Synthesis, characterization, photophysics, and photosensitization studies of squaraine-bipyridinium diads

Article abstract of DOI:10.1021/jp962996z

Two squaraine-bipyridinium diads designed to study electron transfer between the squaraine chromophore and the bipyridinium group have been synthesized. The two diads, denoted as C₄Sq-By and C₁₂Sq-By, consist of amphiphilic squaraines covalently linked to a bipyridinium group by a trimethylene chain. The squaraine chromophores in the diads do not form charge-transfer complexes with the bipyridinium group in the ground state. Both C₄Sq-By and C₁₂Sq-By show fluorescence yields about 100 times weaker than those of alkyl substituted squaraines, which is attributed to quenching by an intramolecular electron transfer from the excited squaraine chromophore to the bipyridinium; the intramolecular electron-transfer rate is estimated to be 4 × 10¹¹ s^-1. Assuming the short lifetime for the undetectable transient produced is due to very fast back electron transfer to the radical cation of squaraine from the reduced bipyridinium ion, the rate of the back intramolecular electron transfer is estimated as >3 ×10¹⁰ s^-1. The two diads synthesized in this work have been used as photocurrent enhancers for the monolayers of DSSQ (4-(distearylamino)phenyl-4′-(dimethylamino)phenylsquaraine). C₄Sq-By as a solution additive is 5 times more effective in sensitizing the cathodic photocurrent generation of the DSSQ monolayer-modified SnO₂ electrode compared to methylviologen. Analogously, C₁₂Sq-By is 3 times more effective in sensitizing the DSSQ monolayer-modified SnO₂ electrode compared to 4-tetradecyl-4′-methylbipyridinium dichloride when it is incorporated upon spreading in the monolayer of DSSQ. The increase in cathodic photocurrent generation efficiency is postulated to result from fast electron transfer from the excited squaraine to the bipyridinium group in the diad. The fast electron transfer facilitates electron separation in the photogeneration step and consequently increases the quantum efficiency of the cathodic photocurrent generation process.

Full text of DOI:10.1021/jp962996z

540
J. Phys. Chem. B 1997, 101, 540-546
Synthesis, Characterization, Photophysics, and Photosensitization Studies of
Squaraine-Bipyridinium Diads
Kangning Liang,† Kock-Yee Law,*,‡ and David G. Whitten*,†
NSF Center for Photoinduced Charge Transfer, Department of Chemistry, UniVersity of Rochester,
Rochester, New York 14627, and Wilson Center for Research and Technology, Xerox Corporation,
800 Phillips Road, 114-39D, Webster, New York 14580
ReceiVed: September 27, 1996; In Final Form: NoVember 19, 1996X
Two squaraine-bipyridinium diads designed to study electron transfer between the squaraine chromophore
and the bipyridinium group have been synthesized. The two diads, denoted as C4Sq-By and C12Sq-By,
consist of amphiphilic squaraines covalently linked to a bipyridinium group by a trimethylene chain. The
squaraine chromophores in the diads do not form charge-transfer complexes with the bipyridinium group in
the ground state. Both C4Sq-By and C12Sq-By show fluorescence yields about 100 times weaker than
those of alkyl substituted squaraines, which is attributed to quenching by an intramolecular electron transfer
from the excited squaraine chromophore to the bipyridinium; the intramolecular electron-transfer rate is
estimated to be 4 × 1011 s-1. Assuming the short lifetime for the undetectable transient produced is due to
very fast back electron transfer to the radical cation of squaraine from the reduced bipyridinium ion, the rate
of the back intramolecular electron transfer is estimated as >3 × 1010 s-1. The two diads synthesized in this
work have been used as photocurrent enhancers for the monolayers of DSSQ (4-(distearylamino)phenyl-4′-
(dimethylamino)phenylsquaraine). C4Sq-By as a solution additive is 5 times more effective in sensitizing
the cathodic photocurrent generation of the DSSQ monolayer-modified SnO2 electrode compared to
methylviologen. Analogously, C12Sq-By is 3 times more effective in sensitizing the DSSQ monolayer-
modified SnO2 electrode compared to 4-tetradecyl-4′-methylbipyridinium dichloride when it is incorporated
upon spreading in the monolayer of DSSQ. The increase in cathodic photocurrent generation efficiency is
postulated to result from fast electron transfer from the excited squaraine to the bipyridinium group in the
diad. The fast electron transfer facilitates electron separation in the photogeneration step and consequently
increases the quantum efficiency of the cathodic photocurrent generation process.
