Inter versus intra-molecular photoinduced charge separation in solid films of donor-acceptor molecules

Article abstract of DOI:10.1039/b808704k

We report on photoinduced charge separation in solid films of two perylene diimides; intramolecular charge separation and recombination is correlated with a reduction in the yield of long-lived, intermolecular charge-separated species. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808704k

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Inter versus intra-molecular photoinduced charge separation in solid films
of donor–acceptor moleculesw
Safa Shoaee,a Mattias P. Eng,a Zesheng An,b Xuan Zhang,b Stephen Barlow,b
Seth R. Marder*b and James R. Durrant*a
Received (in Cambridge, UK) 22nd May 2008, Accepted 13th August 2008
First published as an Advance Article on the web 9th September 2008
DOI: 10.1039/b808704k
We report on photoinduced charge separation in solid films of
two perylene diimides; intramolecular charge separation and
recombination is correlated with a reduction in the yield of
long-lived, intermolecular charge-separated species.
charge separation in many donor–acceptor molecules such as
dyad 2, also results in enhanced intermolecular charge
generation in solid films.
Solid films of both 1 and 2 were prepared by spin coating
(500 rpm for 60 s) from 20 mg mlꢀ1 solutions in chloroform.
The resultant films were approximately 120 nm thick, with
AFM data indicating rms roughness of 5.6 nm, and were
optically nonscattering. UV/visible absorption spectra of the
spin-coated films (see ESIw) were essentially indistinguishable
from the corresponding solution spectra, indicating only
modest intermolecular interactions. As will be discussed in
more detail elsewhere,10 whereas the lowest energy absorption
of 1 has vibrational structure similar to that of unsubstituted
perylene diimides (PDIs) and is only somewhat red-shifted, the
low-energy absorption band for 2 is a considerably red-shifted,
structureless band (with a structured PDI-like absorption at
higher energy) suggesting considerable donor–acceptor
charge-transfer character in the lowest excited state of 2.
There is extensive interest in the synthesis and characterisation
of molecular donor–acceptor structures, arising from the
possibility of mimicking the photochemical processes in
photosynthetic reaction centres and, in part, due to the
potential use of such molecules in applications ranging from
solar energy conversion to non-linear optical absorption.1 The
structure–function relationships which determine the solution
photophysics and photochemistry of such molecules have now
been elucidated in considerable detail.2–5 However, relatively
few studies have addressed the question of whether this under-
standing of solution photochemistry can be extended to the
function of such donor–acceptor structures in solid films. This
issue is of particular importance for technological applica-
tions, which will typically be based upon solid-state, rather
than solution, materials.6–8
Here we report a study of the photoinduced electron-
transfer processes occurring in films of a bis(4-(diarylamino)-
biphenylethynyl)-substituted perylene diimide dyad (2), and
compare these to films of a bis(4-alkoxyphenylethynyl)-
substituted perylene diimide control (1), see Scheme 1. 1 and
2 were synthesized by Sonogashira coupling between the
appropriate dibromoperylene diimide and terminal alkynes
as described in the ESI.w The solution photochemistry of these
molecules, including in particular ultrafast intramolecular
charge separation in 2, will be reported elsewhere.10 In this
contribution, we address the charge photogeneration proper-
ties of thin solid films of these molecules, focusing upon the
generation of the long-lived charge-separated species which
are essential for applications such as solar energy conversion.
In particular, we consider whether the donor–acceptor struc-
tural motif that is found in solution to favour intramolecular
a Department of Chemistry, Imperial College London, Exhibition
Road, London, UK SW7 2AZ. E-mail: j.durrant@imperial.ac.uk;
Fax: (þ44)2075945801; Tel: (þ44)2075945321
b School of Chemistry and Biochemistry and Center for Organic
Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, USA.
E-mail: seth.marder@chemistry.gatech.edu;
Fax: (þ1)404-894-5909; Tel: (þ1)404-385-6048
w Electronic supplementary information (ESI) available: Experimental
details and steady state absorption and emission spectra. See DOI:
10.1039/b808704k
Scheme 1 Structures of the control 1 (left) and the donor substituted
perylene diimide 2 (right) molecules used in this study (top). Also
shown are the calculated LUMOs (middle) and HOMOs (bottom) for
the two molecules.
