Highly dipolar, optically nonlinear adducts of tetracyano-p-quinodimethane: Synthesis, physical characterization, and theoretical aspects

Article abstract of DOI:10.1021/ja963923w

A new series of nonlinear optical molecules are described where the ground state polarization is predominantly zwitterionic when the molecules are dissolved in solution. The molecules, which are derived in general from facile reactions between tertiary amines and tetracyano-p-quinodimethane (TCNQ), are of a type where the stabilization of the charge-separated ground state is favored by an increase in aromaticity over the neutral, quinoidal forms of the molecules. The measured second-order optical nonlinearity of one in the series has been measured by hyper-Rayleigh scattering and a figure of merit value, μβ(0), being the product of the dipole moment and static first hyperpolarizability, is found to be 9500 x 10^-48 esu. This value, which is higher than most other reported values, is taken from studies in chlorinated solvents of relatively low polarity, but the discussion emphasizes the evolution of μβ(0) with solvent polarity, showing that even higher values could be expected with only modest increases in the polarity of the surrounding medium. The analysis of experimental data taken during dipole moment studies is thoroughly examined, and it is concluded that full account must be taken of the molecular shape to correlate the results with theoretical calculations. An ellipsoidal reaction field model is preferred for these highly one-dimensional molecules having strongly anisotropic polarizabilities.

Full text of DOI:10.1021/ja963923w

3144
J. Am. Chem. Soc. 1997, 119, 3144-3154
Highly Dipolar, Optically Nonlinear Adducts of
Tetracyano-p-quinodimethane: Synthesis, Physical
Characterization, and Theoretical Aspects
†
†
†
,†
Marek Szablewski, Philip R. Thomas, Anna Thornton, David Bloor,*
,
†
‡
‡
Graham H. Cross,* Jacqueline M. Cole, Judith A. K. Howard,
§
§,|
§
⊥
Massimo Malagoli, Fabienne Meyers, Jean-Luc Br e´ das, Wim Wenseleers, and
⊥
Etienne Goovaerts
Contribution from the Departments of Physics and Chemistry, UniVersity of Durham,
Durham DH1 3LE, U.K., SerVice de Chimie des Materiaux NouVeaux, UniVersit e´ de Mons
Hainaut, B-7000 Mons, Belgium, Physics Department, UniVersity of Antwerp-UIA,
UniVersiteitsplein 1, B-2610 Antwerp, Belgium, and The Beckman Institute, California
Institute of Technology, Pasadena, California 91125
X
ReceiVed NoVember 12, 1996
Abstract: A new series of nonlinear optical molecules are described where the ground state polarization is
predominantly zwitterionic when the molecules are dissolved in solution. The molecules, which are derived in
general from facile reactions between tertiary amines and tetracyano-p-quinodimethane (TCNQ), are of a type where
the stabilization of the charge-separated ground state is favored by an increase in aromaticity over the neutral, quinoidal
forms of the molecules. The measured second-order optical nonlinearity of one in the series has been measured by
hyper-Rayleigh scattering and a figure of merit value, µâ(0), being the product of the dipole moment and static first
-
48
hyperpolarizability, is found to be 9500 × 10
esu. This value, which is higher than most other reported values,
is taken from studies in chlorinated solvents of relatively low polarity, but the discussion emphasizes the evolution
of µâ(0) with solvent polarity, showing that even higher values could be expected with only modest increases in the
polarity of the surrounding medium. The analysis of experimental data taken during dipole moment studies is
thoroughly examined, and it is concluded that full account must be taken of the molecular shape to correlate the
results with theoretical calculations. An ellipsoidal reaction field model is preferred for these highly one-dimensional
molecules having strongly anisotropic polarizabilities.
Introduction
conjugated π electron system such as a benzene ring or polyene
are classic examples, the simplest of which is p-nitroaniline. In
such a system there is an asymmetry in the polarization of
electrons within the π system leading to a dipole.
Nonlinear optical (NLO) phenomena underpin many of the
operations performed by devices in telecommunications system
switching nodes and provide a means for optical signal
processing in general. Organic materials have now long been
recognized as a potential alternative to inorganic glasses and
semiconductors for device fabrication. Nonresonant optical
nonlinearities are the highest among the organics where control
over the optical nonlinearity has been approached through
The degree to which the two resonance states of the molecule,
one “neutral” and one charge separated or, zwitterionic,
contribute to the ground state structure defines the polarization
+
-
(
i.e., D-π-A to D -π-A ). This is inevitably sensitive to
the surrounding environment which acts to perturb the “vacuum”
polarization. One of the simplest descriptions of environmental
influences over dipole moment lies within Onsager’s reaction
1
-4
understanding the molecular structure/property relationships.
Much activity has concentrated on increasing the magnitude of
8
,9
field theory.
Here, the polarized medium surrounding the
the second-order molecular hyperpolarizability, â, in organic
chromophores.5
-7
molecular dipole exerts a “reaction field” back onto the molecule
and acts through its linear polarizability, R, to further enhance
the dipole. More recently, interest has turned to the evolution
Those chromophores comprising an electron
donor (D) linked to an electron acceptor (A) by means of a
*
Authors to whom correspondence should be addressed.
Department of Physics, University of Durham.
Department of Chemistry, University of Durham.
Universit e´ de Mons Hainaut.
University of Antwerp-UIA.
California Institute of Technology.
of the higher molecular polarizabilities as a function of this
†
10-13
reaction field.
The first hyperpolarizability, â, for example,
‡
§
⊥
in model polyenic systems displays both positive and negative
|
(7) Dhenaut, C.; Ledoux, I.; Samuel, I. D. W.; Zyss, J.; Bourgault, M.;
LeBozec, H. Nature 1995, 374, 339.
(8) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486.
(9) B o¨ ttcher, C. J. F. Theory of Electric Polarization; Elsevier: Am-
sterdam, 1973, Vol. 1; 1978, Vol. 2.
(10) Marder, S. R.; Beratan, D. N.; Cheng, L. T. Science 1991, 252,
1030. Marder, S. R.; Perry, J. R.; Bourhill, G.; Gorman, C. B.; Tiemann,
B. G.; Mansour, K. Science 1993, 261, 186.
(11) Dehu, C.; Meyers, F.; Hendrickx, E.; Clays, K.; Persoons, A.;
Marder, S. R.; Br e´ das, J. L. J. Am. Chem. Soc. 1995, 117, 10127.
(12) Meyers, F.; Marder, S. R.; Pierce, B. M.; Br e´ das, J. L. J. Am. Chem.
Soc. 1994, 116, 10703.
X
Abstract published in AdVance ACS Abstracts, March 1, 1997.
(1) Davydov, B. L.; Kotovshchikov, S. G.; Nefedov, V. A. SoV. J.
Quantum. Electron 1977, 7, 129.
2) Oudar, J. L. J. Chem. Phys, 1977, 67, 446. Williams, D. J. Angew.
Chem., Int. Ed. Engl. 1984, 23, 690.
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Br e´ das J. L.; Pierce, B. M. Science 1994, 265, 632.
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(
(
(
1
(
(
(13) Lu, D.; Chen, G.; Perry, J. W.; Goddard, W. A. J. Am. Chem. Soc.
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S0002-7863(96)03923-6 CCC: $14.00 © 1997 American Chemical Society

Optically Nonlinear Adducts of TCNQ
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3145
(
around 12-13 D) which indicates that the structures are not
strongly polarized even here.
Stabilization of a charge-separated state may be achieved at
a sufficiently high reaction field, i.e. in polar environments. The
dielectric theory indicates that reaction fields are highest when
the solute dipole moment and polarizability and the solvent
dielectric constant are all high. The dipole moment which a
solute molecule therefore has in solution depends on these
factors but also, and significantly, on the stabilization energies
involved in separating charge. In favorable circumstances,
molecules in solution will display all the characteristics of the
polarization responses described on the right-hand side (RHS)
of the BLA diagram. Henceforth therefore we will distinguish
such molecules as RHS types. The merocyanines are such
species while the TCQ derivatives of ref 16 may not be.
Experimentally one might easily identify RHS molecules by
virtue of their negative solvatochromic behavior. The position
of the charge transfer band in the UV/visible absorption
spectrum moves to shorter wavelengths with an increase in
solvent polarity. Incidentally, such molecules will also have
considerably enhanced dipole moments in these media over LHS
(left-hand side) molecules.
