Contrasting photodynamics between C60-dithiapyrene and C 60-pyrene dyads

Article abstract of DOI:10.1002/chem.200700837

The photodynamics of a C₆₀-dithiapyrene donor-acceptor conjugate were compared with the corresponding Cftirpyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C₆₀-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.

Full text of DOI:10.1002/chem.200700837

DOI: 10.1002/chem.200700837
Contrasting Photodynamics between C60–Dithiapyrene and C60–Pyrene
Dyads
Dirk M. Guldi,*[a] Fabian Spänig,[a] David Kreher,[b] Igor F. Perepichka,[b, d]
Cornelia van der Pol,[b] Martin R. Bryce,*[b] Kei Ohkubo,[c] and Shunichi Fukuzumi*[c]
Dedicated to the memory of Janos H. Fendler
Abstract: The photodynamics of a C60–
dithiapyrene donor–acceptor conjugate
were compared with the corresponding
C60–pyrene conjugate. The photoin-
duced charge separation and subse-
quent charge recombination processes
were examined by time-resolved fluo-
rescence measurements on the picosec-
ond timescale and transient absorption
measurements on the picosecond and
microsecond timescales with detection
in the visible and near-infrared regions.
We have observed quite long lifetimes
(i.e., up to 1.01 ns) for the photogener-
ated charge-separated state in a C60–di-
thiapyrene dyad without the need for i)
a long spacer between the two moiet-
ies, or ii) a gain in aromaticity in the
radical ion pair.
Keywords: dyads · electron trans-
fer · fullerenes · photodynamics ·
pyrenes
Introduction
Covalently linked donor–acceptor conjugates including full-
erene C60 have attracted much attention because electron-
transfer reactions to C60 are highly efficient due to the mini-
mal changes of structure and solvation that are associated
with the electron-transfer reduction.[1–5] Such conjugates
have been widely used in photoelectronic devices such as
photovoltaic cells.[6,7] As electron donors for linkage to C60,
porphyrins have frequently been used, because porphyrins,
which contain an extensively conjugated two-dimensional p-
system, are also suitable for efficient electron transfer due
to the minimal structural and solvation changes upon elec-
tron transfer. In addition, rich and extensive absorption fea-
tures of porphyrinoid systems guarantee increased absorp-
tion cross-sections and an efficient use of the solar spec-
trum.[8–10] However, only polar solvents can be used for
charge separation, since the charge-separated state cannot
be observed in non-polar solvents where the charge separa-
tion energy exceeds that of the triplet energy of C60. In such
a case the photoexcitation of the donor–acceptor conjugates
leads to the formation of the triplet excited state of C60
rather than the radical ion pair state.[11] As compared to por-
phyrins, tetrathiafulvalene (TTF) provides an important ad-
vantage due to the stabilization of the radical ion pair state
of the C60 conjugates relative to the triplet excited state of
the C60 moiety.[5,12–16] 1,6-Dithiapyrene (DTP, 1) is also
known to act as an excellent Weitz-type electron donor with
a reduction potential comparable to that of TTF.[17,18] A key
difference between DTP (1) and TTF is that DTP is aromat-
[a] Prof. D. M. Guldi, F. Spänig
Institute for Physical Chemistry
Friedrich-Alexander-Universität Erlangen-Nürnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
Fax : (+49)9131-852-8307
E-mail: dirk.guldi@chemie.uni-erlangen.de
[b] Dr. D. Kreher, Dr. I. F. Perepichka, C. van der Pol, Prof. M. R. Bryce
Department of Chemistry and Centre for Molecular
and Nanoscale Electronics, Durham University
Durham DH1 3LE (UK)
Fax : (+44)191-384-4737
E-mail: m.r.bryce@durham.ac.uk
[c] Dr. K. Ohkubo, Prof. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering, Osaka University
SORST, JAPAN Science and Technology Agency
Suita, Osaka 565-0871 (Japan)
E-mail: fukuzumi@ap.chem.eng.osaka-u.ac.jp
[d] Dr. I. F. Perepichka
On leave from:
National Academy of Sciences of Ukraine
L. M. Litvinenko Institute of Physical Organic and Coal Chemistry
Donetsk 83114 (Ukraine)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author: Synthetic proce-
dures for compounds 1–3, Figures S1–S33, comprising 1D and 2D
NMR spectroscopic data, cyclic voltammetry data, electronic absorp-
tion and differential absorption spectra, DFT calculations including
orbital energy levels diagrams, orbital localization and tables of opti-
mized geometries.
