Exceptional molecular hyperpolarizabilities in twisted π-electron system chromophores

Article abstract of DOI:10.1002/anie.200501581

(Chemical Equation Presented) Nonlinear optics: Twisted π-electron system electrooptic (EO) chromophores with exceptional molecular hyperpolarizabilities (-488 000×10^-48 esu at 1907 nm) were designed and synthesized. Crystallographic analysis r

Full text of DOI:10.1002/anie.200501581

Communications
Electrooptic Systems
structurally, complicating synthesis and introducing potential
chemical, thermal, and photochemical instabilities.[6] Alter-
native routes to verylarge- b chromophores would clearlybe
desirable, and there is growing evidence that simple two-level
systems may not provide access.[7]
Recent theoretical work suggests that unconventional
chromophores with twisted p-electron systems bridging D and
A substituents (TICTOID = twisted intramolecular charge-
transfer; Scheme 1) mayexhibit unprecedented hyperpolar-
DOI: 10.1002/anie.200501581
Exceptional Molecular Hyperpolarizabilities in
Twisted p-Electron System Chromophores**
Hu Kang, Antonio Facchetti, Peiwang Zhu, Hua Jiang,
Yu Yang, Elena Cariati, Stefania Righetto, Renato Ugo,
Cristiano Zuccaccia, Alceo Macchioni,
Charlotte L.Stern, Zhifu Liu, Seng-Tiong Ho, and
Tobin J.Marks*
Molecule-based electrooptic (EO) materials are of intense
research interest for understanding how light interacts with
matter and for applications in photonic technologies such as
high-speed optical communications, integrated optics, and
optical data processing and storage.[1] In such materials, the
second-order susceptibilitytensor governing EO response
(r33), is governed both bythe net polar microstructural order
and the microscopic molecular first hyperpolarizability tensor
(b). Large b values are essential for large EO response, and
the quest for higher performance EO chromophores presents
a daunting challenge.[1] To date, effective chromophores have
been designed according to similar principles embodied in the
classical “two-level” model: conjugated p systems end-
capped with donor (D) and acceptor (A) moieties.[2] Elegant
efforts have sought maximum b byoptimizing D and A
strengths and conjugation pathways,[3] directed by“bond
length alternation”[4] and “auxiliarydonor and acceptor”
models.[5] Such strategies utilize extended planar p-conjuga-
tion, resulting in chromophores that are inherentlyelaborate
Scheme 1. Structures of TICTOID and TMC (twisted p-electron system
molecular chromophore).
izabilities through non-classical mechanisms.[8] These would
have relativelysimple biaryl structures in which b is sterically
tunable through R1, R2 modification of the interplanar
dihedral angle (q). Large b magnitudes are predicted at q
ꢀ 70–858,[8a] with twist-induced reduction in D–p–A conjuga-
tion leading to charge-separated zwitterionic ground states.
The intriguing question is whether such molecules, with small
[*] H. Kang, Dr. A. Facchetti, Dr. P. Zhu, Dr. H. Jiang, Dr. Y. Yang,
C. L. Stern, Prof. T. J. Marks
Department of Chemistryand the Materials Research Center
Northwestern University
numbers of p-electrons, could therebyexhibit far larger
b
2145 Sheridan Road, Evanston, IL 60208-3113 (USA)
Fax: (+1)847-491-2990
E-mail: t-marks@northwestern.edu
values than conventional planar chromophores.
We report here the first realization of such TICTOID
chromophores, that theyhave unprecedented hyperpolariz-
abilities on the order of 10–20 ” larger than previously
observed,[1–5] and that these are not simple two-level systems.
The new chromophores (Scheme 1) were designed according
to the following criteria: 1) Synthetically challenging tetra-
ortho-alkylbiaryl cores should enforce large interplanar
angles, judging from bimesityl (q ꢀ 908).[9] 2) Dicyanometh-
anide and pyridinium groups should be effective D and A
substituents, with dicyanomethanide more stable than phen-
oxide.[10] 3) Pyridine alkylations should enhance solubility and
processability(TMC-1 and TMC-2), with styrenic substitution
further enhancing b (TMC-2).
