Intramolecular charge transfer in bipolar molecules for electron and hole transporting materials

Article abstract of DOI:10.1039/b401255k

The excited state relaxation of 6-(9H-carbazole)hexyl-9,10,10-trioxo-9H-3- thioxanthenecarboxyl (CHTT) molecules containing electron donating and electron accepting groups was investigated in solutions by means of picosecond pump-probe absorption spectros

Full text of DOI:10.1039/b401255k

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R E S E A R C H P A P E R
Intramolecular charge transfer in bipolar molecules for electron and
hole transporting materials
a
R. Karpicz, V. Gulbinas, A. Undzenas, J. V. Grazulevicius and R. Lygaitis
a
a
b
b
a
Institute of Physics, Savanoriu Ave. 231, 02300 Vilnius, Lithuania. E-mail: renata@ar.fi.lt
Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19,
LT-3028 Kaunas, Lithuania
b
Received 26th January 2004, Accepted 4th February 2004
F|rst published as an AdvanceArticle on the web19th March 2004
The excited state relaxation of 6-(9H-carbazole)hexyl-9,10,10-trioxo-9H-3-thioxanthenecarboxyl (CHTT)
molecules containing electron donating and electron accepting groups was investigated in solutions by means of
picosecond pump–probe absorption spectroscopy. The excitation energy and charge transfer between
electrondonating and electronaccepting molecule fragments takes place under molecular excitation and
determine the excited state relaxation dynamics.
and the following esterification of OTC with 6-bromohexyl-
9
Introduction
H-carbazole obtained in the first step.
The chemical structures of the materials are presented in
Charge transport materials are used in a number of organic
optoelectric devices, such as electrophotographic receptors,
1
,2
1
Fig. 1. Acetone, benzene and chloroform solvents were used
for the preparation of solutions. All solvents were reagent
grade and used without additional purification. The concentra-
3
light-emitting diodes, organic field-effect transistors and
,4
5,6
7
,8
solar cells. The hole transporting materials are relatively
easily produced and are commonly used. The production of
ꢁ4
ꢁ1
tion of solutions was about 10 mol l for time-resolved
experiments.
9
–12
electron transporting materials
applications bipolar hole- and electron-, i.e. both charge,
transporting materials are desirable.
is more difficult. For many
The main experimental technique used was transient differ-
ential absorption spectroscopy with ps-time resolution. The
measurements were carried out in the visible spectral range.
The transient absorption study was performed using absorp-
tion pump–probe spectrometer equipped with a home-made
1
3–15
The bipolar conduc-
tivity may be realized by incorporating molecular fractions
with electron-donating and electron-accepting properties into
the material molecules. In the ideal case, both fractions could
be incorporated into a single molecule. Conductive layers hav-
ing such molecules would always lead to the uniform distribu-
tion of electron-donating and electron-accepting fragments.
However, synthesis of such molecules is more problematic.
Furthermore their spectroscopic properties are more complex
because of the interaction between the two fragments. This is
particularly important for application of such materials in
optoelectrical devices, where they function in light. Optical
charge transfer states may be formed if the two molecular frag-
ments are strongly coupled. The charge transfer may also take
place under local excitation of one of the molecule fragments.
Relaxation may be furthermore complicated in a solid state
due to intermolecular exciton and charge transfer interactions.
Here we present steady-state and transient absorption study
of solutions of bipolar molecules, synthesized for electron and
hole transport materials. Here we focus on intramolecular pro-
cesses as a first step of investigation of electronic properties of
the bipolar charge transfer material. We show that intramole-
cular excitation and electron transfer, and the back electron
transfer determine the excited state relaxation dynamics.
þ
low repetition rate (2 Hz) Nd :glass laser generating pulses
of about 2-ps duration. The samples were excited by the third
harmonic of the fundamental laser radiation (351 nm) and
probed by the white-light continuum generated in a water cell.
Transient absorption kinetics at a selected wavelength was
measured by optically changing the delay time between the
excitation and the probe pulses. Transient differential absorp-
tion spectra at fixed delay times were recorded by scanning the
probe wavelength and simultaneously moving the delay line in
order to compensate for the group velocity dispersion effect.
A Beckman UV 5270 spectrophotometer was employed for
studies of the steady-state absorption spectra. The fluorescence
Experimental
N-Ethylcarbazole (N-EtCz) and 9-oxo-9H-tioxanthene-3-
carboxylic acid 10,10-dioxide (OTC) were purchased from
‘
zene (mp 69–70 C), OTC was recrystallized from tetrahydro-
‘Aldrich’’. N-EtCz was purified by recrystallization from ben-
ꢀ
ꢀ
furan (mp 282–283 C).
6
-(9H-Carbazole)hexyl-9,10,10-trioxo-9H-thioxanthene–3-
oate (CHTT) was prepared by the two step synthetic route
including alkylation of 9H-carbazole with 1,6-dibromohexane
Fig. 1 Chemical structures of studied materials.
2
276
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 2 7 6 – 2 2 8 0
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4