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Chem. Eur. J. 2008, 14, 250 – 258

FULL PAPER
ic (18 p-electrons) in the neutral state, whereas TTF has a
non-aromatic neutral state (two 7 p-electron rings) and
gains aromaticity (formation of 6 p-electron dithiolium cat-
ions) upon oxidation. 1,6-Dithiapyrene (1) has more exten-
sive absorption in the visible region, acting as a better chro-
mophore than TTF. However, donor–acceptor dyads com-
posed of dithiapyrene and C60 have yet to be reported, de-
spite the evident interest in combining these moieties.
afforded the mono-adduct 4 (64% yield) (Scheme 1). A sim-
ilar Prato reaction of pyrene-1-carboxaldehyde (5) gave C60–
pyrene dyad 6 in 56% yield. Although electrochemical in-
We report herein the synthesis and photodynamics of C60–
dithiapyrene conjugate 4. The corresponding C60–pyrene
conjugate 6 with virtually the same geometry was synthe-
sized to compare the photodynamics between C60–dithiapy-
rene and C60–pyrene dyads. In order to confirm the photoin-
duced charge separation and subsequent charge-recombina-
tion processes, we employed time-resolved fluorescence
measurements on the picosecond timescale and transient ab-
sorption measurements on the pico- and microsecond time-
scale with detections that range from the visible to the near-
infrared regions. The present study provides deeper insight
into the control of the electron-transfer processes by subtle
changes in the redox potentials and/or solvent polarity.
Scheme 1. Synthesis of the compounds discussed herein.
Results and Discussion
Synthesis: Functionalization of
1 is essentially unex-
plored[17–21] and no 1–acceptor dyads have been reported.
Derivatives of 1 functionalized only at positions 2 and 7 are
known from the literature, which are accessible through lith-
iation chemistry [reaction with nBuLi followed by quench-
ing with an electrophile, for example, C6F13I[19] or N,N-dime-
thylformamide (DMF)[20] resulting in 2-iodo-, 2,7-diodo- or
2-formyl-derivatives, respectively]. We have found that elec-
trophilic substitution in 1 opens an access to 1,6-dithiapy-
vestigations on dyad 6 have been reported recently,[23] no de-
tails on its synthesis and characterization have been given.
Both structures were assigned from 1H NMR spectra,
MALDI-TOF MS and electronic absorption spectra. Dyad 6
was additionally characterized by 13C NMR spectroscopy
(solubility of dyad 4 is very low for 13C NMR, but it showed
satisfactory elemental analyses).
A
Theoretical calculations: Prior to the photophysical investi-
gations we tested the donor and acceptor features in C60–di-
thiapyrene (4) and C60–pyrene (6). In this context, B3LYP/6-
31G(d) calculations emerged as an important reference
point. They reveal that in fully optimized geometries the
HOMO and LUMO are exclusively delocalized over the di-
thiapyrene (À4.58 eV)/pyrene (À5.39 eV) and C60 (À3.05/
À3.06 eV) moieties, respectively (Figure 1; for more details
on DFT calculations see Figures S30–S33 in Supporting In-
formation). This reflects a lack of electronic interactions be-
tween the donor and acceptor fragments in both donor–ac-
ceptors systems, despite imminent close center-to-center dis-
tances between the donor and the acceptor moieties of 8.17
and 8.30 Š in 4 and 6, respectively. The small effects of the
donor units on the LUMO (0.17–0.18 eV) and the acceptor
unit on the HOMO energy levels (0.14 eV) in C60–dithiapyr-
ene (4) and C60–pyrene (6)—as evidenced from comparison
with parent DTP (1) and C60, Figure 2—are in good agree-
ment with the lack of electronic interactions between the
donor and acceptor moieties. Considering the energies of
the two HOMO and LUMO levels we estimate energy gaps
in the gas phase of 1.53 eV for C60–dithiapyrene and 2.34 eV
for C60–pyrene (Figure 2).
phorus oxychloride and DMF under Vilsmeier conditions
gave 5-formyl derivative 2 in 68% yield.
The new 5-formyl derivative 2 has a melting point range
of 255–2578C, which is substantially different from the
known 2-formyl-1,6-dithiapyrene (m.p. 215–2168C).[20] Con-
sidering that other physical data for these two isomers are
similar (e.g. elemental analyses, MS, number of signals in H
and 13C NMR, and splitting in H NMR spectra) and routine
one-dimensional NMR spectroscopy did not confirm the po-
sition of functionalization in the 1,6-dithiapyrene ring, the
structure of 2 was proved by 2D-NMR spectroscopy studies
(COSY, NOESY techniques, see Supporting Information
Figures S1–S9). In order to provide further confirmation of
this structure, aldehyde 2 was converted into the aldimine 3
(see Supporting Information). The increased solubility intro-
duced by the cyclohexyl moiety of 3 enabled more detailed
NMR spectroscopic studies (COSY, NOESY, HSQC,
HMBC techniques) to be preformed to unambiguously
assign the structure (Supporting Information, Figures S10–
S15).
tion protocol with an azomethine ylide generated in situ[22]
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251

D. M. Guldi, M. R. Bryce, S. Fukuzumi et al.
Figure 1. Localization of HOMO and LUMO orbitals in dyads 4 and 6
according to B3LYP/6-31G(d) calculations.
