Photophysical studies on the mono- and dichromophoric hemicyanine dyes I. Photoelectric conversion from the dye modified ITO electrodes

Article abstract of DOI:10.1021/jp020876n

Stilbazolium dimers were designed and synthesized in which methylene groups of different lengths link the two chromophores. The second-order nonlinear optical property has been detected from their Langmuir-Blodgett monolayers. Photocurrent generation as well as other photophysical properties such as absorption and fluorescence were investigated. Comparing the dimers with the stilbazolium monomer, we found strong second harmonic generation (SHG) signals and remarkable enhanced photoelectric conversion (PEC) quantum yields from 1,3-Bis [(E)- 4-(2-(4-(N-methyl, N-octadecylamino) phenyl) ethenyl) pyridinyl]-alkane dibromide (B3) and 1,5-Bis [(E)- 4-(2-(4-(N-methyl, N-octadecylamino) phenyl) ethenyl) pyridinyl]-alkane dibromide (B5). The structures of the Langmuir-Blodgett films of these dyes are assumed. Although these dimers are center-symmetric (or quasi-center-symmetric) molecules, they can form the ordered non-center-symmetric (in normal direction) monolayer by the compress force and the molecular interaction between the amphiphilic dye molecules and the water molecules at the air/water interface. The influences of bias, donor/acceptor, soluble oxygen, and light intensity to photocurrent generation were studied; our results provide a new perspective to improve the performance of functional molecular material without any change of the chemical structure of the chromophore itself.

Full text of DOI:10.1021/jp020876n

10020
J. Phys. Chem. B 2002, 106, 10020-10030
Photophysical Studies on the Mono- and Dichromophoric Hemicyanine Dyes I. Photoelectric
Conversion from the Dye Modified ITO Electrodes
Yanyi Huang, Tianrong Cheng, Fuyou Li, and Chun-Hui Huang*
State Key Laboratory of Rare Earth Materials Chemistry and Applications, and PKU-UHK Joint Lab on Rare
Earth Materials and Bioinorganic Chemistry, College of Chemistry, Peking UniVersity, Beijing 100871, China
Tingjun Hou, Anchi Yu, Xinsheng Zhao, and Xiaojie Xu
Department of Chemistry, College of Chemistry, Peking UniVersity, Beijing 100871, China
ReceiVed: April 3, 2002
Stilbazolium dimers were designed and synthesized in which methylene groups of different lengths link the
two chromophores. The second-order nonlinear optical property has been detected from their Langmuir-
Blodgett monolayers. Photocurrent generation as well as other photophysical properties such as absorption
and fluorescence were investigated. Comparing the dimers with the stilbazolium monomer, we found strong
second harmonic generation (SHG) signals and remarkable enhanced photoelectric conversion (PEC) quantum
yields from 1,3-Bis [(E)- 4-(2-(4-(N-methyl, N-octadecylamino) phenyl) ethenyl) pyridinyl]-alkane dibromide
(B3) and 1,5-Bis [(E)- 4-(2-(4-(N-methyl, N-octadecylamino) phenyl) ethenyl) pyridinyl]-alkane dibromide
(B5). The structures of the Langmuir-Blodgett films of these dyes are assumed. Although these dimers are
center-symmetric (or quasi-center-symmetric) molecules, they can form the ordered non-center-symmetric
(in normal direction) monolayer by the compress force and the molecular interaction between the amphiphilic
dye molecules and the water molecules at the air/water interface. The influences of bias, donor/acceptor,
soluble oxygen, and light intensity to photocurrent generation were studied; our results provide a new perspective
to improve the performance of functional molecular material without any change of the chemical structure of
the chromophore itself.
I. Introduction
on the potential value of these phenomena. For years, we have
devoted ourselves to find effective chromophores for nonlinear
optical and/or photoelectric conversion applications. In the past
several years, we have reported several papers on the photo-
electric conversion of stilbazolium and its derivatives.24-31 We
have found that the chemical modification (even a small change
of the group’s position) of the chromophore can tremendously
alter its photophysical properties and functions, especially for
those properties related to charge distribution (or charge
transfer). A special modification of functional material is
molecular dimerization, through which, as we have reported,
some photophysical functions will be improved remarkably.24,25
We find that a simple dimerization, in which the functional
chromophore is not changed at all and the charge distribution
is almost the same as in monomer, can make a huge difference.
In this article, we attempt to show the relationship between the
structures and properties of the molecules through an example
of a series of stilbazolium dimers.
Organic molecules, which have promising functions for optics
and electronics, are important in various imaginable applications.
Stilbazolium (hemicynane) and its derivatives have been studied
for several decades because of their potential applications to
molecular-based functional materials in opto-electric fields.
Their photophysical properties, that is, absorption, fluorescence,
nonlinear optical phenomena (second harmonic generation
(SHG), two-photon absorption, laser application), and photo-
electric conversion (PEC) have been widely reported.1-14 As
one of the most efficient SHG chromophores, it has many
derivatives that have been designed and synthesized to improve
its SHG property.15,16 Stilbazolium is also a useful chromophore
as a voltage-sensitive membrane probe, especially used in cell
biology.17 The photophysical properties of stilbazolium have
been investigated for years both in theoretical and experimental
regions, and there are still numerous reports on the details of
the picture of photophysics. The most important feature of
stilbazolum is that photoexcitation induces an intramolecular
charge-transfer excited state. This still guides our research on
stilbazolium and its derivatives.
Here, we synthesize a series of stilbazolium dimers: B3, B5,
and B12 (Figure 1) and report their photophysical properties,
especially the SHG from LB monolayer and their PEC abilities
from LB films modified ITO electrodes. Both of these phe-
nomena are related to charge redistribution upon the light
excitation. The strong SHG signal indicates that the molecular
orientation in LB film should not be the center-symmetric. We
assumed that the actual conformation of dimers will be a “folded
shape”. Another important improvement of stilbazolium’s
photoactive properties is that the photoelectric conversion
quantum yields of some dimers are greatly enhanced by
In recent years, molecular aggregates and many molecular
dimers have been synthesized to study the differences between
their novel photophysical properties and those of individual
monomers.18-23 Results show that the interaction between the
monomers (chromophores) in those dimers and aggregates can
change their photophysical behaviors. But rarely are there reports
* To whom the correspondence should be addressed.
