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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 6749–6754 6749
Cite this: Phys. Chem. Chem. Phys., 2012, 14, 6749–6754
Flavin as a photo-active acceptor for efficient energy and charge transfer
in a model donor–acceptor systemw
Xi Yu,za Serkan Eymur,zb Vijay Singh,cBoqian Yang,
dMurat Tonga,
a
Amarnath Bheemaraju,aGraeme Cooke,
eChandramouleeswaran Subramani,
a
Dhandapani Venkataraman,*aRobert J. Stanley*
cand Vincent M. Rotello*
a
Received 9th January 2012, Accepted 20th March 2012
DOI: 10.1039/c2cp40073a
A donor–acceptor dyad model system using a flavin moiety as a photo-active acceptor has been
synthesized for an energy and photo-induced electron transfer study. The photophysical investigations
of the dyad revealed a multi-path energy and electron transfer process with a very high transfer
efficiency. The photo-activity of flavin was believed to play an important role in the process, implying
the potential application of flavin as a novel acceptor molecule for photovoltaics.
Introduction
In organic bulk heterojunction photovoltaic (PV) devices, carefully
selected donor and acceptor molecules with appropriate optical and
electronic properties are keys to effective light absorption, charge
separation, and transport in the solar energy conversion process.1–3
In the initial step of the PV process, a broad absorption spectrum
overlaps with the solar spectrum and high absorption coefficients
are desirable for effective solar energy harvesting.4 It has been
shown that the secondary energy harvesting by acceptors due to
energy transfer from a donor to an acceptor can substantially
increase the efficiency of the solar cell,5–7 which promotes the need
for the further development of photo-active acceptor molecules.
Although a wide variety of donor-incorporating molecules and
polymers have been utilized in the construction of organic solar
cells, comparatively few acceptor systems have been developed. For
example, the fullerene based acceptor [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM) has become ubiquitous in contemporary PV
devices, however, its inherent drawbacks (e.g. poor light absorption
characteristics) are likely to remain a limitation for the development
of next generation systems with significantly higher efficiencies.8
Other acceptor systems have also been investigated including:
perylenediimides,9 vinazenes,10 quinacridones,11 9,90-bifluorenyl-
idenes,12 pentacenes,13 carbon nanotubes,14 diketopyrrolo-
pyrroles,15 and quantum dots.16 However, these systems have
generally failed to provide similar efficiencies to PCBM-based
PV systems and thus there is a need for new acceptor systems to be
developed that can at least rival the properties and efficiencies of
this benchmark acceptor.
Flavins are redox active molecules that serve as enzyme
cofactors, performing redox reactions and working as electron
shuttles in the metabolic cycle.17,18 Furthermore, flavin has a
photo-active center that is thought to be the most likely candidate
for the photo receptor pigment in the ubiquitous ‘‘blue light’’
photoreceptive processes such as the photoaccumulation of
unicellular algae, the photoresponse of fungal sporangiophores
and the DNA repair by photolyase.19 It has been demonstrated
that the optical and redox properties of flavin can be easily tuned
through simple synthetic modification of substituents attached to
this heterocycle. A variety of flavin derivatives with different
LUMO energies and band gaps have been synthesized.20 Despite
the relative ease with which precise adjustments can be made to the
properties of flavin derivatives, however, it is surprising that the
photovoltaic properties have only received limited attention.21,22
To explore the future application of flavins as photo-active
acceptors in PV systems, we report the investigation of the photo-
induced excitation energy (ET) and charge transfer (CT) properties
of an oligothiophene–flavin (OT4–flavin) donor–acceptor (D–A)
model system. In particular, we report a solution phase investiga-
tion of these transfer processes in compound dyads to assess
the feasibility of this D–A pair for future elaboration into PV
devices.23–27 Photophysical studies showed very high efficiencies of
ET and CT from the oligothiophene (OT4) donor to the flavin
acceptor, which effectively leads to a charge transfer state formed in
the dyad. It is believed that the photo-activity of flavin plays a very
important role in the effective formation of the charge transfer state
through combined ET and CT processes.
