Langmuir-Blodgett Films of Charge-Transfer Complexes between an Amphiphilic Monopyrrolo-TTF and TCNQ Derivatives

Article abstract of DOI:10.1021/jp036335v

Electrically active Langmuir-Blodgett (LB) films based on charge-transfer (CT) complexes between an amphiphilic monopyrrolo-tetrathiafulvalene (MP-TTF) electron donor and different derivatives of the electron acceptor 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), namely, 2,5-difluoro-TCNQ, fluoro-TCNQ, TCNQ, decyl-TCNQ, 2,5-dimethyl-TCNQ, and 2-methoxy-5-ethoxy-TCNQ, have been fabricated and investigated systematically. The electronic ground state of the CT complex, (MP-TTF^+??)(TCNQs^-??), in the LB films varied from ionic (?? = 1) to neutral (?? a?? 0) depending on the electron affinity of the TCNQ derivative used. CT excitation energies of the LB films were correlated against the difference between the redox potentials of the MP-TTF and TCNQ derivatives, exhibiting agreement with a neutral-ionic (N-I) phase diagram proposed for mixed-stack CT complexes. Of the six kinds of LB films, the (MP-TTF^+0.3)(decyl-TCNQ ^-0.3) and (MP-TTF^+0.6)(TCNQ^-0.6) complexes were located closest to the N-I phase boundary. In all of the LB films, the CT transition moment was found to be parallel to the substrate surface based on information from polarized UV-vis-NIR, IR transmission, and reflection-absorption spectra. After being transferred onto a mica surface by a single withdrawal, the surface morphologies of the films were found to be uniform or exhibited spongelike domain structures of thickness about 1.8 nm.

Full text of DOI:10.1021/jp036335v

J. Phys. Chem. B 2003, 107, 13929-13938
13929
Langmuir-Blodgett Films of Charge-Transfer Complexes between an Amphiphilic
Monopyrrolo-TTF and TCNQ Derivatives
Tomoyuki Akutagawa,*,†,‡ Masanori Uchigata,§ Tatsuo Hasegawa,† Takayoshi Nakamura,*,†,‡
Kent A. Nielsen,| Jan O. Jeppesen,| Thomas Brimert,| and Jan Becher|
Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 060-0812, Japan,
Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan, CREST, Japan Science and
Technology Corporation (JST), Kawaguchi 332-0012, Japan, and UniVersity of Southern Denmark,
CampusVej 55, DK-5230 Odense M, Denmark
ReceiVed: August 7, 2003; In Final Form: October 7, 2003
Electrically active Langmuir-Blodgett (LB) films based on charge-transfer (CT) complexes between an
amphiphilic monopyrrolo-tetrathiafulvalene (MP-TTF) electron donor and different derivatives of the electron
acceptor 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), namely, 2,5-difluoro-TCNQ, fluoro-TCNQ, TCNQ,
decyl-TCNQ, 2,5-dimethyl-TCNQ, and 2-methoxy-5-ethoxy-TCNQ, have been fabricated and investigated
systematically. The electronic ground state of the CT complex, (MP-TTF+δ)(TCNQs-δ), in the LB films
varied from ionic (δ ) 1) to neutral (δ ∼ 0) depending on the electron affinity of the TCNQ derivative used.
CT excitation energies of the LB films were correlated against the difference between the redox potentials of
the MP-TTF and TCNQ derivatives, exhibiting agreement with a neutral-ionic (N-I) phase diagram proposed
for mixed-stack CT complexes. Of the six kinds of LB films, the (MP-TTF+0.3)(decyl-TCNQ-0.3) and (MP-
TTF+0.6)(TCNQ-0.6) complexes were located closest to the N-I phase boundary. In all of the LB films, the
CT transition moment was found to be parallel to the substrate surface based on information from polarized
UV-vis-NIR, IR transmission, and reflection-absorption spectra. After being transferred onto a mica surface
by a single withdrawal, the surface morphologies of the films were found to be uniform or exhibited spongelike
domain structures of thickness about 1.8 nm.
Introduction
Metallic electrical conduction is usually obtained for δ values
in the range of 0.5-1. For example, the molecular metal
Molecular assemblies held together by charge-transfer (CT)
interactions exhibit peculiar electrical,1 magnetic,2 and optical
properties,3 in which the number of conduction carriers,
magnetic spin, and optical excitation energy are tunable depend-
ing on the magnitude of the intermolecular CT interactions
between the electron donor (D) and the electron acceptor (A)
composed of tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-
p-quinodimethane (TCNQ) exhibits an electronic ground state
of (TTF+0.59)(TCNQ-0.59).1a,4 Therefore, control of the degree
of CT in (D+δ)(A-δ) complexes is important in order to control
the electrical properties of CT complexes. Another important
factor for determining the electronic properties of a (D+δ)(A-δ
)
molecules. The electronic structure of an open-shell (D+δ)(A-δ
)
complex is the molecular orientation within the molecular
assembly.1a Typically, D and A molecules exhibit planar
conjugated π planes, which tend to form one-dimensional π-π
stacking structures. In (D+δ)(A-δ) complexes, two kinds of
π-stacking modes are typically observed. The first one is the
segregated-stack structure, in which the D (A) molecules stack
themselves in a ∼D∼D∼D∼(∼A∼A∼A∼) π-stacking se-
quence.1 Molecular metals such as the (TTF)(TCNQ) CT
complex form uniform segregated-stack structures,4 which are
stabilized by the formation of a π band. The second one is the
mixed-stack structure, in which D and A molecules are stacked
alternately within the same π stack, forming a ∼D∼A∼D∼A∼
π-stacking sequence. Almost all molecular conductors form
segregated-stack structures.1 Although mixed-stack CT com-
plexes do not exhibit metallic electrical conduction, these CT
complexes sometimes exhibit peculiar electronic properties such
as a neutral (N)-ionic (I) phase transition,5 quantum dielectric
properties,6 nonlinear conduction,7 photoinduced phase transi-
tions,8 and optical switching effects.9 The first example of an
N-I phase transition system was the CT complex made from
TTF and chloranil (CA). This system forms a uniform mixed-
stack structure and has a neutral electronic ground state of
(TTF+0.3)(CA-0.3) at room temperature, which changes to ionic
(TTF+0.7)(CA-0.7) when the temperature is reduced to 81 K.5
complex, where δ is the degree of CT from D to A, is principally
determined by the frontier orbitals of the D and A molecules.1a
The difference between the energy level of the HOMO and the
energy of the vacuum level corresponds to the ionization
potential (Ip) of D, whereas the energy difference between the
LUMO and vacuum levels corresponds to the electron affinity
(Ea) of A. The magnitude of δ within the (D+δ)(A-δ) complex
depends on the relative energy difference between Ea of A and
Ip of D. The neutral electronic ground state of δ ∼ 0 is obtained
under condition where Ea > Ip, whereas the completely ionic
state where δ ) 1 is observed when Ea < Ip. Electrical
conduction is largely influenced by the electronic structure of
the (D+δ)(A-δ) complex. The neutral CT complex (D°)(A0) is
an insulator due to a small number of conduction carriers,
whereas the completely ionic CT complex (D+1)(A-1) is also
an insulator due to large on-site Coulomb repulsive energies.2
* To whom correspondence should be addressed. Dr. T. Akutagawa and
Prof. T. Nakamura, Research Institute for Electronic Science, Hokkaido
University, N12W6 kita-ku, Sapporo 060-0812, Japan. Phone: +81-11-
706-2884. Fax: +81-11-706-4972. E-mail: takuta@imd.es.hokudai.ac.jp;
tnaka@imd.es.hokudai.ac.jp.
