Solvent-mediated photoinduced electron transfer in a pyridinium ionic liquid

Article abstract of DOI:10.1021/ja077797f

The dynamics of electron transfer reactions in butyl pyridinium bis(trifluoromethanesulfonyl)imide (BuPyr-NTf₂) and other solvents have been explored using laser flash photolysis. In these experiments, benzophenone (BP), duroquinone (DQ), and 9-cyanoanthracene (9CA) were used as excited-state acceptors, 1,4-diazabicyclo[2.2.2]octane and hexamethylbenzene were used as ground-state donors, and methyl viologen (MV²⁺) was used as a probe molecule. Analysis of kinetic and spectroscopic data from these experiments shows that electron transfer from photoreduced acceptors to the probe occurs via one or more solvent ions in cases where the acceptor anion radical has a reduction potential that is more negative than the solvent ions (BP?- and 9CA^?- in BuPyr-NTf₂). Mediated electron transfer was demonstrated to significantly enhance quantum efficiencies of photoinduced electron transfer in cases where back electron transfer would otherwise predominate. Copyright

Full text of DOI:10.1021/ja077797f

Published on Web 01/15/2008
Solvent-Mediated Photoinduced Electron Transfer in a Pyridinium Ionic Liquid
Rebecca C. Vieira and Daniel E. Falvey*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received October 10, 2007; E-mail: falvey@umd.edu
Chart 1. Structures and Redox Properties of Donors, Acceptor,
Photoinduced electron transfer (PET) is a fundamental process
that is central to a variety of applications including solar energy
conversion,1 photolithography,2 molecular photonics,3 phototrig-
gering,4 and the synthesis of complex molecules.5 Equation 1 depicts
a simple example of this wherein an excited-state electron acceptor,
A, abstracts an electron from a ground-state donor, generating two
radical species, A-• and D+•. Usually, the net efficiency of PET
reactions is determined by the rate of energy-wasting back electron
transfer reactions (eq 2) relative to the completing chemical
reactions or charge separation process (eq 3).
Probe Molecule, and IL Solvents
kET
A* + D 8 A-• + D+•
(1)
(2)
(3)
(4)
kBET
A
-• + D+•
8 A + D
PET reactions using the donors and acceptors shown in Chart 1
were examined. The primary acceptors, BP and DQ, react from
their triplet excited states. Thus, abstraction of an electron from
the donor, DABCO, generates triplet ion radical pairs. The 9CA/
HMB system, in contrast, is known to produce a singlet radical
ion pair, which has a low barrier to back electron transfer and
normally produces ion radicals with low efficiency.9
Five different solvents were employed. It was expected that
BuPyr-NTf2, being more easily reduced than the primary acceptors,
BP and 9CA, would mediate electron transfer to the probe. The
remaining solvents, having more negative reduction potentials,
would be incapable of mediating PET. Thus reduction of the probe
would only occur via diffusion of the photoreduced acceptor.
Benzene and MeCN were chosen as conventional nonpolar and
polar molecular solvents. The ILs, OMIM-PF6 and BMIM-NTf2,
have, respectively, high viscosity and a viscosity similar to that of
BuPyr-NTf2.
Both the triplet and singlet PET reactions were characterized
using laser flash photolysis (LFP), wherein a photoexcited acceptor
(A ) DQ3*, BP3*, 9CA1*) abstracts an electron from a ground-
state donor (D ) DABCO, HMB). The rates and efficiencies for
charge separation and intermolecular electron transfer are monitored
by a subsequent reaction of A-• with a probe ion (P ) MV2+),
whose reduced form is conveniently detected via its strong
absorbance at 610 nm.
In CH3CN, LFP provides transient spectra typical for such
systems in conventional polar solvents. For example, LFP of BP
with DABCO generates the transient spectra of the BP-• (545 nm)
and the DABCO+• (460 nm). Addition of the MV2+ quenches the
BP-• and gives rise to MV+•. Similarly, LFP of DQ/DABCO and
9CA/HMB provide the corresponding ion radical transients, and
the inclusion of MV2+ quenches the anion radicals and generates
the probe ion radical, MV+•. Similar spectroscopic behavior is seen
in BMIM-NTf2 and OMIM-PF6.
kPROBE
A
-• + P2+
8 P+•
kDEC
P
+• + D+•
8 P2+ + D
Following the pioneering theoretical insights of Marcus,6 ex-
perimentalists have sought to optimize the efficiency of PET
reactions. Thus there have been extensive studies on the effects of
the driving force, electronic coupling, and the solvent medium on
the rates of these reactions.6-10
Room temperature ionic liquids (ILs) have found many intriguing
applications in synthetic chemistry,11 electrochemistry,12 and separa-
tion science.13 Their ionic nature implies that they might be able