Figure 3. Cyclic voltammograms of a) C60–dithiapyrene
4 (scan rate
500 mVsÀ1) and b) C60–pyrene 6 (scan rate 100 mVsÀ1) dyads in benzoni-
trile, 0.1m Bu4NPF6.
E2ox1/2) and little cathodic shifts of the reduction potentials
of the C60 moiety compared with 7 (by 0.02 V for both
1/2
1/2
E1red and E2red in PhCN, Table 1). These results are in
good agreement with theoretical predictions from DFT cal-
culations discussed above for the evolution of HOMO–
LUMO energies in 4 compared with 1 and C60. A contribu-
Figure 2. Energy levels of frontier orbitals of dyads 4 and 6 in comparison
with those for DTP (1) and C60 from DFT B3LYP/6-31G(d) calculations
in the gas phase.
1/2
tion to the cathodic shift of E1red in 4 compared with pris-
tine C60 arises from breaking of a C=C double bond in the
1/2
Electrochemistry: The solution redox properties of C60–di-
thiapyrene (4) and C60–pyrene (6) were studied by cyclic
voltammetry (CV) along with compounds 1, C60 and the
model N-methylfulleropyrroli-
C60 system; thus, 7 showed a 90 mV cathodic shift of E1red
compared with C60 (Table 1; cf. ref.[24]).
Reduction of C60–pyrene (6) occurs at almost the same
potentials as for 4 and 7 thus confirming a negligible effect
of the donor units on the LUMO energy level. In contrast,
the oxidation of the pyrene unit in 6 is anodically shifted by
dine (7; Table 1). Compound 1
displayed two reversible, one-
electron oxidation waves at low
pa
0.91 V (from comparison of their E1ox potentials) com-
potentials, due to sequential
pared with the oxidation of the DTP moiety in 4 and the
cation radical and dication for-
process is electrochemically irreversible (Figure 3, Table 1).
mation, as described previous-
ly.[18] C60–dithiapyrene (4), on
the other hand, is highly redox
amphoteric, displaying two
single-electron oxidation waves
Photophysics: Orbital analysis of 4 confirms the expected
localization of the HOMO on the dithiapyrene fragment
and the LUMO on the C60 moiety (Figure 1). There is little
orbital interaction between the donor and acceptor moieties
as indicated by the electrochemical measurements (see
above). This is also confirmed by the UV-visible spectrum,
which is the superposition of that of each component as
shown in Figure 4 (see also Figures S25and S26 in the Sup-
porting Information), in which the spectrum of C60–pyrene 6
is given for comparison.
from the DTP moiety and two
single-electron reduction waves from the fullerene moiety
(Figure 3, Table 1 and Figures S18–S22 in Supporting Infor-
mation). Small donor–acceptor interactions in 4 result in
slightly anodic shifts of the oxidation potentials of the DTP
1/2
moiety compared to 1 (by 0.07 V for E1ox and 0.02 V for
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C60–Dyads
FULL PAPER
Table 1. Cyclic voltammetric data[a] for compounds 1, 4, 6, 7, and C60.
1/2
1/2
1/2
1/2
1/2
CV
Comp. E1ox
E2ox
[V]
E1red
[V]
E2red
[V]
E3red
[V]
Eg
[eV][b]
[V]
1[c]
1[d]
C60
C60
4[c]
6[c]
7[c]
À0.06
À0.11
+0.41
+0.39
[c]
[d]
À0.92
À0.97
À1.33
À1.38
À1.62
+0.01
+0.45[e] À1.03
À1.02
À1.450.87
À1.43
+0.95[e]
À1.82[f]
1.62
À1.01
À1.43
[a] Electrolyte 0.1m Bu4NPF6; scan rate 100 mVsÀ1 (for 1, 6 and C60) and
500 mVsÀ1 (for 4); potentials measured vs. Ag/Ag+ reference electrode
and standardized to Fc/Fc+ couple [EFc/Fc+ =+0.20 V vs. Ag/Ag+
(PhCN); +0.24 V (PhCN/o-DCB, 1:1 v/v)]. [b] HOMO–LUMO gap, esti-
mated from the onsets of oxidation and reduction processes in CV.