10.1021/jp020876n CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/07/2002

Mono- and Dichromophoric Hemicyanine Dyes I
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10021
represents those designed bichromophore stilbazolium molecules
(dimers), and n is the number of methylene groups by which
two chromophores are linked together. The attempts to synthe-
size these dimers are (1) to investigate the SHG from a center-
symmetric (or quasi-center-symmetric) molecule, which can be
changed to a noncenter-symmetric conformation by external
force field; (2) to explore the relationship between the photo-
electric conversion properties and the molecular structure; and
(3) to improve the film stability with dimerization. Also, the
different lengths of linkage are designed to control the distance
between two chromophores in dimer molecules. The 3-meth-
ylene-group is a commonly used linkage for most dichro-
mophoric molecules to induce two chromophores to align in a
parallel fashion and form an excimer;29 whereas the 12-
methylene-group is long enough to make two chromphores
separated to individual under relative free condition such as in
solution phase.
The major synthetic steps are marked in Figure 1. They are
modified from refs 32,33.
Figure 1. Molecular structures and schematic synthesis processes of
dyes. (i) 1,n-dibromoalkane, anhydrous ethanol, reflux; (ii) alkyl iodide,
toluene, NaOH, cat.; (iii) POCl3, DMF, reflux; (iv) anhydrous ethanol,
reflux, cat.
Two series of similar bichromophoric dyes B3, B5, B12, and
B3′, B5′, B12′ have been synthesized (see the Experimental
Section for details). The only difference in the alkyl-amino
groups is that the former series involves long-tail (octadecayl-)
molecules, whereas the latter involves short-tail (methyl-) ones.
The two series of molecules are designed for different usages,
the long-tail amphiphilic ones being employed for monolayer
formation, whereas the short-tail ones being easily dissolved in
most polar solvents for solution investigations.
dimerization with suitable alkyl chains. Most reports on mo-
lecular dimers21-23 are related on the dramatically steady-state
spectroscopic difference of the chromophore after dimerization.
However, in our system, the steady-state absorption spectra
between dimers and monomers are not very different from each
other, neither are their emission spectra. This has not happened
to many dimers reported before.21-23 Having observed the
interesting relationships between their structures and the pho-
tophysical properties, we propose that many property changes
after dimerization cannot be simply explained by the effect on
the intramolecular aggregation. The origin of these property
changes caused by dimerization may be the effect of dynamic
photophysical properties and should be pursued.
1
Figure 2 shows the H NMR spectra of M′, B3′, B5′, and
B12′ in the region δ 6.5-9 ppm. This region contains those
chemical shifts of hydrogen atoms in benzene ring, pyridinyl
ring, and the central double bond. All of the peaks are doublets,
and each peak is assigned to the corresponding hydrogen atoms
in molecule, as shown in Table 1. More details of all of the
Dyes’ chemical shifts and the coupling constants are also listed
in Table 1. The most important common feature of the NMR
data is that the coupling constants of c and d are all around 16
Hz. This value indicates that all dyes are trans-isomers.
Moreover, the spectra of B3, B5, and B12 (or B3′, B5′, and
B12′) are almost the same in the region δ 6.5-9 ppm. The
spectra of dimers is quite similar to the spectrum of monomers,
with the exception that the chemical shift of a in a monomer is
obviously less than that found in dimers. This difference is surely
caused by those different substituent linked to N atom of
pyridinyl ring because the methyl group in M has a weaker
electron-pushing induction effect than that of the longer alkyl
chains in dimers. Furthermore, the induction effect is not valid
II. Results and Discussions
1. Molecular Design and Synthetic Procedures. The
structures of molecules of monomers and dimers, as well as
their schematic synthetic procedures are listed in Figure 1.
Details of the synthetic procedure and structure identification
can be found in the Experimental Section. M is a common
stilbazolium molecule (monomer), which has been investigated
thoroughly in nonlinear optical and molecular electrical fields.
In this article, M is used as a reference material to evaluate the
properties of other new materials. Bn (n ) 3, 5, and 12)
Figure 2. 1H NMR spectra of M′, B3′ B5′ and B12′ in DMSO.

10022 J. Phys. Chem. B, Vol. 106, No. 39, 2002
Huang et al.
Figure 3. Surface pressure (π)-area per molecule (A) isotherms of
dyes. (a) B3, (b) B5, (c) B12, and (d) M.
Figure 4. Possible conformation of B3 (a) and B5 (b) molecules in
LB films.
TABLE 1: NMR Chemical Shifts and Coupling Constants
of Dyes
TABLE 2: Molecular Limiting Areas and Collapse
Pressures of Stilbazolium Dyes
M
B3
B5
B12
1.20
molecular limiting
area (nm2)
collapse pressure
(mN/m)
0.40
0.90
0.80
46
60
60
55
chemical shift (δ, ppm) [coupling constant (J, Hz)]
(hydrophobic parts) point into the air, whereas the most
hydrophilic sites are partly dip into the water. The substrates
for LB film deposition are freshly pretreated by a Piranha
solution (for quartz) or sodium methate (for ITO), which
produces very good hydrophilic surfaces. Thus, the LB films
should have orientations similar to the floating Langmuir films.
Table 2 provides the values of the molecular limiting area of
these four molecules. It is worth noting that the molecular
limiting area of B3 is even larger than that of B5. We assume
that this occurs because the B5 molecule can form a folded
conformation whose two chromophores in one molecule are
nearly parallel, whereas B3 cannot. Under the balance of the
different forces (compression force, the hydrophobic-hydro-
phobic interaction between the long alkyl chains in LB films
and the static electric repulsion between the pyridium moieties),
the B5 is tightly stacked, and the area it occupies is exactly
twice that of M, indicating the package structure of stilbazolium
chromophores in the LB film of B5 is quite similar to that in
the LB film of M. Although a three-methylene-chain is usually
suitable for linking two parallel chromophores,29 B3 is an
exception. The static electric repulsion between the partially
localized positive charges on pyridium moieties of the two
chromophores in B3 separates them, whereas the three-meth-
ylene chain is not long enough for these two charged moieties
to be parallel. Hence, Figure 4 can be used to illustrate our
hypothesis. In the LB film, the B3 molecule has a “V-shape”
conformation and this causes a larger occupation area compared
with B5 which is a “U-shape” folded molecule. Because the
linkage between the two chromophores in B12 is too long (12
methylene groups) and its conformation can hardly be demon-
strated, its image is not shown here. But it is obvious that B12
occupies more space than others because the twelve-methylene-
linkage is closed to hydrophilic sites and occupies some space
when the film is deposited onto a hydrophilic surface.