aDepartment of Chemistry, University of Massachusetts,710 North Pleasant St., Amherst, MA 01003, USA.E-mail: rotello@chem.umass.edu, dv@chem.umass.edu
bDepartment of Chemistry, Middle East Technical University,06800 Ankara, Turkey
cDepartment of Chemistry, Temple University, 1901 N. 13th St.Philadelphia, PA 19122, USA. E-mail: rstanley@temple.edu
dDepartment of Physics, University of Massachusetts, Amherst,MA 01003, USA
eGlasgow Centre of Physical Organic Chemistry, WestCHEM,School of Chemistry, University of Glasgow, Joseph Black Building,Glasgow G12 8QQ, UKw Electronic supplementary information (ESI) available. See DOI:10.1039/c2cp40073az These authors contributed equally to this work.
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6750 Phys. Chem. Chem. Phys., 2012, 14, 6749–6754 This journal is c the Owner Societies 2012
Results and discussion
The synthesis of the D–A dyad 6 is outlined in Scheme 1. We
utilize the N3 position of flavin due to the convenient synthetic
protocols available to introduce functionality into this position. In
this study, flavin bearing a linker molecule with an azide
group, compound 5 (N3-flavin-azide), was synthesized by reacting
7,8-dimethyl-isobutylflavin 3 with the 1-(2-azidoethoxy)-2-
bromoethane 4 in the presence of K2CO3 in dry DMF. The donor
molecule 2 was obtained by removing the triisopropylsilyl (TIPS)
protecting group from compound 1 (see the ESIw for details).
Then, compound 2 was easily coupled with compound 5
under Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAc)
reaction conditions to afford the dyad 6 (N3-OT4-flavin dyad).
The ease of substitution at the N3 position of the flavin
may provide a general synthetic strategy for a series of D–A
dyads using flavin and its derivatives, and can open new avenues
for the systematic investigation of ET and CT processes in these
systems.
We first investigated the steady state absorption of the
dyads by UV-Vis spectroscopy. The absorption spectra of
OT4–flavin, N3-flavin-azide and OT4–TIPS are shown in
Fig. 1. The main absorption peak of flavin is around 450 nm
and OT4 is 370 nm. The spectrum of N3-OT4-flavin can be
considered as a linear combination of those of N3-flavin-azide
and OT4–TIPS, which spans the UV and blue-green part of
their spectrum. There is no extra peak appearing in the
spectrum, which provides evidence for the lack of molecular
orbital reorganization between the flavin and OT4 in the
ground electronic state.
Molecular dynamic simulation (MD) and quantum mechanical
ab initio calculations were used to further examine the electronic
communication between flavin and OT4 in the dyad. As shown in
Fig. 2, the conformational search usingMD simulation (continuum
solvation model in chloroform) revealed a stable conformation
where flavin and OT4 adopt a stacked configuration, which is
possibly due to p–p stacking interaction between flavin and OT4.
Meanwhile, no strong molecular orbital coupling or reorganization
between flavin and OT4 was observed, which is consistent with the
UV spectroscopy result. This further confirmed that flavin and
OT4 molecules in the dyad are discrete and can be treated
independently.
Ab initio calculations also show that the bridge, hydrocarbon
and triazole feature large HOMO–LUMO gaps of 8 eV and 6 eV,
respectively, and do not affect the electronic spectra of D and A in
the energy range close to their HOMO and LUMO levels. This
excludes a through-bond charge transfer coupling of D and A in
the electronic ground state of the dyad.
To explore possible ET or CT between OT4 and flavin in the
dyad, we examined the linear absorption and emission spectra of
N3-flavin-azide and OT4–TIPS (Fig. 3a). The overlap between the
emission spectrum of OT4 and the absorption spectrum of flavin
indicates that Forster resonance energy transfer (FRET) is possible
from OT4 to the flavin. A calculation of the Forster distance for
this dyad yields an R0 of 30.1 A based on the spectroscopy results.
