Photoinduced vectorial electron transfer in multilayered Langmuir-Blodgett films of porphyrin and phtalocyanine derivatives

Article abstract of DOI:10.1134/S0036024410070253

A series of new phtalocyanines and phtalocyanine-fullerene dyads was synthesized. The dyads were transferred to solid substrates in the form of the Langmuir-Blodgett films. Poly(hexylthiophene) (PHT) and/or phtalocyanine (B6PH) were used as secondary electron donors in the multilayered structures, together with molecules of phtalocyanine-fullerene (B6PF) or porphyrin-fullerene (DHD6ee) dyads in a matrix of octadecylamine (ODA) molecules in a ratio of 1: 9. Directed photoinduced electron transfer in films was studied by means of the time-resolved Maxwell displacement charge method. It was established that the addition of a monolayer of molecules of the secondary B6PH donor to a monolayer of molecules of the DHD6ee/ODA (1: 9) dyad resulted in a thirty-fold increase in the sample's photovoltaic response; it did not, however, change the recombination rate of the charges as compared to a single monolayer of the dyad molecules. In the case of bilayer samples consisting of B6PF/ODA (1: 9) and PHT/ODA (3: 2) monolayers, both quantities increased. The absorption of visible light over a wide spectral range was achieved via the use of three-component (PHT, B6PH and DHD6ee) multilayered structures. The relative sensitivities of the samples to excitation radiation were assessed and the efficiencies of their transformation of light energy to electric potential were compared. The largest sensitivity values were obtained for three-component samples prepared from PHT, B6PH, and DHD6ee monolayers in which the sensitivity value was 500 times larger than that for a separate monolayer of DHD6ee dyad molecules.

Full text of DOI:10.1134/S0036024410070253

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 7, pp. 1230–1241. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.S. Alekseev, N.V. Tkachenko, A.V. Efimov, H. Lemmetyinen, 2010, published in Zhurnal Fizicheskoi Khimii, 2010, Vol. 84, No. 7, pp. 1356–1367.
PHOTOCHEMISTRY
AND MAGNETOCHEMISTRY
Photoinduced Vectorial Electron Transfer
in Multilayered Langmuir–Blodgett Films of Porphyrin
and Phtalocyanine Derivatives
A. S. Alekseeva, N. V. Tkachenkob, A. V. Efimovb, and H. Lemmetyinenb
a Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991 Russia
b Tampere University of Technology, Tampere FIꢀ33720, Finland
eꢀmail: alexandre.efimov@tut.fi
Received May 6, 2009
Abstract—A series of new phtalocyanines and phtalocyanine–fullerene dyads was synthesized. The dyads
were transferred to solid substrates in the form of the Langmuir–Blodgett films. Poly(hexylthiophene) (PHT)
and/or phtalocyanine (B6PH) were used as secondary electron donors in the multilayered structures,
together with molecules of phtalocyanineꢀfullerene (B6PF) or porphyrin–fullerene (DHD6ee) dyads in a
matrix of octadecylamine (ODA) molecules in a ratio of 1 : 9. Directed photoinduced electron transfer in
films was studied by means of the timeꢀresolved Maxwell displacement charge method. It was established that
the addition of a monolayer of molecules of the secondary B6PH donor to a monolayer of molecules of the
DHD6ee/ODA (1 : 9) dyad resulted in a thirtyꢀfold increase in the sample’s photovoltaic response; it did not,
however, change the recombination rate of the charges as compared to a single monolayer of the dyad moleꢀ
cules. In the case of bilayer samples consisting of B6PF/ODA (1 : 9) and PHT/ODA (3 : 2) monolayers, both
quantities increased. The absorption of visible light over a wide spectral range was achieved via the use of
threeꢀcomponent (PHT, B6PH and DHD6ee) multilayered structures. The relative sensitivities of the samꢀ
ples to excitation radiation were assessed and the efficiencies of their transformation of light energy to electric
potential were compared. The largest sensitivity values were obtained for threeꢀcomponent samples prepared
from PHT, B6PH, and DHD6ee monolayers in which the sensitivity value was 500 times larger than that for
a separate monolayer of DHD6ee dyad molecules.
DOI: 10.1134/S0036024410070253
INTRODUCTION
method [8–12]. Dyads of phytochlorin fullerene and
porphyrinꢀfullerene have convincingly exhibited phoꢀ
toinduced charge separation, both in solutions and in
solid films [11, 12]. Photoinduced intramolecular
electron transfer in films has been studied by means of
the timeꢀresolved Maxwell displacement charge
(TRMDC) method first described in 1993 [13, 14].
