(
j
) in dilute ethanol solution was located at 560 nm with a
tion spectra were recorded with a Uvikon 930 spectrophoto-
meter and Ñuorescence spectra were recorded with a fully
corrected Perkin-Elmer LS50 spectroÑuorimeter. Solutions for
Ñuorescence studies were adjusted to possess an absorbance of
\0.05 at the excitation wavelength. Singlet excited-state life-
times were measured with a Hamamatsu single-shot streak
camera following excitation by a 30 ps laser pulse at 532 nm.
A narrow bandpass Ðlter was used to isolate Ñuorescence over
the spectral range (600 ^ 10) nm and the laser intensity was
attenuated so as not to saturate the streak camera. Approx-
imately 100 individual laser shots were averaged and the laser
proÐle was deconvoluted from the experimental decay record
prior to data analysis. The time window of the streak camera
was 50 ps to 10 ns.
MAX
molar absorption coefficient (e
) of 187 000 dm3 mol~1
MAX
cm~1. The basic methodology used for preparation of the new
dye BLK is shown below in Scheme 1 and follows our strat-
egy developed earlier for the synthesis of related merocyanine
dyes.8h11 Raw materials were purchased from Aldrich Chemi-
cals or Eastman-Kodak and were used as received. Ethanol
(
Aaper, Absolute grade) was used as received. All other sol-
vents were spectroscopic grade, used as received, or redistilled
under vacuum. Silica gel (40È60 mesh) was purchased from
Phase Separation Limited. Compounds 116 and 211 were pre-
pared according to the literature, the former by a slight modi-
Ðcation of the published method, as described below.
Flash spectrographic studies were made with a frequency-
doubled, Q-switched Nd : YAG laser (pulse width 20 ns; pulse
energy 250 mJ). Solutions were adjusted to possess an absorb-
ance of ca. 0.2 at 532 nm and were purged with N or air,
2
according to the needs of the experiment. Transient di†eren-
tial absorption spectra were recorded point-by-point while
kinetic studies were made at Ðxed wavelength with the data
being analysed by non-linear, least-squares iterative pro-
cedures. The laser intensity was attenuated, as necessary, with
metal screen Ðlters. In several cases it was necessary to restrict
the intensity of the monitoring beam to a low level in order to
avoid photolysis of the photoisomer. Such attenuation was
achieved by placing appropriate neutral density Ðlters before
the sample cell.
Di†erential molar absorption coefficients for the triplet
excited state were measured by the energy-transfer method,
using anthracene as donor. A solution of anthracene in
ethanol was prepared so as to possess an absorbance of 0.3 at
Scheme 1
Preparation of 1
3
55 nm and deoxygenated by purging with N . The triplet
2
2
-Aminophenol (3 g, 27.5 mmol) and 5-bromovaleric acid (7.5
absorption signal was monitored at the peak17 of 422 nm and
the laser intensity was attenuated until the decay proÐle could
be Ðt to a single-exponential process. Various concentrations
of BLK were added to the solution and the lifetime of triplet
anthracene was measured at 422 nm. For each solution, the
rate of formation of the triplet state of the merocyanine dye
was monitored at 680 nm and compared with the rate of
decay of triplet anthracene. The concentration of BLK was
increased until the dye began to compete with anthracene for
absorption of the excitation laser pulse. At moderately high
dye concentrations, triplet energy transfer was quantitative
g, 27.5 mmol) were heated together at 170 ¡C for 2 h before
raising the temperature to 190 ¡C for 5 min (note: overheating
the mixture caused considerable charring, difficulty in puriÐ-
cation, and a decrease in yield). After cooling the reaction
mixture to room temperature, the resultant solid was trit-
urated with dry acetone, removed by Ðltration and washed
successively with acetone (10 cm3) and diethyl ether (10 cm3).
The solid was dried under vacuum. Yield 4.3 g, 63%. 1H
NMR ([D -]acetone): d \ 2.33È2.41 (m, 4H); 3.54È3.60 (t, 2H,
6
J \ 5.0 Hz); 4.62È4.67 (t, 2H, J \ 4.5 Hz); 7.73È7.80 (m, 2H);
8
.00È8.11 (m, 2H). MS (EI): m/z \ 173 (MwHBr)`. IR (KBr
(i.e. decay and formation rates were similar within experimen-
disc) l \ 1604 cm~1 (CxN).
tal error) and the concentration of dye triplet was determined
by reference to triplet anthracene possessing a di†erential
extinction coefficient18 at 422 nm of 52 000 dm3 mol~1 cm~1.
The averaged molar absorption coefficient at 680 nm was
(40 000 ^ 6000) dm3 mol~1 cm~1.
Preparation of BLK
Compounds 1 (38 mg, 0.15 mmol) and 2 (50 mg, 0.15 mmol)
were reÑuxed overnight in methanol (20 cm3) containing a few
drops of triethylamine. During this time the colour of the
solution changed from red to deep purple. The solvent was
removed on a rotary evaporator and the crude mixture was
passed down a silica-gel column, eluting with ethyl acetate to
remove impurities, followed by acetone to a†ord a still slightly
contaminated product. A second chromatographic separation
on silica gel, eluting with a chloroformÈacetone mixture
Di†erential molar absorption coefficients for the cis-isomer
formed under illumination were determined by the previously
described11 complete conversion method, in aerated ethanol.
A dilute solution of BLK was prepared to possess an absorb-
ance at the peak maximum of ca. 0.10 and irradiated with a
single pulse delivered from a mode-locked Nd : YAG pumped
dye laser (40 mJ, 200 ps). The excitation wavelength was 560
nm, while the monitoring beam was passed through a high-
radiance monochromator also tuned to 560 nm. The laser
intensity was varied with crossed-polarizers and the di†eren-
tial absorption signal due to conversion of the ground-state
species into the unstable isomer was measured. The photomul-
tiplier tube was protected from scattered laser light with an
optical shutter that was opened 10 ns after the excitation
pulse. Approximately 100 individual laser shots were averaged
and the decay proÐle extrapolated to zero time. The laser
intensity was calibrated using zinc meso-tetraphenylporphyrin
in benzene as standard.19,20 The size of the bleaching signal
increased with increasing laser intensity before reaching an
optimum value, after which the signal began to decrease with
(
50 : 50), a†orded a purple solid which was recrystallised from
acetone. Yield 69 mg, 40%. 1H NMR (CDCl ): 0.94È0.98 (t,
3
6
2
1
H, J \ 7.2 Hz); 1.39È1.43 (m, 4H); 1.72 (m, 4H); 2.23 (m,
H); 2.83 (m, 2H); 4.08 (m, 2H); 4.46 (m, 4H); 7.15È7.17 (d,
H, J \ 7.2 Hz); 7.33È7.38 (m, 2H); 7.48È7.50 (d, 1H, J \ 7.6
Hz); 7.67È7.75 (t, 1H, J \ 13.2 Hz); 7.79È7.83 (d, 1H, J \ 13.1
Hz); 7.94È7.98 (d, 1H, J \ 12.9 Hz). MS (EI): m/z \ 465 (M)`.
UVÈVIS (C H OH): j
\ 561 nm (e
MAX MAX
\ 77 000 dm3
2
5
mol~1 cm~1).
Methods
1
H NMR spectra were recorded with a Bruker AM360
FT-NMR instrument with TMS as internal standard. Absorp-
1842
J. Chem. Soc., Faraday T rans., 1998, V ol. 94