Charge-transfer complexes between indoline-trieyanoquinodimethane compounds and 7,7,8,8-tetracyanoquinodimethane

Article abstract of DOI:10.1246/bcsj.81.1492

Intramolecular charge-transfer compounds composed of l-alkyl-3,3-dimethyl- 2-methyleneindoline (I_n, donor, n = alkyl chain length; 1 or 6) and 7,8,8-tricyanoquinodimethane (=2-(4'-cyanomethylene-2′,5′- cyclohexadienylidene)- malononitrile, 3CNQ, acceptor) moieties afforded charge-transfer complexes with 7,7,8,8-tetracyanoquinodimethane (TCNQ): (I ₃CNQ)₂(TCNQ) and (I₆-3CNQ)(TCNQ). (I ₁-3CNQ)₂(TCNQ) included two kinds of I₁3CNQ molecules of different molecular conformations. (I₁-3CNQ) ₂(TCNQ) and (I₆-3CNQ)(TCNQ) were constructed from one-dimensional columns of 3CNQ...TCNQ...3CNQ triads and 3CNQ...TCNQ dyads, respectively. These complexes exhibited intermolecular charge-transfer bands extending to the near IR region (1000-1500 nm) in addition to intramolecular charge-transfer bands around 600-900 nm characteristic of I_n-3CNQ compounds. They were characterized as weak charge-transfer solids from estimations of the degree of charge transfer using molecular orbital calculations, bond lengths, and vibration spectra. Intermolecular charge transfer caused a slight increase in the degree of intramolecular charge transfer.

Full text of DOI:10.1246/bcsj.81.1492

1492 Bull. Chem. Soc. Jpn. Vol. 81, No. 11, 1492–1499 (2008)
Ó 2008 The Chemical Society of Japan
Charge-Transfer Complexes between Indoline–Tricyanoquinodimethane
Compounds and 7,7,8,8-Tetracyanoquinodimethane
ꢀ1
Tsuyoshi Murata, Genki Honda,1 Chin-Hong Chong,1 Masaru Makihara,1
Kazukuni Nishimura,1 Salavat Khasanov,1;2 Akihiro Otsuka,3 and Gunzi Saito
ꢀ1;3;4
1Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
2The Institute of Solid State Physics of Russian Academy of Science, Chernogolovka 142432, Russia
3Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8502
4Research Institute, Meijo University, Tenpaku-ku, Nagoya 468-8502
Received March 19, 2008; E-mail: saito@kuchem.kyoto-u.ac.jp, tmurata@kuchem.kyoto-u.ac.jp
Intramolecular charge-transfer compounds composed of 1-alkyl-3,3-dimethyl-2-methyleneindoline (In, donor,
n = alkyl chain length; 1 or 6) and 7,8,8-tricyanoquinodimethane (=2-(40-cyanomethylene-20,50-cyclohexadienylidene)-
malononitrile, 3CNQ, acceptor) moieties afforded charge-transfer complexes with 7,7,8,8-tetracyanoquinodimethane
(TCNQ): (I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ). (I1–3CNQ)2(TCNQ) included two kinds of I1–3CNQ molecules
of different molecular conformations. (I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ) were constructed from one-dimen-
sional columns of 3CNQ TCNQ 3CNQ triads and 3CNQ TCNQ dyads, respectively. These complexes exhibited
ꢁꢁꢁ
ꢁꢁꢁ
ꢁꢁꢁ
intermolecular charge-transfer bands extending to the near IR region (1000–1500 nm) in addition to intramolecular
charge-transfer bands around 600–900 nm characteristic of In–3CNQ compounds. They were characterized as weak
charge-transfer solids from estimations of the degree of charge transfer using molecular orbital calculations, bond
lengths, and vibration spectra. Intermolecular charge transfer caused a slight increase in the degree of intramolecular
charge transfer.