Interlayer directed photoinduced electron transfer in
alternating LB structures of phytochlorin fullerene
dyad and polymer poly(3ꢀhexylthiophene) was studied
in detail in [15]. In bilayer structures consisting of a
monolayer of phytochlorin–fullerene dyad molecules
and a monolayer of diacetylene alcohol molecules, the
amplitude of photovoltaic response and the kinetics of
the recombination of carriers in a state of separated
Over the last ten years, much attention has been
devoted to studying directed photoinduced electron
transfer in organic molecules [1–4]. Different donor–
acceptor (D–A) molecular systems have been studied
with the aim of using such molecules in the form of
thinꢀfilm structures capable of transforming sunlight
into electric energy. Several methods for preparing
molecular films have been developed. One is the Langꢀ
muir–Blodgett (LB) method, which allows one to creꢀ
ate monomolecular films with D–A dyads of uniform
orientation on the surface of a solid substrate [5].
Charge separation between the donor and acceptor
parts of dyad molecules can occur in a film upon phoꢀ
toexcitation [6].
Fullerenes having excellent electron acceptor charges depend on the degree of polymerization (the
properties are often used in the synthesis of D–A
dyads [1–4, 6, 7]. However, fullerene absorbs light
weakly in the visible range of the solar spectrum. To
increase the efficiency of the conversion of light
energy in D–A systems, dyes with properties of good
electron donors and a high absorption of visible light
were used. Several studies have been done with films of
dose of UV irradiation) of the diene monomers and
the type of formed polydiacetylene [16, 17].
Porphyrin chromophores effectively absorb light in
a rather narrow region of the blue part of the spectrum.
The absorption region can be expanded by replacing
the donor porphyrin molecules in a dyad with phtaloꢀ
D–A dyads on a base of fullerene prepared by the LB cyanine molecules, or a monolayer of phtalocyanine
1230

PHOTOINDUCED VECTORIAL ELECTRON TRANSFER
1231
molecules can be used as the layer of the secondary (purity, 99%) was purchased from Sigma, while regioꢀ
electron donor in a bilayer film structure [18].
regular poly(3ꢀhexylthiophene) PHT was purchased
from Aldrich Chemicals. The other reactants and solꢀ
vents used in synthesis were also purchased from Aldꢀ
rich Chemicals and were used without preliminary
purification, if not specified otherwise. Nuclear magꢀ
netic resonance (NMR) spectra were measured on a
Varian Mercury (300 MHz) spectrometer in deuteroꢀ
chloroform, with tetramethylsilane as an internal refꢀ
erence. Mass spectra were obtained on a timeꢀofꢀflight
Waters LCT Premier XE mass spectrometer with elecꢀ
trospray ionization.
This study was aimed at preparing and studying LB
films consisting of phtalocyanine molecules and phtaꢀ
locyanineꢀfullerene and porphyrinꢀfullerene dyads in
order to expand the spectral absorption region of the
film structures, allowing us to increase their sensitivity
to sunlight. The TRMDC method was used to study
directed photoinduced electron transfer in monoꢀ and
multilayer LB films. The relative sensitivities of samꢀ
ples to excitation radiation were assessed from the
results of photovoltaic measurements.
Synthesis
EXPERIMENTAL
The structures of the studied phtalocyanine moleꢀ
cules and dyads are described below. The synthesis and
Reactants and Methods
Chloroform of analytical grade (Merck, Germany) properties of porphyrinꢀfullerene dyad DHD6ee (
1),
was used to prepare and deposit the solutions needed phtalocyanineꢀfullerene dyad B4P2F ( ), and phtaloꢀ
2
to obtain Langmuir films. Octadecylamine ODA cyanine ZnPH4 (
3
) were described earlier in [19, 20].
O
OH
O
O
O
O
O
NH
N
N
N
O
O
NH
N
HN
N
N
O
O
N
O
HN
PhOC H OH
2
4
N
O
O
O
O
O
O
O
O
O
O
O
(2)
(1)
O
O
OH
OH
O
O
O
N
N
O
O
O
N
N
NH
N
N
N
OH
Zn
N
N
N
N
HO
N
HN
O
O
N
N
O
O
O
(3
)
(4)
HO
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1232
ALEKSEEV et al.
O
O
O
O
O
O
O
O
O
O
O
O
N
N
O
O
O
O
NH
NH
N
N
N
N
N
N
N
HN
N
HN
O
O
N
N
O
O
O
O
(5)
(6)
1,4,5,8,9,12ꢀHexabutoxyꢀ14ꢀ[3ꢀ(hydroxymethyl) OCH2C3H7), –0.2 (s, 2H, NH); hydroxyl protons
[phenoxy] phtalocyanine B6PH (4). 4 [3ꢀ(Hydroxyꢀ ArCH2OH were not resolved. Mass spectra
calcuꢀ
m/z
methyl) phenoxy] phtalonitrile (245 mg, 1 mmol), lated for C63H72N8O8 (M + H)+, 1069.5552; found,
1069.5555, 1070.5680.
