury 400 MHz or Varian INOVA 500 MHz spectrometer. Mass
spectra were recorded on a Micromass Quattro II Triple Quadrupole
HPLC/MS/MS mass spectrometer. Elemental analyses were performed
by Midwest Microlabs. UV-vis spectra were recorded on a Cary 1E
or Cary 5000 spectrophotometer. Thermal analysis (TGA) was per-
formed with a TA Instruments SDT 2960 simultaneous DTA-TGA
instrument. Polarized second harmonic generation measurements were
carried out in the transmission mode with a Q-switched Nd:YAG laser
(15) (a) Traber, B.; Wolff, J. J.; Rominger, F.; Oeser, T.; Gleiter, R.; Goebel,
M.; Wortmann, R. Chem.sEur. J. 2004, 10, 1227. (b) Wolff, J. J.; Segler,
F.; Matschiner, R.; Wortmann, R. Angew. Chem., Int. Ed. 2000, 39, 1436.
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11, 1915. (d) Lee, Y.-K.; Jeon, S.-J.; Cho, M. J. Am. Chem. Soc. 1998,
120, 10921. (e) Brasselet, S.; Zyss, J. J. Opt. Soc. Am. B 1998, 15, 257.
(16) (a) Miyata, S.; Sasabe, H. Poled Polymers and Their Applications to SHG
and EO DeVices; Gordon and Breach Science: Australia, 1997. (b)
Yamamoto, H.; Katogi, S.; Watanabe, T.; Sato, H.; Miyata, S.; Hosomi,
T. Appl. Phys. Lett. 1992, 60, 935. (c) Zyss, J.; Oudar, L. L. Phys. ReV. A
1982, 26, 2016.
(19) Kang, H.; Zhu, P.; Yang, Y.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc.
2004, 126, 15974.
(17) (a) Di Bella, S.; Fragala´, I.; Ledoux, I.; Zyss, J. Chem.sEur. J. 2001, 7,
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2000, 87, 3258.
(20) Rivera, J. M.; Martin, T.; Rebek, J., Jr. J. Am. Chem. Soc. 2001, 123, 5213.
(21) Kimura, M.; Narikawa, H.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi,
N. Chem. Mater. 2002, 14, 2711.
(18) Keinan, S.; Ratner, M. A.; Marks, T. J. unpublished results.
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Blue-Shifted Absorption of X-Shaped Electro-Optic Chromophore
A R T I C L E S
operating at 1064 nm, with a pulse width of 3 ns at a frequency of 10
Hz. The details of this setup can be found elsewhere.23
8.258 (d, J ) 5.6 Hz, 4H), 7.817 (d, J ) 16.0 Hz, 2H), 7.707 (s, 2H),
7.453 (d, J ) 8.0 Hz, 4H), 7.181 (d, J ) 16.0 Hz, 2H), 7.120 (d, J )
16.0 Hz, 2H), 6.876 (d, J ) 16.0 Hz, 2H), 6.744 (d, J ) 8.0 Hz, 4H),
3.839 (s, 6H), 3.821 (s, 8H), 3.613 (s, 8H), 0.921 (s, 36H), 0.083 (s,
24H). 13C NMR (100 MHz, CD2Cl2): δ 153.570, 148.766, 144.553,
138.438, 137.037, 132.706, 129.239, 125.745, 125.036, 124.672,
124.371, 119.886, 119.676, 112.033, 61.006, 54.173, 47.640, 26.266,
18.732, -4.970. MS (ESI): m/z 1329.9 [M+ - TfO-, 100], 590.9 [M2+
- 2TfO-, 45]. Anal. Calcd for C72H108F6N4O10S2Si4‚2H2O: C, 57.04;
H, 7.45; N, 3.70. Found: C, 56.81; H, 7.29; N, 3.68.
Synthesis of 1,2-Dibromo-4,5-bis{(E)-2-[p-N,N-bis(2-tert-butyl-
dimethylsiloxyethyl)aminophenyl]ethenyl}benzene (4). To a yellow
solution of 2 (8.04 g, 15.0 mmol) in dry THF (45 mL) was added
sodium methoxide (8.68 g, 28% methanol solution, 45.0 mmol)
dropwise at -15 to -20 °C, and the mixture was stirred at this
temperature for an additional 1 h. Next, benzaldehyde 3 (14.67 g, 33.0
mmol) was slowly added to the reaction mixture at -15 to -20 °C
over the course of 40 min and then stirred at this temperature for an
additional 30 min. The reaction mixture was allowed to warm to room
temperature, covered with aluminum foil, and stirred at room temper-
ature for 10 h. To the resulting orange solution was added saturated
aqueous NH4Cl solution (45 mL) at 0-5 °C in an ice bath. The mixture
was then extracted with diethyl ether (3 × 75 mL), and the ether layer
was successively washed with water, saturated aqueous NH4Cl, and
brine, dried over NaSO4, filtered, and concentrated in vacuo. The
resulting orange oil was purified by flash chromatography on silica
gel, eluting with 10:1 hexane:diethyl ether, to give 8.90 g (53.8%)
of the title compound as a yellow sticky oil. 1H NMR (400 MHz,
CD2Cl2): δ 7.799 (s, 2H), 7.382 (d, J ) 8.4 Hz, 4H), 7.096 (d, J )
16.0 Hz, 2H), 6.928 (d, J ) 16.0 Hz, 2H), 6.695 (d, J ) 8.4 Hz, 4H),
3.778 (t, J ) 6.4 Hz, 8H), 3.547 (t, J ) 6.4 Hz, 8H), 0.892 (s, 36H),
0.040 (s, 24H). 13C NMR (100 MHz, CD2Cl2): δ 148.515, 137.415,
132.736, 130.818, 128.467, 124.911, 122.417, 119.323, 111.967, 60.900,
54.083, 26.294, 18.799, -4.916. MS (FAB): m/z 1103.2 [M+, 100].
MS (high resolution, FAB): m/z 1100.4340 [M+]; calcd, 1100.4344.
Anal. Calcd for C54H90Br2N2O4Si4: C, 58.78; H, 8.22; N, 2.54. Found:
C, 59.01; H, 8.09; N, 2.54.