[c] CV in benzonitrile; see also ref. [23] for CV with the microcell in tolu-
ene/DMF 3:2. [d] CV in benzonitrile/o-DCB 1:1 v/v. [e] Irreversible peak
(E1oxpa). [f] Irreversible peak (E1redpc).
Figure 4. UV/Vis spectra of a) dithiapyrene, b) C60–dithiapyrene 4, c)
pyrene and d) C60–pyrene 6 in toluene.
Preliminary insights into conceivable donor–acceptor im-
plications were gained from steady-state fluorescence ex-
periments (Figure 5). Hereby, the fluorescing features of
pyrene and dithiapyrene are particularly useful, since they
enable the deactivation of the pyrene (3.22 eV) and dithia-
pyrene (2.43 eV) singlet excited states in the references, as
well as in C60–dithiapyrene 4 and in C60–pyrene 6, to be fol-
lowed with ease. Specifically, fluorescence quantum yields of
close to unity and fluorescence lifetimes of the order of tens
of nanoseconds are sensitive markers. Notably, both sets of
systems, that is, C60–dithiapyrene 4 and C60–pyrene 6, exhibit
dual fluorescence. Besides the strong fluorescence of dithia-
pyrene and pyrene in the visible part of the spectrum, C60
derivatives exhibit weak fluorescence in the near-infrared
part of the spectrum (650–850 nm). In fact, the C60-reference
compound 7 (1.78 eV) reveals nearly solvent-independent
fluorescence quantum yields of 6.0”10À4 and fluorescence
lifetimes of 1.5ns.
Figure 5. a) Fluorescence spectra in toluene with matching absorption at
the a) 335nm excitation; b) 460 nm excitation; and c) 325nm excitation.
characteristic *0–0 transition around 715nm, despite exclu-
sive excitation of the pyrene moiety. To unravel the mecha-
nism of producing the C60 fluorescence, an excitation spec-
trum was taken. The excitation spectrum of C60–pyrene was
an exact match of the ground state absorption of the fuller-
ene reference 7 with maxima at 360 and 412 nm, respective-
ly. This implies that there is a rapid transfer of singlet excit-
ed state energy from the photoexcited pyrene to the cova-
lently linked C60. Determining the quantum yield (6.0”10À4)
of the C60 fluorescence shows that its formation is quantita-
tive, despite the exclusive excitation of the pyrene fragment.
Like in C60–pyrene 6, the dithiapyrene fluorescence (i.e.,
0.023) in C60–dithiapyrene 4 is subject to a marked fluores-
cence quenching that intensifies with the solvent polarity
(e.g., toluene: 0.003; THF: 0.002). However, for C60–dithia-
pyrene 4 no particular C60 fluorescence (i.e., !10À5) was
seen in the near-infrared region at all.
Relative to the strong fluorescence of the reference
(0.75), the pyrene fluorescence in C60–pyrene is nearly quan-
titatively quenched. Notably, the quenching is stronger in
THF (i.e., 0.003) than in toluene (i.e., 0.005). Instead, the fa-
miliar fullerene fluorescence spectrum was found with a
To complement the fluorescence studies, the photophysics
of C60–dithiapyrene 4 and C60–pyrene 6 were probed by
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D. M. Guldi, M. R. Bryce, S. Fukuzumi et al.
means of time-resolved transient absorption spectroscopy.
Femtosecond laser flash photolysis allowed for the charac-
terization of the dynamic processes, which are associated
with the generation and the fate of photoexcited states in
these novel donor–acceptor conjugates, but more important-
ly, these data helped to shed light on the nature of the pho-
toproduct, that is, an excited state or charge-separated state
evolving from an intramolecular energy or electron transfer
process, respectively.
Following the conclusion of the femtosecond excitation
(i.e., 355 nm) of C60–pyrene 6 a transient intermediate is
seen that—at first glance—resembles mostly that of the C60
reference. As Figure 6 shows the same broad maximum
First, the reference compounds should be discussed. In
femtosecond-resolved transient absorption measurements,
the pyrene reference gave rise to an instantaneously formed
(i.e., within ꢀ2 ps), broadly absorbing transient with a maxi-
mum around 475nm (Figure S27 in Supporting Informa-
tion). On the timescale of up to 3000 ps no significant decay
of the excited state absorption was observed. It is only on
the hundreds of nanosecond timescale that the pyrene sin-
glet excited state converts slowly (i.e., 4.3”106 sÀ1) to the
corresponding triplet manifold, for which the following fea-
tures were determined in our experiments: a transient maxi-
mum at 430 nm.
