Photoinduced symmetry-breaking charge separation: The direction of the charge transfer

Article abstract of DOI:10.1002/anie.201102601

Even flow: Photoinduced symmetry-breaking charge separation takes place in a few picoseconds in a 1,3-bis(perylene)propane dyad in polar solvents. Polarized transient absorption measurements show that the direction of the charge flow is random and entirely governed by the fluctuations of the solvent orientation around the dyad. Copyright

Full text of DOI:10.1002/anie.201102601

Communications
DOI: 10.1002/anie.201102601
Ultrafast Dynamics
Photoinduced Symmetry-Breaking Charge Separation: The Direction
of the Charge Transfer**
Vesna Markovic, Diego Villamaina, Igor Barabanov, Latevi Max Lawson Daku, and
Eric Vauthey*
Photoinduced symmetry breaking (SB) charge separation
(CS) occurs when an excited chromophore is surrounded by
several identical electron donors or acceptors, but it can also
happen between two identical molecular units when one of
them is in an excited state. Such a process is observed in the
reaction center of photosynthetic purple bacteria, where a
bacteriochlorophyll dimer is surrounded by two almost
identical branches of protein-bound cofactors.[1] Despite this
symmetry, electron transfer takes place almost exclusively
along one branch.[2] It has been shown that because the
constituents of the dimer have slightly different geometries
and local environments, CS along one branch is energetically
more favorable and thus substantially faster than along the
other.[3] Photoinduced SB-CS has also been observed in a few
multichromophoric systems containing two or more identical
molecular units, such as anthracene,[4] stilbene,[5] naphthale-
nediimides,[6] and perylenediimides.[7] Occurrence of SB-CS in
these systems has been deduced from the transient absorption
spectra recorded upon photoexcitation of one of the molec-
ular units, M. However, the MCꢀ and MC+ ions could not be
unambiguously recognized and distinguished, mainly because
their absorption bands overlap or have too dissimilar
intensities. Furthermore, the direction of the charge flow,
that is, whether the electron is transferred from or toward the
excited unit, could not be determined. The CS direction was
supposed to be random and solely determined by solvent
fluctuations, except in one case,[7a] where structural relaxation
of the excited unit was proposed to be at the origin of SB.
Herein, we present investigations into the photoinduced
SB-CS in 1,3-bis(3-perylenyl)propane (Pe-Pr-Pe), a bichro-
mophoric dyad with which we could unambiguously deter-
mine the CS direction, that is, whether photoinduced electron
or hole transfer is taking place. Perylene (Pe) was selected
because this molecule has already been shown to undergo
intermolecular photoinduced SB-CS,[8] and also becuase both
+
PeC and PeCꢀ ions can be easily distinguished by their intense
!
D5 D0 absorption band centered around 540 and 585 nm,
respectively.[9] A propyl bridge was chosen to avoid too strong
an electronic coupling between the two perylene units and to
ensure that they are not parallel to each other. This lack of
planarity was confirmed by conformational analysis, which
also predicted the coexistence of compact and open con-
formers of Pe-Pr-Pe in acetonitrile at 300 K (Figure 1 and
Supporting Information).
Figure 1. Structures of coexisting compact (a) and open (b) conform-
ers of Pe-Pr-Pe in acetonitrile at 300 K. Structures were obtained from
conformational analysis based on well-tempered metadynamics (WT-
MTD) simulations on the isolated molecule and subsequent optimiza-
tion in acetonitrile using the dispersion-corrected DFT-D3 method.
!
The dipole moment associated with the local S1 S0
!
~
transition of perylene, m0, and those related to the D5 D0
~
~
band of the ionic forms, m and m , are all parallel to the
þ
ꢀ
perylene long axis.[10] Therefore, if the excited perylene unit
always acts as electron donor, ~m , but not ~m , is parallel to ~m0
þ
ꢀ
(Figure 2), and the polarization anisotropy of the cation band,
r+, is larger than that of the anion band, rꢀ. On the other hand,
the contrary is true if the excited perylene acts as a hole donor
only. Finally, r+ = rꢀ if CS is bidirectional.
[*] V. Markovic, D. Villamaina, Dr. L. M. Lawson Daku,
Prof. Dr. E. Vauthey
Department of Physical Chemistry, University of Geneva
30 Quai Ernest-Ansermet, 1211 Genꢀve 4 (Switzerland)
E-mail: eric.vauthey@unige.ch
The stationary absorption and emission spectra of Pe-Pr-
Pe in cyclohexane (C6H12) and MeCN are very similar to
those of perylene, except for a small red shift that can be
accounted for by the presence of the propyl bridge (Support-
ing Information, Figure S2). This resemblance points to a
weak electronic coupling between the perylene units and to a
localization of the excitation entirely on one perylene.
However, the fluorescence quantum yield is 0.45 in MeCN
and 0.91 in C6H12, suggesting the existence of an additional
non-radiative deactivation pathway in polar solvents, which is
most probably SB-CS. This is corroborated by time-resolved
Homepage: http://www.unige.ch/sciences/chifi/vauthey
Dr. I. Barabanov
Institute of Chemical Kinetics and Combustion, RAS
Institutskaya str. 3, 630090 Novosibirsk (Russia)
[**] This research has been supported by the Swiss National Science
Foundation (Project Nr. 200020-124393 and the NCCR-MUST) and
the University of Geneva. We acknowledge supercomputer time at
the Swiss National Supercomputing Centre (CSCS) and the Centre
for Advanced Modelling Science (CADMOS).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102601.
7596
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7596 –7598