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(
l
exc ¼ 350 nm) spectra were recorded on LS50B employing a
It should be also noted that the absorption band at around
370–390 nm, observed in N-EtCz solutions, should be attrib-
Perkin Elmer luminescence spectrometer. The spectra were
corrected for the instrument sensitivity. In order to avoid spec-
tra distortion by reabsorption, the solutions used for lumines-
cence investigations had an absorbance at absorption maxima
of about 0.2 in a 1 cm cuvette. In this case the concentration of
solution was about 10 mol l . Solutions of CHTT in three
different solvents were investigated. The CHTT molecules may
be dissolved only in a limited number of solvents.
2
1
uted to the residual antracene. The anthracene molecules
are an inevitable impurity in the N-EtCz synthesis.
Fluorescence spectra of CHTT molecules are independent of
the excitation wavelength and are almost identical to the fluor-
escence spectra of N-EtCz. Fluorescence excitation spectra
under detection at any fluorescence wavelength closely resem-
ble absorption spectra of N-EtCz, however, the fluorescence
yield of CHTT molecules is more than by order of magnitude
lower. Fluorescence of the OTC fragment is completely
quenched in CHTT molecules.
ꢁ5
ꢁ1
Results
Fig. 3 shows the differential absorption spectra of CHTT
and N-EtCz in the chloroform solution obtained at 0 and
The absorption and fluorescence spectra of CHTT, N-EtCz
and OTC in acetone are presented in Fig. 2. Absorption and
fluorescence spectra of CHTT and N-EtCz in benzene and
chloroform are similar, whereas OTC molecules cannot be dis-
solved in these solvents. Absorption spectrum of CHTT mole-
cules closely resembles the sum of the OTC and N-EtCz
spectra indicating that the interaction between the two chro-
mophores is relatively weak, so that it only slightly changes
energetic positions of the ground and excited states of indivi-
dual chromophores. No new absorption bands emerge in this
case. One could expect the appearance of new bands due to the
formation of optically allowed intramolecular or intermolecu-
lar charge transfer states. Such states are formed between chro-
mophores having electronaccepting and electrondonating
properties as is the case in photosensitive carbazolylcontaining
polymers poly-N-vinylcarbazole or poly-N-epoxypropylcarba-
2
00 ps delays after excitation at 351 nm. Since absorption
and fluorescence spectra of both solutions are at shorter wave-
lengths than our investigation region, only excited state
absorption was observed. At the 0 ps delay time, spectra of
both solutions are quite similar. At the 200-ps delay time,
the transient absorption spectrum of N-EtCz remains
unchanged, while that of CHTT changes; the induced absorp-
tion at 650–820 nm increases. Later on the spectrum relaxes
without changing its shape. The transient absorption spectrum
of OTC was too weak for reliable measurements. It showed
only a weak induced absorption below 700 nm. No transient
absorption at longer wavelengths was observed. The transient
absorption spectra of all compounds in benzene and chloro-
form are similar (not presented).
Transient absorption kinetics of N-EtCz and OTC in all
solutions showed only weak decay during 1,5 ns indicating that
excited state lifetime of these compounds is much longer than
our investigation time domain. Transient absorption kinetics
of CHTT in chloroform at different wavelengths is presented
in Fig. 4. The kinetics shows two processes taking place on a
picosecond time scale. The spectrum modification that is
clearly evident in Fig. 3 occurs during the initial 100–200 ps.
Later on, the differential absorption at all probe wavelengths
decays exponentially with the same time constant. Thus, evi-
dently the first process should be attributed to the formation
of some intermediate species from the initially created excited
state. The second process reflects the decay process of the
produced intermediate state to the ground state.
1
6–20
zole mixed with electronaccepting 2,4,7-trinitrofluorenone.
Dynamics of the CHTT transient absorption depends signi-
ficantly on the solvent. Fig. 5 shows the transient absorption
kinetics of CHTT molecules at two probe wavelengths in three
different solvents. Table 1 summarizes the time constants of the
two processes and shows that the decay rate increases in more
Fig. 3 The differential absorption spectra of CHTT (a) and N-EtCz
(b) in chloroform obtained at 0 and 200-ps delay after excitation at
351 nm.
Fig. 2 The absorption and fluorescence spectra of CHTT (a), N-EtCz
b) and OTC (c) in acetone. Fluorescence was excited by 350 nm light.
(
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 2 7 6 – 2 2 8 0
2277