This is quite a large value, which implies a possible very efficient
energy transfer considering the stacking conformation of the dyad
(Fig. 2). We estimated the HOMO and LUMO energy levels of
Scheme 1 Synthesis of oligothiophene (OT4)–flavin dyad.
Fig. 1 UV-Vis spectroscopy of N3-OT4-flavin, N3-flavin-azide, and
OT4–TIPS.
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OT4 and flavin by combining the oxidation potential (HOMO) of
OT4–TIPS and reduction potential (LUMO) of N3-flavin-azide
using cyclic voltammetry and the energy gaps estimated by
UV-Vis absorption spectra. As shown schematically in Fig. 3b,
the HOMO and LUMO of OT4 are offset upward relative to
that of flavin respectively. This arrangement makes ET from
OT4* - flavin thermodynamically possible.
The ET and CT processes in the dyads were investigated by
steady state fluorescence spectroscopy of the dyad (details
are given in the Experimental part). We first examined the
emission spectrum using excitation at 385 nm, since at this
wavelength OT4 has a strong absorption where the extinction
of flavin is at a minimum. As shown in Fig. 4(a), the
fluorescence of OT4 in the dyad is quenched strongly, as
much as 97%, indicating a very efficient deactivation of its
excited state. We believe that both ET and CT processes from
OT4 to flavin account for the fluorescence quenching of the
OT4 moiety in the dyad since the spectral overlap and energy
level offset of the donor and acceptor (Fig. 3b) make both
processes possible.
Although it is hard to separate the ET and CT processes
from OT4* to flavin using steady-state spectroscopy, the ET
process can be explored by fluorescence excitation spectro-
scopy by examining the excitation efficiency of flavin
compared to the dyad. We collected the emission at 650 nm
corresponding to the emission of flavin only and scanned the
excitation wavelength from 330 nm to 530 nm. As shown in
Fig. 5(a), excitation into the OT4 moiety results in lower
emission from the flavin, consistent with highly efficient CT
compared to ET to the flavin itself. The dyad emission has
been multiplied 42� for comparison with the flavin emission.
If OT4* acted as a perfect ET donor, the excitation spectrum
would show a much larger emission at around 360 nm where
the OT4 has its maximal extinction. However, only a small
enhancement is seen. The ratio of Fl emission to dyad emission
is shown in Fig. 5(b), quantifying this observation.
As a photo-active acceptor, the flavin chromophore in
the dyad can be selectively excited using 450 nm light. We
also observed strong quenching of flavin fluorescence
(Fig. 4a red dot line). Since the possibility of FRET from
flavin to OT4 can be excluded because it is endergonic, we
hypothesized that flavin fluorescence quenching in the dyad
is due to the CT from OT4 to the flavin. This is an
important result since it means that the light absorption in
both donor and acceptor can induce charge transfer in the
dyad, which should significantly increase the charge separation
efficiency.
Fig. 2 Molecular orbitals of the OT4–flavin dyad obtained by
ab initio calculation. The conformation of the molecule was obtained
by molecule conformational search. The distance between flavin and
nearest OT4 moiety is estimated to be B8 A.
Fig. 3 (a) Absorption and emission spectra of N3-flavin-azide and
OT4–TIPS. N3-flavin-azide and OT4–TIPS were excited at 450 nm
and 370 nm respectively. Note the overlap between the emission of
OT4 and the absorption of flavin. (b) The energy diagram of HOMO
and LUMO of flavin and OT4.
Fig. 4 (a) Steady state fluorescence spectroscopy of dyads in comparison
with OT4 and flavin, and the schematic representation of ET and CT
processes when OT4 (b) or flavin (c) is excited.
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To further investigate and quantitatively measure CT fromOT4
to flavin, we performed time-resolved fluorescence measurements.
The dyads were excited mainly through the OT4 molecule using a
404 nm pulsed laser. The flavin fluorescence signal at 4550 nm
was collected using an optical filter (Fig. 6). The flavin emission
showed a decay time of 7.2 ns. In contrast, the excited states of
flavin decayed much faster in the dyads. These results support our
assumption that the low fluorescence intensity of flavin in the dyad,
when OT4 was excited, is due to the deactivation of flavin* rather
than the low efficiency of FRET from OT4 to flavin. Identical
results were obtained when using 440 nm laser excitation in which
flavin is primarily excited. This implies that the behaviour of flavin
in both cases is the same and that deactivation of the flavin excited
state is due to hole transfer from flavin to OT4.