2,5ꢀdibutoxyphtalonitrile (270 mg, 1 mmol) and lithꢀ
ium shots (37 mg, 5.1 mmol) were suspended in 7 ml
1ꢀpentanole. The reaction mixture was heat at reflux
for 12 h under argon flow. A dark green solution was
cooled and evaporated under vacuum. The obtained
resinous residue was dissolved in 70 ml of chloroform,
washed out with 100 ml of 10% hydrochloric acid, and
then washed out three times with 100 ml of water. The
organic layer was collected, and the solvent was evapꢀ
orated at +70°С. The mixture of reaction products
was chromatographically separated on Silica 100 silica
gel (Merck.) A mixture of phtalocyanines was obtained
by gradient elution (chloroform, chloroform/ethanol
19/1), and was then separated repeatedly on silica gel.
1,4,5,8,9,12ꢀHexabutoxyꢀ14ꢀ[3ꢀ(ethylmalonateꢀ
oxymethyl) phenoxy]ꢀphtalocyanine (5). Ethylmalonyl
chloride (0.007 ml, 0.05 mmol) and triethylamine
(0.008 ml, 0.005 mmol) was added to a solution of
1,4,5,8,9,12ꢀhexabutoxyꢀ14 [3ꢀ(hydroxymethyl) pheꢀ
noxy] phtalocyanine (21 mg, 0.018 mmol) in 4 ml of
dichloromethane. The reaction mixture was stirred for
14 h at room temperature. After the usual workꢀup, the
reaction mixture was separated on silica gel (eluent
chloroform) and the main green fraction was collected
to yield 20 mg (91%) of 1,4,5,8,9,12ꢀhexabutoxyꢀ14ꢀ
[3ꢀ(ethylmalonateoxymethyl)ꢀphenoxy] phtalocyaꢀ
nine (5). The 1H NMR spectrum (
= 8.2, 1H, 16ꢀH), 8.68 (d,
1 = 2, J2 = 8.2, 1H, 15ꢀH), 7.66–7.10 (m,
(chloroform, chloroform/acetonitrile 475/25, chloroꢀ 10H, phtaloꢀH, ArꢀH), 5.27 (s, 1H, ArCH2OR), 4.86
form/ acetonitrile 275/25, chloroform/ethanol (t, J = 7, 8H, 4,5,8,9ꢀOCH2C3H7), 4.67 (t,
= 6.4, 2H,
= 6.4, 2H, 12ꢀOCH2C3H7),
= 7.2, 2H, OCH2CH3), 3.44 (s, 2H, (с, 2H,
OCOCH2COO), 2.48–1.43 (m, 24H, OCH2C2H4CH3),
.30–0.93 (m, 21H, CH3), 0.02 (s, 2H, NH). Mass
spectra
calculated for C68H78N8O11 (M + H)+,
δ
, ppm,
J, Hz) was
Three phtalocyanine fractions (A (17 mg), B (52 mg), 9.24 (d,
and C (73 mg)) were obtained by gradient elution 7.78 (dd,
J
J
= 2, 1H, 13ꢀH),
J
J
1ꢀOCH2C3H7), 4.56 (t,
4.15 (q,
J
380/25). The Rf parameters for the A, B, and C fracꢀ
tions on thinꢀlayer chromatography (TLC) (alumiꢀ
num plates with Merck Silica gel 60 F254 silica gel)
J
1
1
were 0.86, 0.73, and 0.52, respectively. By H NMR
m/z
spectra, we identified the A fraction as 1,4,5,8,9,12ꢀ
octabutoxyphtalocyanine, the C fraction as a mixture
of 1,4,5,8ꢀtetrabutoxyꢀ10,14ꢀbis [3ꢀ(hydroxymethyl)
phenoxy] phtalocyanine regioisomers, and the B fracꢀ
tion as the target product 1,4,5,8,9,12ꢀhexabutoxyꢀ14
[3ꢀ(hydroxymethyl) phenoxy] phtalocyanine. The 1Н
1183.5868; found, 1183.6012, 1184.5710.
