Photoinduced electron transfer between 2-methylanthraquinone and triethylamine in an ionic liquid: Time-resolved EPR and transient absorption spectroscopy study

Article abstract of DOI:10.1016/j.saa.2014.08.021

Photoinduced electron transfer between 2-methylanthraquinone (MeAQ) and triethylamine (TEA) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), was investigated by comparing the time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and the transient absorption spectroscopy. The results of TR-EPR spectroscopy, in which MeAQ was 8 mmol L^-1and TEA was 150 mmol L^-1, indicated that the transient radical would exist longer time in [bmim][PF₆] than in acetonitrile. At the delay time of 8 μs after laser excitation, the TR-EPR signal transformed from an emissive peak into an absorptive peak when the experiment was performed in [bmim][PF₆]. The results of the transient absorption spectroscopy, in which MeAQ was 0.1 mmol L^-1and TEA was 2.2 mmol L^-1, showed that the efficiency and the rate of the photoinduced electron transfer reaction in [bmim][PF₆] were obviously lower than that in acetonitrile. It was concluded that various factors, such as concentration, viscosity and local structural transformation of the solution, have an influence on the process of photoinduced electron transfer in [bmim][PF₆].

Full text of DOI:10.1016/j.saa.2014.08.021

Accepted Manuscript
Photoinduced Electron Transfer between 2-Methylanthraquinone and Triethyl-
amine in an Ionic Liquid: Time-Resolved EPR and Transient Absorption Spec-
troscopy Study
Guanglai Zhu, Yu Wang, Haiying Fu, Xinsheng Xu, Zhifeng Cui, Xuehan Ji,
Guozhong Wu
PII:
S1386-1425(14)01212-8
http://dx.doi.org/10.1016/j.saa.2014.08.021
SAA 12542
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 November 2013
7 August 2014
18 August 2014
Please cite this article as: G. Zhu, Y. Wang, H. Fu, X. Xu, Z. Cui, X. Ji, G. Wu, Photoinduced Electron Transfer
between 2-Methylanthraquinone and Triethylamine in an Ionic Liquid: Time-Resolved EPR and Transient
Absorption Spectroscopy Study, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014),
doi: http://dx.doi.org/10.1016/j.saa.2014.08.021
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Photoinduced Electron Transfer between 2-Methylanthraquinone and
Triethylamine in an Ionic Liquid: Time-Resolved EPR and Transient
Absorption Spectroscopy Study
Guanglai Zhu, a∗ Yu Wang,a Haiying Fu, b Xinsheng Xu,a Zhifeng Cui,a Xuehan Jia and
Guozhong Wub*
a Institute of Atomic and Molecular Physics, Anhui Normal University, Wuhu 241000, P. R. China
b Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China.
Abstract: Photoinduced electron transfer between 2-methylanthraquinone (MeAQ) and
triethylamine (TEA) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF6]), was investigated by comparing the time-resolved
electron paramagnetic resonance (TR-EPR) spectroscopy and the transient absorption
spectroscopy. The results of TR-EPR spectroscopy, in which MeAQ was 8 mmol L-1 and
TEA was 150 mmol L-1, indicated that the transient radical would exist longer time in
[bmim][PF6] than in acetonitrile. At the delay time of 8 µs after laser excitation, the
TR-EPR signal transformed from an emissive peak into an absorptive peak when the
experiment was performed in [bmim][PF6]. The results of the transient absorption
spectroscopy, in which MeAQ was 0.1 mmol L-1 and TEA was 2.2 mmol L-1, showed that
the efficiency and the rate of the photoinduced electron transfer reaction in [bmim][PF6]
were obviously lower than that in acetonitrile. It was concluded that various factors, such
as concentration, viscosity and local structural transformation of the solution, have an
influence on the process of photoinduced electron transfer in [bmim][PF6].
*Corresponding author. Tel: (86) 553-3869748; E-mail address: zhglai@mail.ahnu.edu.cn, wuguozhong@sinap.ac.cn.
1

