J. Zhao et al. / Spectrochimica Acta Part A 57 (2001) 149–154
151
Table 1
UV–Vis absorption and fluorescence spectra data of BSE and BSHa
UV–Vis absorption spectra data
Fluorescence spectra data
CHCl3 C2H5OH CCl4
CHCl3
C2H5OH CH3CN CCl4
Casting film
Casting film
Solid
BSE
BSH
259
320
255
316
256
318
254
315
255
317
253
314
252
325
265
330
uex 276
uem 370
uex 283
uem 370
357
434
357
435
282
472
280
466
331
494
451
498
472
509
296
506
323
319
a Concentration of the solution: 5×10−5 M; substrate of the casting film: CaF2. uex and uem are optimal excitation and emission
wavelengths (nm).
The original emission peak at 370 nm for BSE
in chloroform solution will decrease with irradia-
tion and a new red-shifted peak centered at 460
nm appeared and intensified (Fig. 1). This is
similar for BSH (Fig. 2). The original emission
peak at 370 nm decreased slightly and the peak at
460 nm increased about ten times. The data in
Figs. 1 and 2 mean that the photochromic ability
of BSE and BSH in chloroform solution is differ-
ent; it is easier for BSH to perform the photo-con-
version, based upon the irradiation time needed in
the photochromism.
More attention should be paid to the pho-
tochromism of solid BSE and BSH if some poten-
tial practical use were considered. Therefore, we
also studied the photochromism of solid BSE and
BSH. The solid emission spectra of BSE would
change slightly when its crystal was irradiated
with light of 472 or 300 nm. However, the solid
emission spectra of BSH change extensively with
irradiation, and the process is so rapid that it is
difficult to monitor the spectral changes by the
usual time-dependent fluorescence spectroscopy.
We therefore use the ‘ time course’ technique to
study the change of the fluorescence intensity at a
fixed wavelength (Fig. 3). The data in Fig. 3 show
that the fluorescence intensity at 506 nm for BSH
decreases sharply with irradiation and becomes
stable after about 10 s of irradiation. But, for
BSE, no substantial change was observed.
decay kinetics is drastically different from that of
BSE. The decay kinetics of irradiated BSH is
nonlinear and is rapid (Fig. 5).
These results are consistent with that deduced
from Figs. 1 and 2, namely the fluorescent ability
of BSH is stronger than that of BSE, and the BSH
can perform the photochromism more quickly
than BSE (for the cause, vide infra)
3.3. Time-dependent UV–Vis spectra
In order to prove the presented change further,
we conducted the UV–Vis absorption spectra
study of the irradiated solution. The experimental
results show that the UV–Vis spectra of BSE or
BSH would change accordingly when it was irra-
diated (Figs. 6 and 7).
For BSE (Fig. 6), the original absorption peaks
centered at 259 and 320 nm decreased with irradi-
ation, and a new red-shifted absorption peak at
The decay kinetics of the irradiated BSE and
BSH are also studied by time-dependent fluores-
cence spectroscopy [6–8]. The results