Dr. Z. Liu, Prof. S.-T. Ho
Department of Electrical and Computer Engineering
Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
Prof. E. Cariati, S. Righetto, Prof. R. Ugo
Dipartimento di Chimica Inorganica Metallorganica e Analitica and
Centro di Eccellenza CIMAINA
dell’Università di Milano
and Unità di Ricerca dell’INSTM di Milano
Via Venezian 21, 20133 Milano (Italy)
Dr. C. Zuccaccia, Prof. A. Macchioni
Dipartimento di Chimica
Università di Perugia
TMC syntheses (Scheme 2) begin with precursor 1,
synthesized as reported elsewhere.[10] Pd-catalyzed
NaCH(CN)2 coupling affords 2, which is then regioselectively
quaternized[11] and deprotonated to afford TMC-1. Precursor
3 is synthesized by Pd-catalyzed coupling of 1-iodo-4-vinyl-
benzene with NaCH(CN)2. Subsequent Heck-coupling of 1
and 3 affords precursor 4, which is then alkylated and
Via Elce di Sotto 8, 06123 Perugia (Italy)
[**] Supported byDARPA/ONR (SP01P7001R-A1/N00014-00-C), the
NSF-Europe Program (DMR-0353831), and MIUR(PRIN 2004–
2005). We thank Dr. S. Keinan and Prof. M. Ratner for computational
collaboration.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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than
in
cyclopentadienylidene-1,4-dihydropyridines.[17]
Together, these metrical parameters confirm a charge-sepa-
rated zwitterionic TICTOID ground state. Importantly, close
correspondence between solid-state 13C NMR and optical
spectra and those recorded in solution[12] argue that this
[18]
structure persists essentiallyunchanged in solution.
As
might be expected, the present zwitterion packs centrosym-
metricallyin pairs (Figure 1), likelya result of electrostatic
dipole–dipole interactions (reasonable considering the large
computed dipole moments).[19]
Optical spectra further support a zwitterionic ground
state, with oscillator strengths unexceptional for two-level
chromophores.[1–5] Thus, TMC-1 exhibits two fairlyshort
wavelength maxima, assigned to pyridinium and phenyl
subfragment high-energyintra-ring excitations, and one low-
energyinter-subfragment charge-transfer (CT) excitation
(Figure 2a).[8] The TMC-1 CT bands exhibit strong negative
Scheme 2. Synthesis of twisted p-electron chromophores. Tf=trifluoro-
methanesulfonyl.
deprotonated to yield TMC-2. The new zwitterions are air-
and moisture-stable, purifiable bycolumn chromatograph,y
[12]
and were characterized byconventional techniques.
Ther-
mogravimetric analysis indicates very high chromophore
thermal stability( Td ꢀ 3068C for TMC-1, ꢀ 3308C for
1
TMC-2),[12] while H NMR indicates stabilityin [D ]DMSO
6
solution at 1508C for periods of hours under air.
Single-crystal diffraction data for TMC-1[13] reveal a large
arene–arene dihedral twist angle (898; Figure 1). The fact that
twisting is governed primarilybytetra -ortho-methylbiaryl
Figure 2. a) Optical absorption spectra of TMC chromophores in
Figure 1. Left: ORTEP drawing (50% probabilityellipsoids) of the
TMC-1 molecule. Hydrogen atoms are omitted for clarity. Right:
Packing diagram.
CH2Cl2 solution. Solid line: TMC-1; dashed line: TMC-2. b,c) Concen-
tration-dependent TMC-1 spectra in CH2Cl2 solution (b) (3.0”10À3
–
2.0”10À5 m) and in CHCl3 solution (c) (5.6”10À4–4.5”10À6 m). Arrows
indicate changes in CT bands upon dilution. The monomer (dashed
line) and dimer (dotted line) spectra were derived from the data at two
different concentrations and Kdimerize. d) Variable-concentration fluores-
cence spectra (lex =300–350 nm) of TMC-2 in CH2Cl2. Intensities are
normalized. I=fluorescence intensity. Solid line: 5”10À4 m, dotted
line: 5”10À5 m, dashed line: 5”10À6 m, dash-dotted line: 2.5”10À6 m.
