Photoinduced electron transfer of double-bridged phthalocyanine-fullerene dyads

Article abstract of DOI:10.1016/j.cplett.2006.08.107

Three new double-bridged phthalocyanine-fullerene dyads were studied with spectroscopic and electrochemical methods. The two-linker strategy was applied for the first time to phthalocyanine-fullerene dyads. Photoinduced electron transfer (ET) of the dyads was found to occur from the excited state of phthalocyanine both in polar and non-polar environment. Examination of ET energetics revealed that the phthalocyanine-fullerene dyads exist mainly in an extended conformation as opposed to a face-to-face orientation common for the similar double-bridged porphyrin-fullerene dyads.

Full text of DOI:10.1016/j.cplett.2006.08.107

Chemical Physics Letters 430 (2006) 36–40
www.elsevier.com/locate/cplett
Photoinduced electron transfer of double-bridged
phthalocyanine–fullerene dyads
a,
a
a
a
*
Marja Isosomppi , Nikolai V. Tkachenko , Alexander Efimov , Heidi Vahasalo ,
a
b
a
Johanna Jukola , Pirjo Vainiotalo , Helge Lemmetyinen
a
Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, 33101 Tampere, Finland
b
Department of Chemistry, University of Joensuu, P.O. Box 111, 80101 Joensuu, Finland
Received 30 June 2006; in final form 21 August 2006
Available online 30 August 2006
Abstract
Three new double-bridged phthalocyanine–fullerene dyads were studied with spectroscopic and electrochemical methods. The two-
linker strategy was applied for the first time to phthalocyanine–fullerene dyads. Photoinduced electron transfer (ET) of the dyads
was found to occur from the excited state of phthalocyanine both in polar and non-polar environment. Examination of ET energetics
revealed that the phthalocyanine–fullerene dyads exist mainly in an extended conformation as opposed to a face-to-face orientation com-
mon for the similar double-bridged porphyrin–fullerene dyads.
ꢀ
2006 Elsevier B.V. All rights reserved.
1
. Introduction
found to undergo ET rapidly [19–21]. Such dyads can also
be modified to form solid films in which the molecules are
oriented controllably, which is of interest when building
photovoltaic devices [22,23].
Photoinduced ET properties of porphyrin–fullerene
dyads have been studied thoroughly by many research
groups during the past decade [1–6]. However, porphyrin
absorbs efficiently only in a rather narrow spectral region
around 430 nm, which is a major drawback considering
applications in photonic devices. Replacing porphyrin with
a phthalocyanine chromophore extends the absorption
range of the dyad considerably matching better with e.g.,
the spectrum of white light and thus solar irradiation.
Despite the obvious advantage of phthalocyanines as elec-
tron donors in donor–acceptor (D–A) systems, they have
been studied to a modest degree as dyads [7–16], because
of their more demanding synthesis compared to porphyrins.
Studies on covalently linked D–A dyads have demon-
strated the importance of close proximity of the donor
and acceptor moieties in achieving efficient ET [5,17,18].
Especially porphyrin–fullerene dyads with a face-to-face
orientation realized with two-linker strategies have been
Two-linker strategy is now applied for the first time to
phthalocyanine–fullerene dyads. Compared to double-
bridged porphyrin–fullerene dyads studied earlier [19,24],
the phthalocyanine–fullerene dyads H PcF-1, H PcF-2,
2
2
and ZnPcF reach the complete charge-separated state from
the first excited singlet state (S ) of phthalocyanine without
1
an observable exciplex intermediate in both polar and non-
polar environment. This behaviour can be explained both
by the different orientation of the donor and acceptor in
the present phthalocyanine–fullerene dyads compared to
several porphyrin–fullerene dyads or by the short lifetime
of the exciplex state in the former case.
2. Materials and methods
2.1. Studied compounds
*
Structures of the studied compounds are presented
in Fig. 1. Synthesis of the molecules is described in the
Corresponding author. Fax: +358 3 31152108.
E-mail address: marja.isosomppi@tut.fi (M. Isosomppi).
0
009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.08.107