Introduction
the holes in the squaraine aggregates. Addition of electron
acceptors, such as methylviologen, enhances the cathodic
photocurrent generation under both ambient and deaerated
conditions. On the other hand, electron donors, such as
hydroquinone, quench the cathodic photocurrent in the ambient
condition and make the photocurrent anodic when the electrolyte
solution is deaerated.
Squaraines have been the subject of many recent investiga-
tions both because of their unusual electronic structures as well
as their properties as semiconductive and photogeneration
materials in electrophotographic photoreceptors.1 In photo-
receptor applications, they are used as microcrystalline solids
embedded in a polymer matrix. Excitation of the microcrys-
talline photoconductor leads to the formation of excitons, which
formally dissociate into electron-hole pairs and subsequently
are used in electrostatic imagings. Using aggregates formed in
monolayers of 4-(distearylamino)phenyl-4′-(dimethylamino)-
phenylsquaraine (DSSQ), we modeled the photogeneration
Very recently, we3 studied electron transfer between the
squaraine exciton and either an electron acceptor or a donor
within a monolayer by incorporating either an amphiphilic donor
or amphiphilic acceptor into the monolayer of DSSQ. From
the effects of the concentration of the electron acceptor/donor
on the intralayer electron-transfer process, we have been able
to estimate the size of the squaraine exciton in the monolayer.
Although transfer of an electron from the exciton of the
squaraine aggregate to O2 has been postulated in these studies,
the thus-generated radical cation has not been detected in
transient absorption experiments.4 In an attempt to gain further
insight on the electron-transfer process, we synthesized two
squaraine-bipyridinium diads, C4Sq-By and C12Sq-By (struc-
tures shown in Scheme 1), by covalently linking an amphiphilic
squaraine group (with butyl or dodecyl chains) and a bipyri-
diniumyl group with a trimethylene chain. We report here a
study on the aggregation and electrochemical, photophysical,
and photoelectrochemical properties of these two diads. Inter-
esting photosensitization results are obtained when the mono-
layers of stearic acid or DSSQ (on SnO2 electrodes) are studied
photoelectrochemically in the presence of C4Sq-By and C12Sq-
By either in the electrolyte solution or in the monolayer. A
photosensitization mechanism is proposed and discussed.
process of squaraine using solution photoelectrochemical experi-
ments.2 For example, we observed generation of a cathodic
photocurrent when a DSSQ aggregate-modified SnO2 electrode
was irradiated with visible light. The cathodic photocurrent
generation was attributed to an electron transfer from the
squaraine exciton to O2 followed by a subsequent transfer of
an electron from the conduction band of the SnO2 electrode to
† University of Rochester.
‡ Xerox Corporation.
X Abstract published in AdVance ACS Abstracts, January 1, 1997.
S1089-5647(96)02996-3 CCC: $14.00 © 1997 American Chemical Society

Squaraine-Bipyridinium Diads
J. Phys. Chem. B, Vol. 101, No. 4, 1997 541
SCHEME 1
Experimental Section
the 532 nm beam in acetone-d6 was used to obtain the 600 nm
excitation pulse used in this work. The excitation beam diameter
was 2 mm. After the delay line, the fundamental pulse was
focused into a 10 cm cell with quartz windows containing H2O/
D2O (1:1) for white light continuum generation. The white light
was then collimated and focused at one end of a 200 µm fused
silica optical fiber. It was split into two beams of equal intensity
at the other end of the optical fiber by a cube beam splitter
after passing through a polarization descrambler. One of the
beams was used as reference, and the other was used as an
interrogation beam. After the absorption of the transient was
recorded, the two beams were then recollimated and focused at
the end of a bifurcated fiber optic pair that was attached to a
SPEX 270 monochromator. The spectra of the white light
continuum for the sample and reference beams with and without
excitation were detected after passing through the monochro-
mator by a cooled dual diode array detector (Princeton Instru-
ments ST121/DDA512), and the corresponding differential
absorption spectra at the set delay time were calculated,
displayed, and stored.