B6PF Dyad. Fullerene С60 (8 mg, 0.011 mmol),
iodine (3.5 mg, 0.014 mmol) and 1.4,5.8,9.12ꢀhexabꢀ
utoxyꢀ14ꢀ[3ꢀ(ethylmalonateoxymethyl)
phenoxy]
phtalocyanine (12 mg, 0.01 mmol) was dissolved in
60 ml of toluene in an ultrasound bath and stirred
under argon for 15 min. 1,8ꢀDiazabicyclo[5.4.0]
undecꢀ7ꢀene (DBU) (0.0042 ml, 0.03 mmol) was
NMR spectrum (
1H, 16ꢀH), 8.68 (s, 1H, 13ꢀH), 7.68 (d,
15ꢀH), 7.62–6.97 (m, 10H, phtaloꢀH, ArꢀH), 4.93–
δ, ppm,
J, Hz) was 9.08 (d,
J = 8.2,
J
= 2.1, 1H,
4.76 (m, 8H, 4,5,8,9ꢀOCH2C3H7), 4.72 (s, 1H, added to the solution and the reaction mixture was
ArCH2OH), 4.58 (t, = 6.4, 2H, 1ꢀOCH2C3H7), 4.50 stirred for 3 h protected from light. The solvent was
(t, = 6.4 Hz, 2H, 12ꢀOCH2C3H7), 2.2–0.8 (m, 42H, evaporated and the mixture of products was separated
J
J
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PHOTOINDUCED VECTORIAL ELECTRON TRANSFER
1233
on silica gel (eluent, chloroform), and the main green with a modulated Q factor (LF114, SOLAR TII,
fraction was collected. The solvent was evaporated and Belarus). The system’s time resolution was deterꢀ
the dry residue was washed with pentane to yield 10 mg mined by the excitation pulse width and was 10 ns. The
(52%) of B6PF dyads. The 1Н spectrum (
Hz) was 9.26 (d,
δ
, ppm,
J
,
samples for photoelectric measurements had the folꢀ
J
= 8.2, 1H, 16ꢀH), 8.87 (d, = 2, lowing structure: glass plate/ITO electrode/9–10 ODA
J
1H, 13ꢀH), 7.79 (dd, J1 = 2, J2 = 8.2, 1H, 15ꢀH), layers/active layers/20–21 ODA layers/InGa elecꢀ
7.67–7.14 (m, 10H, phtaloꢀH, ArꢀH), 5.56 (s, 1H, trode. A monolayer of molecules was transferable from
ArCH2OH), 4.86 (broadened t,
OCH2C3H7), 4.67 (t, = 6.4, 2H, 1ꢀOCH2C3H7), 4.53 moving the latter either from water to air (upward), or
(t, = 6.4, 2H, 12ꢀOCH2C3H7), 4.48 (q, = 7.2, 2H, in the opposite direction (downward). This allowed us
OCH2CH3), 2.51–0.83 (m, 45H, OCH2C2H4CH3, to obtain film structures with opposite orientations of
J = 7 Hz, 8H, 4,5,8,9ꢀ the surface of the water subphase to a solid substrate by
J
J
J
OCH2CH3), 0.07 (with, 2H, NH). Mass spectra
calculated for C128H76N8O11 (M + H)+, 1902.5745
found, 1902.4778, 1903.4824.
m
/
z
;
:
the molecules of donor–acceptor dyads with respect
to the electrode’s ITO surface and different sequences
of monolayers in the samples. Since the dark conducꢀ
tivity of the samples was very low (Rsample > 1010
Ω),
they may be considered as condensers with capacities
of 100–200 pF. Due to the high input resistance of the
Preparation of Films
preamplifier (100 MΩ or 10 GΩ), the TRMDC meaꢀ
KSV LB 5000 and LB Minitrough System devices
(KSV Ltd.) were used to measure isotherms of the
compression and formation of monolayers and their
transfer to solid substrates. MilliꢀQ water containing
phosphate buffer (0.5 mmol Na2HPO4 and 1 mmol
NaH2PO4) of pH 7 was used as a subphase. The temꢀ
perature of the subphase was kept within the limits of
19 0.5°С. The films were transferred to quartz plates
in order to perform spectroscopic studies. Plates were
preliminarily purified according to the standard techꢀ
nology [5] and were then plasma annealed in a nitroꢀ
gen atmosphere at low pressure for 15 min on a
PDCꢀ23G device (Harrick). Samples for photoelecꢀ
tric measurements were prepared on glass plates covꢀ
ered with a semiꢀtransparent layer of indium and tin
surements were performed in the photovoltaic mode at
time intervals of 10 ns to 10 ms or 1.0 s, respectively.
The measured TRMDC signals reflected the photoinꢀ
duced motion of electrons in the layered structures in
the direction normal to the film surface. Signal ampliꢀ
tudes were proportional to the quantity of charges and
their displacement in the direction of the normal to
the film surface. Signal decay over time was deterꢀ
mined by the recombination of the state with the sepꢀ
arated charges in the films under investigation.
RESULTS AND DISCUSSION
Films and Isotherms
oxide (ITO) with a resistance of about 10
Ω. These
substrates were purified with acetone and chloroform
in an ultrasonic bath (30 min in each solvent) and were
then annealed for 10 min in plasma immediately
before they were used to deposit monolayers. Deposiꢀ
tion of 9 or 10 ODA layers on the ITO surface preꢀ
vented any interaction between the active layers and
the ITO electrode. Deposition of 20 or 21 ODA layers
on the structure protected the active layers from direct
contact with the upper electrode (a drop of liquid
InGa metal).
