Electron and energy transfer processes of photoexcited
oligothiophenes onto tetracyanoethylene and C60
Janssen, R.A.J.; Moses, D.; Sariciftci, N.S.
Published in:
Journal of Chemical Physics
DOI:
10.1063/1.467983
Published: 01/01/1994
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Citation for published version (APA):
Janssen, R. A. J., Moses, D., & Sariciftci, N. S. (1994). Electron and energy transfer processes of photoexcited
oligothiophenes onto tetracyanoethylene and C60. Journal of Chemical Physics, 101(11), 9519-9527. DOI:
10.1063/1.467983
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Electron and energy transfer processes
onto tetracyanoethylene
and CGO
of photoexcited
oligothiophenes
R. A. J. Janssen,a) D. Moses, and N. S. Sariciftci
Institute for Polymers and Organic Solids, University of California, Santa Barbara, California 93106
(Received 5 May 1994; accepted 16 August 1994)
We present near-steady-state photoinduced absorption (PIA) studies on photoexcited states of a
series of oligothiophenes (nT, n =6,7,9,11) in solution. Photoexcitation in solution reveals efficient
formation of a metastable triplet state (3nT). In addition, oligothiophene radical cations (nT+.) are
formed from the singlet excited state via electron transfer to the medium in solvents of high electron
affinity. The reactivity of the 3nT state towards acceptor molecules is investigated. In the presence
of Cc0 an energy transfer reaction is observed which quenches the ‘nT state and produces 3C,o via
photosensitization. Addition of tetracyanoethylene (TCNE) results in quenching of the 3nT state via
electron transfer and efficiently produces the corresponding nT+. radical cations, as evidenced from
PIA and light-induced electron spin resonance (ESR). 0 1994 American Institute of Physics.
INTRODUCTION
Many of the interesting electronic and nonlinear optical
properties of novel g-conjugated polymers originate from
the molecular characteristics of small subunits. The uniform
chain length and their well-defined chemical structure make
oligomers instrumental to access properties that critically depend on the conjugation length and thereby serve to establish
the relation between chemical and electronic structure of
r-conjugated materials. As an example, the (cationic) oxidation states of oligothiophenes represent the molecular analogs for polaronic and bipolaronic charge carriers responsible
for electric conductivity in polythiophenes, an important
class of conjugated polymers.‘-‘3
Photoexcitations of oligothiophenes have been the subject of several recent investigations.3*‘4-‘8 Electronic excitation of oligothiophenes across the r-7+* energy gap in fluid
or frozen solutions produces a metastable triplet state via
intersystem crossing from the excited singlet manifold.
These triplet states were first characterized for terthiophene
(3 T) and quaterthiophene (4 T> solutions from their T, -+ T,
absorption spectra.lg Very recently, triplet states have been
identified for longer oligomers with conjugation lengths up
to 11 thiophene units.3”8 The lifetimes of these triplet-states
at ambient temperature in solution are on the order of a few
tens of microseconds and increase to a few hundreds of microseconds at SO K in dilute frozen solutions. The oligothiophene triplet states exhibit a dipole allowed TI-+T2 optical transition which decreases in energy with conjugation
length from 2.70 eV for 3T to 1.54 eV for 11 T. Recent
electron spin echo studies of end-capped oligothiophenes
(nT, n=3-5)
in a toluene matrix provided unambiguous
proof for the formation of a triplet state after photoexcitation
and allowed the determination of the zero-field parameters D
and E.20 The D parameter was found to decrease with oligomer length, reflecting the increased spatial extent of the triplet excitation on the oligothiophene chain.
Photoinduced electron transfer from polythiophenes and
4On leave from the Department of Chemical Engineering, Eindhoven Uni-
versityof Technology,Eindhoven,The Netherlands.
poly(p-phenylene-vinylene)s onto CGohas recently been reported for solid films.21-25 The efficient quenching of the
luminescence and subpicosecond photoinduced absorption
spectroscopy have demonstrated that electron transfer occurs
from the singlet manifold in solid films within a picosecond
after photoexcitation across the energy gap. The fact that in
solution the triplet state of the oligothiophenes is readily
formed, gives a possibility to study the reactivity of the triplet manifold towards acceptor molecules and test the possibility of an electron transfer reaction.
In this paper we explore the photoprocesses of oligothiophenes (nT, n=6,7,9,11) in various solvents and show
that in addition to metastable triplet states, electron transfer
to the medium occurs in solvents of high electron affinity.