Figure 6. Differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond laser flash photolysis (355 nm) of ꢀ1.0”10À5
m
solutions of C60–pyrene 6 in deaerated THF at different delay times; exci-
tation at 380 nm with OD=0.6.
Similarly, dithiapyrene reveals on the femtosecond time-
scale the rapid formation (i.e., within ꢀ2 ps) of a singlet ex-
cited state, for which a maximum evolves at 530 nm (Figure
S28 in Supporting Information). This state is metastable and
another process follows, whose outcome on a timescale of
1500 ps is the formation of a distinct, new species, which is
characterized by bleaching around 475nm and a set of
maxima at 515 and 550 nm. This absorption is in excellent
agreement with that found upon nanosecond excitation,
from which we infer that the underlying reaction is an inter-
system crossing (i.e., 2.3”109 sÀ1) from the dithiapyrene sin-
glet to the energetically lower lying triplet excited state. The
triplet decays (i.e., 3.6”105 sÀ1) to the ground state.
Lastly, C60-reference compound 7 should be discussed
(Figure S29 in Supporting Information). The singlet excited
state of compound 7 displays a distinctive singlet–singlet
transition around 880 nm. The lifetime of the singlet–singlet
intermediate state is relatively short, as C60 and most of its
derivatives convert rapidly to the much longer-lived triplet
excited state with nearly unit yield. The process is a spin-for-
bidden intersystem crossing (ISC) with a high rate of 5.0”
108 sÀ1 driven by an efficient spin-orbit coupling. The spec-
tral characteristics of the triplet excited state are maxima at
360 and 700 nm, followed by a low-energy shoulder at
800 nm.[25]
In the context of the current investigations, important fea-
tures of the electron donors (i.e., dithiapyrene and pyrene)
and the electron acceptor (i.e., C60) are their corresponding
radical cation and radical anion spectra, respectively. Thus,
we have employed radiation chemical means to determine
the spectral characteristics of the dithiapyrene radical
cation, the pyrene radical cation and the fullerene radical
anion. The following peaks evolve for the pyrene radical
cation: 450, 500 and 575 nm.[26] For the dithiapyrene radical
cation, on the other hand, peaks were noted at 560, 630 and
770 nm.
around 880 nm, which is indicative of the C60 singlet excited
state, is seen regardless of the solvent (i.e., toluene and
THF). This singlet excited state decays over the time course
of 3000 ps and concomitant with this decay a maximum at
700 nm grows in. The latter is a clear attribute of the C60
triplet excited state. A closer inspection reveals, however,
features that the C60 reference does not exhibit: maxima in
the visible at 450, 510 and 575 nm and in the near-infrared
at 1010 nm. Upon referral to the pulse radiolytic section we
conclude that the 450, 510 and 575 nm features correspond
to the pyrene radical cation, while the 1010 nm feature re-
lates to the C60 radical anion. Hence, we see besides singlet
and triplet excited states, the formation of an intermediate
radical ion pair state.
Considering the aforementioned analysis of the spectral
evolution it is hardly surprising that the best global fit of the
absorption time profiles comprise a bi-exponential fitting
function. In particular, in both solvents, namely toluene and
THF, a short-lived and a long-lived component were found.
Interestingly, the relative weight of the two contributions
changes with solvent polarity. While, for example, in toluene
the short-lived contribution is on the order of 5% it is
nearly 50% in THF. The short lived component changes
only slightly in the two solvents with values of 46Æ6 ps and
53Æ9 ps in toluene and THF, respectively. Contrasting these
results the long-lived component is solvent independent
and, more importantly, resembles the kinetics seen in the
C60-reference 7. This leads us to conclude that the short-
lived component must reflect the charge recombination,
while the long-lived component is the intrinsic intersystem
crossing affording the triplet excited state (see above). From
such dependence we must assume that the charge recombi-
nation dynamics in C60–pyrene 6 are located in the normal
region of the Marcus parabola.
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C60–Dyads
FULL PAPER
The behavior for C60–dithiapyrene 4 is very different.
Most importantly, with the help of spectroscopic and kinetic
analysis we identified only one photoproduct. Immediately
upon 380 nm photoexcitation the dithiapyrene singlet excit-
ed state features evolve, which attest to the successful di-
thiapyrene excitation despite the presence of C60. C60 exerts,
nevertheless, a profound impact on the singlet excited state
decay. In particular, a rapid conversion (1.3”1012 sÀ1) into a
new transient is registered. Once the rapid disappearance of
the excited dithiapyrene absorption comes to an end (i.e.,
ꢀ1.0 ps after the laser pulse), the following characteristics
remain: maxima at 560, 630, 770 and 1010 nm, see Figure 7.