Keywords: 1-Butyl-3-methylimidazolium hexafluorophosphate; Photoinduced electron transfer;
Time-resolved electron paramagnetic resonance; Transient absorption spectra; Radical
1. Introduction
Room-temperature ionic liquids (RTILs, is simply written as ILs) have received much
attention in past years due to their remarkable properties and their importance in a broad
range of applications [1-3]. The main difference between RTILs and conventional
solvents is that RTILs consist entirely of ionic species that only loosely fit together [4-5].
Properties of RTILs have recently been characterized using various experimental
measures including NMR, IR, Raman, transient absorption spectroscopy, steady-state and
time-resolved fluorescence, etc [6-14]. Many experimental and theoretical efforts were
made to study the photophyical and photochemical behavior of ILs or the reaction
processes of the probe molecules occurring in ILs. Alkyl side chain, hydrogen bonding,
and the presence of other solvent were known to affect the photochemical behavior [6-10].
A lot of these studies focused on solvent relaxation dynamics and energy transfer kinetics.
Effects of RTILs on transient absorption spectra and bimolecular reaction rate constants
of solutes were only partly revealed.
Photoinduced electron transfer (PET) is a fundamental and widespread reaction in
nature and has been extensively applied in various fields such as solar energy conversion,
molecular photonics, and artificial photosynthesis [15-17]. It is noteworthy that there has
been considerable works focusing on photoinduced electron transfer process in RTILs by
laser photolysis transient absorption spectroscopy [11-15]. Furthermore, EPR spectra of
some nitroxide spin probes in RTILs have been investigated [18-19]. However, as for an
effective method that can provide valuable information on the solvent-dependent
2

dynamics of the short-lifetime intermediates in solution, chemically induced dynamic
electron polarization (CIDEP) [20-22], has seldom been utilized to the study of PET in
ionic liquids. Time-resolved electron paramagnetic resonance (TR-EPR) is a suitable
method to distinguish short lifetime intermediate species directly in some photoinduced
reactions. The CIDEP signal obtained by TR-EPR experiments can provide a lot of
information on the photochemical reaction mechanism.
Some representative photoinduced reactions such as photoinduced electron transfer
were studied in 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], which
was compared with volatile organic compounds [11-12]. It was speculated that there were
nanostructures in ionic liquids or their mixtures with molecular dynamics simulations by
some authors [23-24]. Therefore, the change in local structures of the IL/organic mixed
solution tuned by varying the IL fraction may have substantial effects on the reaction
kinetics of transient species in photochemical processes. In this work, we aim to compare
the effects of the ionic liquids on the electron spin polarization and transient absorption
spectroscopy in the photoinduced electron-transfer process. Triethylamine (TEA) was
used as an electron donor and 2-methylanthraquinone (MeAQ) was used as a probe
molecule for its better solubility than 9,10-anthraquinone (AQ). Both time-resolved
electron paramagnetic resonance spectroscopy and transient absorption spectroscopy
were measured for the radicals generated from the photoexcitation of MeAQ in the
presence of TEA. Our results revealed that [bmim][PF6] can affect the TR-EPR spectra of
the radicals produced from the electron transfer between the triplet excited state of
MeAQ (3MeAQ*) and TEA, which was somewhat different from the results observed in
the transient absorption experiment.
3

2. Experimental section
Triethylamine (TEA) and acetonitrile (MeCN) were of HPLC grade and used as
received. 2-methylanthraquinone (MeAQ) was recrystallised through vacuum
sublimation. [bmim][PF6] was purchased from Merck and was dried for 24 hours under
high vacuum at 60-65℃ prior to use. In the TR-EPR experiment, the concentration of
MeAQ was 8 mmol L-1 and the concentration of TEA was 150 mmol L-1. In the
transient absorption spectroscopy experiments, the concentrations of MeAQ and TEA
were much lower than those in the TR-EPR experiments.
The TR-EPR experiments were performed on a home-built TR-EPR spectrometer
with submicrosecond resolution. The experimental instrument consisted of a conventional
X-band EPR spectrometer, a boxcar integrator (Stanford SR 252), a digital oscilloscope
(Philips PM 3350) and a broadband preamplifier with 50 ns time response, which has
been described in detail elsewhere [25-26]. The excitation light is the third harmonic
generation light (355 nm, about 15 mJ) from a Nd:YAG laser. The operation repetition is
10 Hz. The boxcar gate width is 0.3 µs . All the solutions were bubbled with N2 for 20
min, before they were made to flow through a quartz flat cell (optical path: about 0.3 mm)
in the EPR cavity through a peristaltic pump.
Transient absorption experiments were carried out using another Nd: YAG laser
that provides 355 nm laser pulse with a duration of 5 ns and an energy of 10 mJ per
pulse. The probe light source was a pulsed xenon lamp (300 W). The laser and analyzing
light beam passed perpendicularly through a 10 mm×10 mm quartz cell. The transmitted
light signals were collected using a digital oscilloscope and then recorded by a computer.
A detailed description of the system was described in another document [27]. All
4