† Research Institute for Electronic Science, Hokkaido University.
‡ CREST, Japan Science and Technology Corporation.
§ Graduate School of Science, Hokkaido University.
| University of Southern Denmark.
10.1021/jp036335v CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/19/2003

13930 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Akutagawa et al.
SCHEME 1: CT Complexes between the Amphiphilic MP-TTF Derivative 1 and the TCNQ Derivatives 2a-2f
SCHEME 2: Synthesis of the Amphiphilic MP-TTF Derivative 1
In these mixed-stack structures, competition between Ip, Ea, and
electrostatic Madelung energy plays an important role when it
comes to the realization of novel physical phenomena.
inspected by UV-light prior to development with iodine vapor.
Column chromatography was performed using Merck Kiselgel
60 (0.040-0.063 mm, 230-400 mesh AST0000M). Melting
points (mp) were determined on a Bu¨chi melting point apparatus
and are uncorrected. High-resolution matrix-assisted laser-
desorption/ionization mass spectrometry (HiResMALDI-MS)
was performed on an IonSpec Fourier transform mass spec-
trometer, utilizing a 2,5-dihydroxybenzoic acid matrix. 1H NMR
and 13C NMR spectra were recorded on a Gemini-300BB (300
and 75 MHz, respectively) spectrometer. All chemical shifts
are quoted in ppm on the δ scale using TMS or the solvent as
the internal standard. Elemental analyses were performed by
the Atlantic Microlab, Inc., Atlanta.
Electrically active Langmuir-Blodgett (LB) films based on
CT complexes have attracted a lot of attention for application
in nanoscale molecular devices, such as molecular metals in
electrical circuits,10 rectifiers,11 nanowires,12 switches,13 and
sensors.14 However, to realize these electronic properties within
LB films, the electronic structure (δ) and molecular orientation
(segregated or mixed stack) of (D+δ)(A-δ) complexes are
important design factors. Although controlling molecular ori-
entation within LB films is quite difficult at present, control of
the electronic structure can be achieved by suitable chemical
design of the HOMO and LUMO of the D and A, respectively.
In this paper, we describe an efficient method to control the
electronic structure of (D+δ)(A-δ) LB films made from CT
complexes between an amphiphilic monopyrrolo-tetrathiaful-
valene (MP-TTF, 1) derivative and different TCNQ derivatives.
Pyrrolo-TTFs have in recent years been incorporated into a
number of molecular and supramolecular systems, such as
sensors, rotaxane-based switches, and devices.15 Pyrrolo-TTFs
have oxidation potentials in an attractive range and have
therefore a huge potential for forming CT complexes. This
enables a wide range of electron acceptors having different
electron affinities to be used. In the present study, six different
TCNQ derivatives (2a-2f) have been used to control the
electronic ground state of LB films (Scheme 1). The electronic
ground states of (1)(2,5-difluoro-TCNQ) 3a, (1)(fluoro-TCNQ)
3b, (1)(TCNQ) 3c, (1)(decyl-TCNQ) 3d, (1)(2,5-dimethyl-
TCNQ) 3e, and (1)(2-methoxy-5-ethoxy-TCNQ) 3f in LB films
were successfully varied from ionic (D+1)(A-1) to (D0)(A0)
depending on the electron affinity of the TCNQ derivatives.
(i) 2-{4-(2-Cyanoethylthio)-5-octadecylthio-1,3-dithiole-2-
yliden}-5-tosyl-(1,3)-dithiolo[4,5-c]pyrrole (5). A solution of
compound 4 (284 mg, 0.50 mmol) in anhydrous THF (50 mL)
was degassed (N2, 15 min) before a solution of CsOH‚H2O (88
mg, 0.53 mmol) in anhydrous MeOH (1.0 mL) was added
dropwise via a syringe over a period of 1 h. The mixture was
stirred for 15 min, whereupon n-octadecyl bromide (250 mg,
0.75 mmol) was added in one portion to the yellow solution.
The reaction mixture was stirred for 24 h. After removal of the
solvent, the yellow residue was dissolved in CH2Cl2 (100 mL),
washed with H2O (3 × 50 mL), and dried (MgSO4). Concentra-
tion gave an orange powder which was purified by column
chromatography (silica gel, CH2Cl2/cyclohexane 2:1, Rf ) 0.3)
to give 324 mg (84%) of 5 as an orange solid: mp 91∼92 °C.
1H NMR (CDCl3): δ ) 0.88 (t, 3 H, J ) 6.4 Hz), 1.15∼1.45
(m, 30 H), 1.62 (quintet, 2 H, J ) 7.3 Hz), 2.41 (s, 3H), 2.67
(t, 2 H, J ) 7.3 Hz), 2.84 (t, 2 H, J ) 7.3 Hz), 3.01 (t, 2 H, J
) 7.3 Hz), 6.94 (s, 2 H), 7.30 (d, 2 H, J ) 8.2 Hz), 7.73 (d, 2
H, J ) 8.2 Hz). 13C NMR (CDCl3): δ ) 14.1, 18.6, 21.6, 22.7,
28.5, 29.0, 29.3, 29.5, 29.5, 29.6, 29.6, 29.6, 29.7, 29.7 (4 signals
overlapping), 29.8, 31.3, 31.9, 36.4, 111.3, 111.3, 113.5, 117.5,
118.3, 121.5, 126.9 (2 signals overlapping), 127.0, 130.1, 133.5,
135.3, 145.5. HiResMALDI: Calcd for C36H50N2O2S7Na (M+
+ Na), 789.1809; found, 789.1807.
