Crystal structures, degree of charge transfer, and non-linear optical characteristics of intramolecular charge-transfer compounds: Indoline-substituted tricyanoquinodimethanes1

Article abstract of DOI:10.1246/bcsj.81.1131

The substituent effect on the degree of intramolecular charge transfer (δ) and optical properties of donor-π-acceptor compounds comprised of l,3,3-trimethyl-2-rnethyleneindoline (I₁) and substituted 7,8,8-tricyanoquinodimethane (=2-(4-cyanomethylene-2,5-cyclohexadienylidene) malononitrile, 3CNQ-R, R = substituent groups) moieties were investigated (I₁-3CNQ-R). In the crystal structures, I₁-3CNQ-R molecules stacked on indoline and/or 3CNQ-R moieties in a head-to-tail manner to cancel their dipole moments and established segregated or mixed stack columnar motifs. The δ values of I₁-3CNQ-R molecules in solid and solution states were estimated using the bond length ratio in the crystal structures, molecular orbital calculation, and the solvatochromic shift of intramolecular charge-transfer absorption, respectively, and showed significant and reasonable dependences on substituents of the 3CNQ moiety. Structural analysis revealed that molecular conformation and planarity affect the δ values of I₁-3CNQ-R molecules. Molecular orbital calculations revealed that molecular (hyper)polarizabilities can be modulated by tuning δ. I₁ 3CXQ-R exhibited a solvatochromic shift, and the ground state changed from neutral (δ ≤ 0.5) in less-polar solvents to ionic (δ ≥ 0.5) in polar solvents. Two-photon absorption properties of I ₁-3CNQ-R showed a significant substituent effect and indicated that δ is a fundamental parameter for modulating non-linear optical properties.

Full text of DOI:10.1246/bcsj.81.1131

ꢀ 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 9, 1131–1146 (2008) 1131
Crystal Structures, Degree of Charge Transfer, and Non-Linear
Optical Characteristics of Intramolecular Charge-Transfer Compounds:
Indoline-Substituted Tricyanoquinodimethanes1
ꢀ1
ꢀ1;2;3
Tsuyoshi Murata, Gunzi Saito,
Kazukuni Nishimura,1 Chin-Hong Chong,1 Masaru Makihara,1
Genki Honda,1 Yuichiro Enomoto,1 Salavat Khasanov,1;4 Hideki Yamochi,2 Akihiro Otsuka,2
Kenji Kamada,5 Koji Ohta,5 and Jun Kawamata6
1Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
2Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8502
3Research Institute, Meijo University, Tenpaku-ku, Nagoya 468-8502
4The Institute of Solid State Physics of Russian Academy of Science, Chernogolovka 142432, Russia
5Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
AIST Kansai Center, Ikeda, Osaka 563-8577
6Department of Chemistry, Faculty of Science, Yamaguchi University, Yamaguchi 753-8512
Received March 14, 2008; E-mail: saito@kuchem.kyoto-u.ac.jp
The substituent effect on the degree of intramolecular charge transfer (ꢀ) and optical properties of donor–ꢁ–accep-
tor compounds comprised of 1,3,3-trimethyl-2-methyleneindoline (I1) and substituted 7,8,8-tricyanoquinodimethane
(=2-(4-cyanomethylene-2,5-cyclohexadienylidene)malononitrile, 3CNQ-R, R = substituent groups) moieties were in-
vestigated (I1–3CNQ-R). In the crystal structures, I1–3CNQ-R molecules stacked on indoline and/or 3CNQ-R moieties
in a head-to-tail manner to cancel their dipole moments and established segregated or mixed stack columnar motifs. The
ꢀ values of I1–3CNQ-R molecules in solid and solution states were estimated using the bond length ratio in the crystal
structures, molecular orbital calculation, and the solvatochromic shift of intramolecular charge-transfer absorption, re-
spectively, and showed significant and reasonable dependences on substituents of the 3CNQ moiety. Structural analysis
revealed that molecular conformation and planarity affect the ꢀ values of I1–3CNQ-R molecules. Molecular orbital cal-
culations revealed that molecular (hyper)polarizabilities can be modulated by tuning ꢀ. I1–3CNQ-R exhibited a solva-
tochromic shift, and the ground state changed from neutral (ꢀ ꢁ 0:5) in less-polar solvents to ionic (ꢀ ꢂ 0:5) in polar
solvents. Two-photon absorption properties of I1–3CNQ-R showed a significant substituent effect and indicated that
ꢀ is a fundamental parameter for modulating non-linear optical properties.
ꢀꢃ
Since the first proposal in 1974 of a tetrathiafulvalene
(=2-(1,3-dithiol-2-ylidene)-1,3-dithiole, TTF)–tetracyanoqui-
materials. Among these Dꢀþ–ꢁ–A systems, donor–tricyano-
quinodimethane (=2-(4-cyanomethylene-2,5-cyclohexadi-
nodimethane
(=3,6-bis(dicyanomethylene)-1,4-cyclohexa-
enylidene)malononitrile, 3CNQ), D–3CNQ, derivatives have
been studied most intensively.4,6–14,25,30
diene, TCNQ) compound linked through a covalent bond by
Aviram and Ratner,2 many scientists studying functional mo-
lecular materials have been exploring numerous unimolecular
donor (D)–acceptor (A) compounds.3,4 Intramolecular charge-
transfer (CT) compounds, Dꢀþ–ꢁ–A (ꢀ = degree of CT),
where D and A moieties are linked by a ꢁ-bond, have a flexible
electronic structure, amphoteric redox ability, and large dipole
moment. These features of Dꢀþ–ꢁ–A are interesting for use
For binary intermolecular CT complexes composed of elec-
tron-donor and -acceptor molecules, the degree of CT is one of
the most important parameters governing their properties and
functions such as conductivity33–35 and phase transitions (neu-
tral–ionic (N–I), Mott, Peierls, charge order, etc.).36–38 In par-
ticular, to afford highly conducting CT complexes or N–I
phase-transition systems, the selection of D and A moieties
having appropriate donor and acceptor strengths is one of
ꢀꢃ
ꢀꢃ
in molecular optics and electronics such as indicators of solvent
polarity,5,6 molecular rectifiers,4,7–10 organic field-effect tran-
sistors,11 non-linear optics,12–23 photovoltaics,24 photochromic
materials,12,25 organic conductors,26,27 and molecular mag-
nets.28,29 The reaction between amine derivatives (alkylamines,
N-alkyl picolinium, lepidinium, benzothiazolium, etc.) and
strong ꢁ-acceptor molecules (TCNQ,7,14,30 tetracyanoeth-
ꢀꢃ
the most important strategies.34–36 In the case of Dꢀþ–ꢁ–A
compounds, the degree of intramolecular CT (ꢀ), which indi-
cates the charge distribution in a molecule, is an important
parameter governing their (hyper)polarizabilities and non-
linear optical properties.14–16 Therefore, the control and esti-
mation of the ꢀ value are indispensable in molecular design.
Several D–tetrafluoro-substituted 3CNQ (3CNQ-F4) deriva-
tives have been prepared;6,9,25a however, modulation of the ꢀ
ylene,18,31 and p-chloranil32) yields stable Dꢀþ–ꢁ–A com-
pounds, which have been studied as various organic functional
ꢀꢃ
Published on the web September 11, 2008; doi:10.1246/bcsj.81.1131