Materials. N-Methylaniline, 1,3-dibromopropane, N,N-di-
butylaniline, potassium hexafluorophosphate, DMSO-d6, and
1-bromododecane were purchased from Aldrich. Potassium
bicarbonate was from Baker, aluminum trichloride and sodium
nitrate (electrolyte) were from Fisher, aniline was from Mallinck-
rodt, and tributylorthoformate was from Lancaster. All these
materials were used as received. The spreading solvent for the
monolayer work was chloroform (HPLC, pentene stablized) and
was purchased from Fisher. Distilled water was in-house
deionized water purified by passing through a Millipore water
purification system.
General Techniques. FAB-MS analysis of the squaraine
compounds was performed at the Center for Mass Spectrometry
at the University of Nebraska in Lincoln, Nebraska. NMR
spectra were recorded on a GE NMR QE-300 spectrometer.
Fluorescence emission spectra were recorded on a Spex Fluo-
rolog spectrometer, and the spectral response was not corrected.
Monolayers of DSSQ, C12Sq-By, and stearic acid were
obtained by spreading chloroform solutions of the corresponding
compound onto an aqueous subphase in a KSV 5000 film
balance at room temperature (35 °C for DSSQ). Typical transfer
ratios on hydrophilic SnO2 substrates were 1.0 ( 0.1. The
aggregation of the squaraine chromophores in the Langmuir-
Blodgett (LB) films was studied by absorption spectroscopy
using a Hewlett-Packard 8452A diode array spectrophotometer.
The cyclic voltammograms of the squaraine diads were
measured on an electrochemical analyzer, Model BAS 100B,
from Bioanalytical Systems. Acetonitrile was used as solvent,
and tetrabutylammonium perchlorate was used as the supporting
electrolyte. The electrochemical cell consists of three electrodes,
a platinum disk working electrode, a Ag/AgCl (KCl saturated)
reference electrode, and a platinum wire auxiliary electrode.
Photoelectrochemical experiments on various monolayer-modi-
fied SnO2 electrodes were carried out as described elsewhere.2
Transient absorption experiments were performed on an in-
house apparatus using the 1064 nm fundamental pulse (25-35
ps at 10 Hz) of a mode-locked Nd:YAG laser (Continuum
PY61C) as both pump and probe sources. The fundamental
was passed through a computor-controlled variable optical delay
line (Velmex) to achieve the desired delay between the pump
and probe beams. The second harmonic wavelength of the laser
(532 nm) was used as the excitation source. Raman shifting of
Synthesis of 1-(p-Didodecylaminophenyl)-2-hydroxycyclo-
butene-3,4-dione. Aniline (1 g, 10.7 mmol), 1-bromododecane
(10.7 g, 43 mmol), potassium bicarbonate (10 g), iodine (0.18
g), and 100 mL of 1-butanol were added into a 250 mL round-
bottom flask.5 The mixture was brought to reflux for 10 h.
After the mixture was cooled to room temeprature, the solid in
the mixture was removed by filtration and was washed with a
small amount of 1-butanol. The filtrates were combined and
the 1-butanol solvent was removed by evaporation under
vacuum. The product was purified by column chromatography
on silica gel using a mixture of hexane and ether as eluent. A
colorless oil that was identified as N,N-didodecylaniline was
isolated with a yield of 1.7 g (37%). NMR (ppm in CDCl3):
1.02 (t, 6H), 1.42 (m, 36H), 1.72 (m, 4H), 3.37 (t, 4H), 6.77
(m, 3H), and 7.36 (t, 2H).
1-Chloro-2-(p-didodecylaminophenyl)cyclobutene-3,4-di-
one was then synthesized using the above aniline derivative
according to the procedures described by Wendling et al.6 To
a methylene chloride solution containing 0.62 g of squaric acid
dichloride (4.1 mmol, freshly prepared according the procedure
of De Selms and co-workers7) and 0.55 g of AlCl3 (4.1 mmol)
in 50 mL of CH2Cl2, N,N-didodecylaniline (1.7 g, 3.96 mmol)
was introduced dropwise through a dropping funnel. After the

542 J. Phys. Chem. B, Vol. 101, No. 4, 1997
Liang et al.
addition was completed, the mixture was refluxed for 1 h. The
solvent was removed and the product was purified by column
chromatography on silica gel, affording a yellow solid identified
as 1-chloro-2-(p-didodecylaminophenyl)cyclobutene-3,4-dione.
