sensitive tool for monitoring triplet-ketone-derived radicals is to
study H atom abstraction reactions at liquid/ solid interfaces used
for photobinding of molecules to polymer and other surfaces;21,22
the limited number of surface sites at which reactions can occur
in these systems imposes a sensitivity requirement in monitoring
the transient radical population at the interface.
A potentially sensitive means of detecting aromatic radical
intermediates produced in these photoreactions is to photoexcite
the radical to its excited-doublet state23 and monitor fluorescence
from the excited radical. Fluorescence from the diphenylketyl
radical or benzophenone ketyl radical has been studied in order
Fig u re 1 . Laser-induced time-resolved fluorescence experiment.
L is a lens, BS is a beam splitter, M is a mirror, BF is a 514.5-nm
band-pass filter, LF is a liquid filter to block 355-nm irradiation, S is
the sample, D is a beam dump. PT is a vacuum phototube used to
trigger the scope, and PMT is a photomultiplier tube used to detect
emission from the sample.
to investigate the photophysics and photochemistry of the excited-
doublet state.1
5,20,23-37
The fluorescence decay time and quantum
yield of the ketyl radical of benzophenone in toluene have been
measured and were found to be τ
) 3.9 ns32 and Φ ) 0.11,38
respectively. The fluorescence decay time of the radical in more
EXPERIMENTAL SECTION
f
f
Instrumentation. The experimental setup is shown in Figure
1. A Quanta Ray model GCR-11 Q-switched Nd:YAG laser was
operated at 10 Hz and blocked by a shutter to provide a repetition
28
polar solvents is comparable (e.g., τ ) 3.7 ns in acetonitrile ),
f
so the fluorescence quantum yield should also be significant in
these solvents. While the fluorescence yield of the ketyl radical
appears to be high enough for sensitive detection, the radical
fluorescence emission is not resolved from phosphorescence of
the precursor triplet state of benzophenone. If triplet-state
phosphorescence can be independently measured and subtracted
from the total emission signal, the time-resolved fluorescence
transient of the ketyl radical can be isolated, and the kinetics of
radical formation and recombination can be observed. In this
work, time-resolved, continuous wave (cw) laser-excited fluores-
cence was resolved from other emission and scattering and used
to detect nanomolar concentrations of diphenylketyl radicals
derived from H atom abstraction by the triplet benzophenone. The
ketyl radical populations were monitored on submicrosecond and
millisecond time scales, and rate constants for radical formation
and recombination were determined.
rate of 0.1-1.0 Hz; the beam was frequency tripled (λ ) 355 nm),
and the 5-ns UV (100-600 µJ) pulse was weakly focused to a spot
size of 2.2 mm and used to photoexcite the sample. Fluorescence
of transient diphenylketyl radicals was excited by a beam from a
e
Lexel model 95 cw argon ion laser (λ ) 514.5 nm), which was
p
focused to a spot size of 380 µm in the sample with power varied
between 50 and 500 mW.
Fluorescence and phosphorescence were collected at 90° from
the excitation axis and filtered through a 1.0-cm path of a ≈5%
aqueous solution of sodium nitrite and three glass filters (Schott
KV408, KV550, and OG570). The filtered emission was detected
by a Hamamatsu R976 photomultiplier tube, digitized with a
LeCroy 9450 oscilloscope with the input termination at 1 kΩ, and
averaged.
For microsecond radical formation kinetics, the Nd:YAG laser
photolysis pulse energy was 150 µJ, and the cw probe laser power
was 500 mW; the signal voltage was derived from a 1-kΩ
termination resistor, giving an RC time constant of 0.1 µs. The
sample solution was stirred to provide a fresh sample in the
excitation region, and 100 transients were averaged for each data
record. For studies of radical recombination kinetics on a 100-
ms time scale, the effect of stirring could be observed as structure
in the residuals. For these slower experiments, therefore, stirring
was not used, and a lower repetition rate, 0.1 Hz, was chosen to
allow diffusion to replace the reacted population in the beam
between experiments. The signal voltage was derived from a
larger termination resistance of 1 MΩ, which yields an RC time
constant of 100 µs. Ten transients were averaged for each data
record; the excitation laser energy was varied between ≈100 and
(
(
21) Dunkirk, S. G.; Gregg, S. L.; Duran, L. W.; Monfils, J. D.; Haapala, J. E.;
Marcy, J. A.; Clapper, D. L.; Amos, R. A.; Guire, P. E. J. Biomater. Appl.