Furthermore, we investigate the reactivity of photoexcited
oligothiophene triplet states (3nT) in the presence of two
acceptor molecules: Ceo and TCNE. In the presence of Ccc,an
energy transfer reaction occurs which quenches the 3nT state
and produces ?ZGo,contrasting the electron-transfer reaction
observed in oligomer/C!&composite films.‘6 TCNE, on the
other hand, quenches the 3nT state in solution via electron
transfer and produces n T’. radical cations as indicated from
PIA studies and light-induced ESR spectroscopy. The low
concentration of the photo-oxidized products enables a detailed survey of the absorption spectra of the single chain
oligomer radical cations, which reveals the chain length dependence of the polaronic states in these medium sized systems.
EXPERIMENT
The oligothiophenes used in the present study carry a
number of dodecyl side chains that ensure solubility; their
molecular structure is given in Fig. 1. The synthesis of these
oligothiophenes has been described elsewhere by Ten Hoeve
et a1.“*28 Solvents of the highest purity commercially available were dried and deoxygenated by standard methods. Tetracyanoethylene, TCNE (98%, Aldrich) and C,, (99.99%,
Polygon Enterprises) were used as received. Sample solutions were prepared under oxygen-free argon atmosphere and
placedin sealed1 mm cuvettes,Oligothiopheneconcentra-
Q 1994 American Institute of Physics
9519
0021-9606/94/101(11)/9519/9/$6.00
J. Chem. Phys. 101 (il), 1 December 1994
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9520
Janssen, Moses, and Sariciftci: Energy transfer processes of oligothiophenes
FIG. 1. Molecular structures of the 6T, 7T, 9T, and 11 T oligothiophenes investigated in this study.
lions were typically IO-” -10-s M; concentrations of acceptom varied but did not exceed 2.5X 10e3 M for C,, and lo-’
M for TCNE. Samples were checked for photodegradation
by monitoring the optical absorption before and after PIA
experiments. Under the experimental conditions degradation
was small, typically below 1%-2%, over a few hours of
laser irradiation.
Near-steady-state PIA spectra were obtained using a mechanically modulated pump-and-probe technique; the transmission (T> was recorded from 2.4 to 0.64 eV using a
tungsten-halogen white light source (probe), a grating
monochromator, and a Si/PbS two-color detector. Photoexcitation was provided by the 457.9 nm (2.71 eV) line of a cw
argon-ion laser. The power incident on the sample was typically 50-75 mW with a beam diameter of 2 mm. The photoinduced change in transmission (AT) of the sample is measured by chopping the pump beam (w=25 Hz) and the
resulting modulation of the probe beam intensity is detected
using a phase-sensitive lock-in amplifier. The PIA, -A Tf T
=Aad, is obtained from the normalized changes in transmission. PIA spectra are corrected for luminescence.
ESR experiments used an X-band Bruker ER 200D spectrometer and were performed at room temperature. The
samples were illuminated using an optical fiber to bring output from an argon-ion laser into the microwave cavity. Both
steady-state and light-modulated ESR experiments were performed. In the light-modulated version the field- and lightmodulated ESR signal was consecutively amplified using
two lock-in amplifiers referenced to 100 kHz (field modulation) and 25 Hz (light modulation).
RESULTS
AND DISCUSSION
Oligothiophenes
and C,,
The near-steady-state PIA spectra of the oligothiophenes
in p-xylene at 295 K are shown in Fig. 2. The spectra of the
oligothiophenes in p-xylene solution reveal single subgap
absorptions at 1.78 (6T), 1.71 (7T), 1.60 (9T), and’1.55 eV
( 11 T>, respectively, in agreement with previous results for
dichloromethane solutions.‘* These PIA bands are attributed
to the T1 +T2 absorptions of metastable triplet-state oligothiophene photoexcitations. The PIA spectra are completely
quenched after exposing the samples to air. This supports the
assignment of the PIA bands as due to a triplet state being
quenched by molecular oxygen. The PIA bands increase linearly with the pump intensity (I) and are only slightly dependent on the modulation frequency of the pump beam (wj
up to 4000 Hz [Fig. 3(a)]. This behavior is consistent with a
monomolecular decay mechanism of the triplet-state photoexcitation, with a short (compared to l/w) intrinsic lifetime 7.