Figure 1. Evolution of the dipole moment, µ (solid line), and the
molecular hyperpolarizabilities, R (large dashed), â (small dashed), and
γ (dotted), with bond length alternation (BLA) of model donor-
acceptor polyenes.
17
maxima when considered as a function of a polarizing field
3
directed along the dipolar axis (see Figure 1). The evolution
of the hyperpolarizability reflects the changes in the electronic
structure of the molecule as the field is applied. Theoretical
1
0,12
and experimental studies of D-polyene-A systems
have
thus emphasized the extent to which molecular structure affects
the magnitude and sign of â. A convenient measure of the
evolution of the π electron structure is given by the bond length
alternation,10 BLA (i.e., the average difference between the
lengths of adjacent carbon-carbon double and single bonds
along the polyene segment). When such a field is applied to a
polar polyene structure, the geometry of the molecule undergoes
a “cross-over” to a charge-separated “zwitterionic” structure.
Initially the BLA is conventionally taken as positive, becoming
zero at the “cyanine” limit: the point at which there is equivalent
π electron density in the ground state and lowest charge transfer
excited state. When the structure tends toward the charge-
separated state the BLA becomes negative. The two maxima
for |â| occur on either side of the “cyanine” limit, a positive â,
when BLA is positive, and a negative â when BLA is negative
Among the few RHS NLO molecules reported to date are
1
4
the merocyanine dyes (structure 9, Figure 2), heterocyclic
1
8
betaines (structure 10, Figure 2), and a molecule comprising
19
an imidazolidine TCNQ adduct (DCNQI) (structure 11, Figure
20
2). Metzger reported the synthesis and X-ray crystallographic
structure of a TCQ-pyridinium species (structure 13, Figure
2) with a high degree of charge separation in the crystal
environment. Here a calculated value for the dipole moment
was reported to be 26 D, obtained by a closed-shell INDO
calculation using the crystallographic geometry. Second-
21
harmonic generation was observed by Ashwell in Langmuir-
Blodgett films of the related amphiphilic pyridinium, quinolin-
22
ium, and benzthiazolium (structures 14-16, Figure 2) analogues.
A negative solvatochromic shift was noted in these materials.
One other TCNQ-derived zwitterionic adduct (as indicated by
crystal structure) has been reported in which the donor and
acceptor are linked by a σ bond (structure 17, Figure 2).23
The high electron affinity of the TCQ acceptor and the highly
dipolar nature of adducts containing this group has prompted a
(Figure 1).
It has been found both theoretically and by experiment that
different combinations of donors and acceptors with a wide
range of conjugated “bridges” result in differing degrees of
polarization and hence different magnitudes of â. Most
synthesis programs in the field of organic materials for nonlinear
optics have been targeted toward maximizing a positive â, yet
it is clear that in, for example, merocyanine NLO molecules,
stabilization of the charge-separated state is relatively easy to
24
number of theoretical modeling investigations. Honeybourne,
for example, reported calculations on pyridinium-TCQ species
stressing the negative sign of â and the dipole moment decrease
on excitation to the first excited state (i.e., negative ∆µ). More
25,26
recently Broo and Zerner
investigated the nature of the
14
achieve. Merocyanines in this form are extremely susceptible
to protonation however which removes their optical nonlinear-
ground state structure of 13 (Figure 2) and the effects of
environment on the ground state properties and absorption
spectra. It was concluded that the geometry of the species in
liquid solution reflects intermediate BLA, whereas in the crystal
form, the bond-alternated zwitterionic form predominates. Our
15
ity. The merocyanines achieve this zwitterionic state largely
because there is an increase in aromaticity to be gained from
charge separation. In most other molecules studied for nonlinear
optics this feature is not available or not particularly favored.
In a recent report of some tricyanoquinodimethane (TCQ)
molecules,1 related to those which we report herein, the ground
state structure is still predominantly quinoid-like (see, for
example, structure 12, Figure 2). In this particular compound,
however, it is suggested that due to polarization effects the first
maximum in â has been passed. The solution state dipole
moments measured in chloroform were not exceptional however
(
18) Alcalde, E.; Roca, T.; Redondo, J.; Ros, B.; Serrano, J. L.; Rozas,
I. J. Org. Chem. 1994, 59, 644.
19) Lalama, S. J.; Singer, K. D.; Garito, A. F.; Desai, K. N. Appl. Phys.
Lett. 1981, 39, 940.
20) Metzger, R. M.; Heimer, N. E.; Ashwell, G. J. Mol. Cryst. Liq.
Cryst. 1984, 107, 133.
21) Ashwell, G. J.; Dawnay, E. J. C.; Kuczy n´ ski, A. P.; Szablewski,
6
(
(
(
M.; Sandy, I. M.; Bryce, M. R.; Grainger, A. M.; Hasan, M. J. Chem. Soc.,
Faraday Trans. 1990, 86, 1117.
(22) Ashwell, G. J.; Malhotra, M.; Bryce, M. R.; Grainger, A. M. Synth.
(
14) Fort, A.; Runser, C.; Barzoukas, C.; Combellas, C.; Suba, C.;
Thiebault, A. Proc. SPIE. 1984, 2285, 5.
15) Girling, I. R.; Kolinsky, P. V.; Cade, N. A.; Earls, J. D.; Peterson,
I. R. Optics. Commun. 1985, 55, 289.
16) Boldt, P.; Bourhill, B.; Branchle, C.; Yiwen, J.; Kammler, R.; M u¨ ller,
C.; Rase, J.; Wichern, J. J. Chem. Soc., Chem. Commun. 1996, 793.
17) Brooker, L. G. S.; Keyes, G. H.; Heseltine, D. W. J. Am. Chem.
Soc. 1951, 73, 5350.
Met. 1991, 43, 3173.
(23) Miller, J. S.; Calabrese, J. C. J. Chem. Soc., Chem. Commun. 1988,
63.
(24) Honeybourne, C. L. J. Phys. D.: Appl. Phys. 1990, 23, 245.
(25) Broo, A.; Zerner, M. C. Chem. Phys. 1995, 196, 407.
(26) Broo, A.; Zerner, M. C. Chem. Phys. 1995, 196, 423.
(27) Cole, J. C.; Cole, J. M.; Howard, J. A. K.; Cross, G. H.; Szablewski,
M.; Farsari, M. Acta Crystallogr., Sect. B, in press.
(
(
(

3146 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Szablewski et al.
Figure 2. Chemical structures of compounds referred to in the text.
own observations27 confirm that, in molecules of this type,
crystal field effects stabilize the charge-separated structure.
RHS Molecules Formed from Tertiary Amines and
TCNQ. As a result of our discovery of a novel reaction of
TCNQ with triethylamine28 which led to the synthesis of DEMI
the failure to react of (electron-withdrawing) fluorinated and
(diphenylamino)ethyl tertiary amines with TCNQ.
Although the nonlinear and linear optical properties calculated
3
1,32
and determined experimentally
for 1 are favorable in view
of applications, there are several significant difficulties which
have to be overcome in order to make such materials viable
options for use in nonlinear optical devices, namely solubility,
stability to photo-oxidation, and hydration. The planarity and
high dipole moment encourages aggregation while the presence
of ethylenic bonds and a strong optical transition can sensitize
singlet oxygen formation leading to photo-oxidation.33 The
thermal stability of 1 has been investigated by differential
scanning calorimetry, and decomposition occurs at 243 °C in
air. This is similar to other TCQ-type chromophores which have
(structure 1, Figure 2), we have prepared a range of similar
adducts of TCNQ with tertiary ethylamines (structures 1-8).
All of these adducts have similar π electron systems, a TCQ
acceptor separated from an electron-deficient amino moiety. The
spectral properties of these species are therefore very similar,
comprising a broad charge transfer band in the middle of the
visible region with very little absorption to either side of it.
The low optical absorption between 400 and 500 nm prompted
us to describe these molecules as “blue window” chro-
mophores.2
9,30
16
recently been reported.