M. Isosomppi et al. / Chemical Physics Letters 430 (2006) 36–40
37
16. Modelling the structures with CAChe (WorkSystem
Pro Version 6.1.12.33) and estimating the D–A distances,
rDA, revealed that there are two alternative orientations
with different rDA: a face-to-face orientation with rDA
6
=
˚ ˚
.5 A and an extended orientation with r = 12.5 A.
DA
For H PcF-2 the possible isomers (2,6-, 2,7- and 2,15-
2
bisubstituted, Fig. 1) are very similar to each other and
most probably exist in an extended orientation with rDA
=
˚
˚
1
2.0 A. Also a face-to-face orientation with r = 6.5 A
DA
is possible. H PcF-1 may adopt a face-to-face or extended
2
˚
˚
orientation with respective rDA of 6.5 A and 13.0 A.
2.2. Experimental methods
Absorption spectra were measured with a Shimadzu
UV-2501PC spectrophotometer. Fluorescence spectra were
recorded using a Fluorolog 3 (SPEX Inc.) fluorimeter with
a cooled infrared sensitive photomultiplier (Hamamatsu
R2658) and corrected using a correction spectrum supplied
by the manufacturer. Pump-probe and up-conversion tech-
niques for time-resolved absorption and fluorescence,
respectively, were used to detect the fast processes with a
time resolution shorter than 0.2 ps. The instrument and
the used data analysis procedure have been described ear-
lier [25]. Differential pulse voltammetry was used to esti-
mate the energy of the charge-separated state of the
dyads in benzonitrile as described earlier [26]. Tetra-n-
butylammonium tetrafluoroborate (TBABF ) in benzonit-
4
rile was used as the supporting electrolyte and the measure-
ments were done in nitrogen flow.
3. Results and discussion
3.1. Absorption and fluorescence
For the free-base phthalocyanine compounds (Fig. 2)
the Q band at 700 nm was split in two while only one sharp
peak was observed for ZnPc and ZnPcF. The maximum of
the band was clearly red-shifted to 737 nm for H PcF-2 in
2
toluene compared to 716 nm for H PcF-1, because of the
2
different substituents of the phthalocyanine ring (butoxy
or tert-butyl groups). The Q bands were slightly broader
Fig. 1. Studied compounds.
electronic supplementary information. The structure of
H PcF-1 has been determined as the one presented in
2
Fig. 1, but for dyads ZnPcF and H PcF-2 there are a few
2
possible regioisomers with different attachment positions
of the linkers. The asterisks in the structure of ZnPc
(
Fig. 1) mark the two possible attachment points of one
of the substituents resulting in altogether 13 possible iso-
mers of ZnPcF. However, for spatial orientation of phtha-
locyanine and fullerene within the dyad there are only five
alternatives arising from the attachment combinations of
the linkers. The isomers can be obtained from the drawn
structure of Fig. 1 by keeping the linker in position 1 fixed
and changing the other linker to positions 5, 8, 9, 12 and
2 2
Fig. 2. Normalized absorption spectra of H PcF-1 and H Pc in toluene.