steric interactions and is independent of chromophore
architecture is argued bynearlyidentical angles in several
other TMC-type molecules.[10] The TMC-1 (ring)C–C(ring)
distance (1.488(5) Š) is close to that in typical biphenyls
( ꢀ 1.487 Š),[14] indicating steric crowding, consequent reduc-
tion in inter-ring p-conjugation, and departure from quinoidal
[14]
=
structures where (ring)C C(ring) ꢀ 1.349 Š. The phenyl-
solvatochromism – large blue-shifts with increasing the
solvent polarity(Table 1), indicating that the ground state
dipole moment is substantiallylarger than in the excited state,
consistent with the dominant zwitterionic ground state (and a
negative b). TMC aggregation observed in the solid state is
further evidenced in CH2Cl2 solution byconcentration-
dependent optical spectra (Figure 2b), exhibiting a red-shift
and increase of CT band intensityupon dilution in the range
of 10À3–10À4 m (H-aggregation), with an isosbestic point
enedicyanomethanide fragment displays different bond
length patterns than in typical TCNQs;[14,15] the (NC)2C-
bound phenylene ring has less quinoidal character, and the
(dicyanomethanide)C–C(aryl) distance (1.463(5) Š) lacks
TCNQ C C(CN)2 exocyclic character ( ꢀ 1.392 Š).[14] This
=
metrical pattern indicates negative charge localization on the
À
C(CN)2 group, also evident from C CN bond shortening and
ꢁ
C N bond elongation (TCNQ distances are 1.427 Š and
1.144 Š, respectively).[14] Finally, there is significant pyridi- supporting well-defined aggregation equilibria.[20]
nium aromatic character, with molecular dimensions paral-
To estimate binding energetics, the TMC-1 extinction
leling those in N-methyl-p-phenylpyridinium salts[16] rather
coefficient was analyzed[20] as a function of concentration,
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Communications
vely).[21c] The trends in N versus
logc (Figure 3) clearlyshow that
Table 1: Solvatochromic optical spectroscopic data and estimated changes in dipole moment from
ground to excited state for TMC chromophores in selected solvents.[a]
monomer
predominates
in
Solvent (ET)
TMC-1
lmax (e)[c]
TMC-2
[D6]DMSO (er25 = relative permit-
tivityat 25 8C = 46.45) for both
TMC-1 and TMC-2 over the entire
concentration range (see Support-
lmax (e)[b]
Dm[d]
lmax (e)[b]
lmax (e)[c]
Dm[d]
MeOH (0.76)
DMF (0.40)
297
310
451
478
397
427
–
–
64
151
ing Information). In CD2Cl2 (er25
=
CH2Cl2 (0.31)
THF (0.21)
314 (27200)
320
569 (1840)
592
433 (38400)
451
540[e] (2090)
578[e]
8.93), TMC-1 is exclusivelymono-
meric onlyat the lowest concentra-
tions examined (4 ” 10À6 m, Support-
ing Information) while dimers (see
Supporting Information) and even
larger aggregates (Figure 3, Sup-
porting Information) are present at
[a] lmax in nm, e in mÀ1 cmÀ1, Dm in 10À30 Cm. Solvent polaritygiven bythe normalized ET parameter.
[b] Assigned to intra-subfragment excitation. Another high-energysubfragment excitation overlaps with
the solvent (except CH2Cl2) and is not tabulated here. [c] Assigned to low-energyinter-subfragment
charge-transfer (CT) excitation. [d] DFT-derived change in dipole moment from ground to excited state.
[e] Deconvoluted from the subfragment excitation band and assigned to inter-subfragment CT.
assuming a simple dimerization model, and yields Kdimerize
=
the highest concentrations (Figure 3). TMC-2 exhibits a
greater aggregation tendencyand is not exclusivelymono-
meric even at 5 ” 10À6 m (see Supporting Information). For
TMC-2, aggregates larger than dimers are present at the
250 Æ 30mÀ1 and DGdo imerize = À13.6 Æ 0.3 kJmolÀ1 in CH2Cl2.
Concentration-dependent TMC-1 studies (Figure 2c) in less
polar CHCl3 yield larger Kdimerize and DGodimerize values of
13300 Æ 1420mÀ1 and À23.5 Æ 0.3 kJmolÀ1, respectively, not highest concentrations (Figure 3, Supporting Information).
unexpectedlyindicating that the aggregation is weaker in
more polar solvents and also somewhat stronger than in
typical planar merocyanine zwitterions.[20]
Hyperpolarizabilities were measured at 1907 nm in
CH2Cl2 byelectric field-induced second harmonic generation
(EFISH)[22] and track the same concentration-dependence as
observed in the optical and PGSE spectra. Thus, the TCM-1
and TCM-2 mb values show pronounced concentration
dependence due to the aforementioned aggregation effects
The TMC-2 spectrum features an intense band at 433 nm
in CH2Cl2 doubtless involving stilbenyl subfragment excita-
tion,[8b] overlapping a weaker CT band centered at 540 nm
(Figure 2a). Both subfragment and CT bands again exhibit
negative solvatochromism (Table 1). Variable-concentration
TMC-2 fluorescence spectra (Figure 2d) reveal a clear
transition from dimer (515 nm) to monomer (485 nm) in the
range of 5 ” 10À4–2.5 ” 10À6 m, providing confirmation of
aggregation.
(Figure 4). The TCM-1 mb rapidlyincreases as concentration
[1–5]
falls, with mb saturating at a verylarge
À24000 Æ 4320 ”
10À48 esu at 5 ” 10À6 m. The increase with dilution indicates
Additional quantitative information on the state of TMC
chromophore aggregation is provided byPGSE (pulsed field
[21]
gradient spin-echo) NMR spectroscopy(Figure 3).