Langmuir films were formed on the surface of a
water subphase containing phosphate buffer. The soluꢀ
tion of the molecules in chloroform (c = 0.2–
0.35 mmol) was distributed on this surface. The conꢀ
ditions of the formation of monolayers of PHT/ODA
and DHD6ee/ODA mixtures and their transfer to the
solid substrates were described earlier in [12, 15]. For
other substances under study, the speed of the comꢀ
pression (formation) of the Langmuir monolayer was
15 cm2/min. Monolayers were transferred to the solid
substrates at the speeds of the motion of the latter:
7 mm/min (downward) and 5 mm/min (upward).
Figure 1 shows the isotherms of the dependence of
the surface pressure on the area on the molecule (PꢀA)
for phtalocyanines (B6PH, ZnPH4), phtalocyanine
fullerene dyads (B6PF, B4P2F), and the mixture of
B6PF molecules with molecules of the ODA matrix.
Shapes of the PA isotherms for 100% phtalocyanine,
B6PH and B6PF dyads were typical for the expanded
Spectroscopic Study
Stationary absorption spectra of the layered strucꢀ
tures were recorded on a Shimadzu UVꢀ3600 spectroꢀ
photometer.
Photoelectric Measurements
Photoinduced directed electron transfer in the monolayers with an acceptable value of the average area
films was studied by the TRMDC method. Samples per one molecule of about 150 Å2. The isotherm of the
were excited with a pulse (10 ns) from a titaniumꢀsapꢀ mixture of B6PF (10 mol %) and ODA (90 mol %) molꢀ
phire laser (CF125, SOLAR TII, Belarus) at waveꢀ ecules exhibited higher collapse pressure and the presꢀ
lengths of around 770 (basic harmonic) and ~430 nm ence of the condensed state of the monolayer over an
(second harmonic), the pumping of which was perꢀ interval of surface pressure from 25 to 40 mN/m. The
formed by the second harmonic of a Nd:YAG laser areas per molecule obtained for 100% phtalocyanine
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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1234
mN/m
ALEKSEEV et al.
the Langmuir–Schaefer (LS) method. The same
Π
,
method was applied in preparing monoꢀ and multiꢀ
layer films of the B6PF dyad. Langmuir monolayers of
a mixture of molecules of the B6PF dyad (10 mol %)
and the matrix (ODA) were transferred to the solid
substrates by LB method in the form of a Yꢀtype strucꢀ
ture at a surface pressure of 30 mN/m; the transfer
coefficient was close to unity.
60
40
20
ZnPH4
B6PF
B4P2F
B6PH
B6PF/ODA(1/9)
Absorption Spectra
Figure 2 shows steady state absorption spectra of
the substances transferred to the quartz plates by the
LB and LS methods. The wide Qꢀbands of the phtaloꢀ
cyanine free base cover the region from 600 to 850 nm.
The absorption band of ZnPH4 was narrower, with
two maxima at 640 and 710 nm. The wide Qꢀbands of
free compounds base indicate greater interaction
between their chromophores. The use of these comꢀ
pounds allowed us to expand the visible light absorpꢀ
tion region of the films.
0
50
100
150
200
,
Å2/mol
s
Fig. 1. PA isotherms of compression for the compounds
under study; is area.
s
ZnPH4 and B4P2F dyad were much smaller than
those expected for the phtalocyanine monocycle lying
on the water surface, meaning that the films formed on
the subphase surface were not monolayers of homogeꢀ
neously oriented molecules.
Figure 3 shows the absorption spectra of the LB
film of the B6PF (10 mol %) dyad depending on the
number of monolayers. Absorption is proportional to
the number of transferred layers, indicating the reproꢀ
ducibility of monolayer quality. A distinctive feature of
the absorption spectrum of the LB film of molecules of
porphyrin–fullerene dyad DHD6ee (10 mol %) is the
presence of a narrow Soret band with its maximum at
436 nm [12]. The absorption spectrum of the LB film
of polymer PHT (60 mol %) is a wide band with its
The films formed of the B6PH, B6PF, and B4P2F
molecules were transferable by LB method to the solid
substrates at a surface pressure of 18–20 mN/m in the
layer. It was possible to obtain multilayered LB films
only in the case of B6PH molecules. The transfer coefꢀ
ficient of the monolayers to the substrate was 0.8–1.0.
A multilayered structure of the B4P2F dyad was
obtained upon the transfer of layers to the substrate by maximum at 540 nm [15].
