Fluorescence measurement of 3,5-dimethoxyphenol radical cation generated by pulse radiolysis in 1,2-dichloroethane

Article abstract of DOI:10.1246/cl.2000.1126

Radical cation of 3,5-dimethoxyphenol in the excited state generated in 1,2-dichloroethane solution by the tandem irradiation of 28-MeV electron and 532-nm laser pulses emitted fluorescence around 600-750 nm with a quantum yield of 2.0 × 10^-3.

Full text of DOI:10.1246/cl.2000.1126

1126
Chemistry Letters 2000
Fluorescence Measurement of 3,5-Dimethoxyphenol Radical Cation
Generated by Pulse Radiolysis in 1,2-Dichloroethane
Nobuyuki Ichinose,* Sachiko Tojo, and Tetsuro Majima*
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
(Received June 8, 2000; CL-000557)
Radical cation of 3,5-dimethoxyphenol in the excited state
A transient absorption spectrum around 575 nm was
observed immediately after the 8-ns electron pulse irradiation
of DMP (5.0 × 10–3 M) in argon-purged DCE.8 The spectrum
in DCE is similar to that observed by laser flash photolysis of
DMP in acetonitrile and is assigned to DMP•+.9 The time-
resolved transient absorption spectra showed a slow growth of a
new peak at 500 nm (Figure 1). This peak was assigned to 3,5-
dimethoxyphenoxy radical (DMP•) according to Johnston et al.9
The absorption around 575 nm due to DMP•+ decreased with
time and most of DMP•+ disappeared at 2 µs after the electron
pulse to give DMP• through deprotonation of DMP•+. It has
been shown that bimolecular deprotonation of DMP•+ occurs to
give DMP• in the presence of basic reagents.9 The growth of
DMP• monitored at 472 nm corresopnded to the decay of the
transient absorption at 575 nm. A rapid decay of DMP•+ and
growth of DMP• were observed in their time profiles within 20
ns. These would be due to the electron transfer reaction of
DMP with chlorine atom (Cl•) as a byproduct of pulse radioly-
sis of DCE.7b We attribute the basic species for the deprotona-
tion to Cl– which is generated by the radiation chemical primary
processes of DCE. The plausible mechanism involved therein
is summarized in Scheme 1.
generated in 1,2-dichloroethane solution by the tandem irradia-
tion of 28-MeV electron and 532-nm laser pulses emitted fluores-
cence around 600–750 nm with a quantum yield of 2.0 × 10–3.
Radical ions are an important intermediate in various electron
transfer reactions in solution or heterogeneous systems such as
photoinduced electron transfer or radiation chemical rections.
Their chemical and physical properties have been intensively
studied for these three decades. On the other hands, fewer physi-
cal and chemical properties of excited radical ions have been
revealed as compared to excited radicals.1–7 This could be attrib-
uted to the weakly or non-fluorescent nature and extremely short
lifetime of excited radical ions. Whereas lifetime of excited radi-
cals lies in the range of a few tens to hundreds nanoseconds and
many of excited radicals fluoresce, that of excited radical ions is
typically much shorter than 1 ns and fluorescence in solution has
been reported for only a few excited radical ions.2,5–7 Such behav-
ior has been explained by their low excitation energy which
allows fast nonradiative internal conversion for the sake of the
energy gap law.2 We have recently reported laser-induced transient
fluorescence of 1,3,5-trimethoxybenzene (TMB) radical cation
(TMB•+) which is generated in acetonitrile at room temperature by
laser flash photolysis.7a If strong fluorescence of a radical ion is
observed, it will be a help for the monitor of electron transfer phe-
nomena in homogeneous and heterogeneous environment with a
high sensitivity. We have studied laser-induced fluorescence of
radical cations generated by pulse radiolysis in order to find “fluo-
rescent radical ion probe” and have found that 3,5-dimethoxyphe-
nol (DMP) radical cation (DMP•+) showed a fluorescence stronger
than TMB•+ in 1,2-dichloroethane (DCE) solution.
We have tried to measure the fluorescence from excited