The modulation frequency dependence of a photoexcitation
exhibiting monomolecular decay usually shows a characteristic transition from the w&l region (where AT/T is independent of o) to the we 1 regime (where A TIT decreases as
d).2g
Since the onset of this transition for the oligothiophenes in p-xylene only appears at the highest modulation frequencies, it is not possible to obtain an accurate value
of r. A fit of the data to the theoretical expression for monomolecular decay
J. Chem. Phys., Vol. 101, No. 11, 1 December 1994
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Janssen, Moses, and Sariciftci: Energy transfer processes of oligothiophenes
SOX~O-~
12x10-3
1
60
k!
2
”
9521
8
40
20
4
0
0
1
s
2
Q&Q&
-a0
7
i
0 T,
T;
A
1q
8 ..I
12x109
8
$j
4 ‘ 4
0
>
,
I
I
I
15x10”
b
T,,
,
eV
eV
eV
1.55eV
1.78
1.70
1.60
g oT
,
3 4s6
a 1 , ,I
8
,_....,,,,,_,.,
,
,
3 456
loo
, , , ,I .,,_,
78
loo0
.,_,,
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1
9
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8
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%b
0
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OO
0
10 ‘&
=!
61
5
5
4.
0
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T6
0
T7
*
Tl
6~10-~
A
T9
4 c?
0
=-,I
0
I,“”
,*,,
,,,
4
2
cs I I ! I * I I I . . I , I ,I.
1.0
2.0
I.
3
0
P,ne~~$
(ev)
FIG. 2. PL4 spectra of oligothiophenes in p-xylene solution (solid line,
left-hand axis, 3nT triplet state) and p-xylene containing 2X10-’ M C
(dashed line, right-hand axis, ‘C, triplet state). (a) 6T 10d4 M, (b) 7T 10"
M, (c) 9T 10v4 M, and (d) 2X lo-’ M 1 IT. Spectra were recorded at 295 K
by pumping at 2.71 eV with 75 m W and modulation frequency of 25 Hz.
3
~
0
0
45
Ocl
1.55 eV
1.49eV
loo0
Modulationfrequency(Hz)
FIG. 3. Normalized modulation frequency dependence of oligothiophene
PL4 signals shown in Figs. 2 and 4. (a) p-xylene solutions and (b) ODCB
solutions. The relaxation behavior was recorded at 295 K by pumping at
2.71 eV with 50 mW.
(where g the efficiency~ for the creation of the photoexcitation) yields estimated lifetimes on the.order of SO-100 ,LLS,in
agreement with
time-resolved
studies for
shorter
oligothiophenes.3
The PIA spectra obtained for the oligothiophenes in
p-xylene containing 2X10m3 M Cso (Fig. 2) reveal a dramatic decrease of the nT triplet PIA signals compared to
solutions without C,,. For each of the oligothiophene solutions, however, a remaining PIA band is centered at 1.64 eV.
This band is attributed to the triplet-state C,, photoexcitation
(,3C60).In a separate experiment we found that photoexcitation at 2.71 eV of a 2X 10m3 M p-xylene solution of Cc0
produces the same PIA band at 1.64 eV, however with less
intensity. The assignment of the PIA band at 1.64 eV to 3C60
is in excellent agreement with the photomodulation and optically detected magnetic resonance (ODMR) studies of
&,/polystyrene glasses at 4 K.30 From the increased inten-
are prompted to conclude that 3C, is formed via two concurrent processes, i.e., photosensitization via 3nT and intersystem crossing from photoexcited Cco. Cc0 has both a high
electron affinity and a low triplet energy level, and hence
both electron and energy transfer can be anticipated. However, the latter is found to prevail in p-xylene Solution: Both
the quenching of the 3nT bands and increase of the 3C60
signal are the clear signatures of tciplet energy transfer. The
PIA band observed at 1.64 eV in the oligothiophene/C.so mixtures increases linearly with the pump intensity (I) and is
almost invariant to an increase of the modulation frequency
up to 4000 Hz. This demonstrates that 3C60 decays in
p-xylene at ambient temperature via a monomolecular process and has an intrinsic lifetime of less than 50 ,LLS.At the
concentrations employed in the present study, the luminescence of oligothiophenes remains essentially unaltered upon
addition of Cho, which demonstrates that C,, in solution does
not affect the singlet excited state (S,) of nT.