Unimolecular donor (D) and acceptor (A) compounds
linked by a ꢀ-bond (Dꢁþ–ꢀ–Aꢁꢂ, ꢁ = degree of intramolecu-
lar charge transfer (CT)) exhibit flexible electronic structures
and large dipole moments originating from intramolecular
ꢁꢂ
CT processes. Such features of Dꢁþ–ꢀ–A are significant
as potential sources for organic optical materials, such as
non-linear optics,1–11 photovoltaics,12 and photochromics.1,13
ꢁꢂ
In addition to intramolecular CT phenomena, Dꢁþ–ꢀ–A
compounds show amphoteric redox abilities, and have been
widely utilized in research of unimolecular rectifiers14–18 and
organic semiconductors.19 Besides such intramolecular CT
compounds, binary CT complexes composed of D and A mole-
cules have also provided numerous intriguing materials, such
as organic (super)conductors,20–24 magnetic materials,25 and
unusual phase-transition systems (neutral–ionic transition26,27
and photo-induced phase transition28); however, CT com-
ꢁꢂ
plexes of unimolecular Dꢁþ–ꢀ–A compounds have not been
reported previously to our best knowledge.29
We have recently been studying syntheses, crystal struc-
tures, and optical properties of indoline (I)–tricyanoquinodi-
methane (=2-(40-cyanomethylene-20,50-cyclohexadienylidene)-
Chart 1. Chemicals discussed in this study.
malononitrile, 3CNQ) Dꢁþ–ꢀ–A compounds (In–3CNQ-R,
and solvent polarity.31,32 In the study of Dꢁþ–ꢀ–A com-
pounds, we obtained CT complexes between In–3CNQ com-
pounds and TCNQ. In the present study, we present the crystal
structures, estimations of degrees of intra- and intermolecular
CT, and optical properties of (I1–3CNQ)2(TCNQ) and (I6–
3CNQ)(TCNQ). The effect of intermolecular CT on the
degrees of intramolecular CT of In–3CNQ molecules was also
discussed.
ꢁꢂ
ꢁꢂ
n = alkyl chain length and R = substituent group on the
3CNQ moiety, Chart 1).30–32 Our studies demonstrated that
the ꢁ values of In–3CNQ-R are tunable by chemical modifica-
tions and significantly affect the molecular (hyper)polarizabil-
ities and non-linear optical properties.32 Furthermore, we re-
vealed that the ꢁ values are sensitive to environmental pertur-
bations such as molecular conformation, molecular packing,
Published on the web November 12, 2008; doi:10.1246/bcsj.81.1492

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1493
Table 1. Crystallographic Data of (I1–3CNQ)2(TCNQ) and
(I6–3CNQ)(TCNQ)
Experimental
Materials. I1–3CNQ and I6–3CNQ were prepared according
to our previous papers.30,31 TCNQ was purified by sublimation.
Measurements. Melting points were measured with a Yanaco
MP-500D micro melting-point apparatus and were not corrected.
Elemental analyses were performed at the Center for Organic
Elemental Microanalysis, Kyoto University. Ultraviolet–visible
(UV–vis) spectra were measured on a Shimadzu UV-3100 spec-
trometer in acetonitrile (MeCN) solutions or KBr pellets. IR spec-
tra of the samples in KBr pellet were measured using a Perkin-
Elmer PARAGON 1000 FT-IR spectrometer (resolution 4 cmꢂ1).
(I1–3CNQ)2(TCNQ) (I6–3CNQ)(TCNQ)
Formula
Fw
C58H40N12
905.02
reddish purple plate reddish purple plate
C40H32N8
624.74
Crystal habit
Crystal system
Space group
triclinic
ꢀ
P1
triclinic
ꢀ
P1
˚
a/A
8.0500(6)
15.621(2)
20.489(2)
70.920(5)
81.897(5)
87.003(6)
2410.6(4)
2
9.086(1)
13.545(2)
15.155(2)
102.806(7)
97.481(8)
109.230(8)
1674.9(4)
2
˚
b/A
˚
c/A
ꢂ/ꢃ
ꢅ/ꢃ
ꢆ/ꢃ
Preparation of (I1–3CNQ)2(TCNQ).
Hot solutions of
I1–3CNQ (70.0 mg, 0.20 mmol) in MeCN (30 mL) and TCNQ
(20.4 mg, 0.10 mmol) in MeCN (20 mL) were mixed, and then
cooled to room temperature. The resulting crystals were collected
by filtration, to yield the complex (45.0 mg, 50%) as dark reddish
purple plate crystals. Mp 230–232 ꢃC (dec); Anal. Calcd for
(C23H18N4)2(C12H4N4): C, 76.97; H, 4.45; N, 18.57%. Found:
C, 77.02; H, 4.68; N, 18.61%.