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time, which is similar to that of excited N-EtCz, namely the
N-EtCz fragment is predominantly excited by the 351 nm light,
and the lifetime of this excited state in all solvents is about 60
ps. Similarity of the CHTT and N-EtCz fluorescence spectra
confirm this assignment. Much lower fluorescence efficiency
of the CHTT molecules as compared to N-EtCz is in agree-
ment with much shorter lifetime of the initially created excited
state than the lifetime of excited state of N-EtCz.
Judging from the absorption and fluorescence spectra posi-
tions, the lowest excited state of CHTT molecule corresponds
to excitation of the OTC fragment, and one would expect a
rapid excitation energy transfer from N-EtCz to the OTC frag-
ment. However, a long spacer and a low transition dipole
moment of the OTC fragment evidently slows down the energy
transfer rate to at least 60 ps (the energy transfer rate is
approximately proportional the product of transition dipole
moments of energy-donor and acceptor).
Let us discuss the nature of the transient that forms with
about 60 ps time constant. The absence of any fluorescence
features of OTC fragment in CHTT spectrum provides no
arguments to assign it to the excitation of the OTC fragment.
The transient absorption spectra also show no indication of
the population of the OTC excited state; excited OTC mole-
cules show only very weak induced absorption and no induced
absorption above 700 nm, whereas absorption of the observed
transient is very strong in this spectral region. From the close
similarity of the induced absorption spectrum, which appears
with the 60-ps time constant with the absorption spectrum of
Fig. 4 The transient absorption kinetics of CHTT in chloroform at
different wavelengths.
polar solvents, whereas the intermediate state formation rate
within the experimental accuracy remains the same.
Kinetics at 800 nm in acetone is qualitatively different. The
long-lived transient absorption component with the lifetime
much longer than our investigation time domain is evident.
Although determination of the lifetime of this component
was not possible, it should be noted, that this component
decayed completely during the 0.5 s time between laser pulses
and no changes in the sample spectrum before and after experi-
ment were observed. The insert to Fig. 5 shows a spectrum of
the long-lived species. Similarly to the absorption of the tran-
sient photoproduct, the long-lived species exhibit an absorp-
tion maximum at about 800 nm, however, they do not
absorb below 650 nm.
2
2,23
N-EtCz cations reported in literature,
the transient species
may be safely assigned to the charge transfer state. Owning to
the electron donating and accepting properties of N-EtCz and
OTC fragments respectively, excited state electron transfer
from the N-EtCz to the OTC fragment is a very likely process.
Then the relaxation process with a longer time constant should
be assigned to the back electron transfer leading to the mole-
cule relaxation to the electronic ground state The strong
dependence of this relaxation rate on the solvent polarity is
in line with this assignment. According to the Marcus theory,
the electron transfer rate should depend on the difference
between free energies of the initial and final states, which
due to solvation processes is different in solvents of different
polarity. Due to nonpolar nature of the initial state, the for-
ward electron transfer rate may be less sensitive to the solvent
Discussion
According to the spectral position of the excitation pulse, and
judging from the transient absorption spectrum at zero delay
Fig. 5 The transient absorption kinetics of CHTT in various solvents at 600 nm (a) and 800 nm (b) wavelengths. The insert shows the differential
absorption spectrum of CHTT in acetone at 800-ps delay.
2
278
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 2 7 6 – 2 2 8 0
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4