required to stabilize of the charge-separated state relatively to
the local excited state.
Although the propyl bridge should allow formation of an
intramolecular perylene excimer,[11] neither excimer fluores-
cence in the 550–650 nm region[12] nor excimer transient
absorption band at 600 nm[13] was identified in any solvent
investigated. Moreover, the high, perylene-localized, fluores-
cence quantum yield of Pe-Pr-Pe in non polar solvents
indicates that intramolecular perylene excimer is not formed
in a significant amount. Thus, SB-CS in Pe-Pr-Pe does not
occur upon dissociation of an intramolecular perylene
excimer.
Occurrence of excitation energy hopping (EH) between
the two perylene units was examined by measuring the
polarization anisotropy in the 400–440 nm region that corre-
Figure 2. Relationship between the direction of the charge separation
and the polarization anisotropy of the cation and anion absorption
bands.
!
sponds to the bleach of the local S1 S0 absorption band. The
anisotropy first decreases from 0.36–0.4 with a 4–4.5 ps time
constant and then decays to zero with a 75 ps lifetime
(Supporting Information, Figure S5B). The slow component
can be ascribed to the reorientational motion of Pe-Pr-Pe, and
the fast one to EH. As EH is fully reversible, its time constant
is twice the anisotropy decay time, that is, 8–9 ps (Supporting
Information, Eq. S8).[14] The initial anisotropy value close to
the maximum of 0.4 precludes the existence of any significant
EH component faster than the time resolution of the experi-
ment.
fluorescence, which reveals the presence of 10 and 105 ps
decay components in MeCN, with 0.26 and 0.18 relative
amplitudes, along with nanosecond components that are
predominant in C6H12 (Supporting Information, Figure S4).
Insight into the origin of these decay components was
obtained from transient absorption (TA) spectroscopy
(Figure 3). The spectra recorded in MeCN immediately
Information on the CS direction was obtained by compar-
ing the polarization anisotropy of the cation and anion TA
bands (Figure 4). Both r+(t) and rꢀ(t) profiles are very similar
Figure 3. Transient absorption spectra recorded at different time
delays after 400 nm excitation of Pe-Pr-Pe in acetonitrile (GS ground-
state depletion, SE stimulated emission, LES local S1-state absorp-
tion).
after 400 nm excitation are dominated by a band at 700 nm
that can be ascribed to the absorption of the locally excited
perylene unit. This band decreases with time, whereas two
bands, centered at 540 and 585 nm, appear. These two spectral
features, characteristic of the cationic and anionic forms of
perylene, point unambiguously to the occurrence of SB-CS in
Figure 4. Decays of the polarization anisotropy measured with Pe-Pr-
Pe in acetonitrile at a) 540 nm (PeC+ cation) and b) 585 nm (PeCꢀ
anion). Best biexponential fits are also shown.
and can be analyzed using a biexponential function with 4 and
60 ps time constants. The amplitude of the first component is
slightly smaller at 540 than at 585 nm and can be ascribed to
the localized excited state that contributes differently at these
two wavelengths. On the other hand, the amplitudes of the
slow component, that can be assigned to the ions, are the same
within the limit of error at both wavelengths (540 nm: 0.17 ꢁ
0.01, 585 nm: 0.18 ꢁ 0.01). The two time constants are close to
those measured in the 400–440 nm region and can be
+
Pe-Pr-Pe. The growth of both PeC and PeCꢀ bands can be
reproduced with a biexponential function with 12 and 130 ps
time constants, in agreement with the fluorescence lifetimes.
This multiphasic CS dynamics can be explained by the
coexistence of several Pe-Pr-Pe conformers with different
mutual orientations and distances between the perylene units.
These ion bands are not present in C6H12 (Supporting
Information, Figure S5A), indicating that polar solvation is
Angew. Chem. Int. Ed. 2011, 50, 7596 –7598
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7597