1
9 9 1 , 6, 131-155.
22) Ledwith, A. In Photochemistry and Polymeric Systems; Kelly, J. M., McArdle,
C. B., de F.Maunder, M. J., Eds.; Royal Society of Chemistry: London, 1993;
pp 1-14.
(
(
23) Razi Naqvi, K.; Wild, U. P. Chem. Phys. Lett. 1 9 7 6 , 41, 570-574.
24) Hodgson, B. W.; Keene, J. P.; Land, E. J.; Swallow, A. J. J. Chem. Phys. 1 9 7 5 ,
6
3, 3671-3672.
(
(
(
(
25) Mehnert, R.; Brede, O.; Helmstreit, W. Z. Chem. 1 9 7 5 , 15, 448-449.
26) Topp, M. R. Chem. Phys. Lett. 1 9 7 6 , 39, 423-426.
27) Obi, K.; Yamaguchi, H. Chem. Phys. Lett. 1 9 7 8 , 54, 448-450.
28) Baumann, H.; Schumacher, K. P.; Timpe, H.-J.; Reh a´ k, V. Chem. Phys. Lett.
1
9 8 2 , 89, 315-319.
(
(
29) Thurnauer, M. C.; Meisel, D. Chem. Phys. Lett. 1 9 8 2 , 92, 343-348.
30) Hiratsuka, H.; Yamazaki, T.; Takahashi, M.; Hikida, T.; Mori, Y. Chem. Phys.
Lett. 1 9 8 3 , 101, 341-344.
6
00 µJ/ pulse to vary the radical concentration, and the cw probe
laser power was tested at two levels, 50 and 500 mW, to determine
the influence of radical photobleaching on the results.
(
(
(
(
(
(
(
31) Baumann, H.; Merckel, C.; Timpe, H.-J.; Graness, A.; Kleinschmidt, J.; Gould,
I. R.; Turro, N. J. Chem. Phys. Lett. 1 9 8 4 , 103, 497-502.
32) Johnston, L. J.; Lougnot, D. J.; Scaiano, J. C. Chem. Phys. Lett. 1 9 8 6 , 129,
Reagents and Solutions. Benzophenone (Aldrich, +99%),
benzhydrol (Aldrich, 99%), acetonitrile (OmniSolv, glass distilled),
and 2-propanol (OmniSolv, glass distilled) were used without
further purification. Oxygen was removed from the benzophenone
solutions by four freeze-pump-thaw cycles, pumped to a base
pressure of e50 µTorr. Under these conditions, a 100 µM solution
of benzophenone produced an unquenched triplet lifetime of ≈200
µs. The concentration of benzophenone was 100 µM, and
solutions were kept at room temperature unless otherwise
2
05-210.
33) Hiratsuka, H.; Rajadurai, S.; Das, P. K.; Hug, G. L.; Fessenden, R. W. Chem.
Phys. Lett. 1 9 8 7 , 137, 255-260.
34) Johnston, L. J.; Lougnot, D. J.; Wintgens, V.; Scaiano, J. C. J. Am. Chem.
Soc. 1 9 8 8 , 110, 518-524.
35) Yankov, P.; Nickolov, Zh.; Zhelyaskov, V.; Petkov, V. J. Photochem. Photobiol.
A: Chem. 1 9 8 9 , 47, 155-165.
36) Redmond, R. W.; Scaiano, J. C.; Johnston, L. J. J. Am. Chem. Soc. 1 9 9 2 ,
1
14, 9768-9773.
37) Figuera, J. M.; Sastre, R.; Costela, A.; Garcia-Moreno, I.; Al-Hakakk, M. T.;
Dabrio, J. Laser Chem. 1 9 9 4 , 15, 33-46.
Analytical Chemistry, Vol. 70, No. 13, July 1, 1998 2577