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Table 1 Time constants of relaxation processes in solvents
transfer completes the relaxation process. Due to the large
energy difference of the back transfer reaction, it appears in
the inverted region. Therefore, the activation barrier decreases
in polar solvents when the reaction energy decreases.
Solvent
e
t
1
/ps
2
t /ps
benzene
chloroform
acetone
2.28
4.81
66 ꢂ 3
64 ꢂ 3
56 ꢂ 10
7000 ꢂ 1000
475 ꢂ 10
The relaxation process in acetone solution is slightly more
complex. Evidently only a fraction of molecules relax to the
ground state on a picosecond time scale, whereas another frac-
tion forms longer-lived transients, which disappear faster than
over 0.5 s. The absorption spectrum of these transients in the
700–850-nm region is similar to the biradical state absorption,
however, there is a difference at short wavelengths where the
long-lived transients do not absorb. Judging from the absorp-
tion spectrum features of the long-lived transient, it should be
also attributed to the N-EtCz cation radical. Probably some
chemical reaction, like electron transfer between OTC chromo-
phore anion radical and acetone or dissolved oxygen molecule
competes with the intramolecular back electron transfer creat-
ing long-lived transients. Another possible mechanism may be
related to the folding of molecules. If the molecules are folded
so that the two fragments are close to each other, then under
creation of the charge transfer state, solvation forces in highly
polar solvent may unfold the molecule and stabilize the charge
transfer state. This reaction needs additional investigation.
20.7
110 ꢂ 10
polarity. However, almost equal relaxation rates in nonpolar
solvent, benzene, and highly polar acetone, show that the 60
ps relaxation process cannot be identified with the electron
transfer. It is more natural to relate it to the excitation energy
2
4
transfer from N-EtCz to OTC fragment. As was shown in the
charge transfer in intermolecular charge transfer complexes
may take place under excitation of electron-acceptor molecule.
Such process is also typical for sensibilization of Cz containing
2
5
polymers. Thus, energy transfer from N-EtCz to OTC frag-
ment may take place prior to electron transfer. If the energy
transfer rate is slower than the subsequent electron transfer,
then namely the energy transfer determines the charge transfer
state formation kinetics. Since the energy transfer process is
independent, or only weakly depends on the solvent polarity,
the charge transfer state formation rate only weakly depends
on the polarity as well. Population of the OTC fragment
excited state in this case always remains negligible, what
explains the absence of the OTC fragment fluorescence
features in CHTT fluorescence spectrum.
Conclusions
We have examined the photoinduced intramolecular charge
transfer between electrondonating and electronaccepting
fragments of the CHTT molecule in various solutions. The
interaction between the OTC and N-EtCz chromophores is
relatively week so that the chromophores retain their indivi-
dual absorption properties, and the excitation energy transfer
from the EtCz to OTC fragment is relatively slow, taking place
on tens of picoseconds time scale. The charge transfer from the
N-EtCz fragment to the OTC fragment occurs very rapidly
after transfer of excitation energy to OTC fragment. The back
transfer rate is much slower and strongly depends on the
solvent polarity.
The photogenerated charge transfer states in the bipolar
electron-hole transporting material produced from bipolar
molecules may act as efficient charge carrier recombination
centers. This should be accounted for in attempts to use such
materials in electrooptical devices.
The last relaxation process, the electron back-transfer, may
be treated in terms of the Marcus electron transfer theory.
2
6
The energy of the charge transfer state should decrease in polar
solvents due to the increase in the solvation energy. Thus, the
difference between the charge transfer state and the ground
state energies should diminish. In the ‘‘normal’’ electron trans-
fer region, the transfer rate increases with increasing the energy
difference between the reactant and product state. However,
the back electron transfer in CHTT molecules evidently
appears in the inverted Marcus region, and therefore shows
opposite transfer rate dependence on the reaction free energy.
Such dependence is typical of the electron transfer reactions
with large reaction free energy and may be treated in terms of
the energy gap law. Similar dependence of relaxation rates on
the solvent polarity was typically observed in charge transfer
2
7
complexes.
Fig. 6 shows a schematic diagram of the excited state relaxa-
tion in the CHTT compound. After excitation of the molecule
to the second singlet state S
the N-EtCz fragment, the energy transfer to the OTC fragment
forms the S excited state with the time constant k . Later on
an electron is transferred from the N-EtCz fragment to the
OTC fragment. The forward transfer appears in the ‘‘normal’’
2
corresponding to the excitation of
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