High effectiveness of oligothienylenevinylene as molecular wires in Zn-porphyrin and C60 connected systems

Article abstract of DOI:10.1039/b711194k

Photoinduced energy transfer and electron transfer processes have been found between the excited singlet state of Zn-porphyrin and C₆₀ via an oligothienylenevinylene bridge depending on the length of the oligothiophene and solvent polarity. The

Full text of DOI:10.1039/b711194k

Chemical Communications
www.rsc.org/chemcomm
Number 43 | 21 November 2007 | Pages 4425–4548
ISSN 1359-7345
COMMUNICATION
Osamu Ito, Fernando Langa et al.
High effectiveness of oligothienylenevinylene as molecular wires in Zn-
porphyrin and C connected systems
60

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
High effectiveness of oligothienylenevinylene as molecular wires in Zn-
porphyrin and C60 connected systems{
Fre´de´ric Oswald,a D.-M. Shafiqul Islam,bc Yasuyuki Araki,b Vincent Troiani,a Ruben Caballero,a
Pilar de la Cruz,a Osamu Ito*b and Fernando Langa*a
Received (in Cambridge, UK) 23rd July 2007, Accepted 11th September 2007
First published as an Advance Article on the web 21st September 2007
DOI: 10.1039/b711194k
solubility, under careful stoichiometric control to give yields of 42
and 54%, respectively. Compound 1 was prepared according to
the Lindsey method.8 The target molecules 4a,b were prepared by
1,3-dipolar cycloaddition between 3a,b, N-methyl glycine and C60
in chlorobenzene in good yield (49% in both cases) (see ESI{ for
details on the characterization, Fig. S1–S6). Optimized structures
and the HOMO and LUMO of 4a,b were evaluated by Gaussian
calculations (ESI;{ Fig. S7), which suggest that the charge-
Photoinduced energy transfer and electron transfer processes
have been found between the excited singlet state of Zn-
porphyrin and C60 via an oligothienylenevinylene bridge
depending on the length of the oligothiophene and solvent
polarity.
One-dimensionalnanostructuressuchasnanowireshavebecomethe
focus of intensive researches due to their potential uses in devices
including sensors, electronics and optoelectronics. Linear p-con-
jugated oligomers with well-defined chemical structures have been
used as molecular wires in molecular electronics or nanoscopic
systems;1 moreover, photoinduced electron transfer in donor–
bridge–acceptor systems, where the bridge is a p-conjugated
+
?
2
?
.
separated states are ZnP –nTV–C60
The redox potentials, detected by OSWV (vs. Ag/AgNO3 in
o-DCB–acetonitrile (4 : 1) solution (0.1 mol dm23 Bu4NClO4) at
room temperature), of ZnP–2TV–C60 (4a) showed a one-electron
reduction potential of the C60 moiety at 21.07 V whereas the
oxidation potential appeared at 0.35 V. These findings can be
attributed to theoverlapped peaks of both ZnP and 2TV, a situation
oligomer, exhibits weak distance dependence for donor–acceptor
2
distances as long as 40 A. Among the different p-conjugated
˚
+
2
?
?
oligomers, oligothienylenevinylene oligomers (nTVs) have been
describedas excellentwires,3 similar to other conjugated oligomers.4
Molecular architectures formed by porphyrins and fullerenes
linked by various bridging groups have been the subject of
numerous studies in recent years.5 For example, oligothiophene
(OT)6 and oligophenylenevinylene (OPV) moieties7 have been
shown to act as long distance molecular wires in the efficient
photoinduced electron transfer from the porphyrin chromophore
to the fullerene. Although C60–nTV dyads have been investigated
in which nTVs act as electron donors,8 triads in which nTVs act as
wires between porphyrins and fullerene have not been reported to
date despite the potential interest in such systems.
consistent with the possibility of ZnP –2TV–C60 and ZnP–
2TV –C60 2 being present in an equilibrium. In the case of ZnP–
4TV–C60 (4b),one-electronoxidationpotentialsat0.16and0.29Vvs
Ag/AgNO3 were observed. These values can be attributed to 4TV
+
?
?
We report here the synthesis of novel ZnP–nTV–C60 donor–
acceptor systems that incorporate a Zn-porphyrin (ZnP) as the
donor and fullerene (C60) as the acceptor, with these units bridged
by nTV units. In addition, the dependence of the photoinduced
energy-transfer and electron-transfer processes on solvent polarity
was investigated. Charge-separation quantum yields close to unity
and lifetimes of the charge-separated state as long as one
microsecond were found for the ZnP–nTV–C60 triad 4b (Chart 1).
In the first step, ZnP–nTV systems 3a,b were synthesized by
Wadsworth–Emmons olefination between phosphonateporphyrin
19 and bisformyl-oligomers 2a,b,10 bearing alkyl groups to afford
aFacultad de Ciencias del Medio Ambiente, Universidad de Castilla-La
Mancha, 45071, Toledo, Spain
bInstitute of Multidisciplinary Research for Advanced Materials, Tohoku
University, Katahira Sendai, 980-8577, Japan
cDepartment of Chemistry, Jahangirnagar University, Savar, Dhaka,
Bangladesh
{ Electronic supplementary information (ESI) available: Structural
characterization of compounds 4a,b, optimized structures and optical
absorption spectra. See DOI: 10.1039/b711194k
Chart 1 Triads 4a,b and the corresponding starting compounds
(1, 2a,b, 3a,b).
4498 | Chem. Commun., 2007, 4498–4500
This journal is ß The Royal Society of Chemistry 2007