1132 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
Experimental
Measurements and Calculations.
1H NMR spectra were
measured at 400 MHz on a JEOL JNM-FX400 spectrometer using
CDCl3 or DMSO-d6 as a solvent and tetramethylsilane as an inter-
nal standard. Elemental analyses were performed at the Center for
Organic Elemental Microanalysis, Kyoto University. Melting
points were measured with a Yanaco MP-500D micro melting-
point apparatus and were not corrected. Ultraviolet–visible
(UV–vis) spectra were measured on a Shimadzu UV-3100 spec-
trometer. Infrared (IR) spectra of the samples in KBr pellets were
measured using a Perkin-Elmer PARAGON 1000 Series FT-IR
spectrometer (resolution 2 or 4 cmꢃ1). Semi-empirical molecular
orbital (MO) calculations were performed using MOS/F V4 with
INDO/S parameterization coupled with a 20-dimensional CI ma-
trix, which is sufficient to obtain approximately invariant dipole
moments. Geometrical parameters were extracted from the crystal
structures. Two-photon absorption cross-sections (ꢃð2Þ) were
estimated by the Z-scan technique45 employing femtosecond
laser pulses emitted from an optical parametric amplifier (OPA,
SpectraPhysics OPA-800) pumped by a Ti–sapphire regenerative
amplifier system (SpectraPhysics Spitfire, Merlin, Tsunami, and
Millennia). The typical energy and pulse width used for the
measurements were 1 mJ and 100 fs, respectively. Non-linear ab-
sorption behavior was observed with an ordinary optical configu-
ration of an open aperture Z-scan method.45 Laser pulses were
nearly Gaussian in time and space as confirmed by beam profile
and autocorrelation measurements, respectively.46 Reliable results
were obtained for a concentrated solution. To maximize the
concentrations, acetonitrile (MeCN) and CHCl3 were used as
the solvents for R ¼ CF3, F2, and F1 and R ¼ H, Me, (MeO)2,
and (EtO)2, respectively. The recorded traces were fit with the
theoretical equation for normalized transmittance through a pure
two-photon absorption medium.
Chart 1. Chemicals discussed in this study.
value by systematic chemical modifications has not been re-
ported for the D–3CNQ system. In addition, estimation of
the ꢀ value, which is necessary to understand the properties
ꢀꢃ
of a Dꢀþ–ꢁ–A molecule, has rarely been demonstrated.
Broo and Zerner demonstrated the theoretical calculation of
CH3-ꢂP–3CNQ (ꢂP = ꢂ-picolinium) and deduced the ꢀ value
from the sums of atomic charges on both D and A moieties.39
Bell et al. assumed an ionic ground state for CnH2nþ1-BT–
3CNQ (BT = benzothiazolium) based on their solvatochromic
shifts, although quantitative estimation was not performed.6
For several other cases, completely neutral or ionic states
(ꢀ ¼ 0 or 1) have been anticipated.4a,8,14,17,18,40–42
Z
2
1
ð1 ꢃ RÞ
pffiffiffi
TNðꢄÞ ¼
ꢃ1 ln½1 þ qðꢄÞ expðꢃx2Þꢄdx
ð1Þ
ꢁqðꢄÞ
with qðꢄÞ ¼ q0=ð1 þ ꢄ2Þ. Here, TN denotes the normalized trans-
mittance as a function of normalized z-position ꢄ ¼ ðz ꢃ z0Þ=zR
with the position of the beam waist z0. R is the Fresnel reflectance
at the cell wall. The linear absorption can be neglected at the
wavelength of measurement. Curve fitting with eq 1 provides
the two-photon absorption parameter q0, defined as
ꢀꢃ
We have been studying a series of Dꢀþ–ꢁ–A compounds,
N-alkyl indoline–3CNQ derivatives (In–3CNQ-R, Chart 1,
n = N-alkyl chain length and R = substituent groups on
the 3CNQ moiety) prepared by the Stork-enamine reaction
between 1-alkyl-3,3-dimethyl-2-methyleneindoline and R-
TCNQ.1,43,44 In our previous study, crystal structure analyses
of In–3CNQ-H derivatives with various lengths of alkyl chains
(R ¼ H and n ¼ 2{20) revealed that bond length ratio (BLR,
vide infra) shows a good linear relationship with the ꢀ value
and that the molecular conformation (Type I and II, vide infra)
has a notable effect on the ꢀ value.43 In the present study, to
elucidate the effects of chemical modification on the electronic
structure (ꢀ value) and properties of In–3CNQ-R compounds,
we have systematically prepared I1–3CNQ-R derivatives with
various substituent groups (R ¼ F4, CF3, F2, F1, H, Me,
(MeO)2, and (EtO)2). Here, we report the crystal structures
and estimations of the ꢀ values based on the BLR in the crystal
structures and solvatochromic shift of the intramolecular CT
absorption band. We also discuss molecular (hyper)polariza-
bilities and two-photon absorption properties in connection
with the ꢀ value.
q0 ¼ ꢂð2Þð1 ꢃ RÞI0L
ð2Þ
where ꢂð2Þ, I0, and L are the two-photon absorption coefficient, on-
axis peak intensity of the incident pulse (calculated for every set
of Z-scan measurements), and sample length. The power range
was selected by confirming the linearity between q0 and incident
power to exclude the effect of other processes such as three-pho-
ton absorption and excited state absorption. From the ꢂð2Þ value
obtained from the slope of the plot between q0 and incident power,
the two-photon cross-section ꢃð2Þ was calculated with the equation
h!ꢂð2Þ
ꢀ
ꢃð2Þ
¼
ð3Þ
N
where h! is the energy of the incident photon and N is the number
ꢀ
density of solute molecules.47
Preparation of I1–3CNQ-R. I1–3CNQ-R derivatives were
ꢀþ
prepared by the methods used for preparing 3CNQ-type D
ꢀꢃ
–
ꢁ–A systems:6,7,12,25a 1,3,3-Trimethyl-2-methyleneindoline (I)
(1 mol equiv) and R-TCNQ (1 mol equiv) were reacted in dry

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1133
Scheme 1. Synthetic procedures for In–3CNQ-R.
Chart 2. ꢁ-Extended or bulky TCNQ derivatives which did
ꢀꢃ
not yield Dꢀþ–ꢁ–A
.
thoquinodimethane (TNAP, +0.26 V), bis(1,2,5-thiadiazolo)-
TCNQ (BTDA-TCNQ, +0.03 V), 2,3,5,6-tetramethyl-TCNQ
(Me4TCNQ, ꢃ0:05 V), and tetracyano-9,10-anthraquinodimeth-
Scheme 2. Reaction mechanism to produce 2/6- and 3/5-
isomers (denoted (a) and (b), respectively) of mono-substi-
tuted I1–3CNQ-R derivatives.
ꢀꢃ
ane (TCAQ, ꢃ0:32 V) did not afford Dꢀþ–ꢁ–A type products.
Typical Procedure for Preparation: I1–3CNQ-F2.
F2-
TCNQ (480 mg, 2.00 mmol) was dissolved in PhCl (30 mL) under
reflux. I (0.35 mL, 1.98 mmol) in PhCl (30 mL) was added drop-
wise, and the reaction mixture was refluxed for 5 h under nitrogen
atmosphere. The resulting green solution was cooled to room tem-
perature and filtered to yield the product. The resulting crude
product was recrystallized from MeCN to yield (I1–3CNQ-
F2)(MeCN)0:5 (150 mg, 18%) as black rod crystals. Physical
data for the characterization are summarized in Supporting
Information.
chlorobenzene (PhCl) under reflux (Scheme 1). I6–3CNQ-CF3
was prepared by the reaction of CF3-TCNQ and 1-hexyl-3,3-di-
methyl-2-methyleneindoline, which was generated in situ in two
steps from 2,3,3-trimethyl-3H-indole (II). Stork-enamine reaction
of mono-substituted R-TCNQ derivatives (R ¼ Me, F1, and CF3)
is expected to yield 2/6- and 3/5-isomers (denoted as a and b in
Scheme 2) related to substituent positions on the 3CNQ moiety,
where 2/6- and 3/5-indicate the rotational isomers. All mono-sub-
stituted R-TCNQ afforded the 2/6-isomers as major products. The
3/5-isomer was obtained only for R ¼ F1 as the minor product
(a:b ꢂ 10:1 from the 1H NMR spectra of the reaction mixture),
and those of R ¼ Me and CF3 derivatives were not obtained.
The steric repulsion of substituent groups would be the origin of
the selectivity in products. In the intermediate generated by the
nucleophilic addition of the methylene group of I to the dicyano-
methylene group of R-TCNQ, the steric repulsion around the 3/5-
substituent group is larger than that of the 2/6-substituent group,
resulting in the selective formation of the 2/6-isomer (Scheme 2).
The reactions of I with ꢁ-extended or bulky TCNQ derivatives
(Chart 2): tetracyano-1,4-naphthoquinodimethane (TCNNQ, re-
duction potential (EA) = +0.07 V vs. SCE), tetracyano-2,6-naph-
X-ray Crystal Structure Analyses. The intensity data of
structural analyses were collected using a Bruker AXS DIP-
2020K oscillator type X-ray imaging plate or MAC Science
ꢅ
MXC automatic four-circle diffractometer with monochromated
˚
˚
Mo Kꢂ (ꢆ ¼ 0:71073 A, 1 A = 0.1 nm) radiation. Structures
were determined by a direct method using SHELXS-97.48 Least-
squares refinements were performed by the full-matrix least-
squares method on F2 with SHELXL-97.49 All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were includ-
ed without refinement. Positions of the hydrogen atoms were cal-
culated assuming sp3 or sp2 conformations of carbon atoms. Crys-
tallographic data have been deposited with the Cambridge Crys-
tallographic Data Centre: Deposition numbers CCDC-686454–