This yellow solid was then dissolved in a mixture of ethanol
and water (1:1). After a small quantity of concentrated
hydrochloric acid was introduced, the mixture was refluxed for
1 h, resulting in a yellow solid suspension. The yellow solid
was isolated by filtration and was washed throughly with water
until the filtrate was neutral. It was then dissolved in chloroform
and the residual water was removed by adding CaCl2 into the
chloroform solution, yielding pure 1-(p-didodecylaminophenyl)-
2-hydroxycyclobutene-3,4-dione 0.9 g (43%) after removing the
chloroform solvent.
Synthesis of 4-N-Methyl-N-(3-bromopropyl)aminophenyl-
4′-(N′,N′-dibutylamino)phenylsquaraine. The N-methyl-N-
(3-bromopropyl)aniline used in the synthesis was synthesized
by condensing N-methylaniline with 1,3-dibromopropane in
1-butanol in the presence of potassium bicarbonate and iodine.5
The yield was 70%. NMR (ppm in CDCl3): 2.21 (m, 2H),
3.03 (s, 3H), 3.55 (m, 4H), 6.80 (m, 3H), and 7.38 (m, 2H).
Figure 1. Absorption spectrum of C4Sq-By in acetonitrile.
(CD3CN): 0.87 (t, 6H), 1.27 (m, 32H), 1.93 (m, 4H), 2.12 (m,
4H), 2.40 (m, 2H), 3.10 (s, 3H), 3.49 (t, 4H), 3.70 (t, 2H), 4.39
(s, 3H0, 4.69 (t, 2H), 6.85 (m, 4H), 8.15 (m, 4H), 8.30 (m,
4H), 8.85 (m, 4H). FAB-MS: 826.6 (M+). Maximum absorp-
tion: 636 nm (3.1 × 105 M-1 cm-1).
4-N-Methyl-N-(3-bromopropyl)aminophenyl-4′-(N′,N′-di-
butylamino)phenylsquaraine was prepared by condensing 1-(di-
butylamino)phenyl-2-hydroxycyclobutene-3,4-dione (0.48 g, 1.6
mmol, synthesized according to the procedure of Chen et al.8)
with N-methyl-N-(3-bromopropyl)aniline (0.34 g, 1.48 mmol)
in the presence of tributylorthoformate (1 mL) in about 50 mL
of 1-butanol at reflux for 2 h.9 The progress of the synthesis
was monitored by absorption spectroscopy. The product
mixture was cooled in a refrigerator overnight. The solid
product was isolated by filtration and was purified by washing
first with 1-butanol and then ether. The yield was 0.28 g (37%).
NMR (CDCl3): 1.01 (t, 6H), 1.42 (m, 4H), 2.22 (m, 2H), 3.20
(s, 3H), 3.47 (t, 4H), 3.70 (t, 2H), 6.77 (m, 4H), and 8.41 (d,
4H).
The PF6 salt of C12Sq-By, C12Sq-By/PF6, could be prepared
by an ion exchange process using KPF6 and C12Sq-By in
acetonitrile. The spectroscopic properties of C12Sq-By/PF6
were found to be identical with those of the bromide iodide
compound.
Results and Discussion
Synthesis and Characterization of C4Sq-By and C12Sq-
By. The synthetic path for C4Sq-By and C12Sq-By is given
in Scheme 1. The entire synthesis is dictated by our ability to
purify the intermediates and the final products. For example,
we attempted to synthesize C4Sq-By by condensing the
covalently linked aniline-bipyridinium derivative with the
corresponding “half squaric acid”. Although C4Sq-By was
formed and detected spectroscopically and chromatographically,
we failed to isolate any pure material in this study. The
synthetic route in Scheme 1, although not the shortest, offers
the advantage that each intermediate is amenable to purification.
Both C4Sq-By and C12Sq-By are characterized by proton
NMR and FAB mass spectrometric analysis. In the absorption
spectra, both compounds exhibit sharp and intense absorption
very typical of that of the monomer of squaraine (Figure 1).10
The absorption maxmium is at 636 nm in dilute acetonitrile
solution, and the molar extinction coefficient is 3.1 × 105 cm-1
M-1. The absorption data suggest that there is no ground state
interaction between the squaraine chromphore and the bipyri-
dinium group in C4Sq-By and C12Sq-By.