effect efficient charge separation in PET processes via specific ion-
pairing interactions. On the other hand, typical ILs have high
viscosities which tend to slow diffusion rates and reduce the rate
for cage escape. A recent study of PET rate constants in an
imidazolium-based IL showed that the reorganization energies are
not qualitatively different from what is observed in solvents of
moderate polarity.14 This observation is consistent with solvato-
chromic, simulation, and ET studies which indicate that imidazo-
lium-based ionic liquids have polarities comparable to those of
acetonitrile or ethanol.15-22
A pulse radiolysis study by Behar et al. suggested to us that
pyridinium ionic liquids could facilitate rapid intermolecular
electron transfer reactions.23,24 Presumably, the favorable reduction
potential of the pyridinium cation permits electron migration to
occur via a secondary reduction of one or more solvent cations.
The experiments described herein were aimed at determining
whether such a solvent-mediated pathway can be exploited as a
means of segregating charges generated in PET reactions and what,
if any, rate acceleration could be realized relative to comparable
processes that rely on the diffusion of photogenerated radical ions.
It is shown that in cases where the redox potentials are favorable
(BP/DABCO in BuPyr-NTf2) the solvent actively facilitates the PET
process. It is further demonstrated that this solvent-mediated
pathway increases the rates and efficiencies of PET reactions.
Contrasting behavior is seen in BuPyr-NTf2. With BP/DABCO,
no signal for BP-• is observed, even in the absence of MV2+. This
is consistent with the expected secondary electron transfer to the
9
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J. AM. CHEM. SOC. 2008, 130, 1552-1553
10.1021/ja077797f CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
The 9CA/HMB system shows very low ΦSEP in nonpolar solvents
and in ILs incapable of mediating electron transfer (entries 4, 9,
and 11). However, in BuPyr-NTf2, we detect higher yields of MV+•
(entry 7). This is significant as 9CA/HMB gives a singlet radical
ion pair with a very low barrier to back electron transfer.
Nonetheless, this system provides efficient charge separation in
BuPyr-NTf2.
A comparison of entry 6 with 8 and 10 demonstrates that electron
transfer from BP-• to MV2+ (kPROBE) is an order of magnitude faster
in BuPyr-NTf2 than the other ILs, which supports the solvent-
mediated ET pathway for BuPyr-NTf2. A solely diffusive ET
pathway is illustrated in entries 5, 8, and 10. Electron transfer from
DQ-• to MV2+ is approximately 1 × 108 M-1 s-1 in these cases.
The LFP experiments described above show that BuPyr-NTf2
can actively facilitate PET reactions by providing a solvent-mediated
pathway for electron transfer. Further work will be necessary to
fully elucidate the nature of this process. For example, the current
experiments do not distinguish a solvent hopping pathway from a
pathway in which a reduced solvent cation diffuses to the probe.
In the former case, greater rates may be realized in more organized
media such as ionic liquid crystals.25 Results from the latter will
be reported in due course.
Acknowledgment. We would like to thank the National Science
Foundation for financial support. R.C.V. would like to thank the
ARCS Foundation for their generous support.
Figure 1. (A) Transient absorption spectra of BP/DABCO/MV2+ in BuPyr-
NTf2 with inset of waveform at 610 nm, and (B) transient absorption spectra
of DQ/DABCO/MV2+ BuPyr-NTf2 with inset of waveform at 610 nm.
Supporting Information Available: Experimental procedures
including preparation of ILs, viscosity measurements, and general
procedures for LFP experiments, detailed procedure for kinetic simula-
tions. This material is available free of charge via the Internet at http://
pubs.acs.org.
Table 1. Rate Constants and Quantum Yields for PET Reactions
a
a
a
entry
solvent
acceptor
kBET
kPROBE
kDEC
ΦSEP
1
2
3
4
5
6
7
8
9
CH3CN
CH3CN
CH3CN
benzene
BuPyr-NTf2
BuPyr-NTf2
BuPyr-NTf2
BMIM- NTf2
BMIM- NTf2
OMIM-PF6
OMIM-PF6
DQ
BP
9CA
9CA
DQ
BP
9CA
BP
9CA
BP
15.3
47.4
b
38.4
14.2
b
1.74
5.18
b
0.50
0.70
0.25
0.05
0.90
0.89
0.65c
0.85
0.03
N/Ad
0.20
b
b
b
References
0.45
1.03
3.64
0.10
b
0.15
1.00
0.49
0.21
b
0.59
0.12
0.05
0.15
b
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growth of MV+• when the probe is added (Figure 1a). In contrast,
DQ-•, which is not expected to be capable of solvent reduction, is
detected in the DQ/DABCO LFP experiment (at 445 nm). Addition
of the probe also results in formation of MV+• but at a lower rate
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