The efficiency of the -energy transfer depends on the
length of the oligomer. For 6T, we find complete quenching
of the triplet signal at 5 X lo-” M of CGoin p -xylene, whereas
at this concentration triplet signals of 9T and 11 T remain to
a some extend. Even at the highest concentrations employed,
11 T exhibits a small residual shoulder on the low-energy
side of the Ceo triplet signal (Fig. 2). Triplet energy quench-
sity of the 1.64eV bandin the oligothiophenesolutions,we
ing is evidently more effective when the ground-stateto
J. Chem. Phys., Vol. 101, No. 11, 1 December 1994
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9522
Janssen, Moses, and Sariciftci: Energy transfer processes of oligothiophenes
Energy(ev)
FIG. 4. PIA spectra of oligothiophenes in ODCB solution (solid line, lefthand axis, 3~T triplet state) and ODCB containing 2X 10e3 M C,, (dashed
line, right-hand axis, nTf. radical cation). (a) 6T 10M4M, (b) 7T 10T4 M,
(c) 9T 1O-4 M, and (d) 2X 10m5M 11 T. Spectra were recorded at 295 K by
pumping at 2.71 cV with 75 mW and modulation frequency of 25 Hz.
triplet-state energy difference (So-T,)
of the oligothiophene is larger than Se - T, of C6a. Hence, these results
indicate a decrease of the triplet energy level of the oligothiophenes with increasing conjugation length; concurrent
with the energy of the first excited singlet state (S,) which
decreases following a l/n behavior.‘8~28Apparently the
So - Tr energy difference of 11 T is close to that of Cm (estimated to be 1.57 eV),3,1,32making the energy transfer reaction less efficient.
The PIA spectra of the oligothiophenes in 1,2dichlorobenzene (ODCB) (Fig. 4) are similar to those in
p-xylene; the triplet bands are found at 1.75 (6T), 1.67
(723, 1.55 (9T), and 1.49 eV (1 lT),- i.e., O-03-0.06 eV
red-shifted compared to p-xylene solutions. Apart from these
solvent induced shifts, a noticeable difference is the appearance of new, small, PIA bands in the NIR region: 0.82 (6T),
0.78 (77’), CO.64 (9T), and co.64 eV (11T). These PIA
bands are attributed to the Iow-energy features of aT+. radical cations (polarons), formed in a photo-oxidation reaction.
Hence an electron transfer reaction occurs. The assignment is
fully consistent with optical absorption spectra of 6T and 7T
derivatives after chemical oxidation.‘~2*8-*0Compared to the
intensity of the triplet PIA bands, the low-energy radical cation PIA bands are on the order of 6% for 6T, 7T, 9T, and
about 3% for 11 T. For 6T and 7T the corresponding highenergy features of the nT+. radical cations are observed as a
small peak or shoulder on the low-energy edge of the triplet
band at 1.55 and 1.49 eV, respectively. For 9T and 1 1T the
high-energy components apparently overlap with the triplet
absorptions. The presence of two different and partially overlapping photoexcitations can also be inferred from the modulation frequency dependencies of the PIA bands shown in
Pig. 3(b). The relaxation behavior of the PIA signals at 1.55
(6 Tf.) and 1.75 eV (36Z’) is completely different. At 1.75 eV,
the intensity is almost invariant to the modulation frequency
as expected for a triplet state with a lifetime of about 50 JLS.
The PIA band centered at 1.55 eV, on the other hand, initially
strongly decreases but becomes less dependent on w at approximately 500 Hz. This complex behavior is explained by
the superposition of fast (6T+,) and slowly (36T) decaying
components at 1.55 eV. Similar results are observed for T7 at
1.67 and 1.49 eV. For 9T and 1 I T, where the radical cation
and triplet band overlap completely, the modulation frequency dependence is intermediate to that of the two bands
in 6T (and in 7T), and reflects the different ratio of formation of radical cations vs triplet states (9T 6%, 11T 3%).