1134 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
686463. Copies of the data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge, CB2 1EZ, UK; Fax: +44 1223 336033; e-mail: deposit@
ccdc.cam.ac.uk). Crystallographic data are summarized in Sup-
porting Information.
Scheme 3. Two conformational isomers of I1–3CNQ-R
(Type I and II).
Results and Discussion
Molecular and Crystal Structures. Recrystallization of
I1–3CNQ-R (R ¼ F4 (A), F2 (C), F1 (D), (MeO)2 (E), and
(EtO)2 (F)) from MeCN yielded single crystals suitable for
X-ray crystal structure analysis. Crystals of I6–3CNQ-CF3
(B) were obtained from methanol (MeOH). In the cases of
R ¼ F1, (MeO)2, and (EtO)2 derivatives, two or three kinds
of polymorphs (labeled as a–c) were obtained. Polymorphs
of I1–3CNQ-F1 contained different kinds of isomers concern-
ing the position of the fluorine atom on the 3CNQ moiety,
2/6-F (I1–3CNQ-F1a, disordered) (Da) and 3-F isomers (I1–
3CNQ-F1b, Db and Dc) (Scheme 2), respectively, the former
of which included the 3-F isomer as a minor impurity (8–
9%). In the crystal structures, the indoline–indoline and/or
3CNQ–3CNQ ꢁ-stacking structures in a head-to-tail manner
to cancel molecular dipole moments were the most common,
and constructed one-dimensional segregated or mixed stacking
structures. Due to the formation of centrosymmetric ꢁ-stack-
ing motifs, second-order non-linear optical properties such as
second-harmonic generation which need the alignment of the
dipole moments to form non-centrosymmetric systems are
not expected for their bulk crystals. Although several com-
pounds had segregated stacking structures of the indoline
and 3CNQ-R moieties, they were electrically insulators be-
cause they did not have radical electrons as conduction carrier.
In the crystal structures, two molecular conformations, Type
I and II, were observed in which the N-alkyl group is located
close to and far from the CN1 group, respectively (Scheme 3).
The dihedral angle (ꢇ3) between the indoline moiety and the
cyanoethene ꢁ-bridge distinguishes Type I (ꢇ3 < 90ꢅ) and II
(ꢇ3 > 90ꢅ). Dihedral angle between the benzenoid ring in the
3CNQ moiety and the terminal dicyanomethylene group is in-
dicated by ꢇ1 and that between the ring and the cyanoethene ꢁ-
bridge by ꢇ2. The molecular conformations and dihedral an-
gles (ꢇ1–ꢇ3) of the D and A parts are summarized in Table 1.
I1–3CNQ-F4 (A): Crystal A crystallized as a monoclinic
system, where one I1–3CNQ-F4 molecule with a Type I con-
formation (ꢇ3 ¼ 43:2ꢅ) was crystallographically independent
(Figure 1a). The indoline and 3CNQ parts were nearly orthog-
onal to each other (ꢇ2 þ ꢇ3 ¼ 78:9ꢅ). This angle is the largest
among the Type I In–3CNQ-R molecules in the present study,
and reflects the single bond nature of bonds b and d and the
large ꢀ value of this molecule (vide infra). Both indoline and
3CNQ moieties individually formed face-to-face dimers hav-
ing inversion centers and interplanar distances of 3.38 and
Table 1. Conformation and Twist Angles (ꢇ1–ꢇ3) of
In–3CNQ-R Molecules in the Crystal Structures
Compound
Type ꢇ1/ꢅ ꢇ2/ꢅ
ꢇ3/ꢅ
I1–3CNQ-F4
I6–3CNQ-CF3
I1–3CNQ-F2
A
B
C
I
II
I
II
II
I
I
I
I
I
1.4
2.0
9.9
12.0
7.9
11.1
3.8
1.9
1.5
1.4
0.6
35.8
43.2
4.3 162.5
20.3
50.4
4.5 165.0
15.8 177.8
DaI
DaII
Db
Dc
Ea
Eb
Fa
Fb
I1–3CNQ-F1a
19.3
17.9
11.8
7.4
11.1
5.5
48.8
57.3
47.2
I1–3CNQ-F1b
I1–3CNQ-(MeO)2
I1–3CNQ-(EtO)2
a)
—
48.2
39.7
I
a) Due to the rotational disorder of the indoline moiety, ꢇ3 was
not obtained for Eb.
I6–3CNQ-CF3 (B): I6–3CNQ-CF3 crystallized as a triclin-
ic system, and one I6–3CNQ-CF3 molecule with a Type II con-
formation (ꢇ3 ¼ 162:5ꢅ) was crystallographically independent
(Figures 2a and 2b). In this crystal, only one positional isomer
was observed, in which the CF3 group was connected at the 6-
position of the 3CNQ moiety (Chart 1). The small ꢇ1 and ꢇ2
angles (2.0 and 4.3ꢅ, respectively) reflect the planar molecular
structure (Figure 2b). In the crystal structure, the 3CNQ moie-
ty formed a face-to-face dimer in a head-to-tail manner with an
˚
interplanar distance of 3.57 A (yellow areas in Figure 2c). In
addition, the dimers stacked at the indoline and 3CNQ moie-
˚
ties with an interplanar distance of ꢆ3:8 A (green arrows in
Figure 2c) to form a one-dimensional column along the b axis.
Significant interaction between the indoline moieties was not
observed in this crystal. The hexyl group, which extended per-
pendicularly to the molecular plane (Figure 2a), aggregated to
form a one-dimensional structure parallel to the D–ꢁ–A col-
umn (b axis, blue areas in Figures 2c and 2d). Such self-assem-
bling nature involving the cooperation of face-to-face stacks of
the D–ꢁ–A skeleton and aggregation of alkyl chains resembles
those in In–3CNQ-H43 and other D–3CNQ-R derivatives6,50
having long N-alkyl chains.
˚
3.36 A, respectively (red and yellow areas in Figures 1b–1d).
These dimers were arranged alternately along the [101] direc-
tion to form a one-dimensional structure (Figures 1c and 1d).
In addition, a stack between indoline and 3CNQ moieties with
(I1–3CNQ-F2)(MeCN)0:5 (C) and (I1–3CNQ-F1b)-
(MeCN)0:5 (Db): Crystals C and Db were iso-structures. In
crystal C, the I1–3CNQ-F2 molecule had a Type I conforma-
tion (ꢇ3 ¼ 50:4ꢅ, Figure 3a). Fluorine atoms located at the 3-
˚
ꢆ3:5 A interplanar distance was observed (green arrows in
Figure 1b), forming a one-dimensional column along the
ꢀ
[110] and ½110ꢄ directions.

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1135
Figure 1. (a) Molecular structure of I1–3CNQ-F4 in A. (b) One-dimensional column of I1–3CNQ-F4 molecules composed of the
3CNQ–3CNQ (yellow area), indoline–indoline (red areas), and indoline–3CNQ (green arrows) stacks. (c) and (d) Molecular pack-
ing viewed along the b axis at b ¼ 1=2 and 0, respectively. Yellow and red areas represent the dimer units of 3CNQ and indoline
moieties, respectively. Hydrogen atoms are omitted in (b)–(d).
Figure 2. (a) Top view and (b) side view of the molecular structure of I6–3CNQ-CF3 in B. (c) and (d) Molecular packing of B
viewed along the a and b axis, respectively, showing a one-dimensional D–ꢁ–A column and alkyl chain aggregation. Yellow
and blue areas represent the 3CNQ dimers and alkyl chain aggregations, respectively, and green arrows show indoline–3CNQ
stacks. Hydrogen atoms are omitted in (c) and (d).
and 6-positions of the 3CNQ moiety, and positional and rota-
tional disorders were not observed. The short C–H F intramo-
˚
lecular hydrogen bond (2.21 A) stabilized the 3,6-F2 isomer
rather than the 2,5-F2 isomer. In addition, steric repulsion be-
tween CN1 group and fluorine atom (Scheme 3), which is ex-
pected for the 2,5-F2 isomer, is not present in the 3,6-F2 iso-
mer. In the crystal structure of C, the 3CNQ moiety formed
a face-to-face dimer having an inversion center with an inter-
I1–3CNQ-F1b in Db had a Type I conformation (Figure 3b),
and the planarity of the D–ꢁ–A skeleton expressed by the
dihedral angles ꢇ1–ꢇ3 was very close to that of C (Table 1).
This crystal contained only 3-F isomer and showed a short
ꢇꢇꢇ
˚
C–H F intramolecular hydrogen bond (2.19 A). Further, the
molecular packing was very similar to that of C, where the
ꢇꢇꢇ
˚
face-to-face distance within 3CNQ dimer was 3.39 A.
I1–3CNQ-F1a (Da): Crystal Da crystallized as a triclinic
system, and two crystallographically independent molecules
(DaI and DaII) were observed. DaI molecule included two
rotational isomers, where 2-F and 6-F isomers exist in a
0.85:0.15 ratio (Figure 4a). On the other hand, DaII molecule
consisted of 2-F and 3-F positional isomers in a 0.83:0.17 ratio
˚
planar distance of 3.41 A (yellow areas in Figures 3c and 3d).
The dimers interacted with the indoline–3CNQ stacks at a
˚
face-to-face distance of ꢆ3:3 A to form a one-dimensional col-
umn (green arrows in Figure 3d). MeCN solvent molecules
were located between 3CNQ dimers (Figure 3d).