Experimental Section
Synthesis. Solvents were purchased from Aldrich and purified
according to standard procedures. Reagents were purchased from
Aldrich except 4 (Scheme 2) which was synthesized according
to the literature procedure.16 Analytical thin-layer chromatog-
raphy (TLC) was performed on Merck DC-Alufolien Kiselgel
60 F254 0.2 mm thickness precoated TLC plates, which were
(ii) 2-{4,5-Bis(octadecylthio)-1,3-dithiole-2-yliden}-5-tosyl-
(1,3)-dithiolo[4,5-c]pyrrole (6). A solution of compound 5 (269
mg, 0.35 mmol) in anhydrous THF (50 mL) was degassed (N2,

Amphiphilic Monopyrrolo-TTF and TCNQ Derivatives
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13931
15 min) before a solution of CsOH‚H2O (61 mg, 0.37 mmol)
in anhydrous MeOH (1.0 mL) was added dropwise via a syringe
over a period of 1 h. The mixture was stirred for 15 min,
whereupon n-octadecyl bromide (175 mg, 0.53 mmol) was
added in one portion to the yellow solution. The reaction mixture
was stirred for 24 h. After removal of the solvent, the yellow
residue was dissolved in CH2Cl2 (100 mL), washed with H2O
(3 × 50 mL), and dried (MgSO4). Concentration gave an orange
powder which was purified by column chromatography (silica
gel, CH2Cl2/cyclohexane 1:3, Rf ) 0.3) to give 152 mg (45%)
of 6 as a yellow solid: mp 97∼98 °C. 1H NMR (CDCl3): δ )
0.88 (t, 6 H, J ) 6.6 Hz), 1.15∼1.45 (m, 60 H), 1.61 (quintet,
4 H, J ) 7.3 Hz), 2.41 (s, 3H), 2.79 (t, 4 H, J ) 7.3 Hz), 6.92
(s, 2 H), 7.29 (d, 2 H, J ) 8.4 Hz), 7.72 (d, 2 H, J ) 8.4 Hz).
13C NMR (CDCl3): δ ) 14.1, 21.6, 22.7, 28.5, 29.1, 29.4, 29.5,
29.6, 29.7 (2 signals overlapping), 29.7 (7 signals overlapping),
31.9, 36.3, 111.2, 115.3, 116.1, 126.9, 127.3, 127.5, 130.1,
135.4, 145.4. HiResMALDI: Calcd for C51H83NO2S7, 965.4469;
found, 965.4392. Anal. Calcd for C51H83NO2S7 (966.68): C,
63.37; H, 8.65; N, 1.45; S, 23.22. Found: C, 63.49; H, 8.53;
N, 1.44; S, 23.03.
water subphase at 291 K. Surface pressure-area (π-A)
isotherms were recorded at a constant barrier speed of 50 cm2/
min. Langmuir monolayers of 3a-3f were transferred onto a
freshly cleaved mica surface at a surface pressure of 10 mN/m
by a single withdrawal. LB films (20-layer) were transferred
onto hydrophobic substrates at a surface pressure of 10 mN/m
by horizontal lifting. Quartz and CaF2 were hydrophobized by
deposition of five layers of Cd2+-arachidate at a surface
pressure of 30 mN/m.
Atomic Force Microscopy. AFM images were recorded on
a Seiko SPA 400 with an SPI 3800 probe station in tapping
mode (dynamic force mode). Commercially available Si can-
tilevers with a force constant of 20 N/m were used. Surface
morphologies of the films transferred onto mica by a single
withdrawal were observed as topographic images.
Optical Spectroscopy. Polarized UV-vis-NIR (300-3000
nm) and IR (400-7800 cm-1) spectra were recorded on a Per-
kin-Elmer Lambda-19 and a Perkin-Elmer Spectrum 2000 spec-
trometer, respectively. The 20-layer LB films were transferred
onto quartz (20 × 13 mm2) for UV-vis-NIR measurements.
Polarized UV-vis-NIR spectra were measured using p- and
s-polarized light at an angle of incidence of 45° (45p and 45s).
Spectra measured using polarized light both parallel and per-
pendicular to the dipping direction at an angle of incidence of
0° were defined as 90p and 90s, respectively. For IR measure-
ments, transmission (T) and reflection-absorption (RA) spectra
of the 20-layer LB films were measured on CaF2 and evaporated
Au substrates, respectively. IR spectra were recorded by 2000
scans of an MCT detector having a resolution of 4 cm-1. An
angle of incidence of 80° was used for RA measurements.
After the desired product 6 was eluted from the column, a
second band with Rf ) 0.25 containing the detosylated product
1 was isolated. Evaporation of the solvent gave 83 mg (29%)
of 1 as a yellow solid.
(iii) 2-{4,5-Bis(octadecylthio)-1,3-dithiole-2-yliden}-(1,3)-
dithiolo[4,5-c]pyrrole (1). Compound 6 (356 mg, 0.37 mmol)
was dissolved in anhydrous THF/MeOH (3:1 v/v, 100 mL) and
degassed (N2, 15 min) before NaOMe (25% in MeOH, 0.84
mL, 199 mg, 3.68 mmol) was added in one portion. The yellow
solution was stirred at 50 °C for 30 min and cooled to room
temperature, whereupon most of the solvent was evaporated.
The yellow residue was dissolved in CH2Cl2 (100 mL), washed
with H2O (3 × 50 mL), and dried (MgSO4). Concentration gave
a yellow oil, which was subjected to column chromatography
(silica gel, CH2Cl2/cyclohexane 1:2, Rf ) 0.4) to give 272 mg
(91%) of 1 as a yellow solid: mp 92∼94 °C. 1H NMR
(CDCl3): δ ) 0.88 (t, 6 H, J ) 6.6 Hz), 1.15∼1.45 (m, 60 H),
1.60 (quintet, 4 H, J ) 7.4 Hz), 2.81 (t, 4 H, J ) 7.4 Hz), 6.60
(d, 2 H, J ) 2.7 Hz), 8.17 (bs). HiResMALDI: Calcd for
C44H77NS6, 811.4380; found, 811.4308. Anal. Calcd for C44H77-
NS6 (812.48): C, 65.04; H, 9.55; N, 1.72; S, 23.68. Found: C,
64.97; H, 9.33; N, 1.78; S, 23.64.