The visible maxima are ascribed to the radical cation of di-
thiapyrene, because the dithiapyrene radical cation, pro-
duced by the one-electron oxidation of dithiapyrene 1 has
evolve from a transduction of singlet excited state energy
between DTP (1) (2.43 eV) and C60 (1.78 eV). The time pro-
files of the absorbance at 670 nm and 1014 nm in THF and
toluene are shown in Figure 8. The lifetimes are determined
as tCS =0.5ps (2.0”10 12 sÀ1) in DMF, tCS =2 ps (5.0”1011 sÀ1)
in THF, tCS =27 ps (3.9”1010 sÀ1) in toluene and tCS
=
1011 ps (9.8”108 sÀ1) in cyclohexane.
The slower charge-recombination rate in toluene than
that in a more polar solvent (THF) suggests that this process
is in the Marcus inverted region, where the electron-transfer
rate becomes slower with increasing the driving force of
electron transfer as the solvent polarity decreases.[27] This is
demonstrated in Figure 9, which displays the driving force
dependence (ÀDGo) for the charge recombination on the
rate constant for both donor–acceptor conjugates (i.e., C60–
dithiapyrene 4 and C60–pyrene 6), see the Supporting Infor-
mation for details. The parabolic fitting affords a reorganiza-
tion energy (l) of 0.89 eV and an electronic coupling (V) of
32 cmÀ1. Both of these values are well in line with recently
published donor–acceptor conjugates including fullerene
C60.[1–6]
In the corresponding nanosecond experiments C60–dithia-
pyrene 4 and C60–pyrene 6 give rise to contrasting behavior.
While in the former case no appreciable changes were moni-
tored on the nanosecond scale at all, in the latter case triplet
characteristics were found that resemble those of the C60
reference. A high triplet quantum yield—with a value of
0.75Æ0.05in toluene and THF—corroborates the efficiency
of the overall energy transfer. At the same time it points to
the competition between energy and electron transfer when
photoexciting pyrene. The triplets in the C60 reference and
in the C60–pyrene donor–acceptor conjugate 6 decay similar-
ly slow on a time scale of up 100 microseconds to afford
quantitatively the singlet ground state.
Figure 7. Differential absorption spectra (visible and near-infrared) ob-
tained upon femtosecond laser flash photolysis (380 nm) of ꢀ1.0”10À5
m
solutions of C60–dithiapyrene 4 in deaerated toluene at different delay
times; excitation at 380 nm with OD=0.4.
absorption bands at 560, 630
and 770 nm. The absorption
maximum in the near-infrared
region, on the other hand, is an
excellent match to the one-elec-
tron reduced fullerene. We
therefore conclude that photo-
excitation of C60–dithiapyrene 4
yields in THF a radical ion pair
state that evolves from the di-
thiapyrene and/or C60 singlet
excited state. A similar radical
ion pair state is also obtained in
cyclohexane, toluene and DMF.
Please note that no C60 singlet
excited state features are dis-
cernable, neither in toluene nor
THF, at any given time delay
after the photoexcitation. On
Figure 8. Time profiles of the transient absorption at 670 (left side) and 1014 (right side) nm monitoring the
charge separation and recombination in C60–dithiapyrene dyad 4. Upper part in THF and lower part in tolu-
this basis we rule out any signif-
icant contribution that might ene.
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D. M. Guldi, M. R. Bryce, S. Fukuzumi et al.
1
a black powder. H NMR (700 MHz, C6D6): d = 5.94 (d, J=7.7 Hz, 1H),
5.55 (d, J=7.9 Hz, 1H), 5.46 (d, J=10.3 Hz, 1H), 5.28 (d, J=10.1 Hz,
1H), 5.02 (d, J=10.1 Hz, 1H), 4.97 (d, J=10.1 Hz, 1H), 4.93 (s, 1H),
4.35(d, J=9.5Hz, 1H), 4.26 (s, 1H), 3.75 (d, J=9.3 Hz, 1H), 2.40 ppm
(s, 3H); MALDI-TOF MS: m/z: 1015[ M]+; solubility of 4 was too low
to produce an accurate 13C NMR spectrum; elemental analysis calcd (%)
for C77H13NS2 (1016.06): C 91.02, H 1.29, N 1.38; S, 6.31; found C 91.08,
H 1.34, N 1.32, S 6.26; UV/Vis (CH2Cl2): lmax = 256, 305 sh, 322 sh, 394,
404 sh, 430, 704 nm.