1136 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
Figure 3. Molecular structures of (a) I1–3CNQ-F2 in C and (b) I1–3CNQ-F1b in Db. (c) Overlap mode of a one-dimensional struc-
ture in C. (d) Molecular packing of C viewed along the b axis. Yellow areas indicate 3CNQ dimers, and green arrows show indo-
line–3CNQ stacks. Hydrogen atoms are omitted in (d).
Figure 4. (a) and (b) Molecular structures of I1–3CNQ-F1a, DaI and DaII, respectively. (c) and (d) Molecular packing of Da
viewed along the b axis at b ¼ 1=2 (DaI layer) and b ¼ 0 (DaII layer), respectively. Red and yellow areas indicate indoline–indo-
line and 3CNQ–3CNQ stacks, respectively. Hydrogen atoms are omitted in (c) and (d).
˚
lecular hydrogen bond (2.19 A) was observed. In the crystal
structure of Dc, the 3CNQ moiety formed a face-to-face dimer
˚
with an interplanar distance of 3.33 A and an inversion center
(yellow areas in Figures 5b and 5c). The 3CNQ dimers were
connected by the indoline–indoline stacks to form a one-di-
mensional structure along the b axis (Figure 5b). The indoline
moiety formed a segregated column along the a axis with non-
(Figure 4b). The distance between a hydrogen atom of the cy-
anoethene group and a fluorine atom in the 3-F isomer was
I
II
˚
2.16 A (Figure 4b). In this crystal, both Da and Da mole-
cules had a Type II conformation (Figures 4a and 4b), and
their molecular structures were more planar than that of the
Db and Dc molecules having a Type I conformation (Table 1).
DaI and DaII molecules individually formed layer structures
on the b ¼ n þ 1=2 and n þ 0 planes, respectively. In the
DaI layer, the 3CNQ moiety formed an isolated face-to-face
˚
uniform interplanar distances of 3.39 and 3.64 A (Figure 5c).
I1–3CNQ-(MeO)2 (Ea and Eb): Two polymorphs of I1–
3CNQ-(MeO)2 (Ea and Eb) were obtained from the same
batch. In the crystal Ea, the I1–3CNQ-(MeO)2 molecule had
a Type I conformation (ꢇ3 ¼ 47:2ꢅ) and contained two rota-
tional isomers, where 2,5- and 3,6-isomers existed in a
0.40:0.60 ratio (Figure 6a). The terminal dicyanomethylene
group was nearly parallel to the benzenoid plane (ꢇ1 ¼ 1:9ꢅ).
The 3CNQ and indoline moieties individually stacked to form
one-dimensional columns along the b axis and [110] direction,
respectively (yellow and red areas in Figures 6b and 6c). Each
face-to-face stack had a head-to-tail orientation to cancel the
dipole moment. The interplanar distances were 3.51 and
˚
dimer with an interplanar distance of 3.57 A (yellow areas),
whereas the indoline moiety formed a segregated column
along the a axis with non-uniform interplanar distances of
II
˚
3.86 and 4.07 A (red areas in Figure 4c). In the Da layer,
the 3CNQ moiety formed a segregated column along the a axis
˚
with interplanar distances of 3.56 A (yellow areas), whereas
the indoline moiety formed an isolated dimer with a face-to-
˚
face distance of 4.84 A (Figure 4d).
I1–3CNQ-F1b (Dc): Crystal Dc crystallized as a mono-
clinic system, where the I1–3CNQ-F1b molecule with a
Type I conformation is crystallographically independent
(ꢇ3 ¼ 57:3ꢅ, Figure 5a). A fluorine atom was located at the
˚
˚
3.68 A for the 3CNQ column and 3.48 and 3.85 A for the
indoline column.
3-position of the 3CNQ moiety, and a short C–H F intramo-
ꢇꢇꢇ

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1137
Figure 5. (a) Molecular structure of I1–3CNQ-F1b in Dc. (b) and (c) Molecular packing of Da viewed along the a and c axis, re-
spectively. Red and yellow areas indicate indoline–indoline and 3CNQ–3CNQ stacks, respectively. Hydrogen atoms are omitted
in (b) and (c).
Figure 6. (a) Molecular structure of the I1–3CNQ-(MeO)2 in Ea. (b) and (c) Molecular packing of Ea viewed along the [110] di-
rection and b axis, respectively. (d) Molecular structure of the I1–3CNQ-(MeO)2 in Eb. The gray atoms are those of the disordered
indoline moiety. (e) Molecular packing of Eb viewed along the a axis. Yellow and red areas show the 3CNQ–3CNQ and indoline–
indoline stacks, respectively. Hydrogen atoms are omitted in (b), (c), and (e). The disordered atoms of smaller site occupancy
factors are also omitted for clarity in (b), (c), and (e).
Similar to Ea, the I1–3CNQ-(MeO)2 molecule in Eb had a
Type I conformation. A rotational disorder of the indoline moi-
ety (0.90:0.10 ratio) was observed, where dihedral angles (ꢇ3)
were ꢆ41 and 45ꢅ, respectively (Figure 6d). A rotational dis-
order of the 3CNQ moiety was also observed, where 3,6- and
2,5-isomers existed in a 0.90:0.10 ratio (Figure 6d). The termi-
nal dicyanomethylene group was nearly parallel to the benze-
noid plane (ꢇ1 ¼ 1:5ꢅ). In the crystal structure, the 3CNQ moi-
ety stacked in a head-to-tail fashion with interplanar distances
I1–3CNQ-(EtO)2 molecule had a Type I conformation
(ꢇ3 ¼ 48:2ꢅ, Figure 7a). Rotational disorder of the 3CNQ moi-
ety was not observed. The terminal dicyanomethylene group
was nearly parallel to the benzenoid plane (ꢇ1 ¼ 1:4ꢅ), where-
as the ꢁ-bridge was distorted towards the benzenoid plane
(ꢇ2 ¼ 11:1ꢅ). The 3CNQ and indoline moieties stacked indi-
vidually to form one-dimensional columns along the a axis
and [110] direction, respectively (yellow and red areas in
Figures 7b and 7c). Each face-to-face stack had a head-to-tail
orientation to cancel the dipole moment. The interplanar dis-
˚
of 3.35 and 3.38 A to form a one-dimensional column along
˚
the b axis (yellow areas in Figure 6e). In contrast to Ea, the
indoline moiety did not form ꢁ-stacking structures and was
arranged in a herringbone motif in the ab plane (Figure 6e).
(I1–3CNQ-(EtO)2)(MeCN) (Fa) and I1–3CNQ-(EtO)2
(Fb): Two polymorphs of I1–3CNQ-(EtO)2 (Fa and Fb) were
obtained from the same batch. In the solvated crystal Fa, the
tances were 3.43 and 3.52 A for the 3CNQ column and 3.41
˚
and 5.03 A for the indoline column. These structural features
are similar to those of Ea (Figures 6b and 6c).
Similar to Fa, the I1–3CNQ-(EtO)2 molecule in the non-sol-
vated crystal Fb had a Type I conformation with a dihedral an-
gle ꢇ3 ¼ 39:7ꢅ (Figure 7d). Rotational disorder of the 3CNQ