The synthetic procedure used to synthesize the analogues (1-
The nonlinearity of the DEMI chromophore is extremely high,
and it is therefore desirable to limit changes to the molecular
architecture to those which will not decrease this property when
attempting to synthesize variants. As the core of the molecule’s
nonlinearity lies in its conjugated backbone linking the quat-
ernized nitrogen to the negatively charged dicyanovinylidene,
significant changes cannot be made to this part of the molecule.
However either the replacement of the third nitrile group or
substitutions on either the aromatic ring of the acceptor or on
two of the “arms” of the tertiary amine donor can be tolerated.
The aromatic/quinoidal ring is critical in maintaining a charge-
separated state, the stability associated with an aromatic moiety
playing an important part in determining the geometry of the
molecule. The extent to which this effect influences the
nonlinearity and polarity can be seen by comparing 1 to adducts
8
, Figure 2) involves the direct reaction to TCNQ of a tertiary
28
amine derivative, such as triethylamine, in which at least one
of the substituent groups is an ethyl moiety. We initially
reported the synthesis of DEMI (1) to be carried out in
chloroform, but have since found that the preferred solvent of
choice is chlorobenzene, a higher boiling point solvent which
allows the reactions to reach completion in a matter of hours
rather than 3 days as previously reported.28 The reaction
proceeds Via the formation of an enamine which subsequently
attacks the TCNQ in a Stork enamine-type reaction. The critical
step therefore in the synthetic procedure is the formation of the
enamine. The reactivity depends on the degree of stabilization
of the enamine by functionalities adjacent to the site of its
formation in the tertiary amine. We developed a phenomeno-
logical model of reactivity which suggested that electron-
withdrawing groups adjacent to the amino functionality dis-
courage enamine formation by decreasing the electron density
on the amino nitrogen. This principle is clearly illustrated by
34
of TCNE (tetracyanoethylene), which are molecules containing
the tricyanovinyl acceptor group. These molecules are pre-
(31) Ranjel-Rojo, R.; Kar, A. K.; Wherret, B. S.; Carroll, M.; Cross, G.
H.; Bloor, D. ReV. Mex. Fis. 1995, 41, 832.
(32) Healy, D.; Thomas, P. R.; Szablewski, M.; Cross, G. H. Proc. SPIE
1995, 2527, 32.
(
28) Szablewski, M. J. Org. Chem. 1994, 59, 954.
(29) Cole, J. C.; Howard, J. A. K.; Cross, G. H.; Szablewski, M. Acta
Crystallogr., Sect. C 1995, C51, 715.
30) Cross, G. H.; Bloor, D.; Szablewski, M. Nonlinear Opt. 1995, 14,
19.
(33) Prein, M.; Adam, W. Angew. Chem., Int. Ed. Engl. 1996, 35, 477.
(34) Katz, H. E.; Singer, K. D.; Sohn, J. E.; Dirk, C. W.; King, L. A.;
Gordon, H. M. J. Am. Chem. Soc. 1987, 109, 6561.
(
2

Optically Nonlinear Adducts of TCNQ
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3147
to be delocalized over the dicyanomethanide unit and into the
ring. Such assignments do however emphasize the inadequacy
of using conventional Kekul e´ structures to represent such
2
7
species. We have found that the presence of the residual
bridge cyano group is necessary for the retention of quinoidal
character. The extra nitrile group in 2 extends the conjugated
system with respect to the otherwise isostructural 1, resulting
in a molecule which is more quinoidal.
Quantum-Chemical Calculations. There is a growing
interest in applying computational techniques which in some
1
1,36-39
way account for solute environment.
The COSMO
3
8
continuum dielectric model, for example, calculates reaction
fields using solvent dielectric constant, ꢀs, and solute radii as
parameters. Studies on a series of conventional D-π-A
3
9
molecules assuming solvation in DMSO (ꢀs ) 45) show that
Figure 3. Reaction scheme for the synthesis of 6.
modest increases in dipole moment can be expected.
No such assumptions are involved in the following analysis
which simply applies a field to the molecule without regard to
its likely origin. The influence of the effective field at the
molecule is studied at the semiempirical INDO (intermediate
neglect of differential overlap) level. Fields are applied in the
direction favoring charge separation along the molecular dipole
moment axis. Molecular geometries in the presence of the field
have been optimized at the SCF (self-consistent field) level,
and calculations of the dipole moment, µ, polarizability tensor,
R, and dipole-directed first hyperpolarizability component, â,
have been performed by means of the sum-over-states (SOS)
formalism (40 states) on the basis of state energies, state dipole
moments, and transition moments computed with a single
excitation configuration interaction (SCI) calculation.40 Cal-
culations based on the simple two-state model using the lowest
energy charge transfer state were also made. Figure 5 shows
the evolution of µ and â for 1 as a function of the effective
local field. In the absence of externally applied fields, this is
simply the effective reaction field of the polarizable dipole in
its polarizable surroundings. The field strengths used are in
Table 1. Limiting Solubilities (mol/L) Determined by Adherence
to Beer-Lambert Law Behavior for a Selection of Chromophores
chromophore
acetonitrile
chlorobenzene
-
-
-
-
4
4
4
3
-5
1
3
4
6
2 × 10
9 × 10
7 × 10
9 × 10
5 × 10
-4
2 × 10
-5
3 × 10
-3
2 × 10
dominantly non-charge-separated and reside on the left-hand
side (LHS) of the BLA diagrams.
As ether groups are known to aid solubility we thus replaced
one nitrile group at one end of the TCNQ with a methoxide
35
group. Reactions were carried out using diethylamine. In both
cases these TCQ derivatives, on further reaction with triethyl-
amine, resulted simply in 1, the amino or ethoxide group proving
more labile than the remaining nitrile.
Tertiary amine piperidines were used to produce ring-closed
systems (structures 4, 5, and 7, Figure 2). Computer modeling
showed a reduced planarity along the conjugated bridge yet no
significant changes in solubility were observed. Nevertheless,
these “ring-closed” derivatives proved more inert to hydration
in hygroscopic solvents (such as DMF), thus making their
handling in such high-polarity solvents much easier.
A significantly more soluble material (6) was synthesized by
the reaction of 1-piperidineacetaldehyde diethyl acetal with
TCNQ. Some solubilities were determined and are shown in
Table 1.
The use of hydroxy groups on five- and six-membered
saturated N-heterocyclic systems, i.e. 3-hydroxy-1-methylpip-
eridine, provided a starting point for further functionalization.
The hydroxy functionalities were converted to benzyl ethers and
tert-butyl benzyl ethers; however, subsequent reactions with
TCNQ were unsuccessful. The predominant products were, in
both cases, TCNQ radical anion salts.
7
8
41
the range 0.002-0.02 au which correspond to 10 -10 V/cm.
Increasing the strength of the electric field transforms the
geometry of DEMI from neutral to zwitterionic. Of particular
interest is the prediction that even in the absence of a reaction
field (gas phase) the molecule has a structure where the first
maximum in â has been passed. Even so, in the gas phase, 1
starts on the left-hand side of the polyene model diagram but
rapidly crosses over into the right-hand side at only modest
reaction fields.
Interestingly, the solid state geometry (in the crystal) of 1
can be reproduced using these methods by applying a field of
0
.008 au. This implies that the molecular dipole moment in
the crystal is around 35 D (referring to Figure 5).
(
36) Barzoukas, M.; Fort, A.; Boy, P.; Combellas, C.; Thiebault, A.
Nonlinear Optics 1994, 7, 41.
37) Albert, I. D. L.; di Bella, S.; Kanis, D. R.; Marks, T. J.; Ratner, M.
A. Polymers for second-order nonlinear optics. ACS Symp. Ser. 1995, 601,
Solid State Structure
(
The charge-separated ground state has been confirmed by
crystallographic structural studies29 of 1 (Figure 4, structure 1).
The ring system has been found to be predominantly quinoidal
rather than aromatic (Table 2), although the backbone is
conjugated from the nominally “positive” nitrogen to the
5
7
4
7.
(38) Klamt, A.; Sch u¨ u¨ rmann, G. J. Chem. Soc., Perkin Trans. 2 1993,
99.
(39) Allin, S. B.; Leslie, T. M.; Lumpkin, R. S. Chem. Mater. 1996, 8,
28.