Results and Discussion
Design, Synthesis, and Redox Properties. The donor mol-
ecule 1 (Scheme 1) is composed of a redox active TTF unit, a
pyrrole unit, and two hydrophobic n-octadecylthio chains. The
pyrrole unit is hydrophilic as a consequence of its ability to
form hydrogen bonds. The introduction of hydrophobic chains
onto the hydrophilic MP-TTF moiety ensures that the molecule
1 is amphiphilic, a criteria which is necessary to form stable
Langmuir monolayers of 1 at an air-water interface. The HO-
MO coefficients of the MP-TTF moiety have been obtained by
semiempirical PM3 calculations. These calculations indicate that
13% of the HOMO density of the MP-TTF moiety is located
on the pyrrole unit.16 Therefore, it can be expected that the
delocalized π electrons on the MP-TTF moiety can act as the
driving force to form stable CT complexes with TCNQ
derivatives.
The amphiphilic MP-TTF derivative 1 was synthesized as
outlined in Scheme 2. A THF solution of the cyanoethyl
protected MP-TTF building block16b 4 was treated with 1.0 equiv
of CsOH‚H2O. This procedure generated the TTF-monothiolate,
which was alkylated with 1.5 equiv of n-octadecyl bromide
affording compound 5. Subsequently, deprotection/alkylation
of 5 with 1.0 equiv of CsOH‚H2O and 1.5 equiv of n-octadecyl
bromide, respectively, gave (Scheme 2) the TTF derivative 6.
Removal of the tosyl protecting group was carried out by stirring
6 at 50 °C in a 3:1 mixture of THF-MeOH in the presence of
an excess of NaOMe, affording the MP-TTF derivative 1 in
57% overall yield from 4.
Electrochemical Measurements. Redox potentials were
obtained using cyclic voltammetry (CV), and the experiments
were carried out in anhydrous CH2Cl2 solutions. The working
and counter electrode were made of platinum, and a saturated
calomel electrode (SCE) was used as the reference electrode.
The concentration of the examined compounds was 0.1-0.5
mM, and (n-Bu4N)(BF4) (0.1 M) was added as the supporting
electrolyte. The scan rate was 50 mV/s.
Preparation of CT Complexes and LB Films. Spectroscopic
grade acetonitrile and benzene were used as spreading solvents
for preparation of Langmuir monolayers at the air-water
interface. TCNQ and 2,5-dimethyl-TCNQ were purchased from
Tokyo Kasei Inc. and purified by vacuum sublimation prior to
use. Fluoro-TCNQ, 2,5-difluoro-TCNQ, decyl-TCNQ, and
2-methoxy-5-ethoxy-TCNQ were prepared according to the
literature procedures.17 LB films were prepared on aqueous
subphases using an NIMA 632D1D2 trough equipped with two
moving barriers. The CT complexes 3a-3f were prepared in
situ by mixing equimolar (1.0 mM) quantities of donor 1 and
TCNQ derivatives (2a-2f) in a 1:1 mixture of CH3CN and
benzene. These solutions were spread on a milli-Q (>18 MΩ)
The amphiphilic MP-TTF derivative 1 revealed two reversible
redox processes at E1/2(1) ) 0.49 and E1/2(2) ) 0.90 V,
respectively, which can be associated with the first and second
oxidation of 1. The oxidation potentials for 1 are listed in Table
1, together with the reduction potentials for the TCNQ deriva-

13932 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Akutagawa et al.
TABLE 1: Redox Properties of the Amphiphilic MP-TTF
Derivative 1 and the TCNQ Derivatives 2a-2f
TCNQs
E1/2(1), V a
E1/2(2), V a
2a
2b
2c
2d
2e
2f
1
2,5-difluoro
+0.50b
+0.33b
+0.27b
+0.25b
+0.22b
+0.09b
+0.49c
-0.14b
-0.24b
-0.34b
-0.31b
-0.30b
-0.41b
+0.90c
2-fluoro
TCNQ
2-decyl
2,5-dimethyl
2-methoxy-5-ethoxy
a vs SCE in CH2Cl2, n-Bu4NBF4 (0.1 M), Pt working and counter
b
electrodes. Half-wave reduction potentials of TCNQ derivatives.c
Half-wave oxidation potentials of the MP-TTF derivative 1.
tives 2a-2f. Typical electron donors, such as TTF, bis-
(ethylenedioxo)-TTF, and bis(ethylenedithio)-TTF, exhibited
under similar conditions E1/2(1) values at 0.38, 0.45, and 0.56
V, respectively. A comparison of these E1/2(1) values with the
E
1/2(1) value for compound 1 show that the electron donating
ability of 1 is higher than that of bis(ethylenedithio)-TTF and
is lower than that of TTF. The electron donating ability of donor
1 is comparable to that of bis(ethylenedioxo)-TTF, which
previously has been used for the preparation of CT complexes
from a variety of acceptors.18
Six kinds of TCNQ derivatives (Scheme 1), 2,5-difluoro-
TCNQ (2a), 2-fluoro-TCNQ (2b), TCNQ (2c), 2-decyl-TCNQ
(2d), 2,5-dimethyl-TCNQ (2e), and 2-methoxy-5-ethoxy-TCNQ
(2f), were used to fabricate LB films. Although the redox
potentials in solution are affected by the TCNQ solvation energy
and radical anion states,19 the relative magnitude of the electron
affinity of these TCNQ derivatives can be predicted from the
first reduction potential E1/2(1).20 The E1/2(1) values of the
TCNQ derivatives decreased in the order of 2a (0.50 V) > 2b
(0.33 V) > 2c (0.27 V) > 2d (0.25 V) > 2e (0.22 V) > 2f
(0.09 V), as shown in Table 1. The variation in electron affinity
of compounds 2a-2f is about 0.4 eV, assuming that the
solvation energy for all of the TCNQ derivatives are similar.20,21
Therefore, the LUMO level of these TCNQ derivatives could
vary over a range of 0.4 eV with respect to the HOMO level of
the MP-TTF donor 1.
Langmuir Monolayers at the Air-Water Interface. Figure
1 shows the π-A isotherms of Langmuir monolayers of 3a-
3f. Surface pressures of 3a-3f increase by compression at
around A0 ) 0.4 nm2, which is close to the cross-sectional area
of two closely packed alkyl tails.22 Because the estimated surface
area of donor 1 (∼0.7 nm2) is much larger than the A0 values
of the CT complexes, the surface area of the floating monolayers
of 3a-3f needs to be determined by the alkyl-chain moieties.