Preparation of (I6–3CNQ)(TCNQ). Hot solutions of I6–
3CNQ (89.8 mg, 0.21 mmol) in MeCN (30 mL) and TCNQ
(21.4 mg, 0.11 mmol) in MeCN (20 mL) were mixed. The result-
ing mixture was slowly concentrated under atmosphere, to yield
the complex (36.3 mg, 53%) as reddish purple plate crystals. Mp
201–203 ꢃC (dec); Anal. Calcd for (C28H28N4)(C12H4N4); C,
76.90; H, 5.16; N, 17.94%. Found: C, 77.00; H, 5.16; N,
17.96%. The product included un-reacted I6–3CNQ (yellow green
blocks), and the complex was separated by hand.
3
˚
V/A
Z
dcalcd/g cmꢂ1
Temperature
ꢇ (Mo Kꢂ)/mmꢂ1
Unique reflections
No. of reflections
No. of parameters
R1 (I > 2:0ꢈðIÞ)
wR2
1.247
RT
1.239
RT
0.077
8026
0.076
6231
6236
677
3251
409
0.046
0.121
1.021
0.083
0.239
1.066
GOF
red
than that of TCNQ (E1=2 ¼ þ0:22 V) and similar to that of
red
2,4,7-trinitrofluorenone (TNF, Chart 1, E1=2 ¼ ꢂ0:43 V).
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 with monochromated
Electron-donating abilities of I1–3CNQ and I6–3CNQ are
ox
slightly weaker than that of BEDT-TTF (E1=2 ¼ þ0:53 V)
ox
and slightly stronger than that of DBTTF (E1=2
¼
˚
˚
Mo Kꢂ (ꢃ ¼ 0:71073 A, 1 A = 0.1 nm) radiation. Structures were
determined by a direct method using SHELXS-97.33 Refinements
were performed by the full-matrix least-squares method on F2
with SHELXL-97.34 All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were included without refinement.
Positions of the hydrogen atoms were calculated assuming sp3
or sp2 conformations of carbon atoms. Crystallographic parame-
ters are summarized in Table 1. Crystallographic data have been
deposited with Cambridge Crystallographic Data Centre: Deposi-
tion numbers CCDC-232865 and 693070. 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, Cambridge, CB2 1EZ, UK; Fax: +44
1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
þ0:62 V). DBTTF is known to give a CT complex having a
neutral ground state with TCNQ,35 and BEDT-TTF affords
both neutral and mixed-valent CT complexes with TCNQ
depending on the reaction conditions.36 Mixing of a solution
of I1–3CNQ or I6–3CNQ with that of TCNQ in MeCN yielded
reddish purple crystals, which are characterized as 2:1 and
1:1 complexes, respectively, by elemental analyses. Both com-
plexes are insulators (room temperature conductivity <10ꢂ8
S cmꢂ1).
Crystal Structures.
(A) Crystal Structure of (I1–
3CNQ)2(TCNQ): This complex crystallized in a triclinic
system, where two I1–3CNQ (I1–3CNQ-A and I1–3CNQ-B)
and two halves of TCNQ (TCNQ-A and TCNQ-B) molecules
having inversion centers are crystallographically independent.
As shown in Figures 1a and 1b, I1–3CNQ-A and I1–3CNQ-B
have different molecular conformations, where the N-alkyl
group of indoline moiety is located close to (Type I) or far
from (Type II) the CN group of 3CNQ moiety, respectively.
In the crystal structure, 3CNQ moieties of I1–3CNQ-A and
I1–3CNQ-B molecules stack with TCNQ-A and TCNQ-B
molecules, respectively, to form 3CNQ TCNQ 3CNQ triads
Calculation. Intermolecular overlaps were calculated based
on the crystal structures by the extended Huckel method with
¨
single ꢄ parameters. Semi-empirical molecular orbital (MO)
calculations were performed using MOS/F V4 with INDO/S
parameterization coupled with a 20-dimensional CI matrix, which
is sufficient to obtain approximately invariant atomic charge. Geo-
metrical parameters were extracted from the crystal structures.
ꢁꢁꢁ
ꢁꢁꢁ
Results and Discussion
(triad-A and triad-B, respectively, Figure 1c). The triads stack
with twisting by 21.7ꢃ along the c axis to form a one-dimen-
sional column (Figure 1d). Effective intermolecular overlap
integrals are observed only along the stacking direction, where
those within triad-A and triad-B are similar to each other
(16:1 ꢄ 10ꢂ3 and 17:0 ꢄ 10ꢂ3, respectively) and larger than
that between the triads (ꢂ3:6 ꢄ 10ꢂ3). The indoline moieties
Preparation. I1–3CNQ and I6–3CNQ exhibit amphoteric
redox behaviors with irreversible processes in cyclic voltam-
metry (CV), and their redox abilities are very close to each
red
ox
other (Ep ¼ ꢂ0:45 V and Ep ¼ þ0:57 V for I1–3CNQ;
red
ox
Ep ¼ ꢂ0:41 V and Ep ¼ þ0:60 V for I6–3CNQ, V vs.