“negative” carbon; carbon-carbon double bonds are lengthened
(40) The â value calculated from the 40-state model is expected to be
while carbon-carbon single bonds are shortened. Charges are
assigned on the basis of the shortening of the C13-N4 bond to
very close to the converged value. In the case of the p-nitroaniline, we
have previously analyzed the dependence of the polarizabilities as a function
of the number of excited states included in the SOS expression. We found
a fast convergence of the second-order response (within a 10% error) after
only 30 states. (See: Ramasesha S.; Shuai Z.; Br e´ das, J. L. Chem. Phys.
Lett. 1995, 245, 224.)
1
.316 Å and the fact that all the bond angles around N4 are
between 117.4 and 122.6°. The negative charge is considered
(41) Conversion factor: 1 au ) 5.14192 × 10 V m-1
11
.
(35) Acker , D. S.; Blomstrom, D. C. U.S. Patent 3,115,506, 1963.

3148 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Szablewski et al.
Figure 4. X-ray crystal structures of compounds29 1, 3, and 6.
Table 2. Carbon-Carbon Bond Distances (Å) for Compounds 3
and 6 Taken from the X-ray Crystal Structures (150 K)
Cn-Cm
3
6
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C4-C9
1.422(4)
1.370(5)
1.415(5)
1.418(5)
1.368(5)
1.412(5)
1.41(1)
1.36(1)
1.437(9)
1.41(1)
1.36(1)
1.434(9)
Figure 6. UV/vis spectrum of compound 1 in acetonitrile (solid line)
and dichloromethane (dotted line) with labelled peaks.
A in the spectra of some of the compounds is given as a function
of solvent dielectric constant in Table 3. Note that the
monosubstituted amine TCQ species (structure 18, Figure 2) is
included for comparison since this species does not exhibit any
noticeable negative solvatochromism and is thus clearly a LHS
molecule. In the remainder of the compounds, the initial
decrease in transition energy minimizes in chlorobenzene and
then increases in the more polar solvents. Anomalies appear
in the general trend; for example, protic solvents such as alcohols
tend to cause a much greater hypsochromic shift than expected.
It is worth mentioning that the fluorinated analogue of 1
(structure 8) exhibits a dramatic shift of -58 nm for peak A in
comparison to the spectral properties of 1 in acetonitrile. This
suggests that the extra electron-withdrawing moieties on the
TCQ part of the molecule hamper the back charge-transfer from
Figure 5. Evolution versus perturbing field of the ground state
polarization components µ (circles) and â(0) for 1, calculated using a
40-state SOS model (diamonds). Also shown for â(0) are the computed
values using the two-state model (crosses).
Solvatochromism
The electronic (visible absorption) spectra of all the “zwit-
terionic” species reported above are characterized by a broad
band in the region 650-750 nm, with areas of transparency
either side of the main peak. The majority of adducts have a
spectrum similar to that of 1 (Figure 6), where the major band
consists (in the case of 1 in dichloromethane) of two major peaks
at (approximately) 720 nm (A) and 657 nm (B) with two
shoulders at 615 nm (C) and 550 nm (D).
-
+
A to D , thus shifting the band to a higher energy. This trend
is consistent with the effect of applying a larger electric field
to the molecule. Here the higher reaction field caused by the
(presumably) larger dipole in 8 is responsible.
While band B displays only a small positive solvatochromism,
band A displays marked negatiVe solvatochromism over a given
range of solvent environments. The solvatochromism of band
Further evidence of the increase in zwitterionic character of
DEMI analogues in increasingly polar media can be obtained
from H NMR solvatochromic studies of the more soluble
1

Optically Nonlinear Adducts of TCNQ
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3149
Table 3. Position (nm) of the Lowest Energy Excitation (Band A)
versus Solvent Dielectric Constant, ꢀ, for a Selection of Compounds
Referred to in the Text
compound no.
solvent
,4-diox.
benzene
toluene
ꢀ
1
2
3
4
5
6
7
8
18
1
2.2 711
2.3 708
2.4 702
4.3 701
720 712 716 719 604
720
716 724
716 710
571
559
718 648
715
(
2
Et) O
CHCl
PhCl
THF
3
4.8 717 721 726 719 722 721 624 725
5.6 722 725 728 722 726 727 672 730 571
7.6 719 723 708 723 718 667 704
8.9 720 723 725 719 722 718 653 716 570
710 667 584
DCM
C
6
H
10dO 16.1 715 785 723 701
(
Me)
2
CO 20.7 705
712 688 707 697 655 653 565
717 689 709 698 653
709 671 703 678
TM-urea 23.1 708
EtOH
MeOH
3 2
CH NO
24.6 703
32.7 688
35.9 702
700 652 693 658
622
708 664 686 646 649 582
DMF
MeCN
DMSO
36.7 693 802 708 665 697 679 656
37.5 698 785 705 680 702 680 643 640 565
46.7 670
702 662 668 667 652
574
Chart 1
compound 6. The high sensitivity of the NMR technique in
detecting subtle changes in electron density in molecular
1
4
structures makes it valuable for investigations of this type.
Chart 1 gives the structure of 6 again but with relevant protons
identified and indicated on the NMR spectrum of 6 in Figure
7
.
The protons experiencing the largest changes in shielding due
to shifting π electron density will be HA adjacent to the
developing positive charge on the amino nitrogen. The next
most sensitive would be the two aromatic/quinoid ring protons,
(HC), closest to the developing negative charge on the dicya-
nomethanide group; these are seen as a doublet at lowest field
in the fragment of the spectrum considered (Figure 7). The
other doublet HB seen in the spectrum represents the two
remaining ring protons. Proton NMR spectra of 6 were recorded
in deuterated analogues of chloroform, dichloromethane, ac-
etone, DMSO, and MeCN, and all were measured relative to
the scale TMS ) 0 ppm.
Figure 7. Top: 1H NMR of compound 6; typical splitting pattern for
ethylenic and aromatic protons H , H , and H , referred to in the text.
A B C
Bottom: The chemical shift of ethylenic and aromatic protons taken
1
from the H NMR spectrum of compound 6 versus solvent dielectric
constant. Singlet δ
A
, doublet δ
B C
, doublet δ , and the midpoint of the
two doublets, δ , are represented by squares, circles, triangles, and
m
stars, respectively.
We would expect the signal observed for proton HA to shift
to higher field as it is increasingly deshielded by the depletion
of charge on the nitrogen. This is indeed what is seen in
progressively more polar solvents (using the dielectric constant
as a simple index for solvent polarity). The developing negative
charge on the dicyanomethanide group would be expected to
increasingly shield the ring protons HC with increasing solvent
polarity. Such a trend is clearly seen. It is interesting to note
that protons HB shift slightly downfield, apparently more
influenced by the formation of the positive charge rather than
the negative charge.
low solubilities have prevented us from using the conventional
techniques (density studies) to determine the volume of the
solute molecules and their effective radii. In addition, due to
the absorbance over much of the visible region, refractive index
measurements (to yield solute polarizabilities) are further
complicated. We are currently exploring methods to determine
the near-infrared refractive indices of solutions.
There have been a number of methodologies for analysing
the data from experiments of this nature, but we prefer that
4
2
which was outlined by Myers and Birge as the basis for our
own analysis. Here the molecules are treated as polarizable
ellipsoidal particles when considering the directing field acting
on the dipoles. For the induced dipolar contributions, however,
a spherical model is satisfactory and in any case the induced
dipoles are considerably smaller than the permanent dipoles
whose reorientation dominates the dielectric constant.
Solution Dipole Moment Measurements and Underlying
Theoretical Principles. A coaxial capacitance cell was used
to measure the dielectric constant of dilute solutions of
compound 1 at 1 MHz. Low solubility prevented the use of
strictly nonpolar solvents as usually required, but acceptable
results have been obtained using dichloromethane as solvent.
-
4
-3
Concentrations of around 10 mol dm were typical. Such
(42) Myers, A. B.; Birge, R. R. J. Chem. Phys. 1981, 74, 3514.

3150 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Szablewski et al.