The area per molecule at 10 mN/m (A10) of monolayers 3a, 3b,
3c, 3d, 3e, and 3f were 0.31, 0.34, 0.29, 0.40, 0.32, and 0.29
nm2, respectively. A remarkably sharp increase in the π-A
isotherm was observed in the case of 3d, which can be explained
by the presence of an alkyl chain in the acceptor 2d. A larger
A10 value for 3d than for 3a-3c and 3e-3f also suggests that
the n-decyl chains on 2d leads to the formation of a more stable
molecular-assembly. Because the π-A isotherms of 3a-3f are
fundamentally similar, molecular orientations at the air-water
interface are also expected to be similar.
Figure 1. π-A isotherms of 1 and 3a-3f on pure water. The upper
part of the figure shows a Cory-Pauling-Koltun (CPK) representation
of the MP-TTF derivative 1 viewed normal to its π plane.
Surface morphology of the 3c film was slightly different as
compared to the other films. Higher magnification images reveal
a spongelike surface morphology. All of the layers of 3a-3f
on the mica surface have a thickness of ∼1.8 nm, which is
shorter than the calculated molecular length of octadecane (∼2.2
nm),22 indicating that those layers are monolayers. Interlayer
spacing in LB films of orthogonal-packed octadecanoic acid
(C17H35COOH), in which the long axis of the alkyl chains is
tilted at ∼30° with respect to the substrate normal, has been
reported to be ∼2.5 nm.22,23 Therefore, it can be expected that
the n-octadecylthio chains in the LB films of 3a-3f on mica
surface are tilted, which will be discussed later.
Electronic State of LB Films. Table 2 summarizes the CT
transition energies (hνCT) observed in the LB films of 3a-3f
and the difference between the first half-wave oxidation potential
of donor 1 and the first half-wave reduction potential of the
TCNQ derivatives (∆E ) E1/2(1) - E1/2(TCNQ)). Figure 3
shows electronic absorption spectra of 20-layer LB films of 3a-
3f. The low energy transition at around 5.3 ∼ 6.1 × 103 cm-1
(A band) can be associated with the intermolecular CT transition
(hνCT),24 whereas the B and C bands are related to the
intramolecular excitation of the donor 1 and the TCNQ
derivatives, respectively. The solution absorption spectrum of
donor 1 recorded in CH3CN exhibits the intramolecular π-π*
transition at an absorption maximum of 30.7 × 103 cm-1
,
whereas the absorption spectra of CT complexes 3a-3f recorded
in CH3CN exhibited intramolecular π-π* transitions at around
23∼25 × 103 cm-1. Because the absorption intensities of TCNQ
anion radicals in solution are much larger than the absorption
intensity of donor 1 in solution, the C bands in the LB films
can be expected to originate mainly from the intramolecular
excitations of TCNQ anion radicals.24a Two characteristic sharp
bands at around 12∼13 × 103 cm-1 were observed in the
solution absorption spectra of 3a-3c and can be associated with
Surface Morphology of Transferred Films on Mica. Figure
2 shows the surface morphology of films of 3a, 3c, 3d, and 3e
transferred onto freshly cleaved mica by a single withdrawal.
The CT complex covered almost the entire 10 × 10 µm2 surface
area. In higher magnification images (2 × 2 µm2) of 3a, 3c,
and 3e films, domain structures can be observed, whereas a
largely uniform surface was obtained for 3d, reflecting the
stability of the floating monolayer at the air-water interface.

Amphiphilic Monopyrrolo-TTF and TCNQ Derivatives
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13933
Figure 2. Selected AFM images of Langmuir monolayers of (a) 3a, (b) 3c, (c) 3d, and (d) 3e transferred by a single withdrawal onto a freshly
cleaved mica surface (π ) 10 mN/m). Scales of upper and lower images are 10 × 10 and 2 × 2 µm2, respectively.
TABLE 2: CT Transition Energies (hνCT) of LB Films of
3a-3f and the Difference between the First Half-Wave
Oxidation Potential of Donor 1 and the First Half-Wave
Reduction Potential of the TCNQ Derivatives (∆E ) E1/2(1)
- E1/2(TCNQ))
by a much higher concentration of ionized TCNQ species in
the LB films of 3a-3c. The electronic absorption spectrum of
the LB film of 3f has a different spectral shape as compared to
those of 3a-3e, and an A band at 7.3 × 103 cm-1 and two
broad bands (20.2 and 22.7 × 103 cm-1) were observed. The
latter two bands can be assigned to the intramolecular transitions
of neutral 2-methoxy-5-ethoxy-TCNQ in the LB film, In
solution, these bands were observed at 22.8 and 24.2 × 103
cm-1, respectively.
CT complex
hνCT, 103 cm-1
∆E, Va
3a
3b
3c
3d
3e
3f
5.5
5.7
5.8
5.9
6.1
7.3
-0.01
+0.16
+0.22
+0.24
+0.27
+0.40
The electronic ground state of (D+δ)(A-δ) complexes can be
discussed in terms of hνCT and the difference in redox potentials;
∆E ) E1/2(1) - E1/2(TCNQ), between the electron donor 1 and
the electron acceptors (TCNQs) 2a-2f (Table 2) and can be
used to explain the origin of the A bands in electronic absorption
spectra.5,24 Two kinds of methods for predicting the electronic
ground state of CT complexes based on ∆E have been proposed
by Saito et al.25 and Torrance et al., respectively.5 In the former
case, the partial CT state of (D+δ)(A-δ) with 0.5 < δ < 1, which
is necessary to obtain metallic CT complexes with segregated-
stack structures, is achieved under conditions where -0.02 <
∆E < 0.34 V.25 Completely ionic (D+1)(A-1) and neutral (D0)-
(A0) complexes are typically observed under conditions where
∆E < -0.02 V or ∆E > 0.34 V, respectively. On the other
a Difference between the first half-wave oxidation potential of donor
1 and the first half-wave reduction potential of TCNQs.