1138 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
Figure 7. (a) Molecular structure of the I1–3CNQ-(EtO)2 in Fa. (b) and (c) Molecular packing of Fa viewed along the a axis and
[110] direction, respectively. (d) Molecular structure of I1–3CNQ-(EtO)2 in Fb. (e) and (f) Molecular packing of Fb viewed along
the b axis and the molecular short axis, respectively. Red and yellow areas show the indoline–indoline and 3CNQ–3CNQ stacks,
respectively. Hydrogen atoms are omitted in (b), (c), (e), and (f).
˚
Table 2. Selected Intramolecular Bond Lengths (a–e, A) and Bond Length Ratio (BLR) of In–3CNQ-R Molecules
in the Crystal Structures
˚
a/A
˚
b/A
˚
c/A
˚
d/A
˚
e/A
Compound
Type
BLR
I1–3CNQ-F4
I6–3CNQ-CF3
I1–3CNQ-F2
A
B
C
I
II
I
II
II
I
I
I
I
I
1.313(2)
1.348(3)
1.312(2)
1.341(3)
1.348(3)
1.320(3)
1.315(4)
1.349(2)
1.440(3)
1.412(4)
1.435(3)
1.415(3)
1.404(3)
1.428(3)
1.436(5)
1.386(2)
1.375(6)
1.379(7)
1.389(7)
1.350(3)
1.384(4)
1.356(3)
1.372(3)
1.387(3)
1.366(3)
1.358(5)
1.400(2)
1.422(5)
1.391(7)
1.411(7)
1.457(3)
1.439(4)
1.451(3)
1.453(3)
1.441(4)
1.449(3)
1.451(5)
1.416(2)
1.416(5)
1.420(7)
1.423(7)
1.427(3)
1.426(4)
1.423(3)
1.427(3)
1.410(4)
1.419(3)
1.418(5)
1.404(2)
1.400(5)
1.407(7)
1.399(7)
1.624(3)
1.566(4)
1.615(3)
1.583(3)
1.556(4)
1.599(3)
1.611(5)
1.530(2)
DaI
DaII
Db
Dc
Ea
Eb
Fa
Fb
I1–3CNQ-F1a
I1–3CNQ-F1b
I1–3CNQ-(MeO)2
I1–3CNQ-(EtO)2
a)
a)
—
1.357(6)
1.348(6)
—
1.531(8)
1.526(7)
I
a) Not obtained due to the rotational disorder of the indoline moiety.
a methodology to qualitatively estimate the degree of intramo-
lecular CT (ꢀ) of In–3CNQ-R using the bond alternation in
these two forms.43 The bond length ratio defined as (BLR ¼
ðb þ d þ eÞ=ða þ cÞ) is the best experimental parameter to dis-
cuss the ꢀ value in the solid state, and shows a good linear
relationship with the dipole moments and ꢀ values estimated
from MO calculation.43 It should be noted that we can not
quantitatively discuss ꢀ values from BLR analysis, since the
ꢀ ¼ 0 and 1 limits for the In–3CNQ-R system have not been
obtained at present. The BLR values obtained for I1–3CNQ-
R derivatives and I6–3CNQ-CF3 are summarized in Table 2
together with the bond lengths a–e. It is noteworthy that the
BLR value well reflects the ꢀ value; for Type I derivatives, it
decreases in the order R ¼ F4, F2, F1b, (MeO)2, and (EtO)2,
which is in good agreement with the weakening of acceptor
strength of the 3CNQ moiety.
Scheme 4. Bond alternations of I1–3CNQ-R in neutral
(ꢀ ¼ 0) and ionic (ꢀ ¼ 1) forms.
moiety was not observed. The 3CNQ moiety had a planar
structure (ꢇ1 ¼ 0:6ꢅ and ꢇ2 ¼ 5:5ꢅ). In the crystal structure,
the 3CNQ moiety stacked in a head-to-tail manner with inter-
˚
planar distances of 3.43 and 3.47 A to form a one-dimensional
column along the b axis (yellow areas in Figures 7e and 7f). In
contrast to Fa, the indoline moiety did not form ꢁ-stacking
structures.
Structural Parameters Related to Molecular and Elec-
tronic Structures of In–3CNQ-R. (A) Bond Length Ratio
(BLR) vs. Degree of Intramolecular CT: Electronic struc-
ture of an In–3CNQ-R molecule is presented by the mixing
of two resonance structures of neutral (ꢀ ¼ 0) and ionic
(ꢀ ¼ 1) forms (Scheme 4). In a previous report, we proposed
(B) Molecular Conformations and Planarity:
In the
crystal structures, I1–3CNQ-R molecules except for Db, had
a Type I conformation. The molecular conformation of In–
3CNQ-R is determined mainly by the steric repulsion between
the CN1 group and the N-alkyl group or the 3,3-dimethyl
group in the indoline moiety (Scheme 3). Type I conformation

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1139
Table 3. Dipole Moments (ꢈg and ꢈe), Degree of CT (ꢀdipole), Polarizability (ꢂ), and Hyperpolarizabilities (ꢉ and ꢊ)
of In–3CNQ-R Molecules Evaluated by MO Calculation (INDO/S)
Compound
Type ꢈg/D ꢈe/D ꢀdipole ꢂ/10ꢃ24 cm3 ꢉ/10ꢃ30 cm5 esuꢃ1 ꢊ/10ꢃ36 cm6 ergꢃ1
I1–3CNQ-F4
I6–3CNQ-CF3
I1–3CNQ-F2
A
B
C
DaIa)
DaIIa)
Db
Dc
3ab)
10b)
I
II
I
II
II
I
I
I
I
I
27.29
20.04
25.93
19.51
16.75
23.77
25.46
22.59
21.87
15.88
17.07
15.54
10.20
17.87
10.79
16.50
15.68
10.51
8.40
13.14
12.71
15.71
15.18
16.04
0.73
0.53
0.71
0.54
0.52
0.69
0.75
0.63
0.63
0.50
0.53
0.49
67.67
69.81
67.51
74.46
69.24
67.80
64.68
71.01
70.12
64.66
67.14
64.46
ꢃ476:9
ꢃ105:1
ꢃ408:3
ꢃ139:1
ꢃ75:3
57.66
ꢃ264:5
ꢃ38:82
ꢃ320:5
ꢃ270:0
ꢃ96:57
39.13
ꢃ206:1
ꢃ203:2
ꢃ227:3
ꢃ249:3
ꢃ221:4
I1–3CNQ-F1a
I1–3CNQ-F1b
ꢃ363:1
ꢃ432:6
ꢃ298:5
ꢃ291:0
ꢃ55:9
I3–3CNQ-H
I10–3CNQ-H
I1–3CNQ-(MeO)2 Eaa)
Fa
I1–3CNQ-(EtO)2
Fb
I
I
ꢃ94:6
ꢃ37:2
a) Orientation disorders of substituent groups on the 3CNQ moieties were observed. MO calculations were performed using
structures having larger occupancies. b) See, reference 43.
ꢀꢃ
in some Dꢀþ–ꢁ–A compounds.41,51 Figure 8 plots the BLR
value against molecular planarity defined as cos ꢇ1 þ cos ꢇ2 þ
cos ꢇ3 for Type I I1–3CNQ-R molecules and Type I In–3CNQ-
H (n ¼ 3 and 10).43 As expected, the BLR value increases with
the decrease in molecular planarity, indicating that a twisted
D–ꢁ–A skeleton results in a highly ionic form. For the Type
II molecules, we could not find such a relationship between
BLR and molecular planarity because of the small difference
in planarity.
(C) Dipole Moments and Degree of Intramolecular CT:
ꢀꢃ
The ꢀ value of a Dꢀþ–ꢁ–A molecule is related to molecular
properties such as dipole moments (ground state, ꢈg ¼ ꢀed;
excited state, ꢈe ¼ ð1 ꢃ ꢀÞed, where e and d are the elementa-
ry electric charge and the average distance between centers of
positive and negative charges, respectively). In the previous
paper, we demonstrated that the ꢈg values of In–3CNQ-R
molecules obtained by a MO calculation based on the crystal
structures show a good relationship with the BLR values.43
The degree of CT (ꢀdipole) reproducing dipole moments is eval-
uated using eq 4.
Figure 8. Plot of BLR value vs. molecular planarity
(cos ꢇ1 þ cos ꢇ2 þ cos ꢇ3) for Type I I1–3CNQ-R mole-
cules together with I3–3CNQ-H (3; BLR ¼ 1:593ð2Þ, mo-
lecular planarity = 2.70) and I10–3CNQ-H (10; 1.586(2),
2.64) (R ¼ H).43
ꢈg
ꢀdipole
¼
ð4Þ
ꢈe þ ꢈg
Table 3 lists the calculated ꢈg, ꢈe, and ꢀdipole values. The
ꢀdipole value for Type I derivatives increases in the order R ¼
(EtO)2 and (MeO)2, F1b, F2, and F4, which is in good agree-
ment with strengthening of the acceptor strength of 3CNQ
moiety. Similar to the tendency found in the BLR analysis,
the ꢀdipole values for Type II molecules are smaller than those
for Type I molecules. The ꢀdipole value reflects the molecular
shape, conformation, and substitution effects on the spatial
charge distribution, and is regarded as an appropriate parame-
ter to express the ꢀ values.
of I1–3CNQ-R molecules is caused by the smaller size of the
N-methyl group than that of the 3,3-dimethyl group. This is in
contrast to what is observed for most of the long alkyl chain
In–3CNQ derivatives43 and I6–3CNQ-CF3 (B) having a Type
II conformation. In a previous study, we also clarified that
the Type I molecule has a larger ꢀ value than that of Type
II.43 The considerably smaller BLR value of Type II molecule
B (R ¼ CF3, 1.566(4)), which should be similar to that of Type
I molecule C (R ¼ F2, 1.615(3)) according to the acceptor
strength, is also in good agreement with the above tendency.
Molecular (Hyper)polarizabilities and Degree of Intra-
molecular CT. Equation 5 represents the relationship of mo-
lecular polarization (p) under an electric field (E) with molec-
ular polarizability (ꢂ) and hyperpolarizabilities (ꢉ and ꢊ).
ꢀꢃ
The ꢀ value of a Dꢀþ–ꢁ–A compound is expected to re-
late to its molecular planarity. In the ionic form (ꢀ ¼ 1,
Scheme 4), bonds b, d, and e have a single bond character
and can twist to have large dihedral angles ꢇ1, ꢇ2, and ꢇ3, re-
spectively. On the other hand, the neutral form (ꢀ ¼ 0) has a
planar structure because of the double bond character of these
bonds. Such expectation has been theoretically demonstrated
p ¼ ꢈg þ ꢂE þ ꢉE2 þ ꢊE3 þ . . .
ð5Þ
Polarizabilities ꢂ and ꢉ are approximated by the two-level
model,