In a polarized solution containing a polarizable dipolar solute
the total polarization per unit volume, BP tot, may be given by
difficult to define since it should properly represent the distance
from the central point dipole to the point at which the boundary
conditions are applied in solving Laplace’s equation. In
Onsager’s theory, the boundary of the cavity is a discontinuity
in the permittivity, but other arbitrary conditions may be chosen
to acheive the unique solution to Laplace’s equation. For
BP tot ) BP + BP (1)
s
m
where BP s is the polarization contribution from the solvent and
BP m is the total contribution from the dipolar solute. The total
dipolar polarization is further considered to be resolvable into
those contributions arising from induced dipoles, BP R, and
orienting permanent moments, BP µ, as follows:
44
example, Block and Walker allow the permittivity to grow
exponentially (from unity) to the bulk value outside the defining
radius and indeed claim better agreement between gas phase
and liquid dipole moment values. In the following analysis,
two physical limits to the radius are defined: first, that which
may be obtained from the density values obtained from the
crystal structure and, second, that which is represented by the
BP ) BP + BP
µ
(2)
m
R
We immediately state that, in the initial analysis, the dipole
moment, µ, is that which already benefits from enhancement
through the reaction field its ground state moment (µ0) produces.
Thus the analysis which follows will yield solution state dipole
moments.
According to standard methods assuming an isotropic dipole
distribution and low applied electric fields (as in these measure-
ments), we have
1
/3
quantity (abc) . Other than these limits, the radius will be
treated as a parameter to be determined so that it can include
the unknown properties of the solute/solvent boundary.
We need to determine the ellipsoidal shape factor, Aa, and
thus (from eq 5) to determine reasonable values for the semiaxes
a, b, and c. Using a commercial molecular modeling package
4
5
(
Nemesis ) we have modeled compound 1 and calculated the
solvent accessible surface using a probe radius of 1.5 Å (as an
estimate of the radius of a dichloromethane molecule). Using
this we measure the shortest distances between pairs of surface
contact points along the length, width, and thickness of the
molecular surface to yield a ) 7.6 Å, b ) 3.4 Å, and c ) 1.9
Å. Solving eq 5 gives Aa ) 0.106.
2
µ
BP ) N
BE r
(3)
µ
m
3kT
where Nm is the number density of dipolar particles and BE r is
the “directing field”. In the case of polarizable ellipsoidal
particles (with semiaxes labeled a, b, and c), the directing field
may be related to the applied field, BE , by taking account of the
cavity field factor (obtained from solving Laplace’s equation
for vacuum cavities in a dielectric medium), Gell and the
additional applied field-induced reaction field due to the
polarizability of the particle. Thus we follow the method of
The induced dipole contribution, BP R, is considered to be
represented very well by the action of the applied field on a
“
spherical” particle having an average polarizability, Rj . This
4
2
assumption has been made before with success and, although
we are describing considerably more elongated molecules than
those previously, is valid here simply because this contribution
9
is very much smaller than BP µ.
B o¨ ttcher and represent the total directing field as
The induced polarization is given by
1
ꢀs
BE ) F G BE )
BE
(4)
r
ell ell
BP ) N Rj BE
(7)
(
1 - f R ) ꢀ + (1 - ꢀ )A
R
m
i
a
a
s
s
a
where Fell is the ellipsoidal reaction field correction factor which
depends upon the factor of the reaction field, fa, along the dipolar
axis, a, in the molecule whose polarizability along this axis is
Ra. The surrounding dielectric constant, ꢀs, is, in our analysis,
that of the pure solvent. Before proceeding, we are careful to
point out that the dielectric constant used here should strictly
be that of the solution but in using this constant value we
simplify the analysis without sacrificing accuracy. Our justi-
fication lies in the fact that we use only very dilute solutions
and thus observe only rather small changes to the measured
dielectric constant of solutions over these ranges of concentra-
tion.
where we use the average polarizability, Rj ) (Ra + Rb + Rc)/
3
, and the internal field, BE i, is given by
3
ꢀs
1
BE ) FsphGsph BE )
BE
(8)
i
(
1 - fsph Rj ) 2ꢀ + 1
s
where Fsph and Gsph are the spherical model counterparts of the
reaction field factor and cavity field factor described above. The
spherical model factor of the reaction field, fsph, is given by
2ꢀ - 2
1
s
9
fsph
)
(9)
The ellipsoid shape factor, Aa, may be calculated from the
following:
3
2
ꢀs + 1
aj
∞
ds
By noting the definition for induced polarization, BP ) ꢀ0(ꢀ -
1) BE , we can make the approximation for the solvent polarization
contribution such that
abc
2
Aa )
∫
(5)
0
2 3/2
2 1/2
2 1/2
(
s + a ) (s + b ) (s + c )
and is readily determined using commercial mathematics
software (e.g., “Mathematica” 43).
BP ) ꢀ (ꢀ - 1) BE
(10)
s
0
s
The factor of the reaction field, fa, is given by
This polarization is taken to be approximately constant over
the solution concentration ranges used and is measured from
the pure solvent capacitance value. This yields the solvent
dielectric constant contribution to the solution dielectric constant,
A (1 - A )(ꢀ - 1)
3
a
a
s
fa )
(6)
3
ꢀs + (1 - ꢀ )Aa
s
aj
where aj is the radius of a notional sphere occupying the same
volume as the ellipsoidal dipole. This parameter is particularly
ꢀ
, and after making substitutions into eq 1 we obtain
(44) Block, H.; Walker, S. M. Chem. Phys. Lett. 1973, 19, 363.
(43) Mathematica v 2.1, Wolfram Research Inc.
(45) Nemesis v 2.0, Oxford Molecular Ltd.

Optically Nonlinear Adducts of TCNQ
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3151
2
µ
ꢀ
) ꢀ + N FsphGsph Rj + F G
(11)
s
m
[
ell
ell3kT
]
Differentiating with respect to solute number density and
rearranging (11) gives a convenient formula for calculating the
solution state dipole moment:
1
/2
∂
ꢀ
3kT
µ )
- FsphGsphRj
ꢀ0
(12)
{
[
∂Nm 0
|
]
F G
}
ell ell
where µ is in MKSA units if the experimental slope is provided
3
in units of m . The number density used here is the value
obtained from the molar concentration of solute and assumes
that changes in the density of solution with concentration are
negligible when compared with changes in dielectric constant.
This is valid at the concentration of these experiments and
constitutes no more than a 2% error on experimental values.
46
4
7
Figure 8. Dipole moments of 1 obtained by experiment plotted as a
function of reaction field, both obtained using average solute radius as
a parameter. Reaction field values are computed using eq 14 for a range
of solute average radii. Experimental moments and fields using the
INDO calculated gas phase polarizabilities (triangles) and first-order
corrected values (squares) are shown. The theoretical dipole moment
evolution versus field is also provided (circles).
We note that for typical values of average polarizability the
second term in parentheses will often be orders of magnitude
smaller than the slope value. Whilst neglecting this term will
marginally simplify the determination of µ, there is no extra
benefit in reducing the inherent uncertainties in estimating either
molecular dimensions or polarizabilities.
At room temperature, we measured the solvent dielectric
constant (dichloromethane) to be 8.6 ((0.8), which is close to
the quoted textbook value48 of 8.9. This value was used as the
constant solvent contribution to the dielectric constant. The only
parameter for which a measurement has not been obtained is
the polarizability. Values of both the average polarizability and
the polarizability along the dipole axis are required.
R ) f µ
(14)
ell
a
The experimental solution state dipole moment crosses the
theoretical curve when µ ) 33.2 ( 2.5 D and when Rell )
0.0077 au. The gas phase dipole moment can be calculated
for the specific value of fa using the value for Ra given above.
This yields µ0 ) 17.1 D. The value of the solute average radius
which corresponds to this data is 4.13 Å, which is larger than
the average radius obtained from the cube root of the products
of a, b, and c (3.66 Å) yet smaller than that determined from
the crystal structure density data (4.43 Å). We would not wish
to apply any particular physical definition for this radius but
simply note that it lies within reasonable physical limits.