DMSO
M′
B3′
B5′
B12′
a
b
c
d
e
f
8.70 [6.84] 8.85 [6.82] 8.82 [6.80] 8.82 [6.90]
8.06 [6.88] 8.12 [6.92] 8.09 [6.69] 8.09 [6.91]
7.92 [16.12] 7.98 [16.02] 7.97 [16.06] 7.96 [16.03]
7.18 [16.12] 7.22 [16.04] 7.19 [16.07] 7.21 [16.08]
7.60 [8.88] 7.60 [8.92] 7.58 [8.94] 7.61 [8.96]
6.79 [8.92] 6.78 [8.97] 6.77 [8.90] 6.79 [9.02]
CDCl3
M
B3
B5
B12
a
b
c
d
e
f
8.74 [6.92] 9.34 [6.61] 9.49 [6.72] 9.08 [6.45]
7.82 [6.96] 7.69 [6.74] 7.88 [6.76] 7.89 [6.48]
7.60 [15.92] 7.54 [15.85] 7.65 [15.86] 7.64 [15.86]
6.85 [15.88] 6.80 [15.82] 6.91 [15.84] 6.87 [15.85]
7.53 [8.96] 7.49 [8.95] 7.58 [8.89] 7.52 [8.84]
6.66 [8.96] 6.67 [9.00] 6.75 [8.59] 6.66 [8.50]
beyond 4 chemical bonds, so that there is almost no difference
between the electron-pushing abilities of those alkyl linkages
in dimers. For this reason, the NMR spectra of dimers are
extreme similar. This similarity indicates that the dimerization
has not influence the structure of stilbazolium chromophore.
2. Structures of Langmuir-Blodgett Films and Their
Second Harmonic Generation. The Langmuir-Blodgett film
formation and its structure are of considerable importance for
the final functional performance because our SHG signal will
closely relate to the film structure, and our evaluating device is
based on an LB monolayer modified ITO electrode.
The surface pressure (π) - molecular area (A) isotherms of
the long-tail dyes are shown as Figure 3. It is quite clear that
all of these amphiphilic molecules are capable of forming stable
and rigid Langmuir films at air/water interface. Also, the
collapse pressures of these four molecules (B3, B5, B12, and
M) indicate that the dimerization can improve the stability of
the Langmuir monolayer. There is no doubt that the orientation
of the molecules would be like this: the long alkyl tails

Mono- and Dichromophoric Hemicyanine Dyes I
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10023
TABLE 3: Results of SHG of Dyes’ LB Films
M
B3
B5
B12
ø
ø
zzz (pm V-1
zxx (pm V-1
φ (deg)
)
)
216
33
29
148
60
42
207
40
32
173
19
25
The LB monolayers’ SHG signals support our hypothesis very
well. The measurement is taken by means of a standard LB
film method,30 and the results are shown in Table 3. The value
of can be calculated by effective nonlinear coefficient, and the
latter is measured by comparing the SHG signal of the sample
with the Y-cut quartz. It is found that the SHG signal of B5 is
as strong as that of monomer, and the fact that øzzz values from
B3, B12 LB films are also comparable with M indicates that
the molecular orientation in the LB films of dimers should be
a folded shape and noncenter-symmetric. Ashwell et al.31 has
reported an example of the SHG from the LB films of center-
symmetric squaraine compounds to form non symmetric struc-
ture in LB films through the interaction between molecules.
Here, our results provide another example that these dimer
molecules, which can form nonsymmetric conformations in LB
films through intramolecular folding, to give strong SHG signals.
Furthermore, these dimers should provide higher monolayer
stability than monomer.
Figure 5. Crystal structure of the cationic fragment of B5′.
of the samples are too ephemeral under the exposure of X-rays,
the structure determination by X-ray diffraction becomes very
difficult. Finally, there is enough data collected from one crystal
sample. Although the data set is not extensive enough for a
detailed evaluation of those anisotropic displacement coefficients
for all atoms, the framework of the molecule is clear, as shown
in Figure 5: even when there is no compressive force, B5′ tends
to form a folded-shape conformation and the two chromophores
are nearly parallel.
The results of molecular mechanical calculations also support
our assumption of molecular structures in LB films. All of the
structures used in this computational section are short-tail
derivatives, to reduce the difficulty in computation. Conforma-
tion searching (cvff95 field force) reveals that the most favorable
conformation of B3 is an “extended” one (Figure 6(a)); whereas
B5, a “folded” conformation (Figure 6(b)), which is very similar
to the result of the crystal structure. This difference is consistent
with the discussion on the limiting area of occupancy of these
molecules and tilt angles get from SHG results. Obviously, even
though the π-π stacking might exist in the dimers between
the two π-conjugated chromophores, the static electric repulsion
between the two positive charged chromophores of B3 molecule
induces their nonparallelity.
The average tilt angle φ is the angle between the surface
normal and the dipole of the chromophore; it can be calculated
from eq 134-36
p-p 1/2
2ω
-1/2
I
3
2
tan φ )
-
(1)
s-p
(
)
[
]
I2ω
where I stands for the intensity of output light beam, subscript
2ω represents the second harmonic generation output, and
superscript p-p, and s-p represent the relationship between
the polarities of input and output light beams. Here, we find
that the average tilt angles of the dipole of M, B5, and B12 are
all about 30°, whereas the tilt angle of the dipole of B3 is about
40°, indicating that compared with the LB films of other dyes
the chromophores in the B3 LB film are less perpendicular to
the substrate surface. It is evident that if all of the dipoles in
one LB film are nearly parallel (tilted to the same direction),
the LB film will show strong optical nonlinear behavior; whereas
if the directions of the dipoles are not parallel, the chromo-
phore’s optical nonlinear ability cannot be summed up together
effectively. The øzzz values in Table 3 give us the clear idea
that in B3 LB film the parallelity of chromophores is not as
strong as that in M or B5 LB films, because the øzzz value of
B3 LB film is much smaller than that of M or B5 LB films. As
Figure 4 shows, the results of tilt angle and of the øzzz value
both strongly support our interpretation of the structure of LB
films. As for the B12 LB film, that the optical nonlinear signal
is smaller than M or B5 is readily explained by the less-ordered
film structure, which is caused by the long linkage moiety.