Fitting the decay curve of the dyad to a single exponential
curve gave a decay time of 478 ps, indicating a very
fast deactivation process (here a low amplitude long-lived
decay component, o4%, was ignored). Assuming CT, the
charge transfer efficiency (ZCT) then can be calculated by the
equations28:
ZCT ¼kCT
kFl�Thio¼ kFl�Thio � kFl
kFl�Thio
kFl ¼1
tFl
kFl�Thio ¼1
tFl�Thio
where k is the decay kinetic constant, t is the decay life time and
the subscripts Fl and Fl–Thio are flavin and OT4–flavin dyads
respectively. Using these relations and the values in Fig. 6, we
calculated the charge transfer rate constants and efficiencies of
the OT4–flavin dyad to be as large as 1.96 � 109 s�1, or 93.4%,
a very efficient charge transfer process in the dyad.
The above analysis actually suggests a secondary CT process in
the dyad following FRET from the OT4 to the flavin. In Fig. 7, we
summarize the possible energy and CT processes in the OT4–flavin
dyad. After OT4 was excited, it can undergo both energy transfer
and charge transfer. The latter process leads directly to the charge
separation state. Energy transfer, on the other hand, is followed by
the excitation of flavin with a secondary CT thereafter, which
generates charge separation as well. We believe that both primary
and secondary CT are taking place in the dyad and that this is
responsible for the highly efficient quenching.
The charge transfer process was confirmed by using broadband
subpicosecond transient absorption spectroscopy to identify the ion
radicals formed in the dyad molecule after CT. The dyad was
pumped using a 0.6 mJ pulse of 390 nm and the transient absorption
spectrum recorded using a white-light continuum (400–710 nm) at
pump–probe delays of up to 4 ns. Note that at this excitation
wavelength and extinctions, the OT4 has six times the absorption of
the flavin.
As shown in Fig. 8, the absorption peak of the OT4 radical
cation, OT4�+, can be seen clearly at around 648 nm after only
about 2 ps.29 A global target analysis of the transient absorption
data suggests that this fast time constant is due to electron transfer
fromOT4* to the ground state flavin moiety, producing the anionic
flavosemiquinone, which has a modest absorption maximum at
490 nm.30 This radical pair, OT4�+:FlSQ��, recombines in
about 435 ps. A relatively fast FRET process is in competition
Fig. 5 (a) Fluorescence excitation spectra of N3-flavin-azide
(black line) and OT4–flavin dyad (blue line) with emission at
650 nm. The quenched dyad emission is multiplied by a factor of 42
for visual presentation ( ). (b) The ratio IFl/Idyad.
Fig. 6 Time-resolved fluorescence decay of flavin and OT4–flavin
dyad excited at different wavelengths showing essentially identical
decay profiles.
Fig. 7 Schematic representation of ET and CT processes that can
take place in the OT4–flavin D–A dyads upon excitation of the OT4
chromophore.
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with the 2 ps ET channel, resulting in FL�OX. This FRET life
time is about 50 ps. The quantum yield for fast electron
transfer is FET1 = kCT/(kCT + kFRET) = 0.96. The FL�OX
species undergoes electron transfer as an acceptor with OT4 as
the donor in about 29 ps, again leading to OT4�+:FlSQ��.
These results clearly confirm the formation of the charge
separation states in our D–A dyad system, which is a critical
criterion for the possible practical applications of the D–A
pair in the solar cell construction.