Hyper-Rayleigh Scattering (HRS) Measurements. General details
of the hyper-Rayleigh scattering (HRS) experiment have been discussed
elsewhere,24 and the experimental procedure and data analysis protocol
used were also previously described.25 All measurements were per-
formed using the 800 nm fundamental of a regenerative mode-locked
Ti3+:sapphire laser (Spectra Physics, model Tsunami, 100 fs pulses, 1
W, 80 MHz). Measurements were carried out in methanol, with crystal
violet chloride (CV) as an external reference (âxxx ) 338 × 10-30 esu
in methanol at 800 nm), taking into account the difference in symmetry
(octopolar for CV and dipolar for the X chromophore). The sample
was dissolved in methanol and passed through 0.2 µm filters. Dilute
solutions (10-5-10-6 M) were used to ensure a linear dependence of
I
2ω/Iω2 on solute concentration, precluding the need for Lambert-Beer
correction for self-absorption of the second harmonic generation (SHG)
signal. High-frequency femtosecond hyper-Rayleigh scattering (up to
240 MHz) was used to assess any multiphoton fluorescence contribution
at 400 nm. No fluorescence effects were observed at 400 nm for
X-CHR. A relative uncertainty of (4.3% is estimated for the â value
determined in the present experiments. The HRS depolarization ratio26
was determined at 800 nm in methanol according to methodology
reported previously.27
Self-Assembled Film Fabrication. Sodium lime glass, fused quartz,
and silicon wafer substrates were cleaned by immersion in “piranha”
solution (concentrated H2SO4:30% H2O2 7:3 v/v) at 80 °C for 1 h. After
cooling to room temperature, they were rinsed repeatedly with deionized
(DI) water and then subjected to an RCA-type cleaning protocol (H2O:
NH4OH:30% H2O2 ) 5:1:1 v/v/v, sonicated at room temperature for
40 min). The substrates were then washed with DI water and dried in
an oven at 125 °C immediately before coupling agent deposition. Films
were then fabricated via the iterative three-layer process:
(i) Self-Assembly of 4-ICH2C6H4SiCl2I. Freshly cleaned glass or
silicon single-crystal substrates were immersed in a 15 mM solution
of the coupling agent dichloroiodo-(4-iodomethylphenyl)silane in
pentane under rigorously anhydrous and anaerobic conditions for 30
min at room temperature, washed copiously with dry pentane, sonicated
in acetone for 1 min, and then dried under vacuum. A benzyl halide-
functionalized surface is thereby obtained via the self-limiting chemi-
sorption of the silane reagent.
(ii) Self-Assembly of the X Chromophore. The silylated substrates
(after step i treatment) were spin-coated on both sides at 2500 rpm
with a 5 mM solution of BPBAB in toluene inside a class 100 clean
hood (Envirco) and then cured at 120 °C in a vacuum oven (0.4 Torr)
for 40 min. After cooling to room temperature, the samples were washed
with toluene and acetone to remove any residual BPBAB and dried
under vacuum.
Synthesis of 1,2-Bis((E)-2-pyridin-4-ylvinyl)-4,5-bis{(E)-2-[p-N,N-
bis(2-tert-butyldimethylsiloxyethyl)aminophenyl]ethenyl}benzene
(BPBAB). A flame-dried sealable Schlenk tube was charged with 4
(1.0 g, 0.90 mmol), Pd(OAc)2 (20 mg, 0.090 mmol), and PPh3 (23.6
mg, 0.090 mmol), evacuated, and then backfilled with N2. Next,
4-vinylpyridine (0.96 g, 9.0 mmol) and dry Et3N (1 mL) were added
under a N2 flow. The Schlenk tube was then sealed, and the mixture
was degassed by three freeze-thaw cycles and backfilled with N2. The
mixture was then covered with aluminum foil and stirred at 100 °C for
3 days. The resulting yellow solid was next dissolved in CH2Cl2 (50
mL) and washed with water. The organic layer was separated, dried
over MgSO4, filtered, and concentrated in vacuo to give a yellow solid.
The crude product was recrystallized from acetone to give 0.70 g (67%)
1
of the title compound as a yellow solid. Mp 199-200 °C. H NMR
(400 MHz, CD2Cl2): δ 8.585 (d, J ) 5.6 Hz, 4H), 7.843 (s, 2H), 7.705
(d, J ) 16.8 Hz, 2H), 7.442 (d, J ) 8.8 Hz, 4H), 7.423 (d, J ) 5.6 Hz,
4H), 7.284 (d, J ) 16.0 Hz, 2H), 7.076 (d, J ) 16.0 Hz, 2H), 7.052 (d,
J ) 16.0 Hz, 2H), 7.736 (d, J ) 8.4 Hz, 4H), 3.814 (t, J ) 6.4 Hz,
8H), 3.583 (t, J ) 6.4 Hz, 8H), 0.922 (s, 36H), 0.073 (s, 24H). 13C
NMR (100 MHz, CD2Cl2): δ 150.758, 148.520, 144.990, 137.228,
134.125, 132.033, 130.704, 128.803, 128.502, 125.472, 124.681,
121.351, 121.041, 112.124, 60.952, 54.118, 26.285, 18.742, -4.979.
MS (FAB): m/z 1151.5 [M+, 100]. MS (high resolution, FAB): m/z
1150.6976 [M+]; calcd, 1150.6978. Anal. Calcd for C68H102N4O4Si4:
C, 70.90; H, 8.93; N, 4.86. Found: C, 70.82; H, 8.75; N, 4.84.
Synthesis of Methylpyridinium X Chromophore (X-CHR). A
flame-dried Schlenk flask was charged with BPBAB (115 mg, 0.10
mmol) and then evacuated and backfilled with N2. Next, dry methylene
chloride (5 mL) was added via syringe under N2, and the solution was
stirred at 0 °C. Methyl triflate (32.8 mg, 22.6 µL, 0.20 mmol) was
next added dropwise via syringe under N2. The resulting mixture was
stirred at room temperature for 10 min. The solvent was then evaporated
in vacuo to give 147 mg (100%) of analytically pure product as a dark
(iii) Self-Assembly of Octachlorotrisiloxane. The glass or silicon
single-crystal substrates after the steps i and ii treatments were then
(24) (a) Terhune, R. W.; Maker, P. D.; Savage, C. M. Phys. ReV. Lett. 1965,
14, 681. (b) Clays, K.; Persoons, A. Phys. ReV. Lett. 1991, 66, 2980. (c)
Clays, K.; Persoons, A. ReV. Sci. Instrum. 1992, 63, 3285. (d) Hendrickx,
E.; Clays, K.; Persoons, A. Acc. Chem. ReV. 1998, 31, 675.
(25) (a) Clays, K.; Olbrechts, G.; Munters, T.; Persoons, A.; Kim, O.-K.; Choi,
L.-S. Chem. Phys. Lett. 1998, 293, 337. (b) Olbrechts, G.; Strobbe, R.;
Clays, K.; Persoons, A. ReV. Sci. Instrum. 1998, 69, 2233.
(26) Heesink, G. J. T.; Ruiter, A. G. T.; van Hulst, N. F.; Bo¨lger, B. Phys. ReV.