One may ask whether the five-methylene-linkage in B5
molecule is long enough for the two positive charged chro-
mophores to be parallel. The single-crystal structure of B5′
supports our point of view that B5 can form a “U-shape”
conformation to make the two chromophores nearly parallel,
even though the attempts to get single crystals of B3, B5, B12,
and B3′, B12′ have been finally failed. The only difference
between B5′ and B5 is that the long alkyl tail in B5 molecule
is substituted by methyl. The good single crystals of B5′ with
dimensions ca. 0.6 × 0.4 × 0.3 mm3 was obtained from the
mixture solvent of water and ethanol. However, because most
There is another possible reason for the difference between
the dimers and the monomer: the intrinsic values of dimers’
chromophores might be changed by intramolecular coupling due
to dimerization. According to the successful model drawn by
Oudar et al.37,38 that describes the organic chromophores’s NLO
activity
3e2
ωgefge∆µge
2p2 (ωg2e - ω2)(ω2ge - 4ω2)
â )
(2)
where ω is the irradiation frequency, ωge is the frequency
between the ground (g) and excited (e) states; f is the oscillation
strength; and ∆µge is the change of the dipole moment before
and after the light irradiation. The same chromophores should
have the same changes in dipole moments (∆µ) under light
irradiation if the electronic states have not been modified by
outer fields. In our system, if the intramolecular coupling is
existed in the dimer molecules, it will surely affect the electronic
structure of the energy states of chromophores and then influence
the NLO abilities of molecules and also the other photoactive
properties, such as absorption and emission. However, as we
will point out later, the absorption spectra indicate a very light
intramolecular coupling of dimers; therefore, we take structure-
effect as the major factor to influence the molecular/chromo-
phore’s hyperpolarizability.
In summary, the dimers hold the folded conformations in LB
films and the chromophore orientations of dimers are similar

10024 J. Phys. Chem. B, Vol. 106, No. 39, 2002
Huang et al.
Figure 6. Most stable conformation of B3 (a) and B5 (b), and the
model conformation of a folded B3, named “B3f” (c).
to that of M. But evidently, the structures of the dyes’ LB films
are different from each other, and this difference might bring
changes to the photophysical properties of molecules.
Figure 7. Absorption spectra of dyes. (a) chloroform solutions; (b)
3. Steady-State Spectroscopic Properties and Photocurrent
Generation. Absorption spectra always reflect molecular in-
teractions, especially those between two or more chromophores.
If there are molecular aggregates in the system, then the
absorption spectroscopic method is almost the most powerful
technique for identifying them. The absorption spectra of dyes
are shown in Figure 7: (a) is for the spectra of chloroform
solutions while (b) for 1-layer LB films. Evidently, there is no
dramatic difference between the solution spectra of the dyes.
The shift between the dimers and the monomer is very slight
in major peaks around 480 nm. We cannot observe the
absorption shift when the concentration of the solution changes.
That is to say, in solution, the aggregation can be ignored. Our
result of monomer in LB film shows that the aggregation can
also be ignored under our experimental conditions. Song et al.6
have investigated the aggregation of hemicyanine molecule, and
their results show that for some derivatives the formation of
aggregates is very slow at the air/water interface. As we know,
the aggregation process of hemicyanine derivatives can be
dramatically influenced by chemical modification. If the lang-
muir films are deposited onto the substrates once the surface
pressures reach our pre-set values, the LB films will be
nonaggregate dominant. For the monomer molecule in our work,
prolonging the compressing procedure will increase the propor-
tion of aggregate in LB films; meanwhile, the dimer molecules
show the same behavior. These phenomena are similar to the
one-layer LB films on quartz substrates.
reports of Bohn et al.9,39 Because it is difficult to precisely
control the formation of a certain amount of aggregates of the
LB films, we have not investigated the details of those dimers’
photocurrent generation and related photochemistry. Instead,
what we are interested is the monomer-like absorption band in
the visible-light region, not the aggregates’ absorption band in
the UV region. Under the same experimental conditions in our
work, the films of different molecules are all controlled to be
nonaggregate dominant, and the photofunctions are all from the
monomers/isolated molecules. Another supportive evidence is
that the SHG signal intensities of the dimer molecules are
comparable with that of the monomer and all signals are from
the nonaggregates because the aggregate will strongly reduce
the SHG signal intensity.40-42
From Figure 7(b), we see only a minor difference between
the dimers and monomer: all dimers have a weak absorption
peak around 330 nm. This peak should be due to the aggrega-
tion.9 According to the exciton theory, this blue-shifted absorp-
tion is contributed by H-type aggregates, in which the dipoles
are parallel. But we have known the structure of those dyes’
LB films and that the area per chromophore is almost equal for
M, B3 and B5. Then why M has no absorption around 330 nm
under the same film deposit condition? We assume that the

Mono- and Dichromophoric Hemicyanine Dyes I
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10025
TABLE 4: Results of RB3LYP/6-31G(d)//RHF/6-31G(d)
Calculation of Dyes
energy difference (eV)
B3
B5
B3f
(LUMO+1)-(LUMO)
(HOMO)-(HOMO-1)
0.093
0.039
0.032
0.014
0.394
0.160
TABLE 5: Photocurrent Generation of LB Films Modified
ITO Electrodes
M
B3
B5
B12
photocurrent (µA)a
quantum yield (%)
0.67
0.31
1.20
0.63
0.84
0.47
0.85
0.31
a The photocurrent is measured under white light (240 mW/cm2)
irradiation, IR is filtered.
dipole-dipole exciton interaction”, so we calculate (B3LYP/
6-31G(d)//HF/6-31G(d)) the electronic structures of M, B3 and
B5, using their molecular mechanical optimized conformations
to find evidence for the dipole-dipole interaction. To investigate
the electronic structure of the dimer when it is highly com-
pressed in an LB film, we constructed a model by altering the
dihedral angles of the methylene chain of B3 to a “U-shape”
folded conformation, namely B3f (Figure 6(c)). Evidently, it is
not stable in free conditions, but it may simulate the situation
of some dimers in LB films, that is, two chromophores become
much closer than those found in ordinary conditions.