hand, the latter method can be applied to mixed-stack (D+δ)(A-δ
complexes, in which a phase boundary between ionic (D+1)(A-1
)
)
and neutral (D0)(A0) complexes is observed around ∆E ) 0.17
V.5 Ionic (D+1)(A-1) and neutral (D0)(A0) ground states are
typically observed under conditions where ∆E < 0.17 V or ∆E
> 0.17 V, respectively. The N-I transition of (TTF)(CA) was
discovered using this prediction.5
If the molecular arrangement of the CT complexes 3a-3f
consists of segregated π-stacked structures of D and A
molecules, high electrical conductivity can be expected on
account of the presence of mixed valence states and the fact
that ∆E range from -0.01 to +0.40 V in the CT complexes
3a-3f. Highly electrically conducting CT complexes also
exhibit low energy CT excitations in the IR energy region, which
correspond to intra- and inter-band transitions in metal and
narrow band-gap semiconductors, respectively.24 However, the
electrical resistivity of the LB films of 3a-3f was higher than
100 MΩcm, with the hνCT of the LB films observed at energies
Figure 3. UV-vis-NIR-IR spectra of 20-layer LB films of 3a-3f.
the anion radical species of the TCNQ derivatives.24a The
absorption intensities of the B bands in the LB films of 3a-3c
were larger than in the other LB films, which can be explained

13934 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Akutagawa et al.
Figure 4. Plots of hνCT against ∆E for the LB films of 3a-3f and
(TTF)(CA) (closed circle). Equations 1 and 2 are shown as solid lines
in the I and N regions, respectively.
above 5 × 103 cm-1. Therefore, the π-π stacking structures
within the LB films of 3a-3f are mixed stacks rather than
segregated stacks. The A bands observed in the electronic
absorption spectra of the LB films of 3a-3f can be accounted
for by the presence of intermolecular CT transitions from the
HOMO of donor 1 to the LUMO of the TCNQ derivatives.
In cases where the complexes form mixed-stack structures,
the boundary conditions of ∆E ) 0.17 V between the ionic
and neutral ground states can be applied to predict the electronic
structures of the CT complexes. The CT complexes 3a (∆E )
-0.01 V) and 3b (∆E ) +0.16 V) are located in the ionic
(D+)(A-) region, whereas 3c (∆E ) +0.22 V), 3d (∆E ) +0.24
V), 3e (∆E ) +0.27 V), and 3f (∆E ) +0.40 V) are in the
neutral (D°)(A0) region, as shown in Table 2. The CT complexes
3b-3c are in a ∆E region close to the N-I boundary.
The ground state of mixed-stack (D+δ)(A-δ) complexes can
be described by a linear combination of the neutral (D0)(A0)
and the ionic (D+1)(A-1) states. The boundary between the ionic
and neutral electronic ground states is determined from Ip, Ea,
and the electrostatic Madelung energy. Linear hνCT vs (Ip -
Ea) correlations have been proposed for the ionic and neutral
electronic ground states, respectively, according5 to eqs 1 and
2:
e2
d
hνCT(N) ) (Ip - Ea) -
(1)
(2)
e2
d
hνCT(I) ) -(Ip - Ea) + (2R - 1)
Figure 5. Transmission IR spectra recorded of the LB films of 3a-3f
in the frequency ranges of (a) 4000-1400 and (b) 1800-1100 cm-1
.
where R is the Madelung constant and d is the nearest-neighbor
distance between A and D molecules. Equations 1 and 2 meet
at Ip - Ea ) R(e2/d). When the same solvation energies are
applied to the ground and excited-state species, the parameter
Ip - Ea can be replaced by ∆E.
Figure 4 shows plots of hνCT against ∆E for the LB films of
3a-3f, together with the theoretical V-shaped line representing
eqs 1 and 2. The ∆E - hνCT plots of 3a and 3b appeared at a
slightly lower hνCT region than the straight line representing
eq 2, whereas those of 3c-3f are observed to be in agreement
with eq 1. The (TTF)(CA) CT complex is located in the neutral
region close to the N-I boundary.5 The ∆E - hνCT plots of
highly conductive segregated-stack CT complexes are typically
observed in a much lower hνCT region at around 2∼3 × 103
cm-1 24
, in which the CT transition occurs between the same D
or A species. The ∆E - hνCT plots of the LB films of 3a-3f
are consistent with the theoretical predictions for mixed-stack
systems, which further support the assignment of the A bands
in Figure 3 as intermolecular CT transitions between the HOMO
of 1 and the LUMO of the TCNQ derivatives. An inspection
of the V-shape diagram reveals that the ionic electronic ground
state of (D+1)(A-1) is reasonable for the LB films of 3a and
3b, whereas the neutral electronic ground state of (D+δ)(A-δ
for δ < 0.5 is expected for LB films of 3c-3f.
)
Vibrational Spectra of LB Films. Several intramolecular
vibrational modes are sensitive to the CT state. Figure 5 shows

Amphiphilic Monopyrrolo-TTF and TCNQ Derivatives
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13935
TABLE 3: Vibrational Bands Observed in the LB Films of
3a-3f
ν
CN (ag),
ν
CN (b1u),
ν3 (ag),
ν
21 (b1u),
ν5 (ag),
LB film
cm-1
cm-1
cm-1
cm-1
cm-1
3a
3b
3c
3d
3e
3f
2191
2193
2199
2203
2207
2210
2212
2206
2220
1615
1600
1591
1352
1361
1379
1393
1172
1179
1179
transmission IR spectra of the LB films of 3a-3f in the
frequency range from 4000 to 1000 cm-1. In Table 3, typical
vibration frequencies observed in the LB films of 3a-3f are
listed. All LB films exhibited two intense vibrational bands at
2918 and 2850 cm-1, which can be assigned to symmetric and
asymmetric νCH2 modes of the octadecyl chains.22,23 The IR
spectrum recorded of a KBr pellet containing the neutral donor
1 revealed distinctive vibrational modes associated with νCH2
of the alky chains (2918 and 2849 cm-1), νNH of the pyrrole
unit (3400 cm-1), and δCH2 (1470 cm-1) of the alkyl chains.
The tails of the broad CT transitions are extended to the IR
energy region and obscured the νNH modes in the LB films 3a-
3f. All of the LB films exhibited the nitrile stretching band (νCN
expected for TCNQ derivatives at around 2200 cm-1 and some
characteristic vibrational bands at frequencies below 1600 cm-1
)
.
The intensity of some of the vibrational bands in the LB films
of 3a-3c is larger than those in the LB films of 3e-3f, which
can be related to different electronic structures and lattice
distortions.