C60–pyrene dyad 6: C60 (625mg, 0.87 mmol) was heated to reflux in chlor-
obenzene (200 mL). Pyrene-1-carboxaldehyde (5) (100 mg, 0.43 mmol)
and sarcosine (77 mg, 0.86 mmol) were added and the mixture was re-
fluxed overnight. The solvent was removed in vacuo and the residue was
purified by chromatography (silica, carbon disulfide). Solvent was evapo-
rated from the product fraction and the product was dissolved in a small
amount of CS2. Hexane was added to this solution and the formed pre-
cipitate was filtered off, washed with hexane and dried to yield com-
pound 6 (25mg, 56%) as a black powder. 1H NMR (500 MHz, [D4]o-di-
chlorobenzene + CS2 (=10%) + trace C6H6): d = 9.01 (d, J=9.3 Hz,
1H), 8.85(d, J=7.9 Hz, 1H), 8.35(d, J=9.7 Hz, 1H), 8.14–7.95(m, 6H),
6.23 (s, 1H), 5.14 (d, J=9.6 Hz, 1H), 4.54 (d, J=9.2 Hz, 1H), 2.91 ppm
Figure 9. Driving force (ÀDGo) dependence of intramolecular charge re-
^
^
combination in C60–dithiapyrene 4 ( ) and C60–pyrene 6 ( ).
Conclusion
À
(s, 3H). Due to restricted rotation around the exocyclic C C bond be-
tween the pyrrolidine ring and the bulky pyrene ring, two thermodynami-
cally stable (at ambient conditions) rotamers are possible for dyad 6,
which according to DFT calculations are of similar energies (Supporting
Information, Figures S30, S31). They are formed in a ratio of ca 10:1 and
are not separable by column chromatography. The second rotamer is
clearly visible in the 1H NMR spectrum as minor signals: d = 10.56 (d,
J=8.1 Hz, 1H), 8.32 (d, J=9.4 Hz, 1H), 8.24 (d, J=8.2 Hz, 1H), 8.20–
8.17 (m, 2H), 8.14–7.95 (m, 4H), 5.71 (s, 1H), 5.18 (d, J=9.4 Hz, 1H),
4.37 (d, J=9.5Hz, 1H), 2.96 ppm (s, 3H). 13C NMR (125MHz, d-ODCB
+ CS2 + trace C6H6): d = 156.8, 154.22, 154.15, 153.8, 147.5, 147.1,
146.9, 146.5, 146.42, 146.37, 146.3, 146.2, 146.1, 145.9, 145.84, 145.76,
145.72, 145.69, 145.6, 145.5, 145.40, 145.39, 145.3, 144.9, 144.7, 144.6,
144.5, 143.3, 143.2, 142.9, 142.8, 142.69, 142.68, 142.59, 142.56, 142.5,
142.4, 142.23, 142.18, 142.1, 142.0, 141.9, 141.72, 141.65, 140.6, 140.5,
139.8, 137.0, 136.8, 136.2, 136.0, 131.7, 131.4, 131.1, 128.3, 126.5, 126.0,
125.7, 125.5, 125.3, 123.7, 78.7, 70.5, 69.8, 40.4 ppm; ESI-MS: m/z: 976.1
[MÀH]+.
Cyclic voltammetry: Electrochemical experiments in benzonitrile
(PhCN), o-dichlorobenzene (o-DCB) and their 1:1 v/v mixture were car-
ried out using a BAS-CV50W electrochemical workstation with positive
feedback compensation. Cyclic voltammetry was performed in a three-
electrode cell equipped with a platinum disk (1 1.6 or 1.0 mm) as work-
ing electrode, platinum wire as a counter electrode and a non-aqueous
Ag/Ag+ reference electrode (0.01m AgNO3 in dry MeCN). The potential
of the reference electrode was checked against the ferrocene/ferricinium
couple (Fc/Fc+) before and after each experiment, which showed the fol-
lowing average potentials against the reference electrode: +0.201 V (vs.
Ag/Ag+ in PhCN), +0.243 V (vs. Ag/Ag+ in PhCN/o-DCB, 1:1 v/v), and
the values of potentials were then re-calculated versus Fc/Fc+ couple.
Tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) was used as
supporting electrolyte and all experiments were performed under an
argon atmosphere.
In summary, we have observed quite long lifetimes (i.e., up
to 1.01 ns) for the photogenerated charge-separated state in
a C60–dithiapyrene 4 dyad without the need for i) a long
spacer between the two moieties, or ii) a gain in aromaticity
in the radical ion pair. This unexpected result raises issues
of topology and aromaticity, which challenge the current
views on designing C60 derivatives with long lifetimes for
charge-separated species.