side of 2TV. In toluene, the appearance of a peak at 715 nm, due
to the fluorescence of the fullerene, indicates energy transfer (EnT)
from the 1ZnP* moiety to the C60 moiety either directly or
indirectly.11 In PhCN the decrease in the fluorescence was
1
observed without the appearance of C60*-fluorescence; thus, the
fluorescence quenching can be attributed to the charge-separation
+
generated by ZnP –2TV–C60 2 as the first step, a process that has
?
?
a negative DGCS value in PhCN.
The EnT and charge-separation processes in 4a (ZnP–2TV–C60)
in toluene were further confirmed by the time-resolved fluorescence
spectra (ESI;{ Fig. S9), in which the persistent 1C60*-fluorescence
was confirmed in the 700–800 nm region after the decay of 1ZnP*-
fluorescence;11 however, such persistent 1C60*-fluorescence was not
observed in PhCN. In the case of 4b (ZnP–4TV–C60), the
Fig. 1 Absorption spectra in toluene: (i) ZnP–2TV (5a), (ii) 4a (ZnP–
2TV–C60), (iii) magnified spectrum (650) of C60 absorption of 4a and
NMP–C60 (N-methylfulleropyrrolidine).
1
fluorescence intensities of the ZnP* moiety decreased in toluene
andPhCNwithouttheappearanceof1C60*-fluorescence,suggesting
+
2
?
?
andZnP, respectively, andprovideevidencethatZnP–4TV –C60
+
2
?
is more stable than ZnP –4TV–C60
?
.
predominant charge-separation via 1ZnP* in both solvents.
Theelectronicabsorptionspectrumof4aintoluene(Fig.1)canbe
understood asa simple superposition ofthe electronic transitions of
the three constituent chromophores: i.e., the Zn-porphyrin (1)
displays a very strong absorption band at 422 nm (Soret band) and
the p–p* transition of the 2TV appears as a shoulder on the strong
Soret band between 450 and 500 nm. The absorption peaks at 320
and 700 nm can be attributed to the C60 moiety. The sum of the
absorption intensities for ZnP–2TV and NMP–C60 (N-methyl-
fulleropyrrolidine) is slightly lower than that for ZnP–2TV–C60 at
330and500nm,whichcanbeattributedtotheshiftsofthebandsdue
to weak interaction among the chromospheres in the ground state.
In the case of 4b, the 4TV absorption appeared to 500–600 nm and
superimposed over the Q bands (ESI;{ Fig. S8), while other
absorption peaks are similar to those of 4a.
1
The ZnP*-fluorescence lifetimes of ZnP–2TV–C60 (4a) were
evaluated to be 53 ps in toluene and 86 ps in PhCN from the 1ZnP*-
fluorescence decay time profiles (ESI;{ Fig. S10). The rate constant
for the EnT (kEnT) in toluene was evaluated to be 1.8 6 1010 s21. In
PhCNtherateconstantof1ZnP*-fluorescencequenching(kq)canbe
calculated to be 1.2 6 1010 s21, which mainly includes the charge-
separation rate constant (kCS). For ZnP–4TV–C60 (4b) the 1ZnP*-
fluorescence lifetimes have two components, which give short initial