1140 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
Figure 9. Plots of (hyper)polarizabilities ꢂ (a), ꢉ (b), and ꢊ (c) obtained by the MO calculation against ꢀdipole for I1–3CNQ-R, I6–
3CNQ-CF3, and In–3CNQ-H (3: n ¼ 3 and 10: n ¼ 10)43 molecules. Green circles and red triangles show Type I and II molecules,
2
2
respectively. Solid lines are the theoretical curves calculated from eqs 10–12; ꢂ ¼ 3:15 ꢈ 102ꢀ ð1 ꢃ ꢀÞ þ 42:9, ꢉ ¼ 7:17 ꢈ
3
4
2
3
4
104ꢀ ð1 ꢃ ꢀÞ ð1 ꢃ 2ꢀÞ, and ꢊ ¼ 2:14 ꢈ 105ꢀ ð1 ꢃ ꢀÞ ð1 ꢃ 5ꢀ þ 5ꢀ Þ. Dashed lines are the least-squares fittings for each plot;
2
3
4
2
2
3
4
ꢂ ¼ 7:32 ꢈ 101ꢀ ð1 ꢃ ꢀÞ þ 64:3, ꢉ ¼ 1:09 ꢈ 105ꢀ ð1 ꢃ ꢀÞ ð1 ꢃ 2ꢀÞ, and ꢊ ¼ 2:82 ꢈ 105ꢀ ð1 ꢃ ꢀÞ ð1 ꢃ 5ꢀ þ 5ꢀ Þ.
ꢂ / ꢈeg2=Eeg
ꢉ / ꢈeg ðꢈe ꢃ ꢈgÞ=Eeg
ð6Þ
ð7Þ
2
2
Here, Eeg is the energy difference between ground and excited
states, and ꢈeg is the transition dipole moment which is
expressed by eq 8 according to Mulliken’s CT theory.52
ꢈeg ¼ ꢀ1=2ð1 ꢃ ꢀÞ1=2ed
ꢊ is approximated by the three-level model,15
ð8Þ
2
2
4
3
ꢊ / ½ꢈeg ðꢈe ꢃ ꢈgÞ ꢃ ꢈeg ꢄ=Eeg
ð9Þ
þ ꢈeg ꢈe e2=Eeg Ee g
where e0 is the second excited state.
2
2
0
0
It is essential to have a large d value for the improvement in
the ꢂ, ꢉ, and ꢊ values. The ꢀ value is also important according
to the theoretical treatment by Ducuing and Flytzanis based on
the point charge model.53 The ꢂ and ꢉ values can be approxi-
mated using the ꢀ value in eqs 10 and 11, respectively.
Figure 10. Intramolecular CT absorption in UV–vis spec-
tra of I1–3CNQ-R derivatives (R ¼ F4, F2, H, Me, and
(EtO)2) in MeCN solution.
2
2
ꢂ / ꢀ ð1 ꢃ ꢀÞ
ð10Þ
ð11Þ
Figure 9 plots (hyper)polarizabilities (ꢂ, ꢉ, and ꢊ) obtained
by the MO calculation against ꢀdipole value together with the
modified theoretical curves based on eqs 10–12. Relatively
good correlations between calculated values and theoretical
curves confirm that ꢀ is a fundamental parameter to describe
(hyper)polarizabilities. The most important consequence is
3
3
ꢉ / ꢀ ð1 ꢃ ꢀÞ ð1 ꢃ 2ꢀÞ
The ꢊ value cannot be expressed explicitly by the ꢀ value
0
0
without the ꢈe e and Ee g values (eq 9). Since the two-level
model is applicable for the non-centrosymmetric In–3CNQ-R
derivatives, the second term in eq 9 is negligible, leading to
eq 12.
ꢀꢃ
that these non-linear optical parameters of Dꢀþ–ꢁ–A mole-
cule have maximum absolute values at partial ꢀ values
(0 < ꢀ < 1) and that they can be modulated by chemical mod-
ifications of the D- and A-moieties.
2
2
4
3
ꢊ / ½ꢈeg ðꢈe ꢃ ꢈgÞ ꢃ ꢈeg ꢄ=Eeg
ð12Þ
4
2
4
/ ꢀ ð1 ꢃ ꢀÞ ð1 ꢃ 5ꢀ þ 5ꢀ Þ
Solution State Properties. (A) Absorption Spectra and
ꢀꢃ
Equations 10–12 indicate that (1) the ꢂ value has a maximum
at ꢀ ¼ 0:5 and minima at ꢀ ¼ 0 and 1; (2) the jꢉj value has
maxima at ꢀ ¼ 0:31 and 0.69 and minima at ꢀ ¼ 0, 0.5, and
1; and (3) the jꢊj value has maxima at ꢀ ¼ 0:2, 0.5, and 0.8
and minima at ꢀ ¼ 0, 0.28, 0.72, and 1 (Figure 9). These rela-
tionships of ꢂ, ꢉ, and ꢊ values with ꢀ are similar to those ob-
tained against bond length alternation (BLA) demonstrated by
Marder et al.15 or mixing between the limiting-resonance
forms by Barzoukas et al.16
Degree of Intramolecular CT in Solution:
D
ꢀþ–ꢁ–A
compounds derived from amines and TCNQ derivatives are
known to exhibit characteristic broad and intense intramolecu-
lar CT absorption bands in the range of 600–900 nm with one
or two major peaks in the solution state.6,7,14,25a In the I1–
3CNQ-R system, similar spectra were obtained with two major
peaks (Figure 10), which are summarized in Table 4.
The energy of an intramolecular CT for the ionic ground
state (ꢀ > 0:5) (hꢋCTI: Symbol I means the ionic ground state)