Lett. 1993, 71, 999.
1
solid. H NMR (400 MHz, CD2Cl2): δ 8.293 (d, J ) 4.8 Hz, 4H),
(23) Yitzchaik, S.; Roscoe, S. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.;
(27) Boutton, C.; Clays, K.; Persoons, A.; Wada, T.; Sasabe, H. Chem. Phys.
Lett. 1998, 286, 101.
Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem. 1993, 97, 6958.
9
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A R T I C L E S
Kang et al.
Scheme 1. Synthetic Route to the X Chromophore X-CHRa
immersed in a 1:150 (v/v) solution of Si3O2Cl8 in dry pentane for 30
min at room temperature under rigorously anhydrous and anaerobic
conditions, then removed, washed with copious amounts of dry pentane
and methanol, and sonicated in acetone for 1 min.
Specular X-ray Reflectivity (XRR). XRR studies were performed
at beamline X23B of the National Synchrotron Light Source using a
Huber four-circle diffractometer in the specular reflection mode (i.e.,
the incident angle is equal to the exit angle). X-rays of energy E ) 10
keV (λ ) 1.24 Å) were used for all measurements. The beam size was
0.4 mm vertically and 1.5 mm horizontally. The samples were placed
under helium during the measurements to reduce the background
scattering from the ambient gas and radiation damage. The experiments
were performed at room temperature. The off-specular background was
measured and subtracted from the specular counts. Details of the data
acquisition and analysis procedures are given elsewhere.28
Results
The synthesis of the X chromophore via a straightforward
multi-step route is first reported. The new compounds are fully
characterized via conventional analytical/spectroscopic tech-
niques. In addition, thermal stability is quantified by thermo-
gravimetric (TGA). The chromophore linear optical response
properties in solution are studied by optical absorption spec-
troscopy (UV-vis). It will be seen that this chromophore indeed
exhibits a remarkable blue-shifted λmax. The X chromophore
solution structure is next defined by concentration-dependent
optical and NMR spectroscopies. Hyper-Rayleigh Scattering
(HRS) measurements reveal a very large HRS-derived âzzz value.
We further demonstrate that this new chromophore can be
incorporated into self-assembled thin films via a three-step layer-
by-layer self-assembly technique.10 The resulting thin films are
characterized by optical transmission spectroscopy (to character-
ize the fidelity of the assembly chemistry and the microstructural
regularity), synchrotron X-ray reflectivity (XRR; to characterize
film thickness, density, microstructural regularity, and interfacial
roughness), advancing contact angle measurements (CA; to
characterize surface energy), atomic force microscopy (AFM;
to characterize surface morphology and roughness), and angle-
dependent polarized SHG spectroscopy (to characterize NLO/
EO response, microstructure, and polar regularity).
a Conditions: (i) NBS, CCl4; (ii) P(OEt)3; (iii) MeONa, THF; (iv)
Pd(OAc)2/PPh3, Et3N; (v) MeOTf, CH2Cl2.
Figure 1. One-dimensional 1H NMR spectrum (500 MHz, 25 °C) and 1H
2D COSY NMR spectrum (500 MHz, 25 °C; inset) of X chromophore
precursor (BPBAB) in acetone-d6. Signals are assigned as indicated in the
labeling scheme.
X Chromophore Synthesis and Characterization. The
synthetic route to the X chromophore is summarized in Scheme
1. The phosphonate core 2 was obtained from 1,2-dibromo-
4,5-bis(bromomethyl)benzene 1 via a Michaelis-Arbuzov reac-
tion. The double Wittig-Horner reactions between 1 molar
equiv of 2 and 2 equiv of TBDMS-protected 4-[N,N-bis(2-
hydroxyethyl)amino]benzaldehyde 3 afford intermediate 4 in
54% yield. Subsequently, the double Heck coupling between 4
and excess 4-vinylpyridine (10 molar equiv) affords X chro-
mophore precursor BPBAB in 67% yield. The dicationic
methylpyridinium X chromophore (X-CHR) was next obtained
quantitatively via MeOTf alkylation of BPBAB. All new
compounds were fully characterized by conventional analytical/
analysis (TGA) data (Figure 2) show that both BPBAB and
X-CHR have high thermal stability (Td ≈ 340 °C for BPBAB
and Td ≈ 280 °C for X-CHR).
X Chromophore Solution Structure. Optical Spectroscopy.
The X-CHR optical spectrum (Figure 3A) in CH2Cl2 solution
consists of an intense band centered at 357 nm (ꢀ ) 120 000 L
mol-1 cm-1) involving πfπ* CT and a less intense, broad
shoulder band at longer wavelength, ascribed to aggregation as
supported by concentration-dependent optical absorption spectra
(Figure 3B). Here significant spectral changes are observed upon
varying the concentration over the range of 5 × 10-7 to 3 ×
10-8 M, revealing an increase of the 357 nm CT band and
concomitant diminution of the long-λ band upon dilution. A
well-defined isosbestic point is observed in the concentration-
dependent spectra and clearly indicates the presence of ag-
gregation equilibria.29 The bathochromic shifted aggregation
band indicates that J-type aggregation occurs. This strong
aggregation tendency is reasonable considering the large dipole
1
spectroscopic techniques, such as H and 13C NMR spectros-
copy, mass spectrometry, and elemental analysis. Signals from
BPBAB aromatic and vinyl protons in the aromatic region are
difficult to assign due to overlapping of the vinyl and phenyl
protons. Therefore 1H 2D COSY NMR spectroscopy was used
to help confirm the molecular connectivity (Figure 1). Thermal
(28) Evmenenko, G.; van der Boom, M. E.; Kmetko, J.; Dugan, S. W.; Marks,
T. J.; Dutta, P. J. Chem. Phys. 2001, 115, 6722.
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Blue-Shifted Absorption of X-Shaped Electro-Optic Chromophore
A R T I C L E S
Figure 2. Thermogravimetric analysis (TGA) plots of weight percent versus
temperature for chromophore precursor BPBAB (A) and chromophore
X-CHR (B) under 1.0 atm N2. The temperature ramp rate was 1.5 °C/min.
moment of X-CHR (the AM1-computed ground state dipole
moment is 57.4 D).18
X Chromophore Solution Structure. Concentration-De-
pendent NMR Spectroscopy. The strong tendency for X-CHR
aggregation in solution is also observed in concentration-
dependent NMR experiments carried out in CD2Cl2 solution
(Figure 4). The signals from N-methyl protons (Hn), outer
pyridinium rings (Hm and Hl), central arene ring (Hi), and
vinylene fragments (Hk, Hj, Hg, and Hh) are displaced signifi-
cantly upfield in concentrated CD2Cl2 solution due to aggrega-
tion.30 In contrast, the resonances of the protons on the outer
phenylene rings (He and Hf) and ethanolamine groups (Hc and
Hd) undergo negligible shifts with concentration. A similar
phenomenon has also been reported for several 2D-conjugated
poly(p-phenylenevinylene)-based molecules.31 The present re-
sults can be explained if the planar conformation for the central
part and two pyridine vinyl arms results in close stacking in
solution through π-π interactions, while the other two phen-
ylenevinyl arms cannot undergo close stacking due to the bulky
TBDMS protecting groups.