Experimental Section
Materials: Fullerene (C60) was purchased from MER Corporation Nano-
tubes (Tuscon, USA). All other materials and solvents used for the syn-
thesis of these compounds were purchased from Aldrich, Alfa Aesar,
Merck or Acros and used as received. Compound 7 was obtained as de-
scribed previously.[28] Photophysical measurements used C60 purchased
from Kaesdorf (Geräte für Forschung und Industrie, München, Germa-
ny) and Science Laboratories Co., Ltd., Japan, and used as received. Sol-
vents were purchased from Wako Pure Chemical Ind. Ltd., Japan, and
purified according to the standard procedure.[29]
1,6-Dithiapyrene-5-carboxaldehyde (2): Phosphorus oxychloride (160 mL,
1.72 mmol) was added slowly to a solution of 1[18] (280 mg, 1.17 mmol)
and DMF (144 mL, 1.86 mmol) in 1,2-dichloroethane (20 mL) cooled in
an ice bath. This mixture was stirred at reflux 24 h. After cooling, the re-
action mixture was poured into ice-water and neutralized to pH 7 with
10% NaOH solution. The solution was extracted with CH2Cl2, and the
organic phase was dried over MgSO4 and evaporated. The residue was
chromatographed (alumina, CH2Cl2/petrol ether 1:1) to afford compound
2
(220 mg, 68%) as black-violet solid. M.p. 255–2578C; 1H NMR
Pulse radiolysis: The pulse radiolysis experiments were performed by uti-
lizing either 500 ns pulses of 1.55 MeV electrons or about 100 ns pulses of
3.8 V electrons from two different Van de Graff accelerator facilities. De-
tails of the equipment and the analysis of data have been described else-
where.[30]
(400 MHz, CDCl3 + trace Et3N): d = 9.70 (s, 1H), 6.56 (d, J=7.7 Hz,
1H), 6.45(s, 1H), 6.38 (d, J=7.7 Hz, 1H), 6.19 (d, J=9.9 Hz, 1H), 5.98
(d, J=10.1 Hz, 1H), 5.82 (d, J=10.3 Hz, 1H), 5.63 ppm (d, J=10.3 Hz,
1H); 13C NMR (125MHz, CDCl 3 + trace Et3N): d 189.0, 139.4, 134.9,
133.2, 131.6, 130.5, 129.7, 126.7, 126.0, 125.7, 124.9, 124.51, 124.48, 121.42,
120.9 ppm; EI: m/z (%): 268 (100) [M]+; UV/Vis (CH2Cl2): lmax = 328,
392, 412, 554, 582 nm; elemental analysis calcd (%) for C15H8OS2
(268.35): C 67.14, H 3.00, S 23.90; found C 67.10, H 3.02, S 23.89.
Laser flash photolysis: Femtosecond transient absorption studies were
performed with 387 nm laser pulses (1 kHz, 150 fs pulse width) from an
amplified Ti/sapphire laser system (Clark-MXR, Inc.). For all photophysi-
cal experiments an error of 10% must be considered. Fluorescence spec-
tra were recorded with a FluoroMax. The experiments were performed
at room temperature. Each spectrum was an average of at least five indi-
vidual scans and the appropriate corrections were applied. Pulse radioly-
sis experiments were accomplished using 50 ns pulses of 8 MeV electrons
from a Model TB-8/16-1S electron linear accelerator. Nanosecond transi-
C60–Dithiapyrene dyad 4: C60 (190 mg, 0.26 mmol) was heated to reflux
in toluene (80 mL). Compound
2 (35mg, 0.13 mmol) and sarcosine
(30 mg, 0.34 mmol) were added and the mixture was refluxed for 40 h.
The solvent was removed in vacuo and the residue was purified by chro-
matography (silica, CS2 + 2% Et3N) to afford adduct 4 (85mg, 64%) as
256
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C60–Dyads
FULL PAPER
ent absorption measurements were carried out using a Nd/YAG laser
(Continuum, SLII-10, 4–6 ns fwhm) at 355 nm with the power of 10 mJ as
an excitation source. Photoinduced events were estimated by using a con-
tinuous Xe lamp (150 W) and an InGaAs–PIN photodiode (Hamamatsu
2949) as a probe light and a detector, respectively. The output from the
photodiodes and a photomultiplier tube was recorded with a digitizing
oscilloscope (Tektronix, TDS3032, 300 MHz). The transient spectra were
recorded using fresh solutions in each laser excitation. All experiments
were performed at 298 K.
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Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific Re-
search Priority Area (No. 19205019) from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan and by the Office of Basic
Energy Sciences of the US Department of Energy. We thank the ESF
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