38
M. Isosomppi et al. / Chemical Physics Letters 430 (2006) 36–40
for ZnPcF, H PcF-1, and H PcF-2 compared to the corre-
sponding references indicating an interaction between the
chromophores in the dyads.
tion (CR) are very similar for all dyads (Table 1). The
matching rate constants of the two free-base dyads indicate
that orientation of phthalocyanine with respect to fullerene
2
2
Phthalocyanine emission at 730 nm was quenched for
is similar and justifies the estimation of DGCT for H PcF-1
2
H PcF-1 roughly 240 fold in benzonitrile (Fig. 3) and 40
2
as the same as for H PcF-2. The considerably longer life-
2
fold in toluene pointing towards ET from the excited phtha-
locyanine to C . Similar solvent dependence was observed
times of the charge-separated state obtained for all dyads
in non-polar toluene compared to polar benzonitrile are
expected if CR takes place in the inverted region of the
Marcus curve according to electron transfer theory
[29,30,6,21]. Since the charge-separated state energies of
the studied phthalocyanine–fullerene dyads are lower com-
pared to similar porphyrin–fullerene dyads [24], relatively
small reorganization energies of the phthalocyanine–fuller-
ene dyads must account for CR occurring in the inverted
region.
6
0
for H PcF-1 and ZnPcF. In contrast to closely attached
2
phytochlorin– and porphyrin–fullerene compounds studied
earlier [3,5,19,24–27], no clear evidence for formation of an
intermediate exciplex was seen in the emission spectra of the
studied phthalocyanine–fullerene dyads.
3
.2. Differential pulse voltammetry
þ
ꢀ
60
The energies of the charge-separated state Pc ꢀ C for
Decay of S emission of phthalocyanine was monitored
1
ZnPcF and H PcF-2 in benzonitrile were calculated as
at 715 nm or 730 nm for ZnPcF and H PcF-1, respectively.
2
2
1
.16 eV and 1.22 eV from the difference between the first
Similar emission from H PcF-2 could not be measured,
2
oxidation potential of phthalocyanine and the first reduc-
tion potential of C . The S energies of phthalocyanine
because emission of this dyad is too close to the laser fun-
damental harmonic. The decays were bi-exponential in
both toluene and benzonitrile. The rate constants of the
6
0
1
in H PcF-1, H PcF-2, and ZnPcF were estimated from
2
2
1
1
ꢀ1
11 ꢀ1
the absorption and emission spectra to be 1.70 eV,
.65 eV and 1.77 eV, respectively. Knowing this and ignor-
ing Coulombic interaction, the respective charge transfer
CT) free energies, DGCT, of ꢀ0.43 eV and ꢀ0.61 eV from
the S state can be determined for dyads H PcF-2 and
faster components were 0.7 · 10
s
and 2.1 · 10 s
1
for ZnPcF in toluene and benzonitrile, respectively, corre-
sponding well to the rate constants obtained for CT in the
pump-probe measurements (Table 1). A fairly good agree-
ment between the faster components of up-conversion
(
1
2
1
1
ꢀ1
11 ꢀ1
ZnPcF in benzonitrile. Assuming the same DG for both
measurements (1.7 · 10
s
and 13 · 10
s
in toluene
CT
þ
ꢀ
60
free-base dyads, the energy of Pc ꢀ C for H PcF-1 can
and benzonitrile, respectively) and kCT determined in the
2
be estimated as 1.27 eV.
.3. Time-resolved absorption
Pump-probe measurements were carried out with the
pump-probe measurements was found also for H PcF-1.
2
For H PcF-1 the slower component had two times lower
2
3
amplitude than the fast component and lifetimes of >2 ns
and 13 ps in toluene and benzonitrile, respectively. For
ZnPcF the two components had similar amplitudes and
the lifetimes of the slower component were 230 ps and
50 ps in toluene and benzonitrile, respectively. The bi-expo-
excitation at 420 nm promoting phthalocyanine to its sec-
ond excited singlet state (S ). Internal conversion from S
2
2
to S was too fast to be detected for the free-base dyads,
nential decay may indicate equilibrium between S and
1
1
but was resolved (with s = 0.2 ps) for ZnPcF. Formation
of both phthalocyanine radical cation and C60 radical
anion absorptions around the characteristic wavelengths
another state, possibly an exciplex, as was shown earlier
for porphyrin–fullerene and phytochlorin–fullerene dyads
[2,31]. However, the exciplex formation is not evident from
the steady-state emission and pump-probe measurements,
and the origin of the second component in the up-conver-
sion measurements still needs to be clarified.
[
16,28] were detected for all dyads in both benzonitrile
and toluene (around 900 nm and 1000 nm for H PcF-1,
2
Fig. 4). The rate constants for CT and charge recombina-
2 2
Fig. 3. Fluorescence spectra of H PcF-1 and H Pc in benzonitrile.
Spectrum of the dyad has been multiplied for comparison as indicated
in the figure.
2
Fig. 4. Decay component spectra of H PcF-1 in benzonitrile.