Conclusions
We demonstrate in this study a D–A model system using flavin
as a photo-active acceptor. The OT4 donor and flavin are
covalently linked by an inert hydrocarbon and triazole linker
molecule using click chemistry. Photophysical investigations
of the D–A dyad revealed very efficient ET and CT processes
between flavin and OT4. Quenching rates of 1.96 � 109 s�1
and efficiencies of 93.4% were estimated using dynamic
fluorescence spectroscopy, but transient absorption shows that
the initial electron transfer event takes place in about 3 ps. A
mechanism including the combined energy and electron transfer
processes and secondary hole transfer process is proposed to
explain the effective formation of a charge-separated state, where
the photo-activity of the flavin has played an important role. These
results suggested that flavin is a promising photo-active acceptor
candidate for solar energy conversion and storage. Our strategy has
opened an avenue for a series of investigations of the energy and
charge transfer properties using various flavin derivatives. These
studies are underway and will be reported in due course.
Experimental
General procedures
UV/Vis spectra were taken on a HP 8452A spectrometer and
fluorescence spectra were measured with a PTI QuantaMaster
fluorimeter using a 1 cm cuvette. The energy gaps were
determined from the onset of the longest wavelength absorp-
tion band of flavin and OT4.
The flavin reduction and OT4 redox potentials were determined
by cyclic voltammetry on a Cypress System potentiostat with
a three-electrode system using a platinum working electrode, a
platinum wire as the counter electrode, and a silver wire as a
pseudo-reference electrode in dichloromethane solution using
tetrabutylammonium perchlorate (TBAP) as the supporting salt.
The ferrocene–ferricenium couple was used to calibrate and
translate the redox potentials into the HOMOof OT4 and LUMO
of flavin relative to vacuum.31
The conformational search was performed by MacroModel
(Schrodinger, Inc.) using the OPLS_2005 force field and mixed
torsional–low-mode sampling method with chloroform as the
solvent using a continuum solvation model.
The ab initio calculations were performed using Jaguar
(Schrodinger, Inc.) with the initial conformation determined
by a conformational search. Geometry optimizations and
single point energy calculations were carried out using the
B3LYP functional and 6-31G* basis set.
For the time resolved fluorescence measurements, the samples
were excited using a 405 nm/440 nm pulsed diode laser with a
repetition rate of 40 MHz and a pulse width of 50 ps. The
collected flavin fluorescence was filtered through a long-pass
optical filter (550 nm), detected with an avalanche photodiode
(APD, id Quantique id100-50) for photon counting, and
analyzed with a time-correlated single photon counting (TCSPC)
system (PicoQuant PicoHarp 300). The laser and detector
systems provided a 70 ps time resolution in time-resolved
fluorescence measurements.
The optical layout and laser system used for ultrafast transient
absorption spectroscopy has been previously described.32 The
samples had optical densities of 0.4–0.6 in the 2 mm path length
of the quartz cuvette, which was thermostatted at 20 1C. The
pump and probe beams were focused with beam diameters of
B250 and B100 mm, respectively, and a relative angle of 31. The
transmission of the sample was probed at the magic angle (54.71).
The chirp on the white-light continuum (WLC) beam was
measured using the Kerr effect where the dyad sample in the
cuvette was replaced with a naphthalene–toluene solution. The
primary electronic response of the naphthalene molecule gave
the instrument response function with high fidelity. The chirp of
the entire WLC pulse evolved within 1 ps. The data analysis was
truncated at early times to avoid coherent artifacts and stimulated
Raman scattering by included time points only after the chirp fully
developed (41 ps). The fitted lifetimes were found to be robust for
starting time points between 1–4 ps.
Acknowledgements
This material is based upon work supported as part of
Polymer-Based Materials for Harvesting Solar Energy, an
Energy Frontier Research Center funded by the U.S. Depart-
ment of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-SC0001087 (XY, BY,
MT, DV and VMR) and the NSF: CHE-0847855 (RJS and
VS), and facilities MRSEC (DMR-0820506), and the Center
for Hierarchical Manufacturing (DMI-0531171). GC thanks
the EPSRC and Serkan Eymur thanks TUBITAK for
2214-Research fellowship program and the METU-DPT-OYP
program on behalf of Selcuk University.
Fig. 8 Transient absorption spectra of the OT4-N3-flavin dyad with
excitation at 390 nm. Charge separation takes place in picoseconds.
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1039
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