Here,
experimentallydetermined translational self-diffusion coef-
ficients (Dt), hydrodynamic radii (rH), volumes (VH), and the
ratio between VH and the van der Waals volume afford
aggregation numbers (N) readilyidentifying the aggregation
level in solution (N = 1.0, 1.5, or 2.0 means 100% monomer,
50% monomer + 50% dimer, or 100% of dimer, respecti-
&
Figure 4. EFISH-derived mb data for chromophores TMC-1 ( ) and
*
TMC-2 ( ) versus concentration in CH2Cl2 at 1907 nm. Lines are
drawn as guides to the eye. c=concentration [m].
dissociation of (presumablycentrosmy metric) aggregates,
paralleling the PGSE data, as expected. Note that mb
approaches a maximum at concentrations where the PGSE
measurements indicate the presence of more than 80%
monomer. TCM-2 deaggregation occurs at somewhat lower
concentrations (10À5–10À6 m), in good agreement with the
PGSE data, with an unprecedented mb value of À488000 Æ
48800 ” 10À48 esu at 8 ” 10À7 m.[23] From the reliablycomputed
Figure 3. PGSE NMR-derived aggregation numbers (N) as a function
m
[19] we estimate b0.65eV ꢀ 9800 ” 10À30 esu. To our knowledge,
*
of concentration for chromophores TMC-1 in CD2Cl2 ( ) and
these are the largest off-resonance mb and b values ever
~
!
&
[D6]DMSO ( ), and TMC-2 in CD2Cl2 ( ) and [D6]DMSO ( ). Lines
[1–5]
are drawn as guides to the eye. c=concentration [m].
achieved for anymolecule.
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[24]
[8] a) S. Keinan, E. Zojer, J.-L. Bredas, M. A. Ratner, T. J. Marks,
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[9] R. Fröhlich, H. Musso, Chem.Ber. 1985, 118, 4649 – 4651.
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PreliminaryTeng and Man
measurements on poled
TMC-1 and TMC-2 containing guest–host polymers reveal
verylarge EO coefficients ( r33) at 1310 nm. Poly(vinylphenol)
films containing 10 wt% TMC-1 and 5 wt% TMC-2, poled at
100 VmmÀ1, exhibit nonresonant r33 responses of 48 and
320 pmVÀ1, respectively. Lower EO responses are observed
in less polar matrices, presumablydue to the aggregation.
These results suggest obvious molecular and macromolecular
modification strategies, currentlyunder investigation, to
address the aggregation issue and to further enhance r33.
In summary, twisted p-electron system chromophores
have been prepared and exhibit exceptional molecular hyper-
polarizabilities, with nonresonant mb values as high as
À488000 ” 10À48 esu, while preliminarypoled guest–host
experiments indicate promise for EO applications. An
interesting observation here is that the ultralarge hyper-
polarizabilities exhibited bythese unconventional chromo-
phores seem far beyond classical two-level behavior. Kuzyk
argues that the b responses of all organic chromophores
prepared to date fall far short of the theoretical quantum
[11] A. Abbotto, S. Bradamante, A. Facchetti, G. A. Pagani, J.Org.
Chem. 1997, 62, 5755 – 5765.
[12] See Supporting Information.
[13] TMC-1 crystals were grown from saturated acetonitrile solu-
tions. All measurements were made on a Bruker SMART CCD
diffractometer with graphite-monochromated MoKa (0.71073 Š)
radiation at 153(2) K. The structure was solved bydirect
methods and Fourier techniques with SHELXTL. Non-hydro-
gen atoms were refined anisotropically. The hydrogen atoms
were included in idealized positions, but not refined. C28H37N3,
M = 415.60, orthorhombic, Pbca, a = 15.6106(16), b =
16.1279(17), c = 19.714(2) Š, V= 4963.2(9) Š3, Z = 8, 1cald
=
1.110 gcmÀ3, 2qmax = 57.648. Of the 43353 reflections which
were collected, 6031 were independent (Rint = 0.0654), 269
parameters, R1 = 0.0865 (for reflections with I > 2s(I)), wR2 =
0.3100 (for all reflections). CCDC 282594 contains the supple-
mentarycrystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[25]
limits, for reasons that are presentlynot entirelyclear.
Twisted p-electron system chromophores may provide new
insight into the reasons.
[14] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Received: May10, 2005
Revised: September 20, 2005
Published online: November 21, 2005
Orpen, R. Taylor, J.Chem.Soc.Perkin Trans.2
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[17] H. L. Ammon, G. L. Wheeler, J.Am.Chem.Soc. 1975, 97, 2326 –
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[18] Variations in q should significantlyaffect the relative contribu-
tions of quinoid versus zwitterion structures, hence 13C NMR
and optical spectra.
Keywords: chromophores · electrooptics · nonlinear optics ·
.
pi systems · zwitterions
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