M. Isosomppi et al. / Chemical Physics Letters 430 (2006) 36–40
39
Table 1
Rate constants (s ) of charge transfer, kCT, and charge recombination, kCR, for the dyads
ꢀ1
Compound
Solvent
H
2
PcF-1
H
2
PcF-2
ZnPcF
Toluene
Benzonitrile
Toluene
Benzonitrile
Toluene
Benzonitrile
1
1
11
11
11
11
11
k
k
CT
3.3 · 108
<5 · 10
4.0 · 10
1.1 · 10
<5 · 10
3.6 · 10
0.6 · 10
<5 · 10
1.9 · 10
a
10
8a
10
8a
10
CR
a
1.2 · 10
1.9 · 10
1.8 · 10
Detection upper limit of the instrument is 1.5 ns.
3
.4. Energetics of CT
Table 2
Energetics of ET for the dyads in benzonitrile
Energetics of the CT reaction in benzonitrile were calcu-
Compound
H
2
PcF-1
H
2
PcF-2
ZnPcF
˚
lated for the two possible orientations and D–A distances
rDA (A)
6.5
0.54
19000
13.0
0.97
46
6.5
0.54
5370
12.0
0.93
33
6.5
0.54
1160
12.5
0.95
5
˚
k (eV)
CT/kCR
of the dyads. Estimating the donor radius r = 4 A and
D
˚
k
acceptor radius r = 4 A the solvent reorganization energy
A
ks, which accounts for the majority of the total reorganiza-
tion energy k, can be calculated with [30]
as 1160 for the face-to-face and as 5 for the extended orien-
tation. The ratio of the rate constants obtained from the
transient absorption measurements (Table 1) is 11 suggest-
ing the extended orientation, i.e., 1,5- or 1,16-substitution
of the dyad. Furthermore, solving Eq. (2) for V yields a
ꢀ
ꢁꢀ
ꢁ
2
q
1
1
1
1
1
k ¼ 4
pe0 2rD 2rA rDA
þ
ꢀ
eopt es
ꢀ
ð1Þ
as 0.54 eV and 0.95 eV for the face-to-face and extended
orientations of ZnPcF, respectively.
ꢀ
1
reasonable value of 15 cm for the extended orientation.
Equal evaluation for H PcF-1 and H PcF-2 suggests the
2
2
Sketching the potential energy profiles of the CT reac-
tion for ZnPcF (Fig. 5) the difference between the two ori-
entations is evident. For both orientations CT occurs at
almost maximum rate, but for the face-to-face orientation
CR is pushed far to Marcus inverted region slowing down
the reaction considerably compared to the extended
orientation.
extended orientations also for these dyads. Results of the
calculations are summarized in Table 2.
4. Conclusions
For the phthalocyanine–fullerene dyads CT was found
to occur from S of phthalocyanine both in polar and
1
Rate constant of the ET reaction is given by
non-polar solvents and CR was significantly slower in
non-polar environment. The rates of CT and CR for
ZnPcF, H PcF-1, and H PcF-2 were of the same order of
!
3
=2
2
2
2
p
V
ðDG þ kÞ
k ¼
exp
=2
ꢀ
ð2Þ
1
h
4kk T
2
2
ðkk
B
TÞ
B
magnitude. Analysing the energetics of CT revealed that
the phthalocyanine–fullerene dyads studied exist mainly
in extended orientations. Further attempts to synthesize
phthalocyanine–fullerene dyads with face-to-face orienta-
tions are in progress, because significantly longer-living
charge-separated states in polar environment can be
expected for such compounds.
Assuming the electronic coupling, V, to be roughly the
same for CT and CR the ratio of the rate constants for
CT and CR can be estimated from
!
2
2
k
k
CT
CR
ðDGCT þ kÞ
ðDGCR þ kÞ
¼
exp
ꢀ
þ
ð3Þ
4kk
B
T
4kk T
B
Fig. 5. Gibbs free energy surfaces of the CT reaction of ZnPcF in benzonitrile for (a) the face-to-face and (b) the extended orientation.

4
0
M. Isosomppi et al. / Chemical Physics Letters 430 (2006) 36–40
[12] D.M. Guldi, J. Ramey, M.V. Martinez-Diaz, A. de la Escosura, T.
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This work was supported by the National Technology
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Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.cplett.2006.
0
8.107.
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