solutions were bubbled with the high-purity N2 for 20 min prior to laser irradiation. All
the TR-EPR experiments and the transient absorption experiments were carried out at
room temperature (about 25 ℃).
3. Results and Discussion
3.1. Time resolved EPR study
3
Here, we investigated the electron transfer process of MeAQ* using TEA as an
electron donor in the IL/MeCN solutions. No CIDEP signal was observed after the
photoexcitation of a pure solution of MeAQ (8 mmol L-1) in acetonitrile (MeCN), which
indicated that there were not radicals formed. When TEA was added to a MeAQ solution
at the concentration of 150 mmol L-1, a conventional photoinduced CIDEP signal could
be observed at a delay time of 1 µs (Fig. 1). The CIDEP spectrum consisted of only one
central peak which was emissive, similar as the anion radical of AQ [25]. Furthermore,
TEA cannot be excited at 355 nm [28]. Therefore the CIDEP spectrum could be
attributed to the radical anion of MeAQ ( MeAQ•− ) that was mainly formed via the triplet
mechanism (TM) [29-30]. This result indicated that an electron transfer occurred from
TEA to the excited triplet state MeAQ* to generate MeAQ•− and TEA•+ radicals.
3
•+
3
TEA as the electron donor was oxidized by MeAQ* . We couldn’t detect the TEA
signal due to its fast relaxation [25, 31]. When the delay time was set to 4.0 µs , the
central peak divided itself to two very weak peaks, and the CIDEP signal disappeared at
5

longer delay time. The two peaks might be formed from the semiquinone radical
MeAQH• generated by deprotonation [32-33].
Fig. 1.
The general PET reaction scheme of MeAQ and TEA can be described by equation [1].
The contact ion pair [MeAQ− ⋅⋅⋅TEA+ ] is formed via electron transfer (ET) and then
dissociates into solvent separated ions TEA•+ and MeAQ•− . In this case, back electron
transfer (BET) is much slower than electron transfer. A small quantity of MeAQH• may
be produced by deprotonation.
hv ET→
←
BET
Dissociation
[MeAQ ⋅⋅⋅TEA ]←→MeAQ + TEA
−
+
•−
•+
MeAQ + TEA
[1]
Since many reports revealed that ILs are not homogeneous phase and they have some
local nanostructures that affect the reaction in ionic liquids [23-24], what will happen
when ILs are added to this reaction system? Here we report our study on the effects of a
prototype ILs, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6], on the
PET process in MeAQ/TEA system. Similar to the result in MeCN, No CIDEP signal
was observed for the pure MeAQ solution in [bmim][PF6]. As a comparative study, the
experiments were performed at the same concentration of MeAQ and TEA in
[bmim][PF6] or [bmim][PF6]/MeCN solution as in MeCN. In this system, when the
volume ratio of [bmim][PF6]/MeCN is above 1:2, a conventional photoinduced EPR
signal that is similar as the signal in acetonitrile was also observed at a 1 µs delay and
the peak disappeared at a 5 µs delay. However, as shown in Fig. 2(c), when the delay
6