Figure 3. (A) Optical absorption spectrum of chromophore X-CHR in
CH2Cl2 solution. (B) Concentration-dependent optical absorption spectra
of chromophore X-CHR in CH2Cl2 solution. The arrows indicate changes
of the monomer and the aggregation bands upon decreasing the concentration
from 5 × 10-7 to 3 × 10-8 M.
sures in the frequency domain, the apparent first hyperpolar-
izability as a function of amplitude modulation frequency. For
higher frequencies, any multiphoton fluorescence contribution
is diminished in amplitude and acquires a phase delay with
respect to the scattering. The high-frequency limit of the
apparent hyperpolarizability is therefore the accurate, fluorescence-
free value. High-frequency femtosecond hyper-Rayleigh scat-
tering measurements (up to 240 MHz) were carried out to assess
any fluorescence contributions at 400 nm in X-CHR. The
apparent dynamic hyperpolarizability as a function of modula-
tion frequency does not exhibit any appreciable frequency
dependence (with a slope of (0.04 ( 0.5) × 10-30esu/MHz) or
phase accumulation for increasing modulation frequencies,
indicating that there are no fluorescence effects and that the
signal is pure scattering. Therefore, the average value or the
intercept of the hyperpolarizability magnitudes as a function of
frequency, (1840 ( 80) × 10-30 esu, was obtained for the âzzz
of X-CHR, assuming C2V dipolar symmetry with a single major
hyperpolarizability tensor element. This assumption can be
verified by performing depolarization measurements,26,27 where
the depolarization ratio, that is, the ratio of HRS intensity in
polarization paralleling the fundamental laser beam to that of
the HRS in perpendicular polarization, is a sensitive function
of molecular symmetry. The depolarization ratio of 5 is the
Hyper-Rayleigh Scattering Spectroscopy of the X Chro-
mophore. The âzzz component of the hyperpolarizability tensor
for this chromophore has been determined at the molecular level
by the femtosecond HRS techniques24,25 using a 800 nm Ti3+
:
sapphire laser. With the femtosecond pulses, it is possible to
distinguish in the time domain between the immediate nonlinear
scattering and the nanosecond time-delayed multiphoton fluo-
rescence that, if present, results in overestimation of the
hyperpolarizability magnitude.25b The experimental setup mea-
(29) Dipolar molecule aggregation in solution is most frequently studied by
optical spectrospy: Wu¨rthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann,
R. J. Am. Chem. Soc. 2002, 124, 9431.
(30) Kraft, A. Liebigs Ann/Recl. 1997, 1463.
(31) Niazimbetova, Z. I.; Christian, H. Y.; Bhandari, Y. J.; Beyer, F. L.; Galvin,
M. E. J. Phys. Chem. B 2004, 108, 8673.
9
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A R T I C L E S
Kang et al.
and greater signal-to-noise ratio. Note that the X chromophore
is stable under the present HRS measurement conditions, as
verified by UV-vis spectroscopy before and after the measure-
ments, indicating excellent photochemical stability.
X Chromophore Self-Assembled Thin Film Fabrication.
Layer-by-layer self-assembled X-chromophore-derived thin
films were fabricated on a variety of substrates (glass, quartz,
silicon) via an iterative three-step process (Scheme 2): (i) self-
limited chemisorption of coupling agent 4-ICH2C6H4SiCl2I onto
the hydrophilic substrate surface; (ii) self-assembly of chro-
mophore precursor BPBAB onto the benzyl halide-functional-
ized substrate via N-quaternization; and (iii) Cl3SiOSi(Cl2)-
OSiCl3 capping to planarize/cross-link/stabilize the polar structure
into a 3D polysiloxane network and to simultaneously regenerate
an active surface for subsequent layer deposition.
In step i, the freshly cleaned substrates were treated with a
15 mM solution of 4-ICH2C6H4SiCl2I in dry pentane at room
temperature for 30 min under rigorously anhydrous/anaerobic
conditions, followed by washing with copious amounts of
pentane and acetone to remove physisorbed coupling agent.
Previous studies have established that this treatment (step i,
Scheme 2) affords densely packed, self-assembled monolayers
of benzyl iodide coupling agent on the surface.10a,b The self-
assembled coupling layer was characterized by advancing
aqueous contact angle (θa) measurements. The contact angle
changes from 16° for SiO2 to 68° for the silylated surface. This
hydrophobic change in θa is consistent with the presence of
benzyl iodide functionalities on the surface.10a,b
Figure 4. 1H NMR spectra (400 MHz, 25 °C) of X-CHR in CD2Cl2 as a
function of concentration. Signals are assigned as indicated in the labeling
scheme. The unshifted high-field signals from the TBDMS protecting group
protons are not shown here.
upper limit for a purely dipolar symmetry, that is, a single
contributing tensor element in the limit of ideal experimental
conditions, while the ratio if 1.5 is the lower limit for completely
octopolar molecules. The HRS depolarization ratio determined
here for X-CHR is 3.12 ( 0.06, consistent with a largely dipolar
molecule. This value for the depolarization ratio corresponds
to a ratio for diagonal âzzz to off-diagonal âxzz tensor element
of -0.22. In comparison, the HRS depolarization ratio for
Disperse Red 1 (DR1) under identical conditions is found to
be 3.4 ( 0.2. The smaller estimated uncertainty for X-CHR
with respect to DR1 is an indication of the more intense signal
In step ii, the deposition of the X chromophore layers was
achieved in a “topotactic” fashion.10a-f The substrates termi-
nated with the benzyl iodide coupling layer were spin-coated
with a thin layer of chromophore precursor BPBAB, fol-
lowed by vacuum oven treatment at 120 °C/0.4 Torr. The
chromophore deposition kinetics were monitored by SHG
spectroscopy to optimize reaction conditions. Figure 5 shows
the deposition kinetics at 120 °C/0.4 Torr. The 532 nm SHG
output intensity (I2ω) reaches maximum at ∼40 min and
decreases at longer reaction times. The reason for the decline
in I2ω at longer reaction times may be the result of evolution in
Scheme 2. Self-Assembly of the X Chromophore to Prepare Polar Multilayers
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Blue-Shifted Absorption of X-Shaped Electro-Optic Chromophore
A R T I C L E S
Figure 5. Second harmonic generation (SHG) response measurements as
a function of reaction/deposition time for optimizing self-assembled X
chromophore monolayer formation by vacuum oven treatment (step ii,
Scheme 2) at 120 °C/0.4 Torr on a float glass substrate.