The lack of any ground-state charge transfer interaction
between the squaraine chromophore and the bipyridinium group
in C4Sq-By and C12Sq-By is also supported by the cyclic
voltammetry (CV) data. Figure 2 depicts a cyclic voltammo-
gram of C4Sq-By in acetonitrile. The redox potentials are
basically the combination of the two chromphores in the diads,
with the oxdiation potenial at 0.71 and 1.04 V vs Ag/AgCl,
attributable to the squaraine chromophore,11 and the reduction
potential at -0.4 V vs Ag/AgCl is attributable to the bi-
pyridinium group.12
Synthesis of 4-N-Methyl-N-(N′-methyl-N′′-(4,4′-bipyridin-
iumyl)-3-propyl)aminophenyl-4′′-dibutylaminophen-
ylsquaraine Bromide Iodide (C4Sq-By). There was first
prepared 4-N-methyl-N-(4-(4-pyridyl)pyridiniumyl)-3-propyl-
aminophenyl-4′-(dibutylamino)phenylsquaraine by reacting 4-(N-
methyl-N-(3-bromopropyl)amino)phenyl-4′-(dibutylamino)-
phenylsquaraine (0.2 g, 0.39 mmol) with 1.2 g (7.82 mmol) of
4,4′-bipyridyl in 40 mL of acetonitrile at reflux for 2 days. After
the solvent was removed, excess bipyridyl was removed by
extracting with benzene repeatedly. The pure desired squaraine
product was obtained in 69% yield (0.18 g). NMR (DMSO-
d6): 0.88 (t, 6H), 1.26 (m, 4H), 1.53 (m, 4H), 2.33 (m, 2H),
3.12 (s, 3H), 3.53 (t, 4H), 3.75 (t, 2H), 4.71 (t, 2H), 6.68 (m,
4H), 8.06 (m, 2H), 8.11 (m, 4H), 8.63 (m, 2H), 8.81 (d, 2H),
and 9.25 (d, 2H).
C4Sq-By was then prepared by reacting the above squaraine
product (0.03 g, 0.045 mmol) with 2 mL of methyl iodine in
10 mL of acetonitrile at room temperature for 2 days. After
the solvent was removed, the product was purified by column
chromatography on celullose using acetonitrile as eluent,
affording 10 mg of C4Sq-By (27%). NMR (CD3CN): 0.96
(t, 6H), 1.39 (m, 4H0, 1.93 (m, 4H0, 2.42 (m, 2H), 3.11 (s,
3H), 3.50 (t, 4H), 3.71 (t, 2H), 4.40 (s, 3H), 4.74 (t, 2H), 6.88
(m, 4H), 8.15 (m, 4H), 8.35 (d, 4H), and 8.90 (d, 4H). FAB-
MS: 602.3 (M+). Maximum absorption (in CHCl3): 636 nm
(3.1 × 105 M-1 cm-1).
Synthesis of 4-N-Methyl-N-(N′-methyl-N′′-(4,4′-bipyridin-
iumyl)-3-propylaminophenyl-4′′-(didodecylamino)phen-
ylsquaraine Bromide and Iodide (C12Sq-By). The titled
compound was synthesized analogously to C4Sq-By. NMR
Aggregation Studies. Unlike alkyl substitued squaraines,
which are readily soluble in chloroform, C4Sq-By is insoluble
in chloroform and C12Sq-By is only slightly soluble in
choroform, forming a mixture of monomer and blue-shifted

Squaraine-Bipyridinium Diads
J. Phys. Chem. B, Vol. 101, No. 4, 1997 543
Figure 2. Cyclic voltammogram of C4Sq-By in acetonitrile.
Figure 4. Absorption spectra of transferred monolayers of C12Sq-By
on glass substrates as a function of the transferring surface pressure.
TABLE 1: Fluorescence Quantum Yields of C4Sq-By and
C12Sq-By
squaraine
solvent
quantum yield
C4Sq-By
acetonitrile
acetonitrile
acetonitrile
chloroform
1 × 10-3
1 × 10-3
1.2 × 10-3
0.84a
C4Sq-By/PF6
C12Sq-By
bis(4-(octadecylamino)phenyl)-
squaraine
a Reference 16.
intermediate. We have also shown that this aggregation
behavior is a result of the unique stabilzation energy for the
supramolecular structure in the squaraine aggregates.15 At
higher surface pressures, e.g., > 25 mN/m, only the blue-shifted
aggregates are formed.