time was set at 8µs , the EPR signal came forth again and an unusual net-absorptive
CIDEP spectrum was observed from MeAQ•− produced by photoinduced electron
transferring in the TEA/MeAQ system. The above phenomenon would be discussed in
the section 3.3. On the other hand, when the ratio of [bmim][PF6]/MeCN increased，the
maximum intensity of the CIDEP spectra at 0.6 µs decreased gradually and the
maximum intensity of the net-absorptive CIDEP spectra at 8 µs increased but didn’t
change obviously (See Fig. 3.).
Fig. 2.
Fig. 3.
3. 2. Transient absorption study
In order to support the above reaction mechanism, the transient absorption
experiments of this system had been carried out. As shown in Supporting Information, the
absorption spectrum of 3MeAQ* in MeCN was mainly a band centered at about 390 nm
[31]. The spectra of MeAQ/TEA in MeCN were shown in Fig. 4. Here, the concentration
of MeAQ was 0.1 mmol L-1 and TEA was 2.2 mmol L-1, which was much lower than that
in TR-EPR experiments. In the presence of TEA, a new absorption band centered at 530
nm (λmax) quickly appeared at the shorter delay time (even at 0.1 µs) except the peak of
390 nm (Fig. 4.), indicating the formation of new transient species via the reaction of
3MeAQ* with TEA, since TEA itself cannot be excited at 355 nm. The absorption band
7

with λmax at 530 nm could be assigned to MeAQ•− [34]. The reaction rate of MeAQ
with TEA was estimated to be 4.6×109 M −1s−1 based on different concentrations of
TEA.
Fig. 4.
Fig. 5 showed the transient absorption spectra of MeAQ•− produced from MeAQ/TEA
recorded after 355 nm laser excitation in IL/MeCN (volume fraction VIL=0.9) solution
under N2 purging. At a delay time of 0.1 µs, the spectra were similar to the spectra
without TEA (See Fig. S5 in Supporting Information, MeAQ in [bmim][PF6]/MeCN). At
the longer delay time (after 5µs), a new transient absorption band centered at 510 nm
(
max) appeared, which could be assigned to the absorption spectra of MeAQ•− .
Comparing Fig. 4 and Fig. 5, a spectral blue-shift of the absorption band was observed.
λmax of MeAQ•− shifted from 530 nm in MeCN to 510 nm in the IL/MeCN solution,
which might be caused by the interaction between MeAQ•− and cations of ionic liquid
[35-36]. In this case, the signal still existed for a long time. Even at 15 µs after laser
pulse, the peaks due to MeAQ•− were obviously observed. The reaction rate of MeAQ
with TEA in the IL/MeCN solution (VIL=0.9) was estimated to be 1.1×109 M −1s−1 . It
indicated that the growth rate (kgr) was still large although it was obviously lower than
that in MeCN.
Fig. 5.
8

To further understand the effect of IL on the electron transfer process, we compared the
spectra at 510 nm at different VIL. Fig. 6 showed the dependence of the transient
absorption spectra of MeAQ•− on VIL. It showed an obvious change. The intensity of
spectra at 510 nm decreased sharply with increasing VIL. Moreover, we observed that the
intensity of absorption at λmax was much weaker than that in MeCN. According to Fig.
6, relative quantum yields of MeAQ•− in the mixtures can be estimated through
Ф/Ф = ODmax/ OD0 [51]. Ф is the quantum yield and ODmax is the maximal transient
0
absorption. Ф and OD0 represent the values in MeCN. The results are shown in Table 1,
0
where mole fraction as well as mole concentration of [bmim][PF6] was given and the
viscosity of the system was obtained from Ref. [37]. It can be observed that the quantum
yield of MeAQ•− decreased with the increasing of concentration of [bmim][PF6]. This
phenomenon implied that the efficiency of this electron transfer process was much slower
than that in MeCN, which was not consistent with the result of the TR-EPR experiment
carried out at the higher concentration of reactants.
Fig. 6.
Table 1.
3.3. Discussion
Usually, the net-absorptive CIDEP spectra were observed only at the case that the so
called spin-orbit coupling (SOC) interaction enhances the sublevel selective back electron
transferring from the triplet exciplex or contact radical pairs (RPs) to the singlet ground
9