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These experimental data were complemented by DFT and
TD-DFT calculations, described in the ESI.w The calculated
HOMOs and LUMOs of the geometry optimized structures
are shown in Scheme 1 and indicate qualitatively different
electronic structures for the two compounds. In contrast to
molecule 1, molecule 2 shows a clear spatial separation of its
HOMO and LUMO, consistent with optical excitation result-
ing in the formation of an intramolecular charge separated
state.
Fig. 1 shows time-resolved fluorescence decays for the two
molecular films. The control film of 1 shows a monoexponen-
tial, 4.1 ns decay, indicative of formation of a long-lived singlet
excited state. In contrast, the fluorescence for the film of 2 is
strongly quenched, showing only an instrument-response
limited, low-amplitude transient indicative of a singlet excited-
state decay time of o50 ps. These emission dynamics are similar
to those observed in solution. The fluorescence quenching
for the film of 2 is consistent with the observation of ultrafast
(B10 ps) intramolecular charge separation for this molecule in
dilute solution;10 this emission quenching can, therefore, be
attributed to intramolecular photoinduced charge separation in
the film.
These emission data were complemented by transient
absorption studies of photo-induced charge generation, as
shown in Fig. 2. Solution transient absorption studies have
revealed that the intramolecular charge separated state formed
in 2 exhibits a decay time of 750 ps, which is assigned to
intramolecular charge recombination.10 Herein, however, we
focus on the possibility of molecular films of this species
generating long-lived, intermolecular charge separated species
observable on the microsecond timescale. Control data in
dilute solution showed no measurable signals on this time-
scale, consistent with ultrafast intramolecular charge recom-
bination to the ground state. In contrast, solid films of 2 show
clear transient absorption signals, as illustrated in Fig. 2. The
absorption difference spectrum (Fig. 2a) shows a photo-
induced absorption maximum at 740 nm, consistent with
previous reports of the absorption of radical anions of PDI
derivatives9 and distinguishable from the PDI triplet absorp-
tion at 480 nm. This transient spectrum is, therefore, assigned
to a charge-separated state in which the hole and electron are
localised on triarylamine and PDI moieties, respectively.
Fig. 2b shows the corresponding decay dynamics of this
transient absorbance signal, monitored under low excitation
Fig. 2 (a) Transient absorption data for films of 1 and 2, following
excitation at 540 nm. The corresponding transient absorption spectra
measured 1 ms after excitation. (b) The figure shows decay dynamics
monitored at 740 nm.
densities (20 mJ cmꢀ2 at 4 Hz). It is apparent that this
absorbance is long-lived, exhibiting dispersive (approximately
power law) decay dynamics on the microsecond timescale,
with a decay halftime, t50%, of B20 ms. These decay dynamics
were observed to be independent of the presence of oxygen,
confirming they cannot be assigned to triplet states (in agree-
ment with the transient spectrum discussed above). Moreover,
this decay halftime is four orders of magnitude longer than the
intramolecular charge recombination timescale observed in
solution, and therefore, cannot be assigned to the same
intramolecular charge-separated species. Rather, this long-
lived transient signal is assignable to the formation of diꢀs-
1
sociated, intermolecular charge-separated states, i.e. 2ꢂ /2ꢂ
.
The observed recombination halftime, and dispersive, power
law, behaviour of the recombination dynamics, are typical of
those we have reported previously for intermolecular bimole-
cular recombination of dissociated charges in polymer–
fullerene blend films.11
We now turn to our control molecule, 1. Studies in solution
did not reveal any significant charge photogeneration, even on
ultrafast timescales, consistent with the absence of a charge-
transfer type feature in the absorption spectrum and with
the weaker electron-donor properties of the alkoxyphenyl
moiety.10 However, it is apparent from Fig. 2 that solid films
of 1 yield a transient absorption signal that is both larger and
longer lived than that observed for 2. The transient spectrum for
film 1 again exhibited an absorption maximum at B740 nm
indicative of the formation of PDI anions and exhibited
oxygen-independent, dispersive decay dynamics. This
transient signal is, therefore, also assigned to intermolecular
Fig. 1 Time-resolved single-photon-counting data for films of 1 and
2. Data were collected using 467 nm excitation and 670 nm detection,
with a 250 ps instrument response and data collection times corres-
ponding to matched densities of absorbed photons.