lifetimes [138 ps (65%) in toluene and 107 ps (64%) in PhCN] and
longerlifetimes(1065psintolueneand971psinPhCN),whichcanbe
attributedtodifferentconformationssuchasfullyextendstructure.12
The kCS values were evaluated on average to be 6.7 6109 and 8.8 6
109 s21 in toluene and PhCN, respectively. The kCS values in PhCN
decrease with nTV and this gives a small damping factor for the
charge-separation constants (kCS), which shows the effectiveness of
nTV as a molecular wire.
The emission spectra of the triad 4a are shown in Fig. 2 along
with those of the reference compounds ZnP–2TV (5a). The
fluorescence spectra of 4a in toluene (Fig. 2(a)) and in benzonitrile
In an effort to obtain direct evidence for the energy transfer and
charge-separation processes, the transient absorption spectra were
measured. In toluene, the band at 760 nm (with a shoulder at
800 nm) was observed along with the band at 600 nm for 4a in
toluene after the 355-nm nanosecond laser light pulse excitation
(ESI;{ Fig. S11). These bands are attributed to the triplet states of
2TV (32TV*), ZnP (3ZnP*) and C60 (3C60*). This fact is consistent
(PhCN) (Fig. 2(b)) upon irradiation of the Soret band (lex
=
420 nm) of the ZnP moiety are also shown. In 4a the fluorescence
intensity of the 1ZnP* moiety (maxima at 600 and 650 nm in
toluene) was markedly reduced (by a factor of at least 98%)
compared with ZnP–2TV by the fullerene attached on the opposite
1
with the EnT processes occurring from ZnP* to C60, producing
3
1C60*, followed by the generation of C60* via an intersystem
crossing process.
In PhCN (Fig. 3) an additional transient absorption peak was
observed at 1020 nm and this is the diagnostic peak for C60 2. The
?
+
?
shoulder at 620 nm observed at 1000 ns was assigned to ZnP ,
+
suggesting the generation of ZnP –2TV–C60 2. On the other
?
?
hand, the 720 nm band with a shoulder at 800 nm can be
+
+
?
?
attributed to 2TV , suggesting the co-existence of ZnP–2TV –
C60 2. The decay of C60 2 can be fitted with a mono-exponential
function, giving a charge-recombination rate (kCR) of 1.8 6
106 s21, which corresponds to the lifetime of the charge-separated
state as 550 ns.
?
?
Absorption bands appeared at 700, 760 and 1020 nm for 4b
(ZnP–4TV–C60) in toluene and PhCN (ESI;{ Fig. S12) upon
excitation with 355-nm laser light, indicating that the ZnP moiety
and 4TV moiety are excited along with a small amount of the C60
moiety. The 1020 nm band can be attributed to C60 2. The intense
?
Fig. 2 Steady-state fluorescence spectra of 4a (ZnP–2TV–C60) and 5a
(ZnP–2TV) in (a) toluene and (b) PhCN, lex = 420 nm.
absorption band at 760 nm is mainly attributed to the triplet state
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4498–4500 | 4499