It is found that the orbital numbers in a dimers are twice
that of a monomer, but the orbitals are not degenerate. Both
B5 and B3 have two close HOMOs and two close LUMOs,
i.e., there are energy difference between HOMO and HOMO-1
and between LUMO and LUMO+1 (Table 4). However, the
HOMO and HOMO-1 have the same properties and they come
from the same orbital of each chromophore in a dimer molecule.
This is exactly caused by the dipole-dipole interaction,32,33,43,44
and it is an intramolecular interaction. But the energy gap
between the splits in the B3 or B5 is so small that in chloroform
we cannot find any evidence of the absorption shift in their
spectra, indicating the weak coupling between the two chro-
mophores in B3 or B5. As for the B3f conformation, it is a
model that, to a certain degree, simulates the dimers in LB films;
in that case, some molecules are compressed to have two
extremely close chromophores. The calculation shows that both
the energy gaps between HOMO and HOMO-1 and between
LUMO and LUMO+1 are increased. Hence, a shifting absorp-
tion peak (∼330 nm) is detected in experiment. Also, the weak
intensity of the absorption suggests that most of the molecules
in LB films are not compressed as much as B3f, otherwise the
absorption spectra of dimers would be extremely different from
that of monomer and there would be some excimer-like
fluorescence for the dimers’ LB films. All of these electronic
structure calculations indicate our previous assumptions of the
LB films structure and of the absorption spectra are credible.
Figure 8. Fluorescence spectra of dyes. (a) chloroform solutions; (b)
one-layer LB films on quartz substrates.
aggregate-like absorption peak is not the result of the real
molecular aggregates but of the intramolecular interaction
between the two chromophores in some dimers. There might
be a very weak interaction between the two chromophores in
one dimer molecule. This weak interaction might also be
partially due to the exclusive volume effect induced by the
linkage group in dimer species. However, the exclusive volume
effect is not the most important factor because there are some
reports21,22 on the intramolecular interaction of dimers whose
linkage groups are similar to those of our system.
Another possibility for the difference between dimers and
monomer is that the dimers might form excimers after excitation
because their chromophores are close to each other. However,
the results of fluorescence spectra, of both solution samples and
LB films, show no evidence to support the occurrence of
excimer fluorescence radiation in the longer-wave range (Figure
8). Usually, the excimer is formed in the dimers, which are
linked by three-methylene groups,29 the distance (∼3 Å) between
those chromophores is very suitable for excimer formation. But
as we know, B3 can hardly form a conformation in which there
are parallel chromophores, so that the chromophores are not
close enough to form a stable excimer; and the two chromo-
phores in B5, although they are parallel, are also too far away
(∼6 Å) from each other to form the excimer.
A typical three-electrode electrochemical cell is used to
measure the photocurrent generated from the LB-monolayer-
film-modified ITO electrode. Because it is impossible to make
any two LB film samples identical, at least eight individually
prepared samples were employed in each experiment in order
to get the mean value of the results. The mean photocurrent
values are listed in Table 5. The external photoelectric conver-
sion quantum yield (QY) is the criterion for the energy
conversion potentiality, and it is calculated using eq 3
iph
We have suggested that the slight absorption peak around
330 nm in dimers’ LB films are due to the “intramolecular
QY )
(3)
eNp(1 - Aλ)

10026 J. Phys. Chem. B, Vol. 106, No. 39, 2002
Huang et al.
Figure 9. Typical photocurrent generated from B3 one-layer LB films
modified ITO electrode. “ON” and “OFF” indicate the irradiation
situations.
Figure 10. Action spectra of dyes. (solid square: M; open circle: B3;
solid circle: B5; open square: B12)
Here iph is the photocurrent generated from the working
electrode when it is irradiated by the light with wavelength λ.
Np is the number of irradiating photons per second, and Aλ is
the absorption percentage at wavelength λ.
From Table 5, we learn that the QY of B3 and B5 are larger
than that of M. Because the mean occupation area per chro-
mophore of M, B3, and B5 are 40, 45, and 40 nm2, respectively,
the dimerization will not influence the chromophore density of
these LB films much. Therefore, the improvements of the
quantum yields of B3 and B5 are not due to an increase in the
number of the photoactive chromophores. If the photoelectric
conversion efficiency of each chromophore is independent of
the molecular structure or LB film structure, then the M, B3,
and B5 LB films should have shown similar photocurrent
generation and almost the same quantum yields. However, the
remarkable difference between B3 (or B5) and M suggests that
there must be some relationship between the photocurrent
generation and the structure of the molecules. We sum up this
phenomenon in a simple inequality “1 + 1 > 2” because it
seems that when a stilbazolium chromophore is linked with
another one by suitable methylene groups, the phototcurrent
generation is greater than twice that of the monomer.
At first, we changed some experimental conditions to find
the mechanism of the current generation. Under normal condi-
tions (the electrolyte solution is saturated with air and there is
no additional electron donor or acceptor), the photocurrent is a
cathodic one, indicating that the electron is ejected from the
ITO electrode and flows through LB film and then transfers to
the electrolyte. Figure 9 is a typical photocurrent response curve
with the irradiation of white light from a xenon lamp; the current
response is quite prompt.
The photocurrent action spectra of the M, B3, B5, and B12
are shown in Figure 10. Although the points, which are
measured with a set of interference filters, are not continuous,
it is evident that the shapes of these spectra coincide with
corresponding LB films’ absorption spectra (as Figure 7(b)
shows, this absorption is not caused by aggregates but by
individual molecules, see discussion before). This coincidence
indicates that the photocurrent is generated from those molecules
that are deposited on substrates, and it comes from the isolated
molecules. If the intensity of irradiation light is increased, the
photocurrent increases too.
Figure 11. Charge distribution calculation results of M. (a) planar
conformation (optimized conformation for ground state); (b) twisted
conformation (optimized conformation for excited state).
redistribution makes the excited state become an active site for
electron exchange between the electrolyte solution and electrode.
Here, we provide some concise results of our semiempirical
calculations to accurately understand this view of point.