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charge-separated states, in this case corresponding to the
1
coupling for the film of 2, resulting perhaps from the
triarylamine substituent disrupting the molecular packing.
This will act in favour of an intramolecular charge recombina-
tion as opposed to charge dissociation. Indeed, we have
previously reported an enhanced yield of intermolecular
charge separation in solid films of planar TTF : fullerene dyads
relative to their twisted analogues, and have attributed this to
enhanced intermolecular coupling for the planar molecule.15
In either case, it is apparent that whilst solution studies are
essential for our understanding of the intramolecular photo-
chemistry of molecular donor–acceptor systems, the applica-
tion of such structures to solid-state solar energy conversion
requires careful consideration of the film rather than solution
photochemistry.
formation of 1ꢂ and 1ꢂꢀ. The transient signal for the film
of 1 is larger than for the 2 film (normalised to equal densities
of absorbed photons) for all probe wavelengths measured. As
PDI radical cations are reported to show a sharp absorption
maximum at longer wavelengths (900 nm for isolated PDIs12
and presumably expected at still longer wavelengths for ex-
tended species such as 1ꢂ1) the larger amplitude signal for the
film of 1 is unlikely to be due to contributions from PDI1
absorption alone, but rather to an approximately two fold
increase in the yield of charge-separated species. We, thus,
conclude that the film of 1 exhibits an approximately two fold
enhancement of long-lived charge photogeneration relative to
the 2 film.
It is clear from the above data that, at least for the molecules
studied here, the ability of a molecule to achieve efficient
intramolecular charge separation in solution is not in itself
an indicator of the ability of that molecule to achieve efficient
intermolecular charge separation in solid films. Clearly this is
only a limited study covering two such molecules, and further
studies are required to confirm its general applicability.
Notwithstanding this caveat, this conclusion may have im-
portant implications for the relevance of solution studies
of intramolecular electron transfer to the application of
molecular donor–acceptor systems to solid-state solar energy
conversion systems.
The authors are grateful to the AtlanTICC Alliance for
funding from the UK OST. Work at Imperial was also
supported by EPSRC and BP Solar, while work at Georgia
Tech was supported by the STC Program of the NSF
(DMR-0120967) and by the ONR (N00014-04-1-0120). MPE
acknowledges the Knut and Alice Wallenberg Foundation for
funding. We also thank Susan Odom for assistance with
compound characterization.
Notes and references
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In general, it might be expected that the ability of a
molecular structure to achieve an intramolecular charge-
separated state should favour intermolecular charge separa-
tion in the solid state. This intramolecular charge separation
should reduce the coulombic attraction (or ‘binding energy’)
of the electron and hole, thereby facilitating subsequent inter-
molecular charge dissociation. Indeed charge-transfer mole-
cules are employed to achieve efficient charge separation in
both dye-sensitised and polymer–fullerene solar cells.13,14 It is,
therefore, striking that, for the molecules studied in this
communication, intramolecular charge separation appears to
correlate with a reduction in the yield of long-lived, inter-
molecular charge dissociation. The origin of this reduction in
yield can be understood in terms of the relative lifetimes of the
intramolecular photogenerated states for the two molecules
studied. The lifetime of the intramolecular charge-separated
state for the 2 dyad in solution, 750 ps, is, in fact, shorter than
the lifetime of the singlet excited state of the control 1 (4.5 ns).
In this context, the addition of the triarylamine moiety,
enabling intramolecular charge separation, can be regarded
as accelerating the decay of photogenerated states to the
ground state. This acceleration can be understood in terms
of an increase in vibronic coupling resulting from the genera-
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state can be expected to compete with intermolecular charge
dissociation, consistent with the observed lower charge gen-
eration yield. It is also possible that this reduction in yield may
be associated with a reduction in intermolecular electronic
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Chem. Commun., 2008, 4915–4917 | 4917