Absorption
ZnPH4
0.04
B4P2F
B6PH
B6PF
0.02
0
400
500
600
700
800
900
, nm
λ
Fig. 2. Absorption spectra of monolayer films of the compounds under study.
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PHOTOINDUCED VECTORIAL ELECTRON TRANSFER
Absorption
1235
0.16
0.12
0.08
0.04
0
7
5
3
1
400
600
800
λ, nm
Fig. 3. Absorption spectra of the LB layers (curve numbers denote number of monolayers), prepared from a mixture of the molꢀ
ecules of the B6PF dyad and ODA matrix in the molar ratio of 1 : 9.
Photovoltaic (TRMDC) Measurements
(3 : 2) molecules was successfully used earlier as the
secondary electron donor in bilayer structures consistꢀ
ing of monolayers of phytochlorin–fullerene dyad
(PF, 30 mol %) and PHT [15], with interlayer directed
photoinduced electron transfer being observed in such
structures. The distance between the separated charges
and the lifetime of this state increased in comparison
to a monolayer of PF dyad molecules taken separately.
Here, bilayer structures consisting of a monolayer of
B6PF (10 mol %) dyad molecules and a monolayer of
PHT polymer (60 mol %) were studied. Samples with
two types of structure were prepared:
Intralayer electron transfer. It is well known that
the sign of photovoltaic response depends on the oriꢀ
entation of the donor–acceptor complex with respect
to the surface of the ITO electrode [11]. The measured
TRMDC signal therefore depends on the structure
and amphiphilic properties of the dyad molecules. The
photovoltaic responses have opposite signs for the
monolayers of porphyrin–fullerene (DHD6ee) and
phtalocyanine–fullerene (B6PF) dyads, though the
transfer of films to the substrates occurs in the same
direction of their motion through the monolayer
formed on the surface of the liquid subphase (e.g.,
from the subphase to the air). This means that
DHD6ee and B6PF dyads have opposite orientations
to the subphase surfaces. In the Langmuir monolayer
of DHD6ee molecules, the porphyrin (donor) parts of
the dyads are closer to the water’s surface, while the
fullerene (acceptor) parts of these dyads are closer to
water in the case of B6PF molecules. The molecules of
both dyads were mixed with the ODA molecules in a
ratio of 1 : 9, and the films were prepared by the LB
method.
glass / ITO / ODA(10) / B6PF(1 upward) / PHT
(1 downward)/ ODA(21)/InGa
and glass/ITO/ODA(10)/PHT(1 upward)/B6PF
(1 downward)/ODA(21)/InGa.
Figure 5 shows the photovoltaic responses of such
structures. The excitation of primary donor phtalocyꢀ
anine occurred at a wavelength of 764 nm. The ampliꢀ
tude of the responses of such bilayer structures were
five times larger than those for samples with a single
monolayer of B6PF (10 mol %) dyad molecules. The
differences between the bilayer and monolayer samꢀ
ples in their signal amplitudes and decay times were
confirmed in [15] by the twoꢀstep mechanism of
directed photoinduced electron transfer in a bilayer
sample. Primary donor phthalocyanine is first photoꢀ
excited and transferred an electron to the fullerene,
and an electron from secondary donor polythiophene
is then spontaneously transferred to the phtalocyanine
cation. The increase in the distance between the sepaꢀ
rated charges results in an increase in the amplitude of
The photovoltaic signals of the monolayer films
consisting of B6PF/ODA (1 : 9) molecules and transꢀ
ferred to the substrates by their motion upward and
downward deposition are shown in Fig. 4. Pulse exciꢀ
tation was performed at a wavelength of 760 nm.
Amplitudes and kinetics of these samples’ TRMDC
signals were similar to those measured earlier for
DHD6ee/ODA (1 : 9) monolayers [12].
Interlayer electron transfer. The monolayer of the photovoltaic response, while the migration of
polyhexylthiophene (PHT) in the matrix of the ODA holes in the polymer layer increases the time of charge
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ALEKSEEV et al.
U
output, V
0.03
0.01
0.01
0.03
0.05
1
2
0.5
1.0
1.5
2.0
τ, μs
−
−
−
Fig. 4. Photovoltaic signals from monolayers of a mixture of B6PF/ODA (1 : 9) molecules with opposite orientations of dyads on
the substrates: ( ) upward, ( ) downward.
1
2
U
output, V
0.2
0.1
0
0.5
1.0
1.5
2.0
τ, μs
B6PF/PHT
PHT/B6PF
−
0.1
−0.2
Fig. 5. Photovoltaic responses from two types of bilayer samples: B6PF/ODA–PHT/ODA (positive signal), PHT/ODA–
B6PF/ODA (negative signal).
recombination. Samples with repeated bilayer strucꢀ
tures were prepared in order to increase the distances
between the separated charges even more.