The charge distribution (MOPAC 6, PM3 Hamilton) was
calculated with the result that the chromophore in dimers shows
no difference from the monomer. Figure 11 shows the chromo-
phore’s charge distribution (PM3) and the effect of conformation
change. Before excitation, the pyridinium moiety has the most
positive charge (0.817) before excitation, and this causes strong
static electric repulsion, which in turn separates the two
chromophores in B3.
Calculations on the excited state of stilbazolium chromophore
are also shown in Figure 11. The Franck-Condon (FC) state
shows that the charge distribution is more uniform under light
irradiation. The pyridinium moiety, the charge on which is more
positive in the ground state, shares its positive charge with
aminobenzene moiety. Even the polar direction is reversed: the
aminobenzene moiety is more positively charged than the
We have published several papers on the PEC of similar
derivatives before, and a key step in the mechanism of
photocurrent generation is such that the chromophore is excited
by light irradiation and produces an excited state with charge
redistribution. It is well-known that chromophores like stilba-
zolium will form “charge-reversed” excited states. This charge

Mono- and Dichromophoric Hemicyanine Dyes I
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10027
Figure 13. Effluence of bias on photocurrent generation from the dye-
LB-film-modified ITO electrode. White light (110 mW/cm2), 0.5 mol
dm-3 KCl, 298 K. (solid circle: M; open circle: B3; solid square: B5;
open square: B12)
Figure 12. Effluence of light intensity to the photocurrent generation
from the dye LB film modified ITO electrode. Zero bias, 0.5 mol dm-3
KCl, 298 K. (solid circle, M; open circle, B3; solid square, B5; open
square, B12)
Therefore, the investigation of such an effect helps us to better
understand the photocurrent generation process. The donor is
usually a compound with a low oxidation potential, so that it
can easily lose an electron to the electrode; however, the
compound with low reduction potential is a natural acceptor
with strong electron-capturing ability.
Considering the solubility in electrolyte solution, we select
hydroquinone (H2Q) and methyl viologen diiodide (MVI2) as
the electron donor and acceptor, respectively.24 The change in
the photocurrent intensity caused by the electron donor/acceptor
is significant. Because the four dyes show almost the same
behaviors upon the introduction of donor/acceptor, only the
details about B3 are listed here. The introduction of hydro-
quinone and methyl viologen diiodide leads to two opposite
results. The irradiation light is white light (240mW/cm2) radiated
from a Xenon lamp, the infrared light has been filtered out for
protecting the electrode from heating and ruling out the thermo-
effect. Before the addition of the donor/acceptor, the photocur-
rent is cathodic and stable, with a value of 1.25 ( 0.05 µA.
pyridinium does. The dipole moment of ground state is also
greater than in the excited state, which is consistent with our
spectroscopic results (the negative solvatochromic effect).
However, the FC state is not a stable conformation for the
excited state and the semiempirical quantum mechanical opti-
mization of stilbazolium’s excited state gives a twisted state as
shown in Figure 11(b). From the value of the charge distribution
of this conformation, we can find that the twisted conformation
not only stabilizes the excited state but also enhances the charge
reversion of excited state, and of course, the twisted state has
a greater dipole moment than the FC state for the enhanced
charge separation.
Figure 12 shows the relationships between the photocurrent
generation and irradiation light intensity of the dyes. Each curve
is fitted well by a simple exponential eq 4 with suitable value
of k-factor
i ) kI0.5
(4)
I and i represent, respectively, the intensity of irradiation light
and the corresponding photocurrent. The relationship between
the photocurrent generation and the irradiation light intensity
suggests the fundamental character of the photoinduced electron
transfer and recombination.36 The power exponent (0.5) is the
same for all the four dyes, indicating that the pathways of their
photocurrent generation and recombination are similar, and that
the photoactive processes of chromophores in dimers are quite
similar to those in the monomer, that is to say, the dimerization
makes no difference to the photocurrent generation mechanism.
The same mechanism can be used in both monomer and dimer.
It is well-known that a bias voltage can change the photo-
current intensity in this photoelectric conversion system, and
that a negative bias benefits the cathodic photocurrent. It is found
that all the four dyes here follow this rule very well (Figure
13). This suggests once again that the electron is injected into
the dyes from ITO electrode because the electric field caused
by negative bias has the same polarity with the interfield at the
ITO/LB surface. A negative bias accelerates the charge separa-
tion, assists the generation of the electron-hole pairs, reduces
charge trapping during the transmission, and restrains the charge
recombination. If the bias is positive, then the effects are
reversed, and the photocurrent is decreased.
Hydroquinone has an oxidation potential of -4.61 eV
(-0.13V vs SCE) and, thus, easily gives away an electron to
the electrode. It is found that the cathodic photocurrent is
decreased when H2Q is gradually added into the electrolyte
solution. And when the concentration of H2Q is greater than
1.25 × 10-4 mol dm-3, the photocurrent generated from
electrode modified by B3 LB film is turned positive. Then the
anodic photocurrent increases with the addition of H2Q. When
H2Q’s concentration achieves 2 × 10-3 mol dm-3, the positive
photocurrent reaches its maximum (∼1.0 µA).
Although methyl viologen cation (MV2+) has its first reduc-
tion potential of -4.51 eV (-0.23 V vs SCE), it can capture
the electron from the electrode effectively. There is no doubt
that the existence of electron acceptor in the electrolyte solution
accelerates the electron movement from electrode to electrolyte.
Experimental results show an expected tendency of the photo-
current with the addition of MV2+ into electrolyte solution, and
the cathodic photocurrent reaches its maximum (ca. 2.5 µA)
when the concentration of MV2+ is greater than 1 × 10-3 mol
dm-3
.
It is well-known that oxygen, which dissolved in electrolyte,
is also a good electron acceptor in electrochemistry experiments,
and that bubbling nitrogen gas into the electrolyte solution can
drive oxygen out. Our experiments reveal that the oxygen also
Usually, the photocurrent generation is strongly influenced
by electron donors or acceptors in the electrolyte solution.

10028 J. Phys. Chem. B, Vol. 106, No. 39, 2002
Huang et al.
finally arrives at the electrode’s conduction band. This forms
the anodic current.
From those detailed analyses of the photocurrent generation
process and the spectroscopic properties, we can make a
conclusion that the dimerization enhances the PEC efficiency
when the linkage is suitable. However, the reason behind why
the proper linkage benefits the PEC properties is still unclear.