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1141
Table 4. UV–Vis Spectra and the ꢀsolv Values of In–3CNQ-R Derivatives
UV–visb)/nm
R (EA/V)a)
F4 (+0.60)
MeOH
—d), 574
MeCN
—d), 619
Acetone
—d), 636
PhCl
Dioxane
PhH
c)
PhMe
c)
CS2
c)
ꢀsolv
802, 731
796, 725
789, 724
800, 732
791, 718
— —
795sh, 725 800sh, 722
—
—
0.77
0.61
0.54
0.62
c)
CF3 (n ¼ 1) (+0.44) 765sh, 661 770sh, 690 776sh, 708 801, 731
771, 722 780, 728 814, 743
770sh, 657 770sh, 692 775sh, 704 803, 728
790, 720 786, 729 793, 730 809, 735
770sh, 686 765sh, 699 765sh, 705 794, 722
CF3 (n ¼ 6) (+0.44) 756, 716
799, 734
795, 719
797, 732
794, 719
803, 736
801, 727
F2 (+0.41)
F1a (+0.32)
F1b (+0.32)
H (+0.22)
Me (+0.20)
(MeO)2 (+0.05)
(EtO)2 (+0.01)
785sh, 716 795sh, 717 790sh, 712 790sh, 719 0.52
784, 711 785, 714 782, 712 788, 718 0.55
785sh, 710 780sh, 700 785sh, 701 785sh, 707 0.51
779, 720
789, 726
749, 694
750, 694
787, 725
795, 729
752, 696
753, 695
792, 726
800, 731
754, 696
754, 694
794, 726
800sh, 723 790sh, 705 780sh, 703 770sh, 696 775sh, 705 0.50
773sh, 693 —d), 672
771sh, 691 —d), 668
—d), 674
—d), 670
—d), 669
—d), 665
—d), 682
—d), 678
0.50
0.50
a) EA: Reduction potential of R-TCNQ, V vs. SCE in MeCN. b) Peak position. c) Not measured due to too poor solubility. d) Not
found due to overlapping with the second band.
is presented in eq 13,54 where Ip and EA are the ionization po-
tential of D and the electron affinity of A moieties, respective-
ly, and ", r, ꢁG, and X are the dielectric constant of the
solvent, the average distance between D and A moieties, the
difference in solvation energies of D0–ꢁ–A0 and D1þ–ꢁ–
A1ꢃ species, and the resonance stabilization energies of D0–
ꢁ–A0 and D1þ–ꢁ–A1ꢃ species, respectively.
maxima of C16H33-Q–3CNQ-R25a (Q = quinolinium) and
DEMI–3CNQ-R14 (DEMI = diethylammonium, Chart 1) in
MeCN shift to higher energy regions by 147 and 58 nm, re-
spectively, because of the modification of R substituents from
H to F4. In the present work, I1–3CNQ-R derivatives also
showed a blue shift of the intramolecular CT absorption bands
with increasing electron-withdrawing effect of R substituents
from R ¼ Me (729 nm, the second lowest energy peak) to R ¼
F4 (619 nm) in MeCN (Figure 10 and Table 4). For the I1–
3CNQ-R derivatives with strong electron-donating substituents
(R ¼ (MeO)2 and (EtO)2), the bands shifted to higher energy
region (696–695 nm) probably because they have a neutral
ground state in eq 14.
ꢀ
ꢁ
1 e2
I
hꢋCT ¼ ꢃðIp ꢃ EAÞ þ ð2ꢀ ꢃ 1Þ
ꢃ ꢁG þ X
ð13Þ
" r
Similarly, the energy for the neutral ground state (hꢋCTN: Sym-
bol N means the neutral ground state) is presented in eq 14,
where X0 is the resonance stabilization energy.
ꢀ
ꢁ
(C) Solvatochromic Shift: It is known that intramolecular
1 e2
N
ꢃ ꢁG ꢃ X0
ð14Þ
CT absorption bands of a Dꢀþ–ꢁ–A exhibit a hypsochromic
ꢀꢃ
hꢋCT ¼ ðIp ꢃ EAÞ þ ð1 ꢃ 2ꢀÞ
" r
shift with increase in solvent polarity.4a,6–8,54,58,59 A similar
solvatochromism was also observed in the I1–3CNQ-R system.
I1–3CNQ-R derivatives with strong electron-withdrawing
groups (R ¼ F4, CF3, and F2) showed significant solvatochro-
mic shifts (Table 4). For example, I1–3CNQ-F2 exhibited
bands at 770 (shoulder peak) and 657 nm in MeOH, and 803
and 728 nm in PhCl, where the hypsochromic shifts were 33
and 71 nm, respectively (Figure 11a). The shifts became less
pronounced with weakening electron-accepting ability of the
3CNQ-R part, and R ¼ H, Me, (MeO)2, and (EtO)2 derivatives
hardly exhibited a solvatochromic shift (Figure 11b).
Figure 12 plots the transition energy of the second lowest
energy peak (hꢋCT(2nd)), which is more clearly and intensely
observed than the first one, in polar solvents (MeOH, MeCN,
acetone, and PhCl) against transition energy of Reichardt’s be-
tain dye (ET)5b as an indicator of solvent polarity. The plots
showed a good linear relationship (hꢋCT(2nd) ¼ aET þ b),
and the slope a decreased with weakening electron-accepting
ability of the 3CNQ-R moiety.
In previous studies,1b,43 we demonstrated a methodology to
estimate the degree of intramolecular CT in the solution state
(ꢀsolv) using solvatochromic behavior. Considering that the ꢀ
value of an I1–3CNQ-R is subjected strongly to (Ip ꢃ EA) in
eq 13 and that hꢋCT shows a linear relationship with ET, the
ꢀsolv value for ꢀ ꢂ 0:5 is obtained as ða þ 1Þ=2. It should be
noted that the obtained ꢀsolv values bear some ambiguities
due to the difference in the solvation energies (ꢁG) between
Reichardt’s betaine dye and In–3CNQ-R compounds, although
The transition energy (hꢋCT) is mainly affected by three terms:
chemical structure (Ip ꢃ EA), solvent effect (" and ꢁG), and
degree of CT (ꢀ). Furthermore, eqs 13 and 14 indicate that
the CT energy increases with the increase and decrease of sol-
N
vent polarity in the ionic (hꢋCTI, ꢀ ꢂ 0:5) and neutral (hꢋCT
,
ꢀ ꢁ 0:5) ground states, respectively.
I1–3CNQ-R derivatives showed two distinct peaks of which
relative intensity depends on the acceptor strength of the
3CNQ-R moiety and solvent polarity. Compounds derived
from strong or weak R-TCNQ acceptors exhibited two peaks
in less-polar and polar solvents, respectively (Figure 11).
Since electrochemical measurement revealed that the
(IP ꢃ EA) value of a I1–3CNQ-R compound is a linear function
of the ꢀ values,1b the appearance of two separate peaks is
thought to relate strongly to the interaction between the ꢀ
values and solvent dipoles.
The appearance of two separate peaks in some D–ꢁ–A com-
pounds has received a lot of interest, and its origin has been
interpreted in several ways55 such as aggregation phenom-
ena,19,56 conformational isomerism,41,57 or vibronically re-
solved features.40 For I1–3CNQ-R compounds, aggregation
phenomena are not plausible, since Beer’s law is valid. Rota-
tion isomers are not adequate since solid-state spectra exhibit-
ed multiple absorption peaks.
(B) Substituent Effect: Intramolecular CT bands of a D–
3CNQ-R system shift depending on the chemical structure in
accordance with the differences of (Ip ꢃ EA) and ꢀ. Absorption

1142 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
the difference should be small compared to whole intramolec-
ular CT energy.43 The ꢀsolv values obtained for In–3CNQ-R de-
rivatives are summarized in Table 4. These values decrease in
the order R ¼ F4, CF3, F2, F1, and H which is in good agree-
ment with the weakening of acceptor strength of the 3CNQ
moiety, and reach to 0.5 for R ¼ Me. The smaller hꢋCT values
(Figure 10 and Table 4) of R ¼ (MeO)2 and (EtO)2 deriva-
tives than those of R ¼ Me indicate that these derivatives have
smaller ꢀsolv values than 0.5 in the solution state; however,
these values are 0.5 and constant. A reasonable explanation
of this behavior is not given at present, and the estimation of
ꢀsolv value might remain somewhat ambiguous for the ꢀ <
0:5 derivatives. Although the ꢀsolv values of R ¼ (MeO)2
and (EtO)2 derivatives were estimated as nearly 0.5, the slopes
a of the plots of hꢋCT(2nd) of these derivatives vs. ET values
(Figure 12) are small negative values, implying that they have
neutral ground states (ꢀsolv < 0:5) in the solution state. Similar
to the estimation of the ꢀ value in the solid state, comparing
the n ¼ 1 (ꢀsolv ¼ 0:61) and 6 (ꢀsolv ¼ 0:54) compounds for
In–3CNQ-CF3, the increase of alkyl chain length reduced the
ꢀsolv value. It is plausible that the steric repulsion between
the N-alkyl chain and the CN1 group increases with alkyl
chain length (Scheme 2), and the Type II conformation, which
leads to a smaller ꢀsolv value, is also preferable in the solution
state.43
Figure 13 plots the hꢋCT values of I1–3CNQ-R (R ¼ F2, H,
and (EtO)2) against ET values of various solvents including
less-polar solvents (1,4-dioxane, PhH, PhMe, and CS2). The
plots of I1–3CNQ derivatives have positive slopes (a > 0) in
solvents having higher polarity than PhCl (a–d), while the
hꢋCT values in solvents less polar than 1,4-dioxane (e–h) were
larger than that in PhCl (d) and showed a reverse solvato-
Figure 12. Plot of the hꢋCT(2nd) values of I1–3CNQ-R vs.
Reichardt’s ET values5b in various solvents. R ¼ F4 (blue
circle), CF3 (blue triangle), F2 (light blue diamond), F1a
and F1b (green squares), H (orange circle), Me (light red
triangle), (MeO)2 (purple diamond), and (EtO)2 (red
square). Solvents, a; MeOH (ET ¼ 55:5), b; MeCN
(46.0), c; acetone (42.2), and d; PhCl (37.5). Lines are
the least-squares fittings of each plot.
Figure 11. UV–vis spectra of (a) I1–3CNQ-F2 and (b) I1–
3CNQ-(EtO)2 in MeOH, MeCN, acetone, PhCl, 1,4-diox-
ane, benzene (PhH), toluene (PhMe), and carbon disulfide
(CS2). Gray lines show the shifts of absorption peaks.
Figure 13. Plots of hꢋCT(2nd) values of I1–3CNQ-R vs. Reichardt’s ET values in various solvents for (a) I1–3CNQ-F2, (b) I1–
3CNQ-H, and (c) I1–3CNQ-(EtO)2. Solvents: a, MeOH (ET ¼ 55:5); b, MeCN (46.0); c, acetone (42.2); d, PhCl (37.5); e,
1,4-dioxane (36.0); f, PhH (34.5); g, PhMe (33.9); and h, CS2 (32.6).