Photophysics of C12Sq-By and C4Sq-By. Both C4Sq-
By and C12Sq-By fluoresce in the visible region, and the
emission spectra are typical of the monomer of an alkylated
squaraine.10 The fluorescence emission maxima and the
fluorescence quantum yield data are summarized in Table 1. In
comparsion with the fluorescence yields of alkyl substitued
squaraines,10,16 the fluorescence quantum yields for C4Sq-By
and C12Sq-By are about 100 times lower. Since both absorp-
tion and electrochemical data indicate that there is no ground-
state interaction between the squaraine chromophore and the
bipyridinium group, we attribute the significant decrease in
fluorescence yield to fluorescence quenching by electron transfer
from the excited squaraine chromphore to the bipyridinium
group. The radiative decay rate of squaraine was estimated to
Figure 3. Surface pressure-area compression isotherm of a monolayer
of C12Sq-By on water.
aggregates (530 nm) in diluted chloroform solution. A good
organic solvent found for C4Sq-By and C12Sq-By is aceto-
nitrile probably because of the ionic bipyridinium groups in the
diads. The low solubility of C4Sq-By in chloroform has limited
our LB film work to C12Sq-By. Figure 3 shows the surface
pressure-area compression isotherm for the monolayer of
C12Sq-By on water. The monolayer is stable with practically
no hysteresis. The limiting molecular area is 57 Å2/molecule,
and it corresponds to the cross-sectional area of two hydrocarbon
chains,13 suggesting that C12Sq-By lies vertically along the long
molecular axis on the water surface. Figure 4 shows the
absorption spectra of the monolayers of C12Sq-By when they
are transferred to hydrophilic glass substrates at varying surface
pressures. At very low surface pressure, e.g., 2 mN/m, C12Sq-
By exists as squaraine monomers in the monolayer. As the
surface pressure increases, the monomers of C12Sq-By (655
nm) are in an equilibrium with the blue-shifted aggregates (530
nm) in the monolayer. This is very characteristic for a number
of squaraine monolayers, and the results have been summarized
in an earlier publication.14 We concluded earlier that the
squaraine chromophores tend to aggregate as a supramolecular
unit, such as the tetramer, without forming a dimer as an
be 4 × 108 s-1 16
.
From the quantum yield data in this work,
the rate of the intramolecular electron-transfer process can be
estimated to be 4 × 1011 s-1
.
As noted earlier, one of the objectives of this work was to
study the electron-transfer reaction between the squaraine
chromophore and the bipyridinium group and the kinetics of
resulting transients. The transient absorption spectrum for
C4Sq-By is given in Figure 5. The monomer absorption at
638 nm is bleached and recovers very rapidly. Unfortunately,
the reduced bipyridinium radical cation, which should absorb
at around 605 nm,17 was not observable. The lack of a
detectable transient may be due to spectral overlapping between
the bleached peak and the reduced bipyridinium radical cation,
or more likely to the very rapid back electron recombination
between the squaraine radical cation and the reduced bipyri-
dinium radical cation. Kinetic analyses of peaks observed in
the transient experiment suggest that the transient, if formed,

544 J. Phys. Chem. B, Vol. 101, No. 4, 1997
Liang et al.
Figure 7. (a) Action spectra of a stearic acid monolayer-modified SnO2
electrode as sensitized by adding C4Sq-By/PF6 in the electrolyte (1
M NaNO3). (b) Absorption spectrum of the recovered stearic acid layer.
Figure 5. Transient absorption spectra of C4Sq-By in acetonitrile
([C12Sq-By] ) 3.5 × 10-6 M)
excited squaraine to the bipyridinium group is extremely fast,
we suggest that the squaraine chromophore in C4Sq-By is
incorporated into the aggregates of DSSQ on the monolayer-
modified electrode. The presence of C4Sq-By in the DSSQ
aggregate facilitates charge separation, leading to the increase
in the cathodic photocurrent generation. This interpretation is
supported by the action spectrum in Figure 6 (curve c). The
observation of significant photocurrent for the recovered modi-
fied DSSQ electrode indicates that C4Sq-By/PF6 is incorporated
into the aggregates of DSSQ in the first photoelectrochemical
experiment.
To test the hypothesis, we prepared a modified electrode
containing a monolayer of pure stearic acid on the SnO2
substrate. When this modified electrode is introduced into the
photoelectrochemical cell containing 1 M NaNO3 as electrolyte
solution, no photocurrent was observed. On the other hand,
when C4Sq-By/PF6 (1.2 × 10-6 M) is added to the electrolyte
solution, a cathodic photocurrent (600 nA/cm2) was recorded.