The formation of charged photoexcitations in solution is
not limited to ODCB but it is also observed in dichloromethane and chloroform. No nTf. signals, however, are
observed in p-xylene, tetrahydrofuran, or ethyl acetate. This
result is in full agreement with recent observations on
terthiophene3 and poly(3-alkylthiophene)33-“5 and demonstrates that the solvent is crucial in the photo-oxidation reaction. The creation of charged excitations in oligothiophene
and poly(3-alkylthiophene) solutions has been rationalized
by a mechanism in which the solvent is actively involved as
an electron acceptor. It appears that the key parameter for
nT’= formation is not a high solvent polarity but rather the
electron affinity (reduction potential) of the solvent. Recent
photosensitization studies by Wintgens et al. have shown
that in dichloromethane the radical cation of terthiophene
(3Tf.) is not formed from the triplet state (33T) but rather
directly from the excited singlet state (‘3T*).’ In order to
compete with the efficient luminescence and intersystem
crossing processes, electron transfer from the ‘3T* state requires the immediate proximity of an electron acceptor because molecular diffusion is slow compared to the lifetime of
“3T”. In dilute solutions, only the solvent molecules can
fulfill this requirement and thus solvents with a high electron
affinity enhance the quantum yield for nTf. formation. The
fact that in particular chlorinated solvents are found to be
beneficial to nT+. formation can partly be attributed to a
dissociative electron capture reaction which prevents fast
electron backtransfer.
Addition of C,, (2.5X 10B3 M) to the n T solutions in
ODCB quenches the triplet PIA bands (as expected from the
experiments in p-xylenej and reveals much more clearly the
presence of the high-energy transitions of the oligothiophene
cation radicals nT+.: 1.56 (6T), 1.49 (7T), 1.42 (9T), and
1.38 eV ( 11 T) . The nTf. PIA signals increase sublinearly
with pump intensity; best-fit power-law exponents range
J. Chem. Phys., Vol. 101, No. 11, 1 December 1994
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Janssen, Moses, and Sariciftci: Energy transfer processes of oligothiophenes
from 0.55 to 0.80. This indicates that the nT+. radical cations
probably decay via more than one process since the pump
intensity dependence is intermedige to monomolecular (proportional to I) and bimolecular (proportional to Z’.‘) decay.
Interestingly, the PIA signals of n.T*. exhibit an enhanced
intensity as compared to the ODCB solution without Cho.
Concurrent with this increase, a small but distinct signal at
1.14 eV is observed. TQe PIA band at 1.14 eV is readily
assigned to C,$ .36-38The presence of CG and the increase of
nTfm are an indication of photoinduced electron transfer between the oligothiophene and CGoin,ODCB. Several mechanisms, however, can be envisaged to explain the electron
transfer and the presence of C& First, it is possible that in
addition to the energy transfer from “nT to C60, electron
transfer takes place to some extent in this more polar solvent
as compared to p-xylene [E(ODCB)=9.93;
e(p-xylene)
=2.27]. The moderate increase of the nTf. signals in ODCB,
however, suggests that energy transfer remains the most important quenching reaction of 3nT and that electron transfer
is a secondary process. On the Other liand, 3C60possesses an
increased electron affinity as corncared to C60 and is formed
in the mixture, both via direct excitation. of C&, at 2.71 eV
followed by intersystem crossing and via photosensitization
of CGO through triplet energy transfer from the oligothiophene triplet state. In a subsequent electron transfer reaction of 3C60 with ground-state nT, nT+., and C& radical
ions can be formed,3g preferably in a more polar solvent.
Oligothiophenes
and TCNE
An alternative and elegant route to rzT+. radical cation
formation is photoexcitation in the presence of TCNE. TCNE
is a strong electron acceptor and possesses a high-energy
triplet state. Triplet quenching via electron transfer using
TCNE is therefore expected to be highly favored over triplet
energy transfer. Indeed, Scaiano and co-workers have shown
that 3Tf. is readily formed from 3nT in the presence of
TCNE,40 and recently, this method has also been employed
by Wintgens et al. for unsubstituted a-sexithiophene.3 The
PIA spectra of the oligqthiophenes dissolved in dichloromethane containing 10m2 M TCNE are shown in Fig. 5.
The intensity of the nTf. PIA spectra is significantly enhanced with respect to the nT/C60 ODCB solutions which
demonstrates that the 3nT state is quenched by TCNE via
electron transfer. PIA bands due to TCNE-. are expected at
higher energies than the investigated spectral r&ge, and
therefore do not interfere with the present PIA spectra of the
nT*. radical cations.“’ It is important to note that the linear
absorption spectra show that nT and TCNE do not give rise
to a ground-state charge transfer complex in solution. Thus
our observations are solely due to photoinduced electron
transfer.