the chromophore tilt angle or slow structural reorganization of
the uncapped monolayer. Variation in chromophore tilt angle
as a function of coverage during monolayer formation has been
observed in related self-assembling systems.9c,10e The samples
were then washed with methanol and acetone to ensure the
complete removal of excess BPBAB from the surface. Aqueous
CA measurements on the chromophore-functionalized substrates
(step ii, Scheme 2) reveal distinctly hydrophobic surfaces (θa
∼ 76°) in accordance with the formation of densely packed
organic hydrocarbon films, exposing only the trialkylsilyl
protecting groups at the outer surface.9c
The final (capping) step of the deposition sequence (step iii
in Scheme 2) involves treatment of the chromophoric bilayers
with octachlorotrisiloxane. Previous studies established that the
reaction of octachlorotrisiloxane in dry pentane with trialkylsilyl-
terminated chromophore layer surfaces and traces of adsorbed
H2O effects in situ chromophore deprotection and simultaneous
formation of a hydrophilic polysiloxane surface.9a-d This
capping step provides lateral structural stabilization/planarization
via interchromophore cross-linking and also regenerates hy-
droxyl groups for the subsequent iterative superlattice assembly
process.9,10 The deposition of capping layers yields the expected
hydrophilic change in θa; the advancing aqueous contact angle
of the surface decreases from 76 to 18° after capping layer
chemisorption.9,10 This result is consistent with the regeneration
of surface hydroxyl groups as a result of the capping layer
deposition.
Figure 6. Optical absorption spectra of self-assembled X chromophore
films grown on fused quartz as a function of the number of self-assembled
layers. Inset: optical absorbance of the self-assembled films at λmax ) 325
nm as a function of the number of trilayers.
specular reflectivity is a function of the electron density profile
F(z) perpendicular to the sample surface. For the present systems,
a model consisting of a silicon substrate and layers of different
electron densities, Fi, with Gaussian broadened interfaces, σi,
was used (eq 1).28,33c
2
N
R(qz)
(Fi - Fi+1)
-qz2σi+12/2
z
)
e-iq Di e
(1)
∑
|
|
F0
RF(qz)
i)0
Here the wave vector transfer |q| ) qz ) (4π/λ) sin θ is along
the surface normal; F0 is the electron density of the substrate
()FSi), F(z) is the electron density distribution inside the film
averaged over the in-plane coherence length of the X-rays
(usually ∼1-3 µm); RF(qz) is the theoretical Fresnel reflectivity
for an ideally flat substrate surface; N is the number of layers;
Di ) ∑i Tj is the distance from the substrate surface to the
j)1
ith interface, and Ti is the thickness of the ith layer. The
reflectivity data were fit to such a model, the fitting parameters
being the thickness and the electron density of each layer, and
the root-mean-square width of each interface (the film-air inter-
face corresponds to the film roughness). Note that eq 1 is valid
for qz larger than approximately twice the critical wave vector
for total external reflection (qc ) 0.0316 Å-1 for Si), where
refraction effects are negligible. Thus, the fits were performed
using only data for which qz > 2qc. Figure 7A shows normalized
reflectivity data (R/RF) from a typical scan on a chromophore
monolayer film (step ii, Scheme 2) and a one-trilayer SAS film
(step iii, Scheme 2). The solid line shows the best fit using eq
1. The corresponding electron density profiles obtained from
this fit are presented in Figure 7B. Similar reflectivity patterns
and electron density profiles were obtained for all samples (for
example, see the reflectivity data for the five-trilayer sample in
the inset of Figure 8). The data show pronounced “Kiessig
Film Microstructural/Electro-Optic Characterization. The
resulting self-assembled multilayers were characterized by
optical spectroscopy (UV-vis), synchrotron X-ray specular
reflectivity (XRR), angle-dependent polarized second harmonic
generation (SHG), and atomic force microscopy (AFM). The
new films strongly adhere to the hydrophilic substrates and
cannot be removed by the “Scotch tape decohesion test”32 and
are insoluble in common organic solvents. These films exhibit
λmax at the remarkably short wavelength of 325 nm (Figure 6).
The linear dependence of the 325 nm absorbance on the number
of layers (Figure 6 inset) indicates that essentially equal
quantities of equivalently oriented chromophore molecules are
deposited in each layer-forming step.
Specular X-ray reflectivity was used to probe microstructural
evolution of these siloxane-based superlattices. Data for a series
of 1-5 trilayers were obtained by fitting the multilayer
reflectivity data to a Gaussian-step model.28,33 In general, X-ray
(32) (a) Nijmeijer, A.; Kruidhof, H.; Bredesen, R.; Verweij, H. J. Am. Ceram.
Soc. 2001, 84, 136. (b) Krongelb, S. Electrochem. Technol. 1968, 6, 251.
(33) (a) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S. Phys. ReV. B 1990, 41,
1111. (b) Pomerantz, M.; Segmuler, A.; Netzer, L, Sagiv, J. Thin Solid
Films 1985, 132, 153. (c) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.;
Kosowsky, S. D.; Folkers, G. P.; Whitesides, G. M. J. Chem. Phys. 1991,
95, 2854.
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A R T I C L E S
Kang et al.
Figure 8. Specular X-ray reflectivity (XRR)-derived film thickness (Å)
for X chromophore self-assembled films as a function of the number of
trilayers. Inset: XRR data for a five-trilayer sample.
chromophore. The chromophore monolayer (step ii, Scheme 2)
has an electron density of ∼0.48 eÅ-3, thickness of 22.2 Å,
film roughness of 5.4 Å, and a molecular footprint of ∼80 Å2.
This XRR-derived surface roughness is comparable to those of
azobenzene and heterocycle-based monolayers (3.3-6.4 Å)9,10a,b
and to that of highly ordered, self-assembled octadecyltrichlo-
rosilane films on single-crystal silicon.35 From the molecular
footprint of a chromophore monolayer, the chromophore density
(Ns) can be calculated and is found to be ∼1.3 × 1014 molecules/
cm2. The one-trilayer SAS film (step iii, Scheme 2) has a
thickness of ∼35.0 Å, film roughness of 5.7 Å, and a footprint
of ∼80 Å2. The capping layer structure is characterized by the
comparison of electron density profiles of chromophore mono-
layer and one-trilayer SAS films (Figure 7B) to be a densely
packed polysiloxane network with a thickness of ∼15.3 Å,
comprising roughly ∼6 (-Si-O) repeat units.