The action spectrum for the cathodic photocurrent peaks at 530
nm (Figure 7, curve a), suggesting that a blue-shifted aggregate
of the squaraine chromphore is responsible for the observed
photocurrent. Analysis of the recovered stearic acid-modified
SnO2 electrode after the photoelectrochemical experiment by
absorption spectroscopy reveals the presence of squaraine in
the stearic acid monolayer (absorption maximum at 650 nm
corresponding to the monomer of the squaraine chromphore,
Figure 7, curve b). The result suggests that C4Sq-Py/PF6 is
adsorbed and incorporated in the stearic acid monolayer during
the photoelectrochemical experiment. In water, the squaraine
chromophores form the blue-shifted aggregates due to hydro-
phobic interactions,8 and this aggregate produces the cathodic
photocurrent observed in Figure 7. When the stearic acid
monolayer is removed from the electrolyte solution, the ag-
gregate breaks apart and the stearic acid layer exhibits a visible
absorption corresponding to the monomer of the squaraine
chromophore.
Figure 6. Effect of adding C4Sq-By in the electrolyte (1 M NaNO3)
on the action spectra of the cathodic photocurrent generation of a DSSQ
monolayer-modified SnO2 electrode: (a) 1 M NaNO3 as electrolyte;
(b) 3.9 × 10-7 M of C4Sq-By/PF6 added to (a); (c) modified electrode
in (b) recovered and rinsed and then the photocurrent measured
according to conditions in (a).
must have a lifetime shorter than 30 ps. If the radical cation of
the squaraine chromophore is indeed formed, the rate of the
intramolecular charge recomination should be >3 × 1010 s-1
.
Photoelectrochemical Studies. Although C12Sq-By forms
stable monolayers on water and the films are transferrable to
hydrophilic substrates, e.g., SnO2 electrodes, with essentially
unit efficiency, we have been unable to study the photoactivity
of the LB film-modified electrode in solution photoelectro-
chemical experiments because the monolayer respreads on the
water surface when the electrode is introduced to the aqueous
electrolyte solution.
In this work, we use C4Sq-By and C12Sq-By as photo-
current enhancers for the DSSQ monolayers in solution photo-
electrochemical experiments. Previously, we reported that when
methylviologen was added to the electrolyte solution (1 M
NaNO3) in a photoelectrochemical cell, we observed a slight
increase in cathodic photocurrent generation for the DSSQ
monolayer-modified SnO2 electrode.2 The photocurrent en-
hancement was attributed to the electron transfer from the
squaraine exciton to methylviologen. Here, we introduced
C4Sq-By/PF6 (3.9 × 10-7 M) into the electrolyte solution
instead. We observed a drastic increase (×5) in the cathodic
photocurrent generation (Figure 6). Since fluorescence and
transient experiments suggest that electron transfer from the
We3 recently introduced amphiphilic electron acceptors
4-tetradecyl-4′-methylbipyridinium dichloride and 4-octadecyl-
4′-methylbipyridinium dichloride in the monolayers of DSSQ.
We observed intralayer electron transfers when the mixed
monolayers are studied in photoelectrochemical experiments.
Although we have been unable to study the photoactivity of
the monolayer of C12Sq-By directly, we are able to incorporate
it into the monolayer of DSSQ. At a 1:10 ratio (C12Sq-By:
DSSQ), we find that C12Sq-By is 3 times more effective in
sensitizing the cathodic photocurrent compared to a mixed
monolayer containing 4-tetradecyl-4′-methylbipyridinium di-

Squaraine-Bipyridinium Diads
J. Phys. Chem. B, Vol. 101, No. 4, 1997 545
tion range. The results seem to indicate that, in the absence of
an electron acceptor, molecular oxygen plays a dual role in the
cathodic photocurrent generation process. It creates as well as
separates the photogenerated charges.