Figure 6 shows the transmission (T) and reflection-absorp-
tion (RA) spectra recorded of the 3c LB film. In these two
optical arrangements, the transition moment within and normal
to the substrate surface is activated in the T and RA spectra,
respectively.22,23 Molecular orientations in the LB films can be
determined from the dichroism between the T and RA spectra.22,
23, 26
Figure 6. Transmission (T) and reflection-absorption (RA) spectra
recorded of the 3c LB film in the frequency ranges of (a) 3000-1000
and (b) 2300-2100 cm-1. The ag and b1u modes of the νCN band of
TCNQ are also shown. The blue lines in Figure 6b are results of peak
deconvolution for b1u and ag modes.
substrate normal (φalkyl) was calculated to be 30° by applying
the orthogonality relationship cos2(φ(νCs H2)) + cos2(φ(νCa H2)) +
cos2(φalkyl) ) 1.22 Similar tilt angles were observed in LB films
of 3a, 3b, 3d, 3e, and 3f.
Donor 1 contains two hydrophobic octadecyl chains, which
have a tendency to assemble with each other through van der
Waals interactions in the LB film.22 The orientation of the
octadecyl chains was estimated from the intensity of symmetric
(νsCH2) and asymmetric (νCa H2) CH2 stretching modes in the T
and RA spectra.22,23 The enhancement factor m of the RA
spectrum with respect to the T spectrum is given in eq 3:
Significant differences between the T and RA spectra in the
ν
CN band of 3c LB film were observed, as evidenced in Figure
6b. The νCN band of TCNQ is composed of three modes having
b1u, b2u, and ag (ν2) symmetry.28 Although the totally symmetric
ag mode is usually forbidden in IR spectra, lattice distortions
such as dimerization within the π-stacking direction activate
the ag mode in the IR spectra. An intense νCN band was observed
at 2199 cm-1 for the T spectrum of the LB film of 3c, whereas
two νCN bands at 2210 and 2199 cm-1 were observed in the
RA spectrum. The νCN (ag) and νCN (b1u) modes of complete
ionic (K+1)(TCNQ-1) appear at 2200 and 2167 cm-1, respec-
m ) 4n13 sin2 θ/n23 cos θ
(3)
where n1 and n2 are the refractive indices of air and the film,
respectively, and θ is the angle of incidence used in RA
spectroscopic measurements. A value of m ) 6.62 was obtained
for the parameters n1 ) 1.0, n2 ) 1.5, and θ ) 80° in the present
measurements.26d Because the n2 values of fatty acid salt LB
films have been reported to be around 1.5 from elipsometry,27
a value of n2 ) 1.5 was used to obtain the enhancement factor.
The average tilt angle φ of the uniaxially oriented transition
moment from the film normal is given by eq 4:
tively.28 The partially CT (N,N-dimethylphenazine+0.5)(TCNQ-0.5
)
complex exhibited νCN (ag) and νCN (b1u) bands at 2198 and
2205 cm-1, respectively.29 The energies of the two νCN bands
at 2199 and 2210 cm-1 in the LB film of 3c were almost
consistent with those of (N,N-dimethylphenazine+0.5)(TCNQ-0.5
)
rather than those of (K+1)(TCNQ-1). Therefore, the electronic
ground state of the LB film of 3c is close to (1+0.5)(TCNQ-0.5),
and the VCN bands at 2199 and 2210 cm-1 can be assigned to
VCN of the ag and b1u modes, respectively. The ag symmetry
sin2 φ
2m cos2 φ
AT
)
(4)
AR
where AT and AR are the absorbance of T and RA spectra,
respectively. By applying eq 4 to the νaCH2 and νCs H2 modes, the
tilt angles of symmetric and asymmetric CH2 transition moments
of φ (νsCH2) and φ (νaCH2) were estimated to be 70° for the 3c
LB film. The tilt angle of the alkyl chains with respect to the
VCN band was deconvoluted by double Lorentzian (Figure 6b).
The ag band is activated normal to the TCNQ π plane, and the
tilt angle of the transition moment is 80° with respect to the
substrate normal accordingly to eq 4. Thus, the TCNQ π plane
is tilted at an angle of 80° to the substrate surface, which

13936 J. Phys. Chem. B, Vol. 107, No. 50, 2003
Akutagawa et al.
corresponds to a largely parallel arrangement of the π-stacking
direction to the substrate surface.
Several characteristic vibrational bands were observed (Figure
5b) in the frequency range from 1600 to 1200 cm-1. Based on
the vibrational assignments of (N,N-dimethyphenazine+0.5)-
(TCNQ-0.5), the bands at 1591, 1379, and 1179 cm-1 in the 3c
LB film can be assigned to the ν3 (ag), ν21 (b1u), and ν5 (ag)
bands of TCNQ, respectively.29,30 Among them, the ν21 band
at 1379 cm-1 is useful for determining the parameter δ from
the linear correlation between neutral TCNQ0 at 1405 cm-1 to
complete ionic TCNQ-1 at 1361 cm-1. The ν21 band of the 3c
LB film was observed at 1379 cm-1, hence it can be deduced
that the 3c LB film consists of (1+0.6)(TCNQ-0.6). The intensity
of the ag modes (ν3 and ν5) of 3c in the T spectrum was larger
than those of the RA spectra, which is consistent with the
orientation of TCNQ estimated from the νCN bands.
As the energy of the ν21 mode of decyl-TCNQ at 1393 cm-1
exhibits the same linear correlation as that of TCNQ, the
electronic ground state of the 3d LB film is estimated to be
(1+0.3)(decyl-TCNQ-0.3). The tilt angle φalkyl ) 30° and thickness
of the 3d LB film (∼1.8 nm) were similar to those of the 3c
LB film. Although the thickness of the LB films was similar
for 3c and 3d, the intensity of the vibrational bands of the 3d
LB film was lower than that of the 3c LB film. The ag modes
were present in the IR spectra of LB films of 3a-3c, whereas
distinct activation of ag modes was not observed in the LB films
of 3d-3f. These results suggest that lattice distortions within
the π-stacking axis occurred in the LB films that have an ionic
electronic ground state, i.e., the LB films of 3a-3c. The
electronic ground states of the 3c and 3d LB films were
(D+0.6)(A-0.6) and (D+0.3)(A-0.3), respectively. Because the
vibrational spectra suggest that the 3c LB film is in the ionic
ground state, the N-I phase boundary is expected to be located
between the LB film of 3c and 3d. The ∆E - hνCT plot for the
3d LB film was close to that of (TTF)(CA),5 and thus, the
possibility of a temperature induced N-I transition was
examined. However, the temperature-dependent vibrational
spectra of the 3d LB film does not exhibit any evidence of a
N-I transition, even at temperatures below 10 K.