According to our assumption of the structure of LB films, the
motility (vibration, twisting, etc.) of dimers will be much more
difficult than that of the monomer. Although this difference has
no effect on steady-state spectroscopic properties, it will
probably influence the dynamical properties of molecules, both
in ground state and in excited states. We do not have a definitive
conclusion of the fundamental concepts behind the relationship
between the dimers and the monomer, but there is a supposition
in our minds that the difference is not the results of aggregation
or excimer formation, and the energy transfer procedure might
plays an important role here. Further photophysical experiments
are now processing to explore the more convincing evidences
to support our supposition.
Figure 14. Effluence of the oxygen and nitrogen to the photocurrent
generation from a B3-LB-film-modified ITO electrode. Zero bias, white
light (240 mW/cm2), 0.5 mol dm-3 KCl, 298 K.
III. Conclusions
(1) A series of stilbazolium dimers, in which the two
chromophores are linked via variable methylene groups, has
been designed and synthesized. Although the chemical structures
of these dimers are center-symmetric (or quasi-center-sym-
metric), they can be folded in their LB films to form a noncenter-
symmetric orientation monolayer, which give the comparable
SHG signals with the monomer. The surface pressure-area
isotherms indicate the higher stability contributed from the
dimerization. This is a new perspective that one may use external
force to make a molecule to change its conformation to fit one’s
needs.
(2) B3 and B5 have higher photoelectric conversion quantum
yields than M. Both steady-state absorption and fluorescence
experiments indicate that no strong intramolecular interaction
exists in this system, and some computational results also imply
that the common intramolecular dipole-dipole interaction
between the two chromophores in dimer molecules seldom occur
in these stilbazolium dimers. This is due to the static repulsion
between the two closer positive charged sites of the molecule.
Thus, we suppose that the enhanced photocurrent generation
of dimerization probably comes from the structural effect caused
by dimerization, which might influence the energy transfer
procedure.
(3) According to absorption spectra, SHG signals and
computational results, the three-methylene linkage is not long
enough for two stilbazolium chromophores to take parallel
orientation. And in the LB films, B5 has a U-shape, whereas
B3 has a V-shape conformation. The crystal diffraction result
of B5 also partially supports this conclusion.
Figure 15. Schematic mechanism of photocurrent generation.
influence the photocurrent generation process. Figure 14 shows
the relationship between the photocurrent and the bubbling time
of the gas. Obviously, nitrogen restrains the cathodic photo-
current and reduces the value to around 600 nA. Then, if oxygen
is bubbled into the nitrogen saturated electrolyte solution, the
cathodic photocurrent is reconverted to 1200 nA.
In this way, we can propose the mechanism of the photo-
current generation as shown in Figure 15.24,44-46 In this figure,
(a) represents the case of the cathodic photocurrent, whereas
(b) represents the case of the anodic photocurrent. The energy
level of the dye’s ground state (-5.42 eV) is calculated from
M’s first oxidation potential (0.68V vs SCE, and dimers show
electrochemical inertia); the energy level of dye’s excited state
(∼-3 eV) is estimated by absorption spectrum. The energy
levels of the ITO electrode’s conduction and valence bands are
around -4.5 eV and -8.3 eV, respectively. Thus, a continuous
cathodic photocurrent is generated. When there are electron
acceptor molecules (MV2+ or O2, etc.) in the electrolyte solution,
they can obtain electrons from the dyes under irradiation. And
the electrode donates relevant electrons from its conduction band
to those dye molecules. When there are electron donor molecules
(H2Q) in the electrolyte solution, they will replace the electrode’s
conduction band to donate electrons to dyes, and the electron
IV. Experimental Section
1. Synthesis. N-methyl,N-octadecylaniline. 9.88 g (0.092 mol,
10 mL) N-methylaniline reacted with 35.06 g (0.092 mol)
octadecyl iodide in a mixture of 30 mL toluene and 50 mL 30%
sodium hydroxide aqueous solution containing 0.19 g N-
hexadecyl,N,N,N-trimethylammonium bromide (phase transfer
catalyst) at 80 °C under stirring for at least 7 days. The toluene
phase was washed with sodium chloride solution and then
evaporated. The residue was purified on a silica gel column
with pure petroleum ether and then with petroleum ether-ethyl
acetate mixture as an eluent. Yield: 90%.

Mono- and Dichromophoric Hemicyanine Dyes I
J. Phys. Chem. B, Vol. 106, No. 39, 2002 10029
N-methyl,N-octadecylaminobenzaldehyde. 14.36 g (0.04 mol)
N-methyl,N-octadecylaniline was added to 70 mL DMF (freshly
distilled). After the mixture was cooled to 4 °C, 5 mL POCl3
was added dropwise to the mixture. This resulting mixture was
kept stirred at room temperature for 1 h and then at 100 °C for
4 h. The reactant was purified on silica gel column after being
neutralized to pH 7 with sodium hydroxide solution. 12 g target
product was obtained. Yield: 77.5%, Mp: 51 °C.
1,n-Bis [4-methylpyridinyl]alkane(propane, pentane, dodec-
ane) dibromide. 0.12 mol of 4-methylpyridine and 0.04 mol of
1,n-dibromoalkane (n ) 3, propane; n ) 5, pentane; n ) 12,
dodecane) were added to 70 mL anhydrous ethanol. The mixture
was refluxed for 7 d. The product was obtained after evaporation
of solvent and then recrystallized in a mixture solvent of ethanol
and ether. Yield: sustituted propane, 82%; substituted pentane,
85%; substituted dodecane, 87%. Mp: substituted propane, dec
∼215 °C; substituted pentane, 218∼220 °C (dec partly at ∼210
°C); substituted dodecane, 117 °C.
1,n-Bis [(E)- 4-(2-(4-(N-methyl, N-octadecylamino) phenyl)
ethenyl) pyridinyl]-alkane dibromide (n ) 3, alkane ) propane,
B3; n ) 5, alkane ) pentane, B5; n ) 12, alkane ) dodecane,
B12). The mixture of 1,n-Bis [4-methylpyridinyl]alkane dibro-
mide and N-methyl, N-octadecylbenzaldehyde (molar ratio 1:3)
was dissolved in anhydrous ethanol containing piperidine as
catalyst and was kept refluxing for 4 h. After the evaporation
of the solvent, the residue was purified on a silica gel column.