Figure 7. X-ray reflectivity data for films containing chromophore
monolayer (CHR, step ii, Scheme 2) and one trilayer sample (SAS-1, step
iii, Scheme 2). The structural model used for fitting the X-ray reflectivity
data consists of a semi-infinite silicon substrate, a silicon oxide layer, a
coupling layer, a self-assembled chromophore layer, and a capping layer.
(A) X-ray reflectivity data normalized to the Fresnel reflectivity RF for CHR
(0) and SAS-1 (O). The solid line is the best fit to the data. (B) The
corresponding electron density profiles for CHR (solid line) and SAS-1
(dashed line) obtained from this fit.
Polarized angle-dependent second harmonic generation (SHG)
measurements on the X chromophore self-assembled thin films
were carried out at λo ) 1064 nm in the transmission mode.
The characteristic SHG interference pattern from a glass
substrate coated on both sides is shown in Figure 9 inset,
demonstrating that essentially identical film quality and uni-
formity are achieved on both sides of the substrates.7-10 The
presence of near-zero intensity minima indicates that the quality
and uniformity of the deposited films on each side of the
substrate is nearly identical. Moreover, the quadratic dependence
of the 532 nm output intensity (I2ω) on film thickness (Figure
9) further demonstrates polar microstructure preservation as
layer-by-layer assembly progresses.7-10 By calibrating the angle-
dependent SHG data against quartz, a large second-order
fringe” minima, corresponding to destructive interference of
reflections from the top and bottom of the film, and indicate a
narrow distribution of layer thicknesses.8b,9,10,34 The total
thickness of the film, T, can be found simply by using path
length differences: T ) 2π/∆qz, where ∆qz is the spacing
between the minima. The film thickness increases linearly as a
function of the number of trilayers (Figure 8), underscoring the
high structural regularity and efficiency of this approach. From
the slope of the XRR data, an average interlayer spacing of
40.6 Å can be deduced. This thickness is found to be in a
reasonable range using Spartan-level molecular modeling. The
densities of electrons per unit of substrate area, Fexp, for the
films were calculated using the electron density profiles
according to Fexp ) ∫F(z) dz, where the integration is taken over
the entire film. The “molecular footprint” can then be calculated
from Ne/Fexp, where Ne is the total amount of electrons in a single
macroscopic NLO response ø(2) ∼ 232 pm/V at 1064 nm is
333
obtained for X chromophore self-assembled films. Note that
this value was obtained under nominally nonresonant conditions
since the X chromophore has no significant optical absorption
near 532 nm (I2ω).
(35) (a) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner, G.; Heid,
S.; Effenberger, F. Langmuir 1995, 11, 512. (b) Ohtake, T.; Mino, N.;
Ogana, K. Langmuir 1992, 8, 2081. (c) Tidswell, I. M.; Ocko, B. M.;
Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys.
ReV. B 1990, 41, 1111.
(34) (a) Richter, A. G.; Durbin, M. K.; Yu, C.-J.; Dutta, P. Langmuir 1998, 14,
5980. (b) Malik, A.; Durbin, M. K.; Richter, A. G.; Huang, K. G.; Dutta,
P. Phys. ReV. B 1995, 52, 11654.
9
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Blue-Shifted Absorption of X-Shaped Electro-Optic Chromophore
A R T I C L E S
Figure 9. Square root of X chromophore self-assembled film 532 nm SHG
response (I2ω, arbitrary units) as a function of the number of trilayers.
Inset: SHG response as a function of fundamental beam incident angle
from a float glass slide having a film on either side.
Contact mode AFM measurements were performed on each
of five SAS samples, from one to five trilayers. This study
reveals an increased grain texture and an RMS surface roughness
going from 5.6 to 12.4 Å, respectively, for 1 × 1 µm2 scan
areas (Figure 10). Grain formation is a common phenomenon
in layer-by-layer assemblies.9,10 The RMS surface roughnesses
compare favorably with those derived by XRR.
Discussion
Synthetic Strategies. To asymmetrically introduce all (E)-
arylvinylene D/A functionalities, an orthogonal approach is
applied using a sequence of the Wittig-Horner olefination and
Heck coupling reactions. It begins with the separate synthesis
of the aromatic “arms” terminated by aldehyde and vinyl
functionalities and the construction of a “core” containing double
o-phosphonate-activated methylene groups and double o-bromo
groups. The Wittig-Horner reactions between the phosphonate
core 2 and TBDMS-protected 4-[N,N-bis(2-hydroxyethyl)amino]-
benzaldehyde 3 afford the desired two all-(E)-phenylenevinyl
arms in 54% yield. The remarkable E stereoselectivity achieved
with aryl-stabilized phosphonates has been widely demonstrated
in the literature and is commonly employed to prepare (E)-
stilbenes.36 In contrast, conventional Wittig olefination employ-
ing double o-triphenylphosphonium ylides, in the present
synthesis, results in a mixture of Z/E-isomers and in significantly
lower yield. This is clearly a result of the weak stereoselectivity
of triphenylphosphonium ylides bearing mildly conjugating
phenyl substituents,36 whereas the Heck reaction is known to
be highly E stereoselective.37 The subsequent double Heck
coupling reaction results in the other two all-(E)-pyridine-vinyl
Figure 10. AFM images at 1 × 1 µm scan areas of X chromophore self-
assembled films composed of one (A) and five (B) trilayers.
(325 nm), consistent with the AM1/ZINDO computed HOMO-
LUMO CT excitation of λmax ) 369 nm for this chromophore.18
The blue-shifted X chromophore HOMO-LUMO CT excitation
can be understood to first order in terms of the excitonic
coupling of transition dipoles. According to molecular exciton
theory,38 the angle between the transition dipoles and their
interconnected axis, θ, is a key parameter defining the direction
and magnitude of the band shift (see Figure 11A). The excitonic
coupling of transition dipoles leads to a first excited state
composed of a pair of excitonic states (Figure 11A). In the case
of θ > 54.7°, excitation to the upper excitonic state is optically
allowed, and optical transitions from the ground state populate
the upper excitonic state, resulting in the spectral blue-shift.
When θ < 54.7°, excitation to the lower excitonic state is
optically allowed, and optical transitions from the ground state
populate the lower excitonic state, resulting in the spectral red-
shift. In the X chromophore, intramolecular coupling of
transition dipoles in 2D is therefore sufficiently strong as to
afford a dramatically blue-shifted optical maximum.