Further evidence for the formation of nT+. radical cations is obtained from light-induced ESR experiments. Figure
6 shows the ESR signals obtained upon illuminating a nT1
TCNE dichloromethane solution with light corresponding to
the ~-7ix energy gap, The spectra show ESR signals extending over approximately 14 G, with some resolved nuclear
hyperfine splitting in case of 6T. The hyperfine splitting ob-
9523
0.08
0.06
0.04
0.02
80~10‘~
60x10"
w
Energy (eV)
PIG. 5. PL4 spectra of oligothiophenes in dichloromethane solution containing IO-’ M TCNE. (a) 6T low4 M, (b) 7T 10e4 M, (c) 9T low4 M, and (d)
2X10e5 M 11 T. Spectra were recorded at 295 K by pumping at 2.71 eV
with 75 mW and modulation frequency of 25 Hz.
and TCNE-‘, as inferred from spectrum simulation with
available literature data.8v42For the longer oligothiophenes
no nuclear hyperfine couplings could be resolved and the
overlapping nT+. and TCN!-. spectra give rise to single
broad transition. The almost constant width of the ESR signal, independent of the conjugation length, is likely to result
from a reduction of the hyperfine couplings concurrent with
an increased delocalization of the unpaired electron, which
explains the loss of hyperfine resolution for the longer oligomers. The light-induced ESR signals can be observed during
steady-state illumination as well as modulated excitation
(and double lock-in detection). The latter technique eliminates any persistent or background ESR signals and exclusively detects the light-induced ESR spectra at the frequency
of light modulation.
The increased PIA intensity of the nT+. radical cations
in the y1T/TCNE solutions as-compared to the nT/C6,-,/ODCB
solutions and the complete absence of oligothiophene and
Cc0 triplet signals allow a more detailed analysis of the radical cation absorption bands. Each of the PIA spectra is characterized by two absorption bands, whose positions depend
on the oligomer chain length. Each of the principal absorp-
served
for 6T is consistent
withthepresence
of both6T+’ tionbands
exhibits
additional
finestructure,
in theformof an
J. Chem.
Phys., Vol. subject
101, No.to11,
December
1994
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or copyright,
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Janssen, Moses, and Sariciftci: Energy transfer processe”s of oligothiophenes
9524
l/n
3345-
3350
3355
3360
3365.
3370
I3 (Gauss)
FIG. 6. Light-induced ESR spectra of aT/TCNE solutions in dichloromethane obtained upon illuminating at 2.54 eV with an argon-ion laser.
additional peak or shoulder on the high-energy side. The intensity of this high-energy shoulder increases for the longer
oligomers. The observed peak positions (Table I) are plotted
vs l/n in Fig. 7 and reveal that the PIA energies linearly
depend on the reciprocal chain length.
To assign the nTf. bands to specific transitions it is useful to consider the molecular symmetry of the oligothiophenes. Assuming a transoid conformation of the
thiophene rings and neglecting the dodecyl side chains, the
molecular point groups of the oligothiophenes nT are Czh
(n=even) and CZU (n=odd). Since the symmetry of the
highest occupied molecular m orbitals is 6, and a2, respectively, oxidation produces the radical cations in 2B, and ‘A2
states (Fig. 8). From the ‘Bg or ‘A2 state a dipole allowed
transition can occur to 2A, or ‘B2, respectively, which is
polarized along the Iong axis of the molecule and thus favorable for a large transition dipole moment and large oscillator
strength. For n=even,. the mb,-+(k+
!>a, and ka,--+mb,
excitations correspond to the dipole allowed 2Bg+2AU tranfor n==odd, ma,-y(k+
l).bs and
sitions. Similarly,
kbz-+maz excitations involve the dipole ,allowed 2A2--‘2H,
TABLE I. PIA band energies (eV) of oligothiophene photoexcitations in
dichloromethane solution at 295 K.
Radical cation
6T
7T
9T
llT
‘Reference 18.
Triple?
1
2
3
1.80
1.72
1.60
0.84
0.7
0.63
CO.63
1.00
0.94
0.79
0.69
1.58
I.,52
1.45.
1.41
1.54
4
1.80
1.72 .
1.61
1.57
FIG. 7. Energies of PIA bands of oligothiophenes in dichloromethane solution at 295 K vs the reciprocal number of thiophene units showing chain
length dependence of nTf. radical cation (solid markers) and 3nT triplet
state (open markers) optical absorption energies. Lines are best fits to linear
dependence.
transition. The two strong PIA features are readily assigned
to these transitions. The high-energy-band corresponds to
excitation of an electron from the singly occupied molecular
to the lowest unoccupied level
orbital (mbgfmaa)
l)b,]
whereas the low-energy band arises
[(k+ l)a,l(k+
from excitation from the doubly occupied level (ka,lkb,)
into the singly occupied orbital (Fig. S). However,~ since
these’excitations result in states of identical symmetry it is
likely that some configurational mixing occurs.