Summary and Remarks
This work reports the synthesis of two squaraine-bipyri-
dinium diads. Both absorption and electrochemical results
indicate that there is no intramolecular ground-state charge-
transfer interaction between the squaraine chromophore and the
bipyridinium group. In the fluorescence emission spectra, a
significant decrease in fluorescence yield was observed for the
diads. The low fluorescence yield was attributed to fluorescence
quenching due to intramolecular electron transfer. From the
radiative rate constant, the forward intramolecular electron-
transfer rate is estimated to be 4 × 1011 s-1. In the transient
absorption experiment, no long-lived transient was detected and
any transient, if formed, should have a lifetime shorter than 30
ps. This leads to an estimated return electron-transfer rate of
Figure 8. Cathodic photocurrent generation action spectra for various
DSSQ-modified SnO2 electrodes in solution photoelectrochemical
experiments: (a) C12Sq-By incorporated in the DSSQ monolayer at a
1:10 ratio and 1 M NaNO3 used as electrolyte; (b) 7.9 × 10-7 M of
C4Sq-By/PF6 added into the 1 M NaNO3 electrolyte solution; (c) the
electrolyte solution in (b) deaerated by nitrogen.
>3 × 1010 s-1
.
C12Sq-By was shown to form stable monolayers on water,
and the monolayers are transferrable to hydrophilic substrates
with nearly unity efficiency. Photoelectrochemical experiments
with the monolayer-modified electrode of C12Sq-By were
unsuccessful because the monolayer was found to respread on
the water surface. However, we have been able to study both
C4Sq-By and C12Sq-By as sensitizers in photoelectrochemical
experiments involving the monolayer-modified electrodes of
DSSQ. We found that C4Sq-By/PF6 is a much more effective
sensitizer in the electrolyte solution compared to methylviologen.
Evidence is provided that C4Sq-By is incorpoated into the
monolayer of DSSQ. The incorporated C4Sq-By serves as a
site for charge separation because of the fast intramolecular
electron transfer between the excited squaraine chromophore
and the bipyridinium group. Similarly, C12Sq-By is a more
effective sensitizer compared to 4-tetradecyl-4′-methylbipyri-
dinium dichloride in the monolayer of DSSQ. Again, the
improved effectiveness in sensitization for C12Sq-By can be
attributed to the fast electron transfer between the excited
squaraine chromphore and the bipyridinium group, which
facilitates the charge generation.
chloride. For instance, the maximun photocurrent observed for
the C12Sq-By sensitized electrode is 1500 nA/cm2 (Figure 8,
curve a) whereas one that is sensitized by 4-tetradecyl-4′-
methylbipyridinium dichloride is only about 500 nA/cm2 under
similar conditions.3 We attribute the higher sensitization
efficiency for the C12Sq-By-sensitized electrode to the very
fast intramolecular electron transfer from the squaraine
chromphore to the bipyridinium group. This is because the
squaraine chromophore in C12Sq-By is part of the squaraine
aggregate in the monolayer-modified electrode and the presence
of C12Sq-By facilitates charge separation in the photogeneration
step. A further increase in cathodic photocurrent generation,
as high as 2600 nA/cm2, was obtained when C4Sq-Py/PF6 is
added into the electrolyte solution (Figure 8, curve b). This
corresponds to a quantum yield of 1.5% for the squaraine
monolayer-modified SnO2 electrode.
The estimated photocurrent generation efficiency for the
DSSQ-C12Sq-By monolayer electrode and the estimated rate
constant of 3 × 1011 s-1 for return electron transfer in the
C12Sq-By diad allow estimation of the rate constant for the
interfacial electron transfer between the monolayer and the
electrode (and/or monolayer-oxygen) be in the range (2-5)
× 109 s-1. We assume in these calculations that the return
electron transfer rate is the same in the LB film as in solution
and that the photocurrent generation efficiency is limited only
by return electron transfer. Consequently, the estimated rate
constants may be taken as lower limits. In any event, our
estimate is in the same range compared to that of the electron
transfer rate constants from excited states of J-aggregate of
cyanine dyes to AgBr microcrystals, which were reported to
be 109-1010 s-1 by Tani et al.18 and Muenter et al.19
This work also demonstrates that molecular oxygen is
essential to the cathodic photocurrent generation process even
in the presence of electron acceptors such as the bipyridinium
group. We hypothesize that oxygen plays a dual role in the
cathodic photocurrent generation process. It facilitates the
charge generation step and then serves as an electron shuttle to
move the generated charge pairs out of the recombination range.
Acknowledgment. The authors thank the National Science
Foundation for the financial support in the form of a grant from
the Center for Photoinduced Charge Transfer (CHE-9120001).
References and Notes
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