3
arms. The observed coupling constant J values for the vinyl
1
protons in the BPBAB and X-CHR H NMR spectra are all
∼16.0 Hz, consistent with an all-E configuration.
Optical Properties. The X-CHR chromophore exhibits a
remarkably blue-shifted optical absorption maximum both in
CH2Cl2 solution (357 nm) and in the self-assembled superlattices
(37) (a) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009. (b)
Cabri, W.; Candiani, I. Acc. Chem. Res. 1995, 28, 2. (c) de Meijere, A.;
Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379.
(38) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965,
11, 371.
(36) Maryanoff, B. E.; Reitz, A. B. Chem. ReV. 1989, 89, 863.
9
J. AM. CHEM. SOC. VOL. 128, NO. 18, 2006 6203

A R T I C L E S
Kang et al.
length (400 nm) is close to the wavelength of the electronic
resonance in X-CHR (340 nm in methanol), the resonance
enhancement effect on the dynamic âzzz value obtained at 800
nm must be taken into account. Applying the simple two-level
model,41 the static, intrinsic first hyperpolarizability â0 can be
estimated from the experimental HRS âλ according to eq 2:
â0 ) âλ(1 - (2λmax/λ)2)(1 - (λmax/λ)2)
(2)
Here λ is the HRS fundamental wavelength, and λmax is the
maximum absorption wavelength. We derive a resonance
enhancement factor of 4.40. â0 can be then estimated to be (420
( 17) × 10-30 esu, in excellent agreement with the AM1/
ZINDO computation-derived âω)0 of 407 × 10-30 esu.18
The X chromophore self-assembled thin films exhibit a
second-order NLO macroscopic response ø(2) ∼ 232 pm/V
333
at 1064 nm, larger than that of related azobenzene-based self-
assembled superlattices (ø(2) ∼ 180 pm/V at 1064 nm).9c
333
Moreover, these X-chromophore-based films are completely
transparent at the fundamental laser wavelength and at the 532
nm SHG output wavelength (2ω) (Figure 6). Therefore, resonant
enhancement due to overlap between the film CT absorption
(λmax ) 325 nm) and the second harmonic wave should be
relatively small, while it is known that such resonant effects
Figure 11. (A) Exciton band energy diagram for a molecular dimer with
coplanar transition dipoles inclined with respect to the interconnected axis
by the angle θ.38 (B) Schematic description of presumed aggregation of
X-CHR in solution with θ < 54.7°, resulting in a blue-shift. Arrows show
the directions of dipole moments. (C) Schematic depiction of the organiza-
tion of the X chromophore in self-assembled superlattices with θ > 54.7°,
resulting in a blue-shift. Arrows show the directions of dipole moments.
can enhance experimental azobenzene-based SAS film (λmax
)
575 nm) nonlinearities by as much as 1 order of magnitude.42
The EO coefficient r33 can be related to ø(2)333 according to eq
3,43 reasonably assuming a two-level dispersion model and
neglecting dispersion of the local field factors.
In X chromophore solutions, concentration-dependent 1H
NMR spectroscopy (Figure 4) reveals that the chromophore
molecules are aggregated primarily between the pyridine vinyl
arm portions through π-π interactions, while the other two
phenylenevinyl arm portions cannot undergo close stacking due
to the bulky TBDMS protecting groups, thus resulting in an
antiparallel aggregate with two considerably slipped chromo-
phore molecules (e.g., Figure 11B). The chromophores presum-
ably aggregate in a “head-to-head”-like manner (θ < 54.7°), a
type of J-aggregation, consistent with the X-CHR UV-vis
spectra in CH2Cl2 solution, where a bathochromically shifted
aggregation band is observed (Figure 3).
The additional blue-shifting of the film λmax versus that in
solution (Figures 3 and 6) is consistent with intermolecular
dipole-dipole coupling within the closely packed chromophore
layers, where strong π-π interactions are likely.39 The AM1
semiempirical quantum mechanical energy-minimized structure
of a “bidentate” quaternized X chromophore unit, with two
covalently anchored siloxane functionalities fixed on an imagi-
nary plane, yields a thickness of 24.5 Å. Comparing this
thickness with the XRR-derived chromophore monolayer thick-
ness of 22.2 Å argues that the chromophores are more or less
perpendicular to the surface and θ reaches a value of ∼65° (see
Figure 11C). In this case, a blue-shift of the electronic transition
is expected. In addition to θ, intermolecular distances and
aggregation number will influence the spectral evolution. In
particular, the magnitude of the band shift is also an index of
the structural order within the monolayer. Such spectral shifts
are well-known for azobenzene and other dyes in closely packed
self-assembled monolayers and LB films.40
(3ω02 - ω2)(ω02 - ω′2)(ω02 - 4ω′2)
2
n4
(2)
333
r33
)
ø
(3)
2
3ω0 (ω02 - ω2)2
Here ω and ω′ are EO and SHG fundamental frequencies,
respectively (i.e., 1310 and 1064 nm, respectively); ω0 is the
first resonance frequency (corresponding to λmax), and n is the
refractive index. Reasonably assuming an index of refraction
of 1.6 for self-assembled organic films of this type,10b,c r33 at a
1310 nm EO working fundamental is estimated to be ∼45 pm/V
for X-chromophore-derived SAS films versus ∼9.4 pm/V for
the azobenzene-based SAS film. It can be seen that the electro-
optic response of X-chromophore-based film is comparable to
those of other high-efficiency organic-based films44 and inor-
ganic materials, such as LiNbO3 (r33 ) 31 pm/V at 1310 nm).2c,d
Structural Characteristics of Self-Assembled X Chro-
mophore Superlattices. Our previous studies established that
the present layer-by-layer self-assembly procedure affords
microstructurally regular polar multilayer superlattices without
loss of acentricity (randomization of chromophore orientation)
(39) (a) Gunatatne, T.; Kennedy, V. O.; Kenney, M. E.; Rodgers, M. A. J. J.
Phys. Chem. A 2004, 108, 2576. (b) Engelking, J.; Wittemann, M.; Rehahn,
M.; Menzel, H. Langmuir 2000, 16, 3407.
(40) (a) Nieuwkerk, A. C.; Marcelis, A. T. M.; Sudho¨ter, E. J. R. Langmuir
1997, 13, 3325. (b) Everaars, M. D.; Marcelis, A. T. M.; Sudho¨ter, E. J. R.
Langmuir 1996, 12, 3964. (c) Shimomura, M.; Aiba, S.; Tagima, N.; Inoue,
N.; Okuyama, K. Langmuir 1995, 11, 969.
(41) Qudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664.