Fig. 4 Energy diagram and proposed photophysical processes.
Fig. 3 Transient absorption spectra of 4a (0.1 mM) in Ar-saturated
PhCN obtained by 355-nm laser light irradiation. Inset: Absorption time
profiles.
over energy transfer. The small damping factor for the charge-
1
separation via ZnP* in PhCN suggests the effectiveness of nTV
3
3
of 4TV (34TV*) and the absorption bands of ZnP* and C60*
as molecular wire. We are currently extending the nTV unit in
ZnP–nTV–C60.
may be overlapped with this. The absorption bands of 4TV + are
expected to appear at 660, 950 and 1120 nm13 but they were very
?
Financial support from Ministerio de Educacio´n y Ciencia of
Spain and FEDER funds (Project Consolider-HOPE CSD2007-
00007 and project CTQ2007-63363/PPQ) is gratefully acknowl-
edged. This work was also supported by the 21st Century Center
of Excellence Program ‘‘Giant Molecules and Complex Systems’’
of Tohoku University.
weak in the observed transient absorption spectrum at 100 ns. This
+
2
?
indicates that ZnP–4TV –C60 generation is minor; this species
?
is generated either by hole-shift (HS) from an initial charge-
+
separated state such as ZnP –4TV–C60 2 via 1ZnP* or by charge-
?
?
1
separation directly via C60*. This HS process in ZnP –4TV–
C60 2 may be competitive with the charge-recombination process,
+
?
?
generating the triplet states of ZnP* and 3C60*. The kCR value was
evaluated to be 0.6 6 106 s21 from the decay at the 1020-nm band.
Thus, the lifetime of the charge-separated state was evaluated to be
1670 ns in toluene.
Notes and references
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C. B. Gorman, Angew. Chem., Int. Ed., 2002, 41, 4378.
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1998, 396, 60.
3 I. Jestin, P. Fre`re, P. Blanchard and J. Roncali, Angew. Chem., Int. Ed.,
1998, 37, 942.
4 (a) J.-F. Nierengarten, New J. Chem., 2004, 28, 1177; (b) N. Mart´ın,
Chem. Commun., 2006, 2093; (c) T.-M. Figueira-Duarte, A. Ge´gout and
J.-F. Nierengarten, Chem. Commun., 2007, 109.
In PhCN, the combination of the 1020 nm band with the 660,
+
2
?
?
860 and 1200 nm bands shows the presence of ZnP–4TV –C60
even after 100 ns. This species could be generated from the initial
+
charge-separated state (ZnP –4TV–C60 2) via HS. A kCR value of
?
?
0.5 6 105 s21 was evaluated on the basis of the time profile at
5 (a) H. Imahori and S. Fukuzumi, Adv. Funct. Mater., 2004, 14, 525; (b)
H. Imahori, Org. Biomol. Chem., 2004, 2, 1425; (c) D. M. Guldi,
G. M. A. Rahman, V. Sgobba and C. Ehli, Chem. Soc. Rev., 2006, 35,
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S. A. Vail, D. I. Schuster, D. M. Guldi, M. Isosomppi, N. Tkachenko,
H. Lemmetyinen, A. Palkar, L. Echegoyen, X. Chen and J. Z. H. Zhang,
J. Phys. Chem. B, 2006, 110, 14155.
1020 nm in PhCN (ESI;{ Fig. S12b) and, from this constant, the
+
lifetime for ZnP–4TV –C60 2 was evaluated as 2000 ns. The kCR
?
?
value in PhCN was also slightly lower than that in toluene.
+
2
?
?
Compared with the kCR value for ZnP–2TV –C60 in equili-
+
brium with ZnP –2TV–C60 2 in PhCN, the kCR value of ZnP –
?
4TV–C60 is markedly smaller (by a factor of 1/3), although
+
?
?
?
2
6 (a) J. Ikemoto, K. Takimiya, T. Otsubo, M. Fujitsuka and O. Ito, Org.
Lett., 2002, 4, 309; (b) T. Nakamura, M. Fujitsuka, Y. Araki, O. Ito,
J. Ikemoto, K. Takimiya, Y. Aso and T. Otsubo, J. Phys. Chem. B,
2004, 108, 10700; (c) T. Oike, T. Kurata, K. Takimiya, T. Otsubo,
Y. Aso, H. Zhang, Y. Araki and O. Ito, J. Am. Chem. Soc., 2005, 127,
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K. Takimiya, Y. Aso and T. Otsubo, J. Phys. Chem. B, 2005, 109,
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therein.
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the charge-recombination processes are different. The ratios of
kCS/kCR were evaluated to be ca. 105, indicating that 4a,b (ZnP–
nTV–C60) are efficient charge-separation systems that stabilize the
charge-separated states.
In summary, an energy diagram can be depicted as shown in
Fig. 4. Since the four chromophores ZnP, C60, 2TV and 4TV have
their triplet states, which are generated through intersystem
crossing from the excited singlet states and charge-recombination
of the charge-separated states higher in energy than these triplet
states,11 the transient spectra became complicated due to overlap
+
2
?
with the absorption bands of radical ions such as ZnP , C60
?
,
+
+
1
?
?
2TV and 4TV . The processes that occur via ZnP* are clearly
revealed on combination of the fluorescence data and transient
absorption data. For short nTV units in a nonpolar solvent, energy
transfer takes priority over charge separation, whereas in PhCN
the charge-separation predominantly occurs because of the
stabilization of the charge-separated state. In the case of longer
nTV units in a nonpolar solvent, the charge-separation prevails
13 J. J. Apperloo, C. Martineau, V. A. P. Hal, J. Roncali and
R. A. J. Janssen, J. Phys. Chem. A, 2002, 106, 21.
4500 | Chem. Commun., 2007, 4498–4500
This journal is ß The Royal Society of Chemistry 2007