[(E)-N-methyl-4-(2-(4-(N-methyl, N-octadecylamino) phenyl)
ethenyl) pyridinium] iodide (M). The mixture of 193.5 mg (0.5
mmol) N-octadecyl,N-methylamino- benzaldehyde, 117.5 mg
(0.5 mmol) 1,4 dimethylpyridinium Iodide, 0.2 mL piperidine
and 20 mL ethanol was refluxed for 6 h. The resulting system
was cooled, and the precipitate was collected. Recrystallization
was done twice with ethanol.
1, n-Bis [(E)-4-(2-(4-(N,N-dimethyl)phenyl)ethenyl)pyridinyl]-
alkane dibromide (n ) 3, alkane ) propane, B3′; n ) 5, alkane
) pentane, B5′; n ) 12, alkane ) dodecane, B12′). The
synthetic procedures are similar to the BC3, BC5, and BC12.
B3′, B5′, B12′, and M′ were synthesized for improving the
solubility in some high polar slovents.
2. Spectroscopic Measurement. The UV-vis absorption
spectra of B3, B5, B12, and M in solutions and LB films on
quartz slides were recorded by a Shimadzu UV-3100 Spectro-
photometer, with blank chloroform (A.R grade) or a quartz slide
as a reference, respectively. Static-state fluorescence excitation
and emission spectra were measured by a Hitachi F4500
fluorescence spectrophotometer, using proper excitation cutoff
filters. Excitation and emission bandwidths of 5 nm were used,
and the fluorescence spectra were corrected for nonlinear
instrumental response and blank solvents.
3. Surface Pressure-Area Isotherm and Film Deposition.
A computer-controlled 622 Nima Langmuir trough (Nima Co.,
England) was used for π-A isotherm measurements and
monolayer deposition. The surface pressure-area per molecule
(π-A) isotherm was obtained by compressing the spreading
sample, which was added dropwise from a micro-injector onto
deionized water (18.0 MΩ cm, pH 5.6) surface, at a barrier
speed of 50 cm2/min. The LB trough was controlled at 20 (
0.5 °C. Monolayers of B3, B5, B12, or M were obtained by
spreading the four corresponding chloroform solutions (0.44 mg
ml-1 for B3, 0.54 mg ml-1 for B5, 0.68 mg ml-1 for B12, 0.54
mg ml-1 for M) onto the water surface. The floating layer was
compressed to a surface pressure of 30 mN/m for each of B3,
B5, B12 or M. The compressed floating layer was kept at this
pressure during the deposition procedure, the monolayer was
transferred at a speed of 5 mm/min onto the hydrophilically
pretreated quartz or transparent ITO glass (∼50 Ω/0) substrates
in upstroke. The transfer ratio were 1.0 ( 0.1.
4. Photoelectrochemical Measurements. The conventional
three-electrode setup with a 0.8 cm2 effective contact area was
used in all irradiation experiments. This experiment used ITO
glass slides modified by the monolayer LB films as working
electrode (WE), a platinum electrode as the counter-electrode
(CE), a SCE as the reference electrode (RE), and a 0.5 mol
dm-3 KCl aqueous solution as supporting electrolyte solution
in photocurrent generation measurements. All measurements
were carried out at 25 ( 1 °C on a Model 600 Voltametric
Analyzer (CH Instruments, USA). The excitation source for the
photochemical experiments was a 500 W Xenon Lamp (Ushio
Electric, Japan). The incident white light was perpendicular to
the contact area of the working electrode. The intensity of the
incident beam was checked by a model LM-91 Photopower
meter (National Institute of Metrology, Beijing, China). Different
wavelengths were obtained by using a series of interference
filters (Toshiba, Japan) with certain band-passes. IR light was
filtered out by a Toshiba IRA-25S filter (Toshiba Co., Japan)
in all experiments to avoid thermal effects. Photocurrents were
the average values of at least six independent measurements.
5. SHG Measurement. The SHG signals of the LB films of
B3, B5, B12, and M were measured in transmission with a Y-cut
quartz plate as a reference (d11 ) 1.2 × 10-9 esu). The intensity
of the incident Q-switch Nd:YAG laser (GCR-4, 1064 nm,
Spectra-Physics) was 140 mW cm-2, pulse duration was 10 ns
and repetition rate was 10 Hz. A 1/2 λ plate and a Glan-Taylor
polarizer were used to vary the polarization direction of the laser
beam. The linearly polarized laser beam was directed at an
incident angle of 45°onto the vertically mounted sample. The
output beam was filtered and monochromated, and then the
signal was recorded by a digital oscilloscope (HP 54510A). The
second harmonic intensities (I2ω) obtained from the monolayer
LB film were analyzed by the general procedure described by
Ashwell.35
6. X-ray Crystal Structure Determination. A dark red color
crystal of B5′ with dimensions 0.60 × 0.40 × 0.30 mm3 was
selected for structure determination. It was mounted on a Rigaku
AFC65 diffractometer with highly oriented graphite crystal
monochromated MoKR radiation. The reflection data were
collected in the range 2θ 4.0∼40.0°. The cell parameters were
calculated by the least-squares method; 2406 independent
reflections were collected, from which 862 reflections with F
> 4.0σ(F) were used in the refinement. Intensities were
corrected. The final R and Rw were 0.095 and 0.076, respec-
tively.
7. Computational Chemistry. A conformation search was
carried out by a SGI O2 workstation using a Cerious2 software
package and cvff95 force field. Semiempirical quantum chemical
calculations were carried out by MOPAC 6.0 or HyperChem5.1
by means of an Intel Pentium workstation. The PRECISE option
was used as a convergence criterion for calculation of geometry
for all MOPAC calculations. DFT calculations were operated
by Gaussian 98 on a Linux based workstation. PM3 hamilton
was used for semiempirical calculations, and for more precise
calculations, such as molecular energy levels, B3LYP/6-31G-
(d)//RHF/6-31G(d) was performed. In semiempirical calcula-
tions, a configuration interaction (CI) related to 10 energy levels
was taken into considered.
Acknowledgment. The authors thank the State Key Project
of Fundamental Research (Grant Nos. G1998061308 and

10030 J. Phys. Chem. B, Vol. 106, No. 39, 2002
Huang et al.
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