Communications
Received: April 14, 2011
Published online: July 4, 2011
interpreted likewise, namely EH/CS and reorientational
motion.
The very similar r+(0) and rꢀ(0) values indicate that the
Keywords: charge transfer · energy transfer ·
initially excited perylene unit has the same probability to be
oxidized as to be reduced. However, this does not necessary
means that CS is bidirectional. Indeed, if EH were much
faster than CS, the initial anisotropy of both ions bands would
also be the same even if CS were unidirectional. However, our
results shows that the EH time constant is around 8–9 ps,
whereas a large fraction of the excited-state population
undergoes CS with a 10–12 ps time constant. Numerical
simulations described in the Supporting Information reveal
that if CS were unidirectional, r+(0) and rꢀ(0) would differ
sufficiently to be distinguished even with EH five times as fast
as CS. Consequently, both directions are operative, that is,
both electron and hole transfer take place in Pe-Pr-Pe.
The break of symmetry introduced by optical excitation of
Pe-Pr-Pe is not enough to favor a unique CS direction. This
can be explained by the absence of dipolar interaction
between the solvent and perylene in both ground and excited
states. Thus, the time-averaged orientations of the solvent
around Pe*-Pr-Pe and Pe-Pr-Pe* are the same. However, the
instantaneous solvent orientation around Pe-Pr-Pe is not
symmetrical and therefore CS is energetically more favorable
in one direction than in the other. As the solvent orientation
fluctuates, the preferred CS direction changes accordingly.
Although solvent fluctuations have already been invoked as
being at the origin of SB-CS, the present study represents a
straightforward evidence of this effect.
This result should also hold for SB-CS in any multi-
chromophoric systems with apolar units in a homogenous
environment. Similarly, solvent fluctuations most probably
govern the direction of CS in systems with a chromophore
symmetrically surrounded by several electron donors or
acceptors. On the other hand, the situation could differ with
systems composed of chromophores experiencing a substan-
tial change of electric dipole moment upon excitation. If the
electronic structures of the ground and excited states are
sufficiently dissimilar to polarize the solvent differently, CS
should be energetically more favored in one direction.[7a] In
such a case, the break of the symmetry originates from the
optical excitation of one of the chromophoric units.
.
photoinduced electron transfer · ultrafast dynamics
[1] G. Feher, J. P. Allen, M. Y. Okamura, D. C. Rees, Nature 1989,
339, 111.
[2] H. Levanon, K. Mꢀbius, Annu. Rev. Biophys. Biomol. Struct.
1997, 26, 495.
[3] L. L. Laporte, V. Palaniappan, D. G. Davis, C. Kirmaier, C. C.
Schenck, D. Holten, D. F. Bocian, J. Phys. Chem. 1996, 100,
17696.
[4] a) H. Yao, T. Okada, N. Mataga, J. Phys. Chem. 1989, 93, 7388;
b) J. J. Piet, W. Schuddeboom, B. R. Wegewijs, F. C. Grozema,
J. M. Warman, J. Am. Chem. Soc. 2001, 123, 5337; c) S. A.
Kovalenko, J. L. P. Lustres, N. P. Ernsting, W. Rettig, J. Phys.
Chem. A 2003, 107, 10228.
[5] F. D. Lewis, P. Daublain, L. Zhang, B. Cohen, J. Vura-Weis, M. R.
Wasielewski, V. Shafirovich, Q. Wang, M. Raytchev, T. Fiebig,
J. Phys. Chem. B 2008, 112, 3838.
[6] a) S. Bhosale, S. A. L. , P. Talukdar, A. Fꢁrstenberg, N. Banerji,
E. Vauthey, G. Bollot, J. Mareda, C. Rꢀger, F. Wꢁrthner, N.
Sakai, S. Matile, Science 2006, 313, 84; b) N. Banerji, A.
Fꢁrstenberg, S. Bhosale, A. L. Sisson, N. Sakai, S. Matile, E.
Vauthey, J. Phys. Chem. B 2008, 112, 8912; c) N. Banerji, G.
Duvanel, A. Perez-Velasco, S. Maity, N. Sakai, S. Matile, E.
Vauthey, J. Phys. Chem. A 2009, 113, 8202.
[7] a) J. M. Giaimo, A. V. Gusev, M. R. Wasielewski, J. Am. Chem.
Soc. 2002, 124, 8530; b) M. W. Holman, P. Yan, D. M. Adams, S.
Westenhoff, C. Silva, J. Phys. Chem. A 2005, 109, 8548.
[8] E. Vauthey, P. Suppan, E. Haselbach, R. S. Davidson, Helv.
Chim. Acta 1986, 69, 430.
[9] T. Shida, Electronic Absorption Spectra of Radical Ions, Vol. 34,
Elsevier, Amsterdam, 1988.
[10] T. M. Halasinski, J. L. Weisman, R. Ruiterkamp, T. J. Lee, F.
Salama, M. Head-Gordon, J. Phys. Chem. A 2003, 107, 3660.
[11] F. Hirayama, J. Chem. Phys. 1965, 42, 3163.
[12] P. C. Johnson, H. W. Offen, Chem. Phys. Lett. 1973, 18, 258.
[13] R. Katoh, S. Sinha, S. Murata, M. Tachiya, J. Photochem.
Photobiol. A 2001, 145, 23.
[14] A. Morandeira, E. Vauthey, A. Schuwey, A. Gossauer, J. Phys.
Chem. A 2004, 108, 5741.
7598
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7596 –7598