Comparative TRMDC measurements of these
samples were performed upon light pulse excitation of
primary donor phtalocyanin at a wavelength of
764 nm. For the samples with one and two active bilayꢀ
ers, the energy density of optical excitation was
Repeated bilayer structures. Three samples with
PHT–B6PF active bilayer numbers of 1, 2, and 10
were prepared under identical experimental condiꢀ
tions. The samples had the following structure:
glass/ITO/ODA (10)/[PHT (1 upward)/B6PF
3.75 μ μ
J/cm2, while it was 1.12 J/cm2 for the sample
with 10 bilayers. Figure 6 shows the photovoltaic sigꢀ
nals of these samples. The signal amplitude was proꢀ
portional to the number of bilayers. We may assume
that the measured photovoltaic signal in the experiꢀ
ment was the sum of signals from each bilayer. Charge
(1 downward)]n/ODA (21)/InGa, where
ber of bilayers in the structure.
n is the numꢀ
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PHOTOINDUCED VECTORIAL ELECTRON TRANSFER
1237
0
0.5
1.0
1.5
2.0
τ, μs
1
0.02
0.04
2
3
0.06
–Uoutput, V
Fig. 6. Photovoltaic signals from samples of bilayer structure PHT/ODA–B6PF/ODA (
1), repeated on the substrates 2 (2) and
10 times ( ). See text for details.
3
separation occurred in each bilayer between the PHT substrates covered with ITO film. As in the previous
and B6PF layers as a result of light excitation, and experiments, the active bilayers were insulated from
there was no charge transfer between the bilayers. Figꢀ the ITO and InGa electrodes by ODA multilayers.
ure 7 shows the photovoltaic signal for the sample with Upon light excitation of the porphyrin chromophores
10 bilayers measured over two time intervals. The lifeꢀ
(λ = 436 nm, E = 150 μ
J/cm2) the two types of samples
time of the state with separated charges is noticeably exhibited photovoltaic signals of opposite sign and the
longer than the singleꢀbilayer sample. The signal same amplitude (Fig. 9). Upon excitation of secondary
amplitude decreased by almost half 2 ms after the exciꢀ donor phtalocyanine (λ = 766 nm, E = 150 μ
J/cm2),
tation pulse was switched off, while the signal the shapes of the photovoltaic responses remained virꢀ
decreased by a factor of 10 in the singleꢀbilayer samꢀ tually the same and their amplitudes were oneꢀthird of
ple. This indicates the possibility of the electrons tranꢀ those where porphyrin was excited.
sitioning not only inside the bilayers, but also between
the bilayers. The time recombination of the charges
The application of external electric voltage ( 0.5 V)
to samples barely influenced the amplitude of the
increases both due to their migration along the polyꢀ
measured signal (~10%), indicating a rather high
meric polythiophene chains and as a result of the tranꢀ
homogeneity in the orientation of donors and accepꢀ
sition of charges from one bilayer to another.
tors in the film.
The dependence of the photovoltaic response of
The photovoltaic response of the bilayer B6PH–
the sample with 10 PHT–B6PF bilayers on the energy
DHD6ee structure was 30 times greater than the analꢀ
density of optical excitation (λ = 764 nm) is shown in
ogous response from a separate monolayer of the dyad
= 436 nm). However, the signal decay was the same
Fig. 8. The signal increased linearly to a value of
(
λ
15
μ
J/cm2, and it remained unsaturated at 80 J/cm2.
μ
in both cases. The addition of a B6PH (100%) monoꢀ
layer to a monolayer of the DHD6ee (10 mol %) dyad
did change the form of the photovoltaic response in
the time scale of 0–2 ms. Considering that the speed of
charge recombination in the monolayer of the dyad
should be controlled by the motion of electrons within
the monolayer between the macrocycles, we may conꢀ
clude that there was no migration of charges in the
layer of the secondary donor in the bilayer sample. The
According to the data obtained by studying phtalocyꢀ
anine films in [21], signal saturation can be expected at
an excitation density exceeding 100 μ
J/cm2, due to
exciton–exciton annihilation between phtalocyanine
chromophores. Under the conditions of this study, the
highest excitation density at which signal intensity
reached the limit of the preamplifier’s linear response
region was 80 μ
J/cm2.
Phtalocyanine layer as the secondary donor. Two addition of a B6PH monolayer thus considerably
types of samples were obtained by the transfer of increases the photovoltaic signal from the monolayer
DHD6ee (10%) (downward)/B6PH (100%) (upward) of the dyad, but does not change the recombination
or B6PH (downward)/DHD6ee (upward) bilayers to rate of the charges. At the same time, the addition of a
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ALEKSEEV et al.