SCE).31 Their electron-accepting abilities are much weaker

1494 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
CT Complexes of Indoline–3CNQ and TCNQ
Figure 1. (a & b) Molecular structures of I1–3CNQ-A and I1–3CNQ-B, respectively. (c) One-dimensional column comprised of
3CNQ TCNQ 3CNQ triads. (d) Molecular packing viewed along the a axis. TCNQ molecules are drawn in thin colors. Yellow
ꢁꢁꢁ
ꢁꢁꢁ
and green areas show 3CNQ TCNQ 3CNQ triads of I –3CNQ-A and I –3CNQ-B, respectively, and red area represents the
1
ꢁꢁꢁ
ꢁꢁꢁ
1
assembly of indoline moieties. Hydrogen atoms are omitted in (c) and (d).
Figure 2. (a) Molecular structure of I –3CNQ. (b) One-dimensional column composed of 3CNQ TCNQ dyads. (c) Molecular
ꢁꢁꢁ
6
packing viewed along the a axis. TCNQ molecules are drawn in thin colors. Yellow and red areas show 3CNQ TCNQ columns
ꢁꢁꢁ
and indoline dimers, respectively. Hydrogen atoms and hexyl groups are omitted in (b) and (c).
of I1–3CNQ-A and I1–3CNQ-B molecules individually stack
˚
with interplanar distances of 3.49 and 3.82 A, respectively,
to form dimers, and the dimers assemble along the c axis to
form a columnar structure (Figure 1d).
(B) Crystal Structure of (I6–3CNQ)(TCNQ): This com-
plex crystallized in a triclinic system, where one I6–3CNQ
and one TCNQ molecule are crystallographically independent.
The I6–3CNQ molecule has Type II molecular conformation
(Figure 2a), which corresponds to those in the crystal struc-
tures of I6–3CNQ itself.31
Estimation of Degrees of Inter- and Intramolecular CT.
In (I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ) complexes,
the majority of intermolecular CT interactions occur between
the 3CNQ moiety and the TCNQ molecule within the one-
dimensional columns, since significant overlap integrals are
observed only within these columns. Therefore, CT processes
in the complexes are presented as Scheme 1, where the degree
of intramolecular CT between indoline and 3CNQ moieties is
ꢁ, and that of intermolecular CT from 3CNQ moiety to TCNQ
is ꢄ. Indoline and 3CNQ moieties have charges of þꢁ and
ꢂðꢁ ꢂ ꢄÞ, respectively. Since (I1–3CNQ)2(TCNQ) complex
has a 2:1 ratio, its TCNQ molecule has a minus charge of ꢂ2ꢄ.
(A) Molecular Orbital Calculation: The degrees of CT of
The 3CNQ moieties stack with TCNQ molecules to form a
one-dimensional alternating column (Figure 2b). The interpla-
˚
nar distances of the stacks are 3.31 and 3.52 A, and the column
ꢁꢂ
composed of 3CNQ TCNQ dyads. The overlap integral
D
ꢁþ–ꢀ–A compounds and CT complexes express the charg-
ꢁꢁꢁ
within the dyad is 28:6 ꢄ 10ꢂ3 and larger than that between
neighboring dyads (1:0 ꢄ 10ꢂ3). These columns are arranged
along the b axis to establish a layered motif (Figure 2c). The
indoline moiety also stacks with an interplanar distance of
es on donor and acceptor moieties. We performed an MO cal-
culation to investigate the degrees of CT. The summation
of atomic charges on each moiety gives the degrees of CT:
indoline (þꢁcharge), 3CNQ (ꢂðꢁcharge ꢂ ꢄchargeÞ), and TCNQ
(ꢂXꢄcharge) moieties. In the crystal structures, significant over-
lap integrals are observed only in the 3CNQ TCNQ 3CNQ
˚
3.72 A to form a dimer motif (Figure 2b), and the dimers are
separated by an alkyl chain.
ꢁꢁꢁ
ꢁꢁꢁ

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1495
Table 2. Charges of Indoline, 3CNQ, and TCNQ Moieties
Obtained by the MO Calculation
Indoline
3CNQ
TCNQ
(ꢁcharge) (ꢂðꢁcharge ꢂ ꢄchargeÞ) (ꢂXꢄcharge
)
Scheme 1. Schematic presentation of CT processes in
(In–3CNQ)X(TCNQ) complexes. X shows the component
ratio.