The INDO calculated gas phase value is 14 D, and this
indicates that there is a small overestimation in the experimental
dipole moment and reaction field values calculated using the
above parameters. There are two possible reasons for this. First,
the values of polarizability assumed are the gas phase values
yet we have calculated an evolution in these parameters with
field. Thus better estimates of dipole moment would use these
Measurements of refractive index of solutions can yield the
average polarizability from which the a axis component can be
9
obtained, but suitably chosen molecular dimensions are again
4
2
required. However, Myers and Birge have suggested that it
is better to use the actual value for Ra if it is available. Here
we use the calculated zero field values for molecule 1 obtained
from the INDO calculations outlined above. Thus we use Ra
-
30
3
-30
3
)
121 × 10
m , Rb ) 12 × 10
m , and due to the
planarity, Rc ) 0. Therefore the average polarizability Rj ) 44
-
30
3
×
10
m .
Using this data and our experimental slope value, we can
determine the solution state dipole moments where the average
solute radius is the unknown variable. In parameterizing the
radius we are exploring the possible reaction fields implied from
our experimental data. Despite the experimental results relying
on particular values for Ra and Rj obtained theoretically, the
preferred radius (which is the quantity of least certainty) would
be defined when there was self-consistent agreement between
the experimental solution state moment and theoretical data at
a common value of field. Thus we show a plot of these dipole
moments versus field in Figure 8. The solution state dipole
moments are obtained using eq 12 but where the field factors
are parameterized using a range of aj . The reaction field is
obtained from the following:
(higher) values which are more relevant to experimental
conditions. Second, this discrepancy could be due to the
49
Kirkwood “solvent cage” effect. Dichloromethane is a slightly
polar solvent, and we might expect an enhancement to the
reaction field from this effect.
As a first-order correction to these results, we can use the
INDO calculated values of Ra and Rj versus field and re-compute
the experimental data as a function of solute radius. Using the
gas phase values for µ0 and Ra in eq 13, we obtain a field of
0.0062 au at which the calculated evolved values for Ra and Rj
-
30
3
are 137 and 50 × 10
m , respectively. The experimental
data obtained for these values is also shown in Figure 8. Here
the crossing point occurs at R ) 0.0071 au, where µ ) 31.3 (
2.4 D and µ0 ) 14.1 D. Clearly this is in rather better agreement
with the theoretical data.
fa
R )
µ0
(13)
ell
(
1 - f R )
a a
It is of interest to compare the spherical model theory when
applied to the experimental results. Here we find that an average
radius of 4.2 Å would be required to give the INDO gas phase
moment. However, using the spherical model for this radius
but noting that µ0 ) (1 - faRa)µ which gives
(
(
(
46) Scholte, Th. G. Recueil 1951, 70, 50.
47) Singer, K. D. University of Pennsylvania Ph.D. Thesis, 1981.
48) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic SolVents,
Physical properties and methods of purification, 4th ed.; John Wiley and
(49) See, for example: Fr o¨ lich, H. Theory of Dielectrics; Oxford Science
Son Inc.: New York, 1986.
Publications: Oxford, U.K., 1986.

3152 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Szablewski et al.
-
30
3
and with the average polarizability Rj ) 44 × 10
m and µ0
are, fortuitously, nearly equivalent to the simple two-level model
results at this value of reaction field, we can use the two-level
correction factor, Fâ:
)
14 D, the reaction field value becomes Rsph ) 0.018 au (using
an equation analogous to eq 13). This is clearly unrealistic and
serves to reinforce the argument for careful use of the ellipsoidal
field theory.
4
ω0
Finally, despite the apparent success in its use, we acknowl-
edge that this analysis places heavy reliance on the quality of
quantum-chemical calculations. Measurements of density (and
therefore of the average radius of the solute-occupied volume)
coupled with solute refractive index measurements in an
appropriate spectral region are still to be preferred, where these
are possible.
Measurements of First Hyperpolarizability, â. The mea-
surement of µâ by the usual EFISHG (electric field induced
SHG) method using 1.064 µm radiation turns out to be difficult
for these molecules due to the finite optical absorption at 532
nm and to problems of aggregation at the concentrations suitable
for this measuring technique. This problem was circumvented
by the application of the hyper-Rayleigh scattering (HRS)
technique, in which lower concentrations can be employed
because the signal is linear with concentration (instead of
quadratic as for EFISHG).50
Fâ )
(15)
2
2
2
2
[
ω - ω ][ω - (2ω) ]
0 0
where ω and ω are the frequency of the lowest energy optical
0
transition (here λmax in CHCl ) 717 nm) and the experimental
3
frequency, respectively, to determine that the experimental
-30
|â(0)| ≈ (350 ( 11) × 10 esu. This is in excellent agreement
with theory.
The solution state dipole moment of 1 has not been measured
in chloroform (due to solubility difficulties), but we can assume
that the theoretical predicted value of 27 D in a field of 0.0058
au is a good approximation in view of the foregoing. We can
therefore report that, in CHCl3, the molecular figure of merit,
-48
52
µâ(0), for 1 is 9450 × 10
esu.
Conclusions
HRS measurements with a fundamental wavelength at 1.064
µm were performed on dilute solutions (number density, 1-2.6
Tertiary amine adducts of TCNQ prepared by a simple one-
step facile reaction exhibit a charge-separated ground state
structure indicated by their negative solvatochromism. These
materials are expected to show higher figures of merit for
second-order optical nonlinearity, µâ(0), than LHS molecules,
not especially because of their high hyperpolarizability, but
because of the inevitably high dipole moments when in solution.
The polarization properties of the adducts evolve with increasing
solvent polarity, and we have observed that all of them quickly
become what we have termed RHS molecules, being on the
right-hand side of the cyanine limit in diagrams depicting bond
length alternation versus polarization. The increasing dipole
1
6
-3
×
10 cm ) of 1 in chloroform. The solutions were
systematically passed through 500 nm microporous filters.
Laser pulses (energy, 20-25 µJ; width, 70 ps; repetition rate,
2
kHz) from a Nd:YAG regenerative amplifier were focused
into a rectangular glass cell by a 10 cm lens. The scattered
harmonic light was collected at 90° and filtered by a mono-
chromator with 1 nm bandwidth. Single-photon pulses from a
photomultiplier were detected in a 5 ns time gate around the
laser pulse. The count rates were corrected for pile-up errors
at increasing count rates and for absorption (<5%) of the
scattered light. Systematic scanning of a narrow region around
1
moment has been confirmed from shifts in the H NMR spectra
5
32 nm showed no significant photoluminescence background
as a function of solvent polarity. Measurements of the polariza-
tion properties have required close attention to the properties
of the local field description. Particularly where static directing
fields are concerned, as in the dipole moment measurements,
we show in some detail that it is essential to consider the
molecules as ellipsoidal volumes in the surrounding continuum
rather than as notional spherical volumes. Detailed theoretical
calculations of the ground state polarization properties as a
function of perturbing field only agree with experimental data
when account is taken of the molecular shape. The value for
the moment in dichloromethane of one in the series is 31 D
and its expected value in chloroform reduces to 27 D. The
measured value for the static hyperpolarizability measured by
for solutions of 1 in these circumstances. In a reference arm,
a fraction of the laser light was frequency-doubled and this
intensity was used to correct for slow laser fluctuations.
Polarized measurements with analyzer perpendicular and
parallel to the laser polarization, corrected for the relative
2
2
monochromator transmission, show a ratio of 〈âXZZ 〉/〈âZZZ 〉 )
.21 ( 0.01, which agrees, within experimental accuracy, with
0
that of a molecule with only one non-zero diagonal tensor
component, âzzz. Note that the use of upper case subscripts
denotes laboratory coordinates, and lower case, molecular
coordinates. This ratio is to be expected for this compound
with its linear conjugated backbone. Using the internal reference
method we obtained a ratio âHRS/[âHRS(CHCl3)] of 1600 (where
-
30
hyper-Rayleigh scattering in chloroform is 350 × 10
esu,
2
2 1/2
âHRS stands for [〈âZZZ 〉 + 〈âXZZ 〉] ). Applying the previously
giving one of the highest reported values for µâ(0). We note
that, with only a slight increase in reaction field, this value could,
based on the quantum-chemical calculations, be increased to
used approximation50 that the â-tensor of chloroform is also
dominated by âzzz and adopting the EFISH value for âCHCl of
3
-
30
51
0
(
.49 × 10
esu, the measured near-resonance value of â-
-
48
around 17 500 × 10
esu. Problems in confirming this
-30
-2ω;ω,ω) in this approximation is |âZZZ| ) 780 ( 25 × 10
exceptional nonlinearity are related to those of interpreting
dipole moment measurement data taken from studies using
highly dipolar solvents. Furthermore, the utility of these
materials relies on their being hosted in solid polymeric matrices
where the reaction fields are not expected to be high. Measure-
ments of the dipole moment of these molecules in polymer films
esu. The usual spherical local field models were applied in
this analysis, and we are thus able to compare the present results
with other published work.