DMP•+ (DMP•+*) to examine whether the symmetrical substitu-
tion as in TMB is required for fluorescence emission and uni-
molecular deprotonation from DMP•+* competes with radiative
transition or not. Excitation of DMP•+ at 532 nm with the 2nd
harmonic pulse of Nd:YAG laser gave broad fluorescence
around 600–750 nm (Figure 2). The fluorescence spectrum
showed a mirror image symmetry to the absorption spectrum of
DMP•+. The integrated fluorescence intensity decreased with the
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1127
The fluorescent nature of TMB•+* was previously attrib-
uted partially to its pseudo-D3h structure which makes the D0
and D1 states degenerate to allow the D2–D0 fluorescece as has
been discussed for 1,3,5-trifluorobenzene radical cation in the
gas phase.12 The fluorescent nature of DMP•+* ruled out the
necessity of complete C3-symmetric substitution of the ben-
zene-ring and suggested a possibility in expansion of the 1,3,5-
trioxybenzene family as fluorescent radical cation probes for
the detection of hole or electron transfer phenomena.
We thank all the member of the Radiation Laboraotry of
the ISIR, Osaka University for the operation of the linear accel-
erator. This work was partly supported by a Grant-in-Aid for
Scientific Research (Nos. 09226223, 10132237, 09450319, and
09875209) from the Ministry of Education, Science, Sport and
Culture of Japan.
References and Notes
1
2
3
4
5
6
7
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An argon-purged 4-mL portions of DCE solution of DMP
was placed in a quartz cuvette (1 × 1 × 4 cm3) and was irra-
diated with an electron pulse (28 MeV, 8 ns). The radiolyt-
ically generated radical cations were excited with a second
harmonic pulse (532 nm, 130 mJ pulse–1) of an Nd:YAG
laser (Quantel Brilliant) with a delay time of –100–2000 ns
relative to the electron pulse. The obtained fluorescence
spectra are not corrected for the wavelength dependence of
the system. The details are described in the previous
paper.7b
increase of the delay time of the laser pulse relative to the elec-
tron pulse in a similar manner to the temporal profile of the tran-
sient absorption monitored at 575 nm (Figure 2). This also sup-
ports the assignment of the fluorescence to that of DMP•+*.
When DMP•+ was excited with the 532-nm laser pulse, the
absorption of DMP•+ was unchanged and no increase of that of
DMP• was observed.
8
The fluorescence quantum yield (Φf) of DMP•+ in DCE
was independent of the delay time being (2 0.3) × 10–3 except
for that at 30 ns (1.3 × 10–3). The low fluorescence quantum
yield at 30 ns may be resulted from intra-cage quenching of
[DMP•+* Cl–]. The Φf = (2 0.3) × 10–3 for DMP•+* was higher
than that for free TMB•+* (1.1 × 10–3) in DCE.10 We also esti-
mated the radiative rate constant (kf) for DMP•+* to be kf = 5.8
× 106 s–1 as did in the previous report using the Strickler–Berg
relationship.7a Although the value of kf is smaller than that for
TMB•+* (9.5 × 106 s–1), Φf was larger by a factor of 1.8. It is
suggested that nonradiative process in DMP•+* is slower by a
factor of 3.0 as compared to TMB•+*. Since deprotonation
from DMP•+* is not an important process for the nonradiative
process, methyl group would play an important role in the non-
radiative decay of DMP•+* and TMB•+* as reported for alkyl-
benzenes in the singlet excited states.11 Lifetime of DMP•+*
can be estimated from Φf to be ≈350 ps from eq 1,
9
T. A. Gadosy, D. Shukla, and L. J. Johnston, J. Phys.
Chem. A , 103, 8834 (1999).
10 Since TMB•+ tends to form an ion pair with Cl–, free
TMB•+ was generated by secondary electron transfer to
biphenyl radical cation (BP•+) by the addition of BP 10
times as much as TMB.7b The excitation was performed
within 100 ns after the electron pulse.
11 W. W. Scloman Jr. and H. Morrison, J. Am. Chem. Soc.,
99, 3342 (1977).
12 M. Allan and J. P. Maier, Chem. Phys. Lett., 34, 442
(1975).
where knr is the rate constant for the nonradiative decay.