(I1–3CNQ)2(TCNQ)a)
(I6–3CNQ)(TCNQ)
A
B
+0.65
+0.51
+0.47
+0.38
+0.37
+0.40
ꢂ0:56
ꢂ0:43
ꢂ0:42
ꢂ0:38
ꢂ0:37
ꢂ0:40
ꢂ0:19
ꢂ0:16
ꢂ0:05
—
I6–3CNQb)
—
31
triad for (I –3CNQ) (TCNQ) and the 3CNQ TCNQ dyad
1
ꢁꢁꢁ
2
—
for (I6–3CNQ)(TCNQ); therefore, the MO calculations were
performed for the isolated pairs extracted from the crystal
structures. Considering the values of atomic charges, the
boundary between D and A parts of an In–3CNQ is assumed
to be located between indoline and cyanoethylene moieties
(Table 2). The obtained CT degrees are summarized in
Table 2 together with those for I6–3CNQ crystals.31
a) Calculated for a (I1–3CNQ)2(TCNQ) triad. Calculated values
for two I1–3CNQ molecules in each triad are very close to each
other within the differences of 0.001. b) Three kinds of poly-
morphs have been found, where all I6–3CNQ molecules have a
Type II conformation.
(B) Bond Length Ratio (BLR): It is well known that bond
lengths (a–c in Table 3) of TCNQ molecules exhibit bond
alternation between neutral and radical anion states. The ratio
of single and double bond lengths defined as r ¼ b=ða þ cÞ
shows a good linear relationship with the degree of CT (ꢄ),
and has been utilized in the estimation of charges on TCNQ
molecules in CT complexes.37 Table 3 summarizes the bond
In the (I1–3CNQ)2(TCNQ) complex, the ꢁcharge value of I1–
3CNQ-A (+0.65, Type I conformation, Figure 1a) is larger
than that of I1–3CNQ-B (+0.51, Type II conformation,
Figure 1b). This result corresponds to our previous study,
where an In–3CNQ molecule of Type I conformation has a
larger ꢁ value.31 Compared with the difference in the ꢁcharge
values between triad-A and triad-B, the difference in charges
on TCNQ-A and TCNQ-B moieties (ꢂ2ꢄcharge ¼ ꢂ0:19 and
ꢂ0:16, respectively) is small, indicating that the effect of
molecular conformations of I1–3CNQ molecules on the inter-
molecular CT is small. The ꢁcharge value of the I6–3CNQ mole-
cule (+0.47) in the complex is larger by 0.07–0.10 than those
in I6–3CNQ crystals (0.37–0.40). Similarly, the charge on the
3CNQ moiety (ꢂðꢁcharge ꢂ ꢄchargeÞ) in the (I6–3CNQ)(TCNQ)
complex is larger than those in I6–3CNQ crystals, although
the difference is small compared with that in the ꢁcharge values.
These results indicate that the intermolecular CT between
the 3CNQ moiety and TCNQ molecule increases the ꢁ
value. Larger amounts of charges on TCNQ moieties in
(I1–3CNQ)2(TCNQ) (ꢂ2ꢄcharge ¼ ꢂ0:19 and ꢂ0:16) than that
in (I6–3CNQ)(TCNQ) (ꢂꢄcharge ¼ ꢂ0:05) well reflect the
component ratios.
lengths a–c and the estimated charges on TCNQ (ꢂXꢄBLR
)
in (I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ). The charges
on TCNQ molecules are relatively small; ꢂ2ꢄBLR ¼ ꢂ0:21
and ꢂ0:25 for TCNQ-A and TCNQ-B molecules, respectively,
in (I1–3CNQ)2(TCNQ) and ꢂꢄBLR ¼ 0:00 for (I6–3CNQ)-
(TCNQ), indicating the neutral ground states (ꢂ0:5 <
ꢂXꢄ ꢅ 0) of these CT complexes. The smaller charge on
the TCNQ molecule of (I6–3CNQ)(TCNQ) than those of
(I1–3CNQ)2(TCNQ) coincides with that obtained by the MO
calculation.