We can turn to the reaction field model and compare this
resonant value with that predicted and shown in Figure 5. For
chloroform (ꢀs ) 4.8), we calculate (from eq 13) Rell ) 0.0058
5
3
is continuing and will be reported on separately.
-
30
3
au, using µ0 ) 14.1 D, Ra ) 137 × 10
m , and aj ) 4.13 Å.
At this value of field we see from the INDO results that â(0) )
-320 ( 50) × 10 esu. Since the 40-state SOS model results
(52) Some authors prefer to take account of the volume-normalized
-30
(
figure-of-merit by simply dividing by molecular weight. On this basis, we
-
48
-1
can quote a value for compound 1 of 9450 × 10 /276 esu g mol.
(53) Cross, G. H.; Healy, D.; Szablewski, M.; Bloor, D.; Malagoli, M.;
Kogej, T.; Beljonne, D.; Br e´ das, J.-L. Chem. Phys. Lett. 1997, in press.
(
(
50) Clays, K.; Persoons, A. Phys. ReV. Letts. 1991, 66, 2980.
51) Kajzar, F.; Ledoux, I.; Zyss, J. Phys. ReV. A 1987, 36, 2210.

Optically Nonlinear Adducts of TCNQ
J. Am. Chem. Soc., Vol. 119, No. 13, 1997 3153
Chart 2
Chart 3
1
aromatic H (adjacent to dicyanomethanide)], δ 7.6 ppm [doublet, 2 ×
1
aromatic H (adjacent to ethylenic bridge)], δ 8.8 ppm [singlet (ethylenic
proton nearest positively charged N)]. For species B: δ 2.15 ppm
[
-
triplet, ring -(CH
(CH
multiplet, ring -(CH
2
)- (nearest CdC bridge)], δ 2.83 ppm [triplet, ring
+
2
)- (adjacent to N )], δ 3.74 ppm [singlet CH
3
δ 3.8 ppm,
2
)- (â to ring N)], δ 6.9 ppm [doublet, 2 ×
Experimental Section
1
aromatic H (adjacent to dicyanomethanide)], δ 7.4 ppm [doublet, 2 ×
1
General Procedure for Preparation of Dipolar TCNQ Adducts.
A solution of TCNQ (2 mol equiv) in chlorobenzene was heated under
reflux for 0.5 h. Tertiary amine (1 mol equiv) in chlorobenzene solution
was added dropwise. The reaction mixture was heated at reflux from
aromatic H (adjacent to ethylenic bridge)], δ 8.6 ppm [singlet, (ethylenic
proton nearest positively charged N)]. IR (KBr disc): ν
(
nitrile)
2188.7,
-1
2161.6 cm . Microanalysis. Calcd for C H N : C, 74.43; N, 20.42;
17
14
4
H, 5.14%. Found: C, 74.08; N, 20.37; H, 5.10%.
3
h up to 60 h dependent on the tertiary amine used. The reaction
5. (N-Methyl-2-pyrolidinol)-DEMI-3CNQ. TCNQ (0.49 mmol,
2 mol equiv) and 1-methyl-3-pyrolidinol (0.24 mmol, 1 mol equiv)
refluxed for 6 h gave 5 a dark green powder (150 mg) in 22.6% yield.
solution was monitored at intervals by UV/vis spectroscopy. When
no TCNQ or TCNQ peaks were seen and the characteristic “blue”
-
2
6
1
band was observed, the now blue/turquoise solution was removed
from the heat. Filtration of the reaction mixture yielded a blue solution
and a black residue. The solution was reduced to dryness under vacuum
giving the crude product, more of which was obtained by extraction of
the residue with acetonitrile.
The combined crude solids were recrystallized from acetonitrile three
times to yield the product. The product was filtered under suction and
washed with toluene and ether.
Due to the highly insoluble nature of 5 a poor-quality H NMR in CD -
3
CN was obtained, it showed features in common with the other
analogues described herein, namely: δ 7.0 and 7.65 ppm [doublet of
doublets, p-substituted benzene ring], δ 8.2 ppm [ethylenic proton, δ
+
3.85 ppm, (OH)]. Mass spectrum: m/z (FABHI) 277.09 (3.81%) (M
+ 1). IR (KBr disc): ν(nitrile) 2186, 2153.4 cm , ν(OH str.) 3416 cm-1.
UV/vis spectra were consistent with other compounds described herein.
6. N-Acetaldehyde Diethyl Acetal-Piperidyl-DEMI-3CNQ. TCNQ
(0.98 mmol, 2 equiv) and 1-piperidineacetaldehyde diethyl acetal (0.49
mmol, 1 equiv) gave 6, lustrous emerald green platelets (540 mg) in
25% yield. 6 was recrystallized from hot acetonitrile, and a trace of
an orange TCNQ decomposition product (λmax ) 480 nm in MeCN)
was found to be present. Column chromatography performed on neutral
silica gel with 1:9 acetonitrile:ethyl acetate eluent was used to purify
the product. The purified compound was again recrystallized from
acetonitrile. Crystals grown for X-ray structural determinations were
obtained by slow recrystallization from hot dichloromethane. The data
-1
1. DEMI-3CNQ (4-[1-Cyano-3-(diethylamino)-2-propenylidene]-
2
2
4
7
,5-cyclohexadiene-1-ylidenepropanedinitrile). TCNQ (0.98 mmol,
mol equiv) and triethylamine (0.49 mmol, 1 mol equiv) refluxed for
h gave 1, metallic-like green-gold needle-like crystals (990 mg) in
1
3% yield. H NMR (DMSO-d
6
): δ 0.8 ppm [quintet, -(CH
3 2
) , δ 3.8
ppm, quintet, -(CH
p-substituted benzene ring, δ 7.3 ppm doublet, ethylenic proton] δ 8.3
ppm [doublet, ethylenic proton nearest positively charged N]. Mass
)
2 2
-, δ 6.9 and 7.8 ppm, doublet of doublets,
+
spectrum: m/z 276 (M ) (100%, molecular ion). Decomposition tem-
perature: 243.28 °C. IR (KBr disc): ν(nitrile) 2185.7, 2155.6 cm-1
obtained indicated that one molecule of dichloromethane was present
1
(characteristic of CtN stretch in such zwitterionic species), 1588.1
for every molecule of 6 in the crystal. H NMR (CD
2
Cl
-)], δ 2.79 ppm
-)], δ 3.5-3.8 ppm [multiplet [triplet (-CH -) +
-) + quartet 2 × (-CH -)]], δ 4.70 ppm [triplet
2
): δ 1.23
-
1
cm (CdN str). Microanalysis. Calcd for C17
0.27; H, 5.84%. Found: C, 73.78; N, 20.41; H, 5.76%. The structure
of 1 was confirmed by X-ray crystallography; see ref 29.
. CN-DEMI-3CNQ (4-[1,3-Dicyano-3-(diethylamino)-2-propen-
H N
16 4
: C, 73.89; N,
ppm [triplet, (2 × CH
[triplet (-CH
doublet (-CH
3
)], δ 2.02 ppm [quintet (-CH
2
2
2
2
2
2
2
(-CH-)]. δ ) 7.02 ppm, doublet 2 × (-aromatic H-)], δ 7.49 ppm
+
13
ylidene]-2,5-cyclohexadiene-1-ylidenepropanedinitrile). 2 was the
unexpected result of a preparation of 1 that was refluxed for an
excessive period. TCNQ (0.49 mmol, 2 mol equiv) and triethylamine
[doublet 2 × (-aromatic H-)], δ 8.08 ppm [singlet (-N dCH)].