The presence of fine structure on both the high- and
low-energy PIA bands is similar to the one observed for
chemically oxidized oligothiophenes. However, various different interpretations concerning its origin have been pro_~b pi
C 2v
c2h
(killa;
1-
@+l) bz
I
I
f
I
I
mbg-Gka
ma2
n
FIG. 8. Schematic molecular orbital diagram for nT+. radical cations showing frontier moIecular orbitals and dipole allowed transitions. Cab point
group for nT with n =even, Cz,, when n =odd.
J. Chem. Phys;, Vol. 101, No. 11, 1 December 1994
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Janssen, Moses, and Sariciftci: Energy transfer processes of oligothiophenes
A
KJ6
1tP
lOA
l.OOeV
A
1o-3
ConcentrationT, (MJ
FIG. 9. Intensity of the hT+. PIA bands: high-energy PIA bands (solid
markers) and low-energy PL4 bands (open markers) vs the 6T concentration
in dichioromethane containing 10 -* M TCNE obtained upon excitation at
2.71 eV. The inset shows the ratio of the two vibronics of the iiigh- and
.
low-energy PIA bands vs the 6T concentration.
Modulation freqency (Hz)
..
posed. One interpretation, originally formulated by Miller
et CZZ.~-~and later confirmed by Bauerle et CZZ.,~,~
attributes
the fine structure to two pairs of peaks corresponding to radical cations (nT+.) and radical cation dimers [(nT)z+]. Studies of the temperature dependence of the VIS/NIR absorptions in combination with ESR revealed that the high-energy
shoulders are due to the rr dimer. For 6T,, the bands at 0.87
and 1.60 eV were assigned to 6 Tf. whereas the bands at 1.14
and 1.80 eV were attributed to (ST);+ .’ The blue shift of the
+rr-dimer bands as compared to the radical cation bands is a
well-known phenomenon’3-4” (Davydov shift) and arises
from the interaction between the transition dipole moments
on adjacent radical cations. For a stacked rr dimer composed
of two identical parallel chromophores this interaction results
in a splitting of the electronic levels. For a symmetric dimer
the transition to the highest level (parallel transition dipole
moments) is allowed, whereas the transition to the lower
level (antiparallel transition dipole moments) is forbidden. In
contrast, Diaz et al. favor an alternative interpretation, of
these tine structure bands in terms of a vibrational coupling
of the C=C stretching mode to the electronic structure of the
oligothiophene backbone.“,”
Although ‘the low concentration of radical cations under
photo-oxidation conditions argues against rr dimerization in
the present case, we investigated the concentration dependence of the PIA spectra for 6T in order to test whether rr
dimers do occur during our experiments: The equilibrium
between the oligothiophene radical cation and its 7~ dimer
has been shown to depend strongly on concentration and
temperature. ‘,s Therefore, the concentration of 6T was varied
from 10e6 to 10v3 M in a dichloromethane solution containing 10-s M TCNE. The resulting concentration dependence
(I!&. 9) revealsthat the relative intensity of the PIA bands
FIG. 10. Normalized modulation frequency dependence of the PIA signals
of the photoexcited oligothiophene solutions in dichloromethane’containing
lo-* M TCNE shown in Fig. 5. For each of the oligothiophenes the two
features of the high-energy PIA band are included in the graph, arbitrarily
normalized to 1.0 and 0.1, respectively, for clarity of presentation.
and their fine structure are nearly independent of the 6T
concentration over three orders of magnitude. This ,behavior
is incompatible with the dimerization model since the dimer
concentration is expected to increase quadratically with the
concentration of the radical cation. We conclude that our
spectra correspond to a single species, and that rr dimers are
absent under the present conditions of near-steady-state
photo-oxidation. We attribute the line structure of the radical
cation PIA bands in our experiments to a vibronic origin.
All PIA.,bands due to the oligothiophene cations have the
same pump-intensity dependence and best-fit exponents
(ranging from 0.45 to 0.49, i.e., about square-root behavior).