In a previous study, we demonstrated that C–N and C–C
bond lengths defined as a0–e0 exhibit a bond alternation be-
tween neutral and ionic forms (Scheme 2), and that the bond
length ratio (BLR) value defined as BLR ¼ ðb0 þ d0 þ e0Þ=
ða0 þ c0Þ shows a good linear relationship with the degree of
intramolecular CT (ꢁ) for In–3CNQ-R molecules.31 Since the
Table 3. Intramolecular Bond Lengths (a–c),a) r, and Charges on TCNQ Molecules in the CT Complexes
c)
rb)
Charge (ꢂXꢄBLR
)
˚
a/A
˚
b/A
˚
c/A
(I1–3CNQ)2(TCNQ)
A
B
1.440(2)
1.437(3)
1.435(2)
1.448(4)
1.423(4)
1.385(2)
1.382(3)
1.373(3)
1.374(3)
1.420(4)
1.440(3)
1.428(3)
1.449(2)
1.441(3)
1.416(4)
0.4810(9)
0.482(1)
0.476(1)
0.476(1)
0.500(2)
ꢂ0:21
ꢂ0:25
0.00
0.00
ꢂ1:00
(I6–3CNQ)(TCNQ)
TCNQ0 d)
RbþTCNQꢆꢂ d)
a) A (1 A = 0.1 nm). b) r ¼ b=ða þ cÞ. c) ꢂXꢄBLR ¼ ðr ꢂ r0Þ=ðr1 ꢂ r0Þ, where r0 and r1 are the r values of
˚
˚
TCNQ0 and RbþTCNQꢆꢂ. d) See, Ref. 37.

1496 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
CT Complexes of Indoline–3CNQ and TCNQ
Scheme 2. Bond alternation of In–3CNQ in neutral and ionic forms. a0–e0 show the bond lengths sensitive to the ꢁ value.
Table 4. Intramolecular Bond Lengths (a0–e0)a) and r0 of the 3CNQ Moieties in the CT Complexes
0
0
˚
0
0
0
˚
r0c)
˚
a /A
˚
b /A
˚
c /A
d /A
e /A
(I1–3CNQ)2(TCNQ)
(I6–3CNQ)(TCNQ)
A
B
1.322(2)
1.342(2)
1.353(4)
1.347(3)
1.355(4)
1.351(3)
1.427(2)
1.411(2)
1.411(5)
1.397(3)
1.389(4)
1.403(3)
1.368(2)
1.383(2)
1.381(5)
1.398(3)
1.394(4)
1.388(3)
1.439(2)
1.444(2)
1.436(5)
1.427(3)
1.431(4)
1.435(3)
1.419(2)
1.415(2)
1.420(5)
1.408(3)
1.407(5)
1.417(3)
2.089(4)
2.067(4)
2.068(8)
2.028(5)
2.036(6)
2.055(5)
I6–3CNQb)
31
˚
˚
a) A (1 A = 0.1 nm). b) Three kinds of polymorphs have been found, where all I6–3CNQ molecules have the Type II
conformation. c) r0 ¼ ðd0 þ e0Þ=c0.
charges on indoline (ꢁ) and 3CNQ (ꢂðꢁ ꢂ ꢄÞ) moieties are dif-
ferent, the estimations of the degrees of CT should be per-
formed individually for each moiety. We used the bond length
a0 and BLR defined as r0 ¼ ðd0 þ e0Þ=c0 to discuss the charges
on indoline and 3CNQ moieties, respectively (Table 4). Since
ꢁ ¼ 0 and 1 limits for the In–3CNQ compounds have not been
obtained at present, quantitative estimation of the ꢁ values
cannot be obtained from the BLR values.
In the (I1–3CNQ)2(TCNQ) complex, the I1–3CNQ-A
molecule has a shorter a0 bond and a larger r0 value than
those of I1–3CNQ-B. This result indicates that the I1–
3CNQ-A molecule of a Type I conformation has larger ꢁ
and ꢂðꢁ ꢂ ꢄÞ values than those of I1–3CNQ-B of a Type II
conformation, which is consistent with our previous study31
and the MO calculation (Table 2). In the case of the I6–
3CNQ molecule, a notable difference in the bond length a0
is not found between molecules in the complex and I6–
3CNQ crystals; however, the r0 value of the complex is larger
Figure 3. IR spectra (KBr pellet) of (I1–3CNQ)2(TCNQ)
and (I6–3CNQ)(TCNQ) with those of related compounds
in the CꢇN stretching region (2100–2250 cmꢂ1).
than those of I6–3CNQ crystals to some extent, implying that
the ꢂðꢁ ꢂ ꢄÞ value is increased by complex formation.