Figure 7. Polarized UV-vis-NIR spectra of a LB film. (a) Config-
uration of the measurements. The dipping direction of the substrate is
nominally abbreviated to as p in normal incidence. (b) UV-vis-NIR
spectra recorded of the 3c LB film for light incident at (i) 45s and (ii)
45p.
Figure 8. Schematic illustration of the molecular orientation of the
LB films on a hydrophilic substrate.
Although complete vibrational assignments of 2,5-difluoro-
TCNQ and fluoro-TCNQ have not been reported yet, the
vibrational features of the 3a and 3b LB films in the frequency
range from 1600 to 1200 cm-1 resembled that of the 3c LB
film quite significantly. The ν21 band of the 3b LB film was
observed at 1361 cm-1, which is consistent with completely
ionic TCNQ-1 (1361 cm-1). On the other hand, the ν21 band of
the 3a LB film at 1352 cm-1 was located at an intermediate
frequency between completely ionic TCNQ-1 (1361 cm-1) and
F4-TCNQ-1 (1344 cm-1).28-31 Because the frequency shift of
the ν21 band is dependent on the substituents on TCNQ, an exact
determination of the parameter δ was impossible for the 3a and
3b LB films. However, the results indicate that the electronic
ground state of the LB films of 3a and 3b are considerably
close to the completely ionic (D+1)(A-1) state. Because the
vibrational spectra of the 3e and 3f LB films are superimposi-
tions of neutral 1 and TCNQs, the electronic ground state of
these is the neutral (D+δ)(A-δ) state with δ ∼ 0.
are shown in Figure 7b. From the intensities of the spectra with
normal incidence of 90p and 90s, the 3c LB film was estimated
to be optically isotropic in the film plane. The intensity of the
A band at 5.8 × 103 cm-1 with s polarized light of 45° incidence
(spectrum ii) is larger than that of p polarized light (spectrum
i), which can be accounted for by the largely parallel orientation
of the CT transition moment to the substrate surface.32 The
polarization dependence is small for intramolecular transition
moments at 17 and 27 × 103 cm-1. Because the intramolecular
transition moments are along the long axes of TCNQ and donor
1, the long axes of these molecules are arranged near parallel
to the substrate surface. This polarization dependence is
consistent with the results obtained from the T and RA IR
spectra.
From the polarized IR and UV-vis-NIR spectra and AFM
height images, a possible model for molecular packing on the
hydrophilic substrate surface was proposed and is shown in
Figure 8. The height of a single domain measured by AFM gave
a film thickness of ∼1.8 nm. The octadecyl chains and π plane
of TCNQ are tilted at ∼60° and 10° with respect to the substrate
surface. Considering the direction of CT transitions parallel to
the substrate surface, the π-stacking direction between D and
Molecular Orientation in LB Films. Polarized UV-vis-
NIR spectra were used to get information about the molecular
orientation within the LB films.22,32 Figure 7a shows the optical
arrangement of the measurements, where the dipping direction
is nominally abbreviated as p for the 90 incidence. All of the
LB films of 3a-3f revealed similar polarization dependence in
the UV-vis-NIR energy region. The results for the 3c LB film

Amphiphilic Monopyrrolo-TTF and TCNQ Derivatives
J. Phys. Chem. B, Vol. 107, No. 50, 2003 13937
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electrical conductivity and hνCT - ∆E plot in the V-shape
diagram strongly suggest a mixed-stack structure of 1 and
TCNQ. Several ag modes were activated in the IR spectra of
the LB films of 3a-3c. However, the activation of ag modes
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Summary
CT complexes consisting of an amphiphilic MP-TTF deriva-
tive 1 and one of six kinds of TCNQ derivatives were used to
fabricate LB films. The electron affinity of the TCNQ deriva-
tives can be varied by more than 0.4 eV by changing the
substitutents at the 2 and 5 positions of TCNQ from 2-methoxy-
5-ethoxy-TCNQ to 2,5-difluoro-TCNQ. Monolayers transferred
onto mica surfaces by a single withdrawal were found to be
rather uniform, except for the TCNQ complex, which exhibited
a spongelike structure. The UV-vis-NIR spectra of these LB
films exhibited CT transitions in the NIR-IR energy region,
with energies (hVCT) corresponding to the difference in redox
potential (∆E) between the first oxidation potential of the MP-
TTF derivative 1 and the first reduction potential of the TCNQ
derivatives. The ∆E - hVCT plots of the LB films were observed
to be in good agreement with the theoretical predictions for
mixed-stack CT complexes, an observation which also is
consistent with the insulating electrical conductivity of the LB
films. From the optical properties, it was deduced that the CT
complexes of 2,5-difluoro-TCNQ, fluoro-TCNQ, and TCNQ
have ionic electronic ground states, whereas those of decyl-
TCNQ, 2,5-dimethyl-TCNQ, and 2-methoxy-5-ethoxy-TCNQ
have neutral electronic ground states. Among these LB films,
the electronic ground states of (1)(TCNQ) and (1)(decyl-TCNQ)
were estimated to be (D+0.6)(A-0.6) and (D+0.3)(A-0.3), respec-
tively, which are located around the neutral-ionic phase
boundary. The parallel arrangement of the CT transition moment
to the substrate surface was confirmed by polarized UV-vis-
NIR, transmission IR, and reflection-absorption IR spectra for
these LB films. Systematic modulation of the electronic ground
state of the CT complexes (1)(TCNQs) is possible by controlling
the electron affinity of the TCNQ derivatives, which changes
the dipole interaction of the D-A pairs in the thin-film
structures. The LB films composed of mixed-stack CT com-
plexes with tunable electronic structures are attractive candidates
for the formation of nonlinear optical and dielectric materials.
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Acknowledgment. This work was partly supported by a
Grant-in-Aid for Science Research from the Ministries of
Education, Culture, Sports, Science and Technology of Japan
and by the Carlsbergfondet in Denmark.
Supporting Information Available: The AFM images of
the films transferred onto mica surface by a single withdrawal
(3b and 3f), transmission and reflection-absorption IR spectra
(3a, 3b, 3d, 3e, and 3f), and polarized UV-vis-NIR spectra
(3a, 3b, 3d, 3e, and 3f). This material is available free of charge
via the Internet at http://pubs.acs.org.
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