0.5
1.0
1.5
2.0
τ, μs
0.01
0.03
0.05
Time,
1.5 2.0
μs
0.5
1.0
0
−
−
−
0.02
0.04
0.06
–Uoutput, V
0.07
–Uoutput, V
Fig. 7. Photovoltaic signals from a (PHT/ODA–B6PF/ODA) × 10 sample.
monolayer of PHT polymer (60 mol %) to the monoꢀ
layer of the B6PF (10 mol %) dyad results in a noticeꢀ
able increase in the time constant of the recombinaꢀ
tion of charges, due to their migration along the polyꢀ
mer chains.
Threeꢀcomponent layered structures. To increase
the distance between the separated charges and the
lifetime of each state, a monolayer of PHT polymer
(60 mol %) is added to the bilayer structure
DHD6ee–B6PH. Samples with the following strucꢀ
ture were prepared by the LB method: ITO/ODA
(9)/PHT (downward, upward)/B6PH (downward)/
DHD6ee (upward)/ODA (20)/InGa.
U
output, V
The photovoltaic responses were measured upon
the excitation of either primary donor porphyrin (
λ
=
=
436 nm) or secondary donor phtalocyanine (
λ
2
767 nm). The signal amplitude of each sample was
three times greater than that from the bilayer structure
upon excitation at 767 nm. However, the difference
was even more noticeable upon the excitation of the
primary donor (λ = 436 nm) (Fig. 10). In our experiꢀ
ment, the density of the excitation energy for the
bilayer sample was 5.3 times greater than that for the
sample with the addition of a PHT layer. As a result,
the signal amplitude of the threeꢀcomponent sample
was 15 times greater than the one of the bilayer and
almost 500 times greater than the one from the monoꢀ
layer of DHD6ee (10 mol %) dyad molecules. We can
explain such an increase in signal amplitude by the
presence of a threeꢀstep directed photoinduced charge
transfer. The first step was the intramolecular electron
transfer from the photoexcited primary donor porphyꢀ
rin to the fullerene. The second and third steps were
the intermolecular transitions of the electron, first
from phtalocyanine to the porphyrin cation and then
from the polythiophene to the phtalocyanine cation.
1
0
25
50
75
I, μ
J/cm2
Fig. 8. Dependence of the amplitude of photovoltaic
response on the energy density of optical excitation (
763 nm) for a sample with (PHT/ODA–B6PF/ODA)
structure.
λ
=
× 10
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PHOTOINDUCED VECTORIAL ELECTRON TRANSFER
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Uoutput, V
0.2
0.1
0
1
0.5
1.0
1.5
2.0
τ, μs
2
−
0.1
−
0.2
Fig. 9. Photovoltaic signals ((
DHD6ee/ODA ( ).
λ = 436 nm) from two types of bilayer samples: DHD6ee/ODA–B6PH (1); B6PH–
2
–
Uoutput, V
0
1
0.02
0.04
0.06
0.08
2
0
0.5
1.0
1.5
2.0
τ, μs
Fig. 10. Photovoltaic signal from the bilayer LB structure B6PH–DHD6ee/ODA (
1) and its change upon the addition of two
PHT/ODA monolayers ( ).
2
The electrons were thus finally located on fullerene tovoltaic responses were normalized by the excitaꢀ
tion energies. Figure 11 summarizes the obtained
results. In the alternating structures, the sensitivity
of the samples was higher upon excitation of the priꢀ
mary donors (porphyrin in DHD6ee or phtalocyaꢀ
nine in B6PF). For the bilayer B6PH/DHD6ee and
PHT/B6PF samples, the results of the measureꢀ
ments were the same. The highest sensitivity values
and the holes were on polythiophene. Therefore, the
lifetime of the charge separated state in the sample
considerably exceeded those for the monolayer and
bilayer samples.
Relative sensitivity of samples. The samples were
compared by calculating their relative sensitivity to
the excitation radiation. The amplitudes of the phoꢀ were obtained for the samples prepared using three
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1240
ALEKSEEV et al.
2PHT + B6PH + 10% DHD6ee,
760 nm
2PHT + B6PH + 10% DHD6ee,
436 nm
B6PH + 10% DHD6ee,
436 nm
PHT + 10% B6PF,
760 nm
10% B6PF,
760 nm
10% DHD6ee,
436 nm
−2
−
1
0
1
2
logσ mJ]
[V сm2/
Fig. 11. Relative sensitivity (
σ) of samples with different compositions to excitation laser radiation, as calculated from the results
of photovoltaic measurements.
active compounds: DHD6ee, phtalocyanine B6PH,
and polythiophene PHT dyads. Upon optical exciꢀ
tation of the primary donor at a wavelength of
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ
tion for Basic Research and the Academy of Finland
436 nm, the sensitivity of the samples was as high as (grant 07ꢀ02ꢀ91016ꢀAF_a).
27 V cm2/mJ.
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