(C) IR Spectra: In the solid-state IR spectra, complexes
(I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ) exhibit single
CꢇN stretching modes at around 2210–2220 cmꢂ1 and
multiple bands at around 2150–2200 cmꢂ1 (Figure 3). Table 5
summarizes the CꢇN stretching frequencies of the complexes
together with those of related compounds.
The ꢉCꢇN (b1u) mode of TCNQ in CT solids softens linearly
with the degree of CT (0–0.5), and has been utilized in the
estimation of the degree of CT: XꢄCꢇN ¼ ðꢉ0 ꢂ ꢉCꢇNÞ=
ðꢉ0 ꢂ ꢉ1Þ, where ꢉ0 and ꢉ1 are the ꢉCꢇN values of neutral
(2227 cmꢂ1) and radical anion species (2183 cmꢂ1).38 Such
modes of (I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ) are
observed at 2212 and 2216 cmꢂ1, respectively, and exhibit
low-frequency shifts of 15 and 11 cmꢂ1 from that of TCNQ
(2227 cmꢂ1). This result indicates the occurrence of intermo-
lecular CT between In–3CNQ and TCNQ molecules and gives
the degrees of CT; ꢂ2ꢄCꢇN ¼ ꢂ0:34 for (I1–3CNQ)2(TCNQ)
and ꢂꢄCꢇN ¼ ꢂ0:25 for (I6–3CNQ)(TCNQ). The smaller
value of (I6–3CNQ)(TCNQ) than that of (I1–3CNQ)2(TCNQ)
corresponds to those estimated by the MO calculation and
BLR (Tables 2 and 3). The difference in the estimated ꢁ
values between these methods would be caused by the pertur-
bation of crystal packing to the ꢉCꢇN values.38 The CꢇN
stretching modes of the 3CNQ moieties in the complexes have
multiple peaks, which show low-frequency shifts of 3–8 cmꢂ1
(Figure 3). Assuming that such modes of the 3CNQ molecules
soften with increasing the degree of CT similarly to TCNQ CT
complexes, the low frequency shifts indicate that the intermo-
lecular CT interaction increases the degree of intramolecular
CT of In–3CNQ molecules. The complicated pattern in the
(I1–3CNQ)2(TCNQ) complex would originate from the exis-
tence of two kinds of I1–3CNQ molecules having different
molecular conformations (Figures 1a and 1b).

T. Murata et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1497
Table 5. CꢇN Stretching Wavenumbers in the IR Spectraa) and Intra- and Intermolecular CT Absorption
Bands in the Solid-Statea) and Solution-Stateb) (Bracket) UV–Vis–NIR Spectra of (I1–3CNQ)2(TCNQ)
and (I6–3CNQ)(TCNQ) and Related Compounds
CꢇN stretching modes/cmꢂ1
UV–vis–NIR spectra/nm
TCNQ
I1–3CNQ
Intermolecular CT
Intramolecular CT
(I1–3CNQ)2(TCNQ)
I1–3CNQ
2212
2185, 2175,
2163, 2152
2193, 2156
1156
858sh, 730
(786, 725)
865sh, 730
(787, 725)
842sh, 748
(806, 735)
844, 748
(I6–3CNQ)(TCNQ)
I6–3CNQ
2216
2227
2185, 2160
2190, 2163
1132
(805, 734)
TCNQ
a) Measured in KBr pellet. b) Measured in MeCN solution.
of the CT absorption bands, 1156 nm (8:7 ꢄ 103 cmꢂ1) for
(I1–3CNQ)2(TCNQ) and 1132 nm (8:8 ꢄ 103 cmꢂ1) for (I6–
3CNQ)(TCNQ), agree with the neutral ground state from
the estimations of degrees of intermolecular CT. The intermo-
lecular CT energies are higher than those of (BEDT-
TTF)(TCNQ) complex in the neutral phase (5:7 ꢄ 103 cmꢂ1
36
)
and (DBTTF)(TCNQ) (7:4 ꢄ 103 cmꢂ1),39 whose donor mole-
ox
cules have similar electron-donating abilities (E1=2 ¼ þ0:53
and +0.62 V vs. SCE, respectively) to those of In–3CNQ.