(42) (a) Lundquist, P. M.; Yitzchaik, S.; Zhang, T.; Kanis, D. R.; Ratner, M.
R.; Marks, T. J.; Wong, G. K. Appl. Phys. Lett. 1994, 64, 2194. (b) Wang,
L.; Yang, Y.; Marks, T. J.; Liu, Z.; Ho, S.-T. Appl. Phys. Lett. 2005, 87,
161107/1.
(43) Singer, K. D.; Kuzyk, M. G.; Sohn, J. E. J. Opt. Soc. Am. 1987, 4, 968.
(44) (a) Penner, T. L.; Motschmann, H. R.; Armstrong, N. J.; Ezenyilimba, M.
C.; Williams, D. J. Nature 1994, 367, 49. (b) Robinson, B. H.; et al. Chem.
Phys. 1999, 245, 35.
X Chromophore NLO/EO Properties. The HRS measure-
ments provide the principal molecular tensor element âzzz
)
(1840 ( 80) × 10-30 esu. Since the second harmonic wave-
9
6204 J. AM. CHEM. SOC. VOL. 128, NO. 18, 2006

Blue-Shifted Absorption of X-Shaped Electro-Optic Chromophore
A R T I C L E S
and diminution of SHG efficiency,9,10 and that the efficient
“topotactic” approach10c-f dramatically increases the rate of the
related azobenzene chromophore deposition and results in self-
assembled chromophoric films with greatly enhanced structural
regularity and second-order optical nonlinearity. In the present
study, it can be further seen that this topotactic approach
successfully provides 2D X chromophore deposition, where
double quaternization is crucial. Incomplete, single quaterniza-
tion, which would result in alteration of the chromophore optical
properties, disorder of chromophore alignment, and diminution
of chromophore NLO response, is apparently minimal. The
XRR-derived molecular footprint of the X chromophore mono-
layer is found to be ∼80 Å2, approximately twice that of the
self-assembled 4-ICH2C6H4SiCl2I coupling layer (∼40 Å2, step
i, Scheme 2).10a,c It therefore appears that each X chromophore
is favorably anchored at two adjacent surface benzyl iodide
functionalities of the coupling layer (step ii, Scheme 2), and an
almost complete surface quaternization/coverage is achieved.
This surface chromophoric coverage is slightly larger than those
of related 1D azobenzene and heterocycle-based chromophore
SA films (footprint ∼ 47-57 Å2) deposited on the similar
coupling layer surface, where the degree of surface quaterniza-
tion of chromophore precursor is found to be 80-90% of the
available surface benzyl halide groups.10a,c This result suggests
that the double pyridylvinyl functionalities in X chromophore,
a bidentate-like form, may facilitate the self-limiting chemi-
sorption of the chromophore units via a type of “chelate effect”,
providing a greater packing density than those of related 1D
chromophores. It is well-known in coordination chemistry that
metal-ligand complexes possessing chelating ligands exhibit
enhanced stabilities versus nonchelating analogues.45 This
entropy-driven chelate effect is also found to provide well-
packed and highly oriented self-assembled monolayers on gold
generated from bidentate thiols46 and on ITO surfaces formed
from multi-chlorosilane-tethered molecules.47 In addition, likely
π-π interactions between X chromophore π system planes (as
seen in the blue-shifted λmax of the X chromophore SA film,
vide supra) may also facilitate greater structural regularity/
packing density. Addition of subsequent layers results in an
equal molecular footprint of ∼80 Å2 (step iii, Scheme 2),
indicating that the footprint remains constant upon multilayer
formation.
by the linear dependence of X-ray reflectivity-derived multilayer
thicknesses on the number of assembled trilayers (Figure 8).
Multilayer structural uniformity is further evidenced in the AFM
images that show that the present multilayer films have very
smooth surface morphologies (Figure 10). Finally, for a micro-
structurally regular polar multilayer, with minimal self-absorp-
tion, the SHG intensity (I2ω) is expected to scale quadratically
with the number of chromophore deposition cycles because the
incident light wavelength (1.06 µm) is large compared to the
SA film thickness.9,10 Indeed, the observed linear dependence
of (I2ω 1/2
on the number of trilayers demonstrates (Figure 9)
)
that approximately equal densities of uniformly polar-oriented
chromophores are deposited in each assembled trilayer while
maintaining good structural regularity.
Conclusions
We have shown that a novel type of X-shaped 2D EO
chromophore with extended conjugation displays a markedly
blue-shifted optical maximum (357 nm in CH2Cl2) while
maintaining a very large first hyperpolarizability. HRS measure-
ments on the new chromophore at an 800 nm fundamental
wavelength provide a â value of 1840 × 10-30 esu. Both the
experimental linear and nonlinear optical response properties
exhibit good agreement with computational predictions. The new
chromophore can be successfully integrated into structurally
regular and acentric self-assembled multilayer films via a layer-
by-layer chemisorptive siloxane-based approach. The chro-
mophoric film exhibits a dramatically blue-shifted optical
maximum (325 nm) while maintaining a large EO response
(ø(2) ∼ 232 pm/V at 1064 nm; r33 ∼ 45 pm/V at 1310 nm).
333
This work demonstrates an attractive approach to developing
EO materials offering improved nonlinearity-transparency
trade-off characteristics. Further studies will focus on optimizing
â response and optical properties, as well as on an in-depth
understanding of this special nonlinearity-transparency rela-
tionship by a combination of molecular modeling and modifica-
tion.
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 North-
western University MRSEC for support of characterization
facilities (DMR 0076077). X-ray reflectivity measurements were
performed at Beam Line X23B of the National Synchrotron
Light Source, which is supported by the U.S. Department of
Energy. We thank Dr. S. Keinan and Prof. M. Ratner for
computational collaboration, and Drs. P. Zhu and A. Facchetti
for helpful discussions.
The uniform growth of the self-assembled multilayers is
indicated by the linear dependence of the X chromophore
longitudinal HOMO-LUMO charge-transfer absorbance at 325
nm on the number of assembled trilayers (Figure 6) as well as
(45) Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; W. B. Saunders:
Philadelphia, 1977.
Supporting Information Available: Complete ref 44b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(46) (a) Grag, N.; Lee, T. R. Langmuir 1998, 14, 3815. (b) Shon, Y.-S.; Lee, T.
R. Langmuir 1999, 15, 1136. (c) Park, J.-S.; Vo, A. N.; Barriet, D.; Shon,
Y.-S.; Lee, T. R. Langmuir 2005, 21, 2902.
(47) Huang, Q.; Evmenenko, G. A.; Dutta, P.; Lee, P.; Armstrong, N. L.; Marks,
T. J. J. Am. Chem. Soc. 2005, 127, 10227.
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