state in photochemical reactions [38-41], such as photoinduced electron and hydrogen
transfer reaction [42-46]. The generation of the triplet exciplexes or contact RPs is due to
the depopulated-type triplet mechanism (d-type TM) or spin-orbit coupling mechanism
(SOCM) and has been widely studied [42-43], because they play an important role in
photoinduced electron transfer reactions. On the other hand, CIDEP effects due to the
d-type TM or SOCM can be observed only in the condition that the rate of intersystem
crossing (ISC) first excited triplet (T1) states between and the ground (S0) states can
compete with both radical formation and spin relaxation between the triplet sublevels
[38-41]. Usually such condition is hard to be satisfied, therefore CIDEP due to the d-type
TM had been observed only for two limited cases, one is the radicals generated through
triplet exciplexes of some excited dye molecules with heavy atom substituted electron
donors in polar solvents [47-48], another is the radicals from intra-molecular reactions for
photo-excited triplet states of azoalkanes and triarylphosphines found by Paul’s group
[49].
In our case, the PET reaction performed between MeAQ and TEA without heavy atom
substituted electron donors. Furthermore, many studies have indicated that the PET was
restricted in imidazolium ionic liquids due to their high viscosity [50-52], which was
inconsistent with our present result of TR-EPR. How to explain a theory for CIDEP due
to the d-type TM in ionic liquids?
Although many ILs can be dissolved in water or organic solvents, such IL/organic
solutions may not be homogeneous and should be regarded as nano-structured solvents
indicated by numerous works based on the theoretical calculations and experimental
studies [23-24]. When the concentration of IL reached the certain value (It could be
10

concluded that the critical VIL value was about 0.3 from our previous work [51])，a
special structure similar to reverse micelle formed. It is well known that there exists the
hydrogen bond network and in some cases generate nanostructures with the polar regions
and the nonpolar regions in imidazolium ionic liquids [23].
In the transient absorption experiments, MeAQ was 0.1 mmol L-1 and TEA was 2.2
mmol L-1, the solute was uniformly dispersed in the media. In this ionic media, the
diffusion of solute must be very slow due to the high viscosity and possible interaction
with ions. The high viscosity (about 201
at 20 ℃) and slow diffusion (about
mPa ⋅s
6.8×10−12 m2s−1 for bmim) [53-54] make the lifetime of ion pairs of [MeAQ− ⋅⋅⋅TEA+ ]
longer and result in higher efficiency of the back electron transfer. Therefore, we can
conclude that PET was restricted from the results of the transient absorption spectroscopy
because the viscosity of the solution increased and the diffusion rate of the solutes
decreased. The low yield of the photoinduced electron transfer products might be due to
the slow diffusion in [bmim][PF6]. Therefore, the results of transient absorption showed
that the PET reactions were restricted at the low concentration of solutes.
On the other hand, the maximum intensity of CIDEP signals at 1 µs in [bmim][PF6] is
almost equal to that in MeCN, indicating that the efficiency was not much lower than in
MeCN. Maybe the effects of IL on photoinduced electron transferring are due to various
factors, e.g. concentration, viscosity, and local structural transformation of the solution.
In the TR-EPR experiments, the concentrations of solutes (8 mmol L-1 for MeAQ and
150 mmol L-1 for TEA) were much higher than that in transient absorption studies. The
large concentration made it possible that the probability of encounter of donors and
acceptors increased. Furthermore, we consider that the polar regions were beneficial to
11

charge separation to produce radicals, and the nonpolar regions were beneficial to back
electron transfer resulting in a long charge separated state. Therefore we can observe the
CIDEP due to the d-type TM. We call for more theoretical and empirical studies to
examine the above speculation in the future.
4. Conclusions
In summary, we have shown for the first time the CIDEP spectrum of radical MeAQ•−
formed by PET in [bmim][PF6]. It was found that the lifetime of the transient radical in
[bmim][PF6] was longer than that in acetonitrile, and the CIDEP signal changed from a
net emissive signal at shorter delay times to a net absorptive signal at longer delay times
when the TR-EPR experiment was performed in [bmim][PF6]. The concentrations of the
electron donor and the acceptor, the high viscosity and the local structure of the
[bmim][PF6] in molecular solvents, might be the main factors that affected the PET
process. Our results provide a valuable addition to the application of common ionic
liquids, especially they are helpful for the application of ILs in the DSSC and the related
fields.
Acknowledgements
This work was supported by the National Science Foundations of China (No. 21173002)
and the Science Foundation of the Education Committee of Anhui Province of China
(No. KJ2010A145).
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16