This gives additional evidence to our assertion that they arise
from the same species. A square root intensity dependence is
characteristic for a bimolecular decay mechanism, due to the
fact that an incre~aseof the concentration of photoproducts
will result in an-increased decay rate. The bimolecular decay
is consistent with an electron backtransfer reaction from the
TCNE-. anion to the nT+. radical cations.3v40Further evidence for bimolecular decay comes from the relaxation behavior of the PIA bands. Again, both vibronics of the PIA
band exhibit exactIy the same characteristics (Fig. 10). This
relaxation behavior .is completely different from the data
shown in Fig. 3(a) for the 3nT states. The combined square
root intensity dependence and relaxation behavior show that
the high-energy PIA bands in oligothiophene/TCNE mixtures
J. Chem.
Phys., Vol.
101, No.
11, license
1 December
1994
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9526
Janssen, Moses, and Sariciftci: Energy transfer processes of oligothiophenes
I
nT
tluoreswnw
T
I ,.,T -ho+
lnT+ ----‘__,
‘&, electrontransfer
intersystem
crossing
+
=S’
I
3
nT
TcNE
---__c
electmntransfer
2
nT* f ‘TCNE’
CC4
energytransfer
I
1
nT
+
3c,
--electrontransfer
*nT”
+
,‘Cam’
Addition of CeDresults in a quenching of the triplet state via
energy transfer and produces 3C,,. Hence, we estimate that
the triplet energy of the oligothiophenes (n T, n =6, 7, 9, and
11) lies between the the triplet energy of Cc0 (1.57 eV)30.31
and triplet energy of 3 T (1.71 eV).47 In polar solvents the
triplet energy transfer reaction appears to be followed subsequently by electron transfer from the 12T ground state to 3C60
(Fig. 11).
In the presence of TCNE, 3nT transfers an electron to
TCNE to produce rzT+. radical cations (polarons). This reaction was readily detected by light-induced ESR and PIA
spectrbscopy at very low concentrations of the oligothiophene (10e6 M). It is shown that the two dipole allowed
polaronic absorption bands exhibit vibrational fine structure
and that their energies scale linearly with I/n.
ACKNOWLEDGMENTS
FIG. 11. Photophysics and photochemistry of oligothiophenes in solution in
acceptor
the
presence
of
molecules
(S=solvent,
TCNE
=tetracyanoethylene).
are not due to the ‘nT states, although their energies are
virtually identical (Fig. 7).
The data plotted in Fig. 10 are consistent with bimolecular decay, and exhibit a more gradual transition from the
w-independent to the 110 behavior at UET=I than expected
for monomolecular decay. At a laser power of 50 mW the
steady-state lifetime 7s is on the order of I ms as determined
from a fit of the~data to the theoretical curve for bimolecular
decay?
where a= &(w~~):,),r.v= (gpl) -“.5, and P=bimolecular decay
rate constant.
Further evidence for bimolecular decay can be inferred
from the overall spectral intensity upon increasing the concentration of 6T (Fig. 9). For samples of low concentration
(OD*l), the PIA spectra increase’with increasing concentration, reflecting the higher number of photons absorbed. For
the most concentrated (10T3 M) and highly absorbing
(ODB 1) sample, however, the PIA intensity has decreased as
a result of the reduced laser penetration depth and hence the
higher (local) concentration of photoproducts which results
in enhanced decay.
CONCLUSIONS
The various photophysical and photochemical processes
observed upon excitation of oligothiophenes in solution
across the r-n=* energy gap are summarized in Fig. 11.
Apart from fluorescence, we have shown that independent of
the nature of the solvent, the excited singlet state of the oligothiophenes can decay via intersystem crossing to form a
3nT triplet state. In high electron affinity solvents the singlet
excited state also produces nT+. radical cations via electron
transfer to the. medium.
The reactivity of the 3nT state has been probed by the
addition of two different acceptor molecules, C,, and TCNE.
We gratefully acknowledge Philips Research Laboratories for generously providing the oligothiophenes synthesized by Dr. W. ten Hoeve, Professor H. Wynberg, Dr. E. E.
Havinga, and Professor E. W. Meijer. We would like to thank
Professor A. J. Heeger and Professor F. Wudl for stimulating
discussions and valuable cotnments. R. A. J. J. acknowledges
support from the Eindhoven’university of Technology, The
Netherlands. The research was supported by the Office of
Naval Research (Grant No. ONR NOOO14-91-J-1235).
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