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M. Yamaji et al. / Chemical Physics Letters 368 (2003) 41–48
2. Experimental
backand forward energy transfer in the triplet
equilibrium are related to DET by Eq. (1).
a-Naphthylmethyl phenyl sulfide (NMPS) was
synthesized by refluxing a-chloromethylnaphtha-
lene with thiophenol in a mixture of toluene and
water with KOH for 12 h. The organic layer was
washed with water and evaporated, leaving crude
NMPS, which was purified by repeated recrystalli-
zations from hexane. a-Naphthyl phenyl ketone
(NPK) was prepared by usual Friedel–Crafts acyl-
ation of a-naphthoyl chloride in benzene, and re-
crystallized from hexane. Benzophenone (BP) and
9-fluorenone (9-F) were recrystallized from ethanol.
Acetonitrile was used as a solvent after distillation.
All samples in a quartz cell with a 10 mm path
lengthweredegassedbyfreeze-pump-thawcycles on
a high vacuum line. Laser flash photolysis was car-
ried out at room temperature (295 K) or in the
temperature range between )6 and 60 °C. The
temperature of the sample in a quartz dewar was
kept with hot water (>295 K) or a mixture of
methanol and liquid nitrogen (<295 K) within a
precision of Æ0.5 °C during the measurement. Third
and fourth harmonics (355 and 266 nm) of a nano-
second Nd3þ:YAG laser (JK Lasers HY-500) were
used for flash photolysis as the excitation sources.
The details of the detection system for the transient
absorption have been reported elsewhere [23]. The
kinetic analysis of the transient data obtained by
laser photolysis was carried out by the least-squares
best-fitting method within Æ5% experimental er-
rors. Thetransientabsorptionspectrawererecorded
on an USP-554 system, which can take a transient
absorption spectrum with single laser pulse.
kf kbꢀ1 ¼ expðꢀDET=RT Þ:
ð1Þ
In an earlier Letter, we have shown by means of
laser flash photolysis that the rate constant of an
endoergonic triplet energy transfer upon triplet
equilibrium formation (DET ¼ 0:3 kcal molꢀ1) is
in the magnitude of 109 dm3 molꢀ1 sꢀ1 in solution
at room temperature [15], which is contrary to the
well-known relationship between the transfer rate
constant and the triplet energy gap. As to the ki-
netic and thermodynamic profiles of endoergonic
triplet energy transfer, it is predicted that triplet
energy transfer still occurs in endothermic systems
at a much slower rate, and that such an endo-
thermic transfer requires thermal activation [4].
However, it is difficult to determine the bimolec-
ular rate constants and corresponding activation
energies for endoergonic energy transfer processes
because the backtransfer rate constant is much
larger than the forward one in a triplet equilib-
rium. The rate constants of the endoergonic energy
transfer processes in the triplet equilibrium have
been relatively evaluated by using Eq. (1) on the
assumption that the rate constant of the exoerg-
onic energy transfer process is the same as the
diffusion limit of the solvent. To our best knowl-
edge, there is no report on the absolute bimolec-
ular rate constant and the activation energy for
endoergonic triplet energy transfer in solution.
In the present work, we show one solution to
ignore the large rate constant of backenergy
transfer in an endoergonic donor–acceptor system.
In order to prevent the backenergy transfer, the
sensitized triplet acceptor shall vanish by a chem-
ical reaction immediately after the preceding en-
ergy transfer. As a chemical reaction to deactivate
the sensitized triplet acceptor, cleavage of the
carbon–sulfur bond in naphthylmethyl phenyl
sulfide (NMPS) is used since the enthalpy of the
carbon–sulfur bond is estimated to be smaller than
the triplet energy of NMPS. By means of laser
flash photolysis, bimolecular rate constants of
endoergonic triplet energy transfer from aromatic
ketones to NMPS are determined in acetonitrile as
well as the corresponding activation energies for
the first time.
The triplet energies (ET) of NMPS, BP, NPK
and 9-F were determined to be 59.2, 69.1, 57.5 and
50:3 kcal molꢀ1, respectively, from the phospho-
rescence origins obtained in a mixture of methanol
and ethanol (1:1, v/v) at 77 K by using a Hitachi
F-4010 fluorescence spectrophotometer.
3. Results and discussion
3.1. Triplet sensitization of NMPS by benzo-
phenone: exoergonic system
Fig. 1a shows the transient absorption spec-
tra observed after 355 nm laser pulsing in a BP