C
NMR (see numbered structure in Chart 3): C1, δ 15.8 ppm; C2, δ
51.3 ppm; C3, δ 100.8 ppm; C4, δ 61.8 ppm; C5, δ 129.5 ppm; C6, δ
119.9 ppm; C7, δ 25.9 ppm; C8, δ 21.49 ppm; C9, δ 64.9 ppm; C10,
δ 125.5 ppm; C11, δ 116.2 ppm; C12, δ 121.8 ppm; C13, δ 133.9
ppm; C14, δ 158.9 ppm; C16, δ 119.3 ppm; C17, δ 153.2 ppm. No
(0.24 mmol, 1 mol equiv) refluxed for 18 h gave 2, dark green crystals
-
1
(59 mg) in 8.15% yield. IR (KBr disc): ν(nitrile) 2199.61 cm with
-
1
small shoulder at approximately 2001 cm . The small quantity of
product did not allow an adequate microanalysis to be obtained. The
structure of 2 was confirmed by X-ray crystallography; see ref 27.
-1
peak was assigned for C15. IR (KBr disc): ν(nitrile) 2181.2, 2146.8 cm
.
Microanalysis. Calcd for C22 (recrystallized from acetoni-
24 4 2
H N O
3
. Dicyclohexyl-DEMI-3CNQ (4-[1-Cyano-3-(dicyclohexylamino)-
-propenylidene]-2,5-cyclohexadiene-1-ylidenepropanedinitrile). TCNQ
0.98 mmol, 2 equiv) and dicyclohexylethylamine (0.49 mmol, 1 equiv)
refluxed for 5 h gave 3, metallic-like dark green-gold needle-like crystals
trile): C, 70.19; N, 14.88; H, 6.43%. Found: C, 69.85; N, 14.93; H,
6.19%.The structure of 6 was confirmed by X-ray crystallography; see
attached structural data.
2
(
7. (N,N-Dimethylpiperazinyl)-DEMI-3CNQ. TCNQ (0.98 mmol,
2 mol equiv) and 1,4-dimethylpiperazine (0.49 mmol, 1 mol equiv)
was heated at ∼105 °C for 60 h. 7 was collected in the form of green
crystals. These were found not to be of analytical purity, however,
the material was sufficiently soluble to obtain meaningful NMR data.
1
(
1130 mg) in 62% yield. H NMR could not be recorded due to the
highly insoluble nature of 3. IR (KBr disc): ν(nitrile) 2188.49, 2160.58.
-
1
cm . Microanalysis. Calcd for C25
28 4
H N : C, 78.09; N, 14.57; H,
7
.34%. Found: C, 77.74; N, 14.60; H, 7.27%. The structure of 3
was confirmed by X-ray crystallography; see attached structure and
data.
-
1
Yield: 60 mg (42%). IR (KBr disc): ν(nitrile) 2190, 2160 cm , ν(imine)
-
1
1
1598 cm
.
6
H NMR (DMSO-d ): δ 2.05 ppm [singlet, 2H; δ 2.85
4
. (N-Methylpiperidyl)-DEMI-3CNQ. TCNQ (0.98 mmol, 2 mol
equiv) and 1-methylpiperidine (0.49 mmol, 1 mol equiv) refluxed for
h gave 4, metallic-like green-gold powder (428 mg) in 31.8% yield.
ppm, singlet, 2H; δ 3.4 ppm, singlet (broad), 6H], δ 6.90 ppm [doublet,
1
3
2H; δ 7.70 ppm, doublet, 2H: δ 8.00 ppm, singlet, 1H]. C NMR
1
3
1
5
(see Chart 4) (assigned with the aid of a 2D heteronuclear C- H
correlation spectrum): δ 30.7 ppm, C3; δ 42.1 ppm, C2; δ 49.0 ppm,
C4; δ 68.6 ppm, C1; δ 115.3 ppm, C5; δ 115.8 ppm, C7; δ 115.9
ppm, C8; δ 119.3 ppm, C10; δ 119.9 ppm, C14; δ 129.5 ppm, C6; δ
130.9 ppm, C11; δ 131.8 ppm, C9; δ 152.4 ppm, C12; δ 166.7 ppm,
1
H NMR (DMSO) identified two isomers, A (60%) and B (40%)
assigned structures shown in Chart 2). These two species could not
be separated by column chromatography. For species A: δ 2.00 ppm
triplet, ring -(CH
(CH
multiplet, ring -(CH
(
[
-
[
2
)- (nearest CdC bridge)], δ 2.9 ppm [triplet, ring
+
2
)- (adjacent to N )], δ 3.78 ppm [singlet, CH
3
], δ 3.8 ppm
C13. Microanalysis. Calcd for C17
15 5
H N : C, 70.6; N, 24.2; H, 5.2.
2
)- (â to ring N)], δ 6.9 ppm [doublet, 2 ×
Found: C, 67.0; N, 26.8; H, 3.9.

3154 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Szablewski et al.
Chart 4
6.1 and 6.6 ppm [doublet of doublets, p-substituted quinoidal ring]. IR
-
1
(KBr disc): ν(nitrile) 2188.7, 2161.6 cm .Microanalysis: calculated for
17 16 4
C H N : C, 73.89; N, 20.27; H, 5.84%. Found: C, 73.83; N, 20.30;
H, 5.81%. The structure of 18 was confirmed by X-ray crystallography,
see ref 27.
Acknowledgment. We thank the U.K. E.P.S.R.C. for support
for M.S., A.T., J.M.C., and P.R.T. J.M.C. also receives support
from the Institut Laue-Langevin (Grenoble, France). This work
was also supported by funds provided under the EU Human
Capital and Mobility Programme, Research Network, “Novel
third-order nonlinear optical molecular materials” [CHRX-
CT93-0334 (DG XII)]. The work in Mons is part funded by
the Belgian Federal Government, “P oˆ le d’Attraction Interuni-
versitaire en Chimie Supramoleculaire et Catalyse”, FNRS/
FRFC, and an IBM Joint Study. We acknowledge the assistance
given in NMR studies by Dr. R. S. Matthews. IR spectra were
carried out by Mr. Y. Kagawa. The work in Antwerp is part
funded by the Flanders Government in its action for the
promotion of participation in EU-research programmes and also
by the Belgian Fund for Scientific Research (F.W.O.).
8
.
Tetrafluoro-DEMI-3CNQ (4-[1-Cyano-3-(diethylamino)-2-
propenylidene]-2,3,5,6,-tetrafluoro-2,5-cyclohexadiene-1-ylidenepro-
panedinitrile). TCNQF (0.036 mmol, 2 mol equiv) and triethylamine
0.018 mmol, 1 mol equiv) refluxed for 4 h gave 8, a purple powder
4
(
(
[
1
27 mg, 21% yield). Very poor H NMR (CD
quintet, -(CH , δ 3.85 ppm, quintet, -(CH -, δ 8.4 and 7.2 ppm
Cl showed
three singlets at δ -146.9, -137.8, and -136.3 ppm, integration in
3
CN): δ 1.35 ppm
3
)
2
2 2
)
1
19
very weak signals, ethylenic H]. F NMR spectra in CD
2
2
1
9
4
the ratio of 1:1:1.3; a F NMR spectra of TCNQF showed only one
singlet at δ -132.2 ppm. UV/vis absorption spectra were in accordance
with other analogues described above and displayed the characteristic
bands expected; no TCNQF
spectra; the origin of the third singlet in the F NMR spectra of 8 is
unexpected and could possibly be due to the species TCNQF
-
4
or TCNQF
4
bands were observed in these
1
9
4
2
H .
3
5
18. 7-(4-Methylpiperidino)-7,8,8-tricyanoquinodimethane. To
3
a warm solution of TCNQ (0.49 mmol) in 100 cm of tetrahydrofuran
was added 0.6 cm of 4-methylpiperidine. The initially green solution
3
became purple. After being cooled overnight to room temperature,
the solution was cooled in an ice bath and filtered to give a fine purple
Supporting Information Available: Details of instrumenta-
tion, reagents, and X-ray crystal structure determinations of 3
and 6 (18 pages). See any current masthead page for ordering
and Internet access instructions.
solid. Recrystallization from acetonitrile yielded fine purple needles,
1
5
31 mg (39% yield). H NMR (DMSO-d
6
): δ 0.08 ppm [doublet
quintet, -CH
3
], δ 0.65, 1.00, 2.95 and 3.40 ppm [multiplets (integration
1
2
, 3, 2, and 2 × H, respectively), aliphatic piperidine ring protons], δ
JA963923W