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1143
Figure 14. Two-photon absorption spectra of (a) I1–3CNQ-CF3 in MeCN and (b) I1–3CNQ-(EtO)2 in CHCl3. Shaded zones cover
the wavelength dependence of ꢃð2Þ. (c) Plot of ꢃð2Þ
vs. ꢀsolv of I1–3CNQ-R derivatives. Vertical lines are the error bars, and the
max
shaded zone covers the wavelength and ꢀ dependences of ꢃð2Þmax. The dotted line in (c) represents the modified eq 16
2
(ꢃð2Þ
¼ 4:03 ꢈ 103ꢀð1 ꢃ ꢀÞð1 ꢃ 2ꢀÞ þ 60:3).
max
Table 5. Two-Photon Absorption Properties of I1–3CNQ-R
Compounds
a)
ꢀsolv
"ph/L molꢃ1 cmꢃ1
ꢆðm1aÞx/nm
ꢃðm2aÞx/10ꢃ50 cm4 s photonꢃ1 moleculeꢃ1
ꢆðm2aÞx/nm
I1–3CNQ-CF3
a)
0.61
0.62
0.52
0.51
0.50
0.50
0.50
2:36 ꢈ 104
3:73 ꢈ 104
7:07 ꢈ 104
7:09 ꢈ 104
4:73 ꢈ 104
4:58 ꢈ 104
6:38 ꢈ 104
691
692
729
724
725
695
693
152
76
82
54
47
70
45
1358
1348
1455
1455
1455
1406
1400
I1–3CNQ-F2
I1–3CNQ-F1aa),c)
I1–3CNQ-Hb)
I1–3CNQ-Meb)
I1–3CNQ-(MeO)2
b)
b)
I1–3CNQ-(EtO)2
a) Measured in MeCN solution. b) Measured in CHCl3 solution. c) I1–3CNQ-F1a used for the two-photon absorption mea-
surement contained several percent of I1–3CNQ-F1b as minor impurity.
chromism. According to eqs 13 and 14, the hꢋCT values of an
I1–3CNQ-R derivative show a linear relationship with ET val-
ues with a positive slope (a ꢂ 0) for an ionic molecule
(0:5 ꢁ ꢀ ꢁ 1), and a negative slope (a ꢁ 0) for a neutral mole-
cule (0 ꢁ ꢀ ꢁ 0:5). Therefore, this result indicates that the
ground state of I1–3CNQ-R changes from ionic in polar sol-
vents (ꢀ ꢂ 0:5) to neutral in less-polar solvents (ꢀ ꢁ 0:5). Sim-
ilar reverse solvatochromism in less-polar solvents has also
for I1–3CNQ-F4 was not obtained, and thus, not included in
Table 5.
According to Kogej et al.,21 the imaginary part of ꢊ is attrib-
2
2
uted mainly to the dipolar term (ꢆꢈeg ðꢈe ꢃ ꢈgÞ =Eeg3) in
P
addition to the two-photon term (ꢆ ꢈeg ꢈe0e2=Eeg Ee e),
which involves the two-photon excited states denoted as e0
(eq 9). In measurements of I1–3CNQ-R, although the results
2
2
0
showed a large deviation, the ꢆð2Þ
value is close to twice
max
the value of ꢆð1Þmax, and the dependence of ꢆð2Þ
against
max
ꢀꢃ
6
been observed for D–3CNQ-R type Dꢀþ–ꢁ–A
and phen-
oxide–pyridinium derivatives.60
the ꢀsolv value looks similar to that of ꢆð1Þmax; ꢆð2Þmax increases
with acceptor strength of the 3CNQ-R moiety from R ¼ CF3
to Me and then shows a blue shift for R ¼ (EtO)2 (Table 5).
These observations indicate that the one- and two-photon ab-
sorption processes have the same excited state. Therefore,
we hypothesized that the main contribution to ꢃð2Þ for non-cen-
trosymmetric I1–3CNQ-R compounds is the dipolar term, and
thus, the maximum ꢃð2Þ (ꢃð2Þmax) is approximated by eq 15,66
(D) Two-Photon Absorption and Degree of Intramolecu-
lar CT: The two-photon absorption phenomenon of an organ-
ic chromophore, a third-order non-linear optical effect, has
been attracting much interest from the viewpoints of three-di-
mensional microfabrication, optical data storage, optical limit-
ing, and biological caging.61,62 The two-photon absorption
cross-section (ꢃð2Þ) is proportional to the imaginary part of
ꢊ.63 Parameters that enhance the third-order optical non-linear-
ity are the increase in conjugation length, donor and acceptor
2
2
ꢈ2egðꢈe ꢃ ꢈgÞ
ꢌeg
ꢈ2egðꢈe ꢃ ꢈgÞ
ꢌeg
16ꢁ2
ð2Þ
ꢃ
¼
/
;
ð15Þ
max
5hc2n2
ꢀꢃ
strengths of a Dꢀþ–ꢁ–A compound, and charge defects.64,65
ꢀ
where c, n, and ꢌeg are the velocity of light, the refractive
index of the medium, and the damping constant, respectively.
Two-photon absorption was measured by the Z-scan tech-
nique at wavelengths where all compounds are non-resonant
for the linear absorption.45 Figures 14a and 14b show the rela-
tionships between ꢃð2Þ and wavelength for I1–3CNQ-CF3 in
MeCN and I1–3CNQ-(EtO)2 in CHCl3. The positions of
Assuming ꢌeg does not depend on the ꢀ value, the ꢃð2Þ
max
value is expressed by ꢀ as
2
2
2
ꢃð2Þ / ꢈ ðꢈe ꢃ ꢈgÞ / ꢀð1 ꢃ ꢀÞð1 ꢃ 2ꢀÞ ;
ð16Þ
max
one- (ꢆð1Þmax) and two-photon absorption maxima (ꢆð2Þ
)
eg
max
and the two-photon absorption coefficient ("ph) are summariz-
ed in Table 5. Because of the low solubility, reliable data
which shows a similar ꢀ dependence to that of ꢉ. Accordingly,
the ꢃð2Þ
value is proportional to the product of squares of
max

1144 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Indoline-Substituted 3CNQ D–ꢁ–A Compounds
the transition dipole moment (ꢈeg) and the difference of dipole
moments in the excited (ꢈe ¼ ð1 ꢃ ꢀÞed) and ground
(ꢈg ¼ ꢀed) states. In the ꢀ ¼ 0 state, the HOMO (highest oc-
cupied molecular orbital) and LUMO (lowest unoccupied mo-
section (ꢃð2Þ), and I1–3CNQ-CF3 having the largest ꢀsolv value
in the present measurements exhibited a relatively high ꢃð2Þ
value.
Our investigation showed that (1) the ꢀ values of I1–3CNQ-
R derivatives in the solid and solution states can be controlled
by chemical modification; (2) molecular (hyper)polarizabili-
ties, which are the key parameters of non-linear optical proper-
ties, can be tuned and enhanced by controlling the ꢀ values;
and (3) the two-photon absorption properties show significant
dependence on the ꢀ values. We emphasize that our research
has derived the basic and intriguing parameters of a Dꢀþ–ꢁ–
ꢀꢃ
lecular orbital) of a Dꢀþ–ꢁ–A molecule are allocated at the
ꢀꢃ
D and A moieties, respectively, while those of a Dꢀþ–ꢁ–A
molecule in the ꢀ ¼ 1 state are allocated at the A and D moie-
ties, respectively. Therefore, the ꢀ values close to 0 and 1 re-
sult in the decrease in the overlap integral between HOMO and
LUMO, leading to a small ꢈeg value. The charge distribution
ꢀꢃ
of a Dꢀþ–ꢁ–A molecule in the ground state changes to
ꢀꢃ
system, which is a potential system for non-linear optical
D
ð1ꢃꢀÞþ–ꢁ–Að1ꢃꢀÞꢃ in the excited state. The ꢈg and ꢈe values
A
of a ꢀ ¼ 0:5 molecule are close to each other (ꢈg ¼ ꢈe ¼
0:5ed), and the jꢈe ꢃ ꢈgj value (¼jð1 ꢃ 2ꢀÞedj) has a mini-
mum at ꢀ ¼ 0:5. Accordingly, eq 16 indicates that completely
neutral (ꢀ ¼ 0) and ionic (ꢀ ¼ 1) states as well as charge delo-
materials and will provide a new strategy in material design for
the future development of Dꢀþ–ꢁ–Aꢀꢃ based organic function-
al materials.
calized state (ꢀ ¼ 0:5) are not favorable for the ꢃð2Þ
value,
This work was partly supported by a Grant-in-Aid (21st
Century COE programs on Kyoto University Alliance for
Chemistry and 15205019) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. T. M. is a
recipient of a Japan Society for the Promotion of Science
(JSPS) research fellowship.
max
and the partially ionized state with a moderately small or large
ꢀ value is necessary to obtain a large ꢃð2Þ
value. From
max
eq 16, ꢃð2Þ
exhibits maxima at ꢀ ¼ ꢆ0:15 and 0.85 and
max
minima at ꢀ ¼ 0, 0.5, and 1 (Figure 14c).
The magnitude of ꢃð2Þ was the largest for I1–3CNQ-CF3,
which has a large ꢀsolv value; whereas that of I1–3CNQ-
(EtO)2, which has the smallest ꢀsolv value, was the smallest
observed thus far (Figures 14a and 14b). A rough tendency
that the ꢃð2Þ value increases with the ꢀsolv value of I1–
3CNQ-R is in agreement with the theoretical curve expected
from eq 16 (Figure 14c).
Supporting Information
Selected physical data, crystallographic data, UV–vis, and two-
photon absorption measurements for In–3CNQ-R. This material is
available free of charge at http://www.csj.jp/journals/bcsj/.
References
Conclusion
ꢀꢃ
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