FIGURES:
Fig. 1
Fig. 2
17

2.5
2.0
1.5
1.0
0.5
0.0
a)
b)
0.0
0.2
0.4
0.6
0.8
1.0
V
IL
Fig. 3
0.3
0.2
0.1
0.0
-0.1
350
400
450
500
550
600
650
Wavelength/nm
Fig. 4
18

0.08
0.06
0.04
0.02
0.00
-0.02
350
400
450
500
550
600
650
Wavelength/nm
Fig. 5
0.6
VIL
0
0.2
0.4
0.6
0.9
0.4
0.2
0.0
0
5
10
15
20
25
Time/µs
Fig. 6
19

Figure captions:
Fig. 1 Time-dependence of the TR-EPR spectra of MeAQ anion radical generated by the
photoinduced electron transfer from TEA in acetonitrile at the delay time a) 1.0µs ; b) 2.0µs ; c)
4.0µs ; d) 8.0µs . [MeAQ]= 8 mmol L-1 and [TEA] =150 mmol L-1.
Fig. 2 Time-dependence of the TR-EPR spectra of MeAQ anion radical generated by the
photoinduced electron transfer from TEA in [bmim][PF6] at the delay time a) 1.0µs ; b) 4.0µs ; c)
8.0µs . [MeAQ]= 8 mmol L-1 and [TEA] =150 mmol L-1.
Fig. 3 Dependence of the intensity of TR-EPR spectra of MeAQ•− on the volume fraction of
[bmim][PF6] (VIL) at the delay time a) 0.6 µs ; b) 8.0 µs .
Fig. 4 Transient absorption spectra recorded at（■）0.1 µs；（●）5 µs；（▲）15 µs after 355 nm
excitation of the N2-saturated neat MeCN containing 0.1 mmol L-1 MeAQ and 2.2 mmol L-1 TEA.
Fig. 5 Transient absorption spectra recorded at（■）0.1 µs；（●）5 µs；（▲）15 µs after 355 nm
excitation for N2-saturated IL/MeCN solution (VIL=0.9) containing 0.1 mmol L-1 MeAQ and 2.2 mmol
L-1 TEA.
Fig. 6 Time profiles of MeAQ•− observed at 510 nm in IL/MeCN mixed solutions under N2
purging with different VIL values.
20

Tables:
Table 1
a
Relative quantum yields of MeAQ•− estimated through
Φ/Φ0=∆ODmax/∆OD0
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
VIL
x1
0.027 0.059 0.098 0.145 0.168 0.277 0.371 0.504 0.695
0.48 0.96 1.44 1.92 2.40 2.88 3.36 3.84 4.32
c [mol/L]
η [mPa⋅s]
0.339 0.46 0.61 0.83 1.17 1.45 3.32 6.09 15.12 44.93
0.81 0.73 0.63 0.54 0.47 0.35 0.26 0.19 0.16
1
Φ/Φ
0
a
VIL is the volume fraction and x1 is the mole fraction of [bmim][PF6] in the mixture; c is the mole
concentration of [bmim][PF6] in the mixture; η is the viscosity of the system, which was estimated
from Ref. [37].
21

PET in [bmim][PF6]
22

Highlights
MeAQ•−
● TR-EPR and transient absorption spectra of
in [bmim][PF6] were recorded.
● CIDEP signal was changed to net absorption at longer delay times in [bmim][PF6].
● Lifetime of transient radical in [bmim][PF6] was longer than that in acetonitrile.
● Various factors have an influence on photoinduced electron transfer in [bmim][PF6].
23