Conclusion
Indoline–tricyanoquinodimethane type intramolecular CT
compounds (In–3CNQ, n ¼ 1 and 6) gave CT complexes with
TCNQ at 2:1 and 1:1 ratios, respectively. Structural analyses
elucidated that In–3CNQ molecules interact at the 3CNQ moi-
eties with TCNQ through ꢀ-stacking interaction to establish
one-dimensional columns. The estimations of degrees of intra-
and intermolecular CT based on the MO calculation, bond
lengths, and vibration spectra indicate that (1) the CT com-
plexes between In–3CNQ and TCNQ have neutral ground
states, (2) intra- and intermolecular CT coexist in the CT com-
plexes to give weakly ionized TCNQ molecules, and (3) inter-
molecular CT processes increase the degree of intramolecular
CT of In–3CNQ. Intermolecular CT processes in the com-
plexes are evidenced by the solid-state UV–vis–NIR spectra,
where low-energy absorptions assignable as intermolecular
CT bands between In–3CNQ and TCNQ are observed around
1000–1500 nm.
Both intramolecular CT compounds and CT complexes
have been intensively studied in the development of mole-
cule-based materials; however, they exhibit significantly dif-
ferent physical properties and functions. We emphasize that
the present study shows not only the simple combination of
two kinds of phenomena, but also will provide new strategies
in the development of multifunctional organic materials. To
make deeper discussion on the correlation between intra- and
intermolecular CT processes and to discover novel functions
of this kind of materials, the exploration of complexes having
larger degrees of intermolecular CT by complex formation of
In–3CNQ-R with strong acceptor and donor molecules is indis-
pensable. The next target of this kind of material is complexes
with mixed-valent intermolecular CT states exhibiting high
electrical conductivity, where the flexible electronic state and
Figure 4. UV–vis–NIR spectra (KBr pellet) of (I1–
3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ) with those of re-
lated compounds.
Intra- and Intermolecular CT Absorption Bands in UV–
Vis–NIR Spectra. The UV–vis–NIR spectra in an MeCN so-
lution (see Supporting Information) of (I1–3CNQ)2(TCNQ)
and (I6–3CNQ)(TCNQ) are explained by simple superimposi-
tions of those of neutral TCNQ and In–3CNQ, where two
major bands at around 400 and 500–900 nm, the latter of which
has two major peaks, are observed. The former band is ascrib-
able to the intramolecular excitation of neutral TCNQ, and
the latter one originates from the intramolecular CT process
of In–3CNQ. Absorption bands originating from TCNQ radical
anion species and intermolecular CT were not observed in the
solution state.
Figure 4 shows the solid-state UV–vis–NIR spectra of the
complexes, and Table 5 summarizes the CT transition bands
in the solution and solid states. In the solid state, the intramo-
lecular CT bands of both complexes as well as In–3CNQ show
red shifts from those in the solution state due to the highly po-
larized environment of bulk crystals (Table 5). The solid-state
spectra exhibit additional broad absorption bands at 1000–
1500 nm, which are ascribed to the intermolecular CT bands
between 3CNQ and TCNQ, similar to those observed in CT
complexes with alternating D A stacks.39 The peak energies
ꢁꢁꢁ

1498 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
CT Complexes of Indoline–3CNQ and TCNQ
Hasan, J. Chem. Soc., Faraday Trans. 1990, 86, 1117. b) Y.
Onganer, M. Yin, D. R. Bessire, E. L. Quitevis, J. Phys. Chem.
1993, 97, 2344.
large dipole moment of the intramolecular CT compounds
sensitive to the environmental perturbations, would control
the degrees of intermolecular CT and their functions.
14 Comprehensive reviews for unimolecular rectifiers based
ꢁꢂ
on Dꢁþ–ꢀ–A molecules: a) R. M. Metzger, Acc. Chem. Res.
This work was partly supported by a Grant-in-Aid (21st
Century COE programs on Kyoto University Alliance for
Chemistry and No. 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.
1999, 32, 950. b) R. M. Metzger, J. Mater. Chem. 1999, 9,
2027. c) R. M. Metzger, Chem. Rev. 2003, 103, 3803. d) R. M.
Metzger, Anal. Chim. Acta 2006, 568, 146.
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Liq. Cryst. 1984, 107, 133.
16 R. M. Metzger, B. Chen, U. Hopfner, M. V.
¨
Lakshmikantham, D. Vuillaume, T. Kawai, X. Wu, H. Tachibana,
T. V. Hughes, H. Sakurai, J. W. Baldwin, C. Hosch, M. P.
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Supporting Information
Dihedral angles of In–3CNQ molecules, overlap mode in the
crystal structures of (I1–3CNQ)2(TCNQ) and (I6–3CNQ)(TCNQ),
UV–vis–NIR spectra in the solution state. This material is availa-
ble free of charge at http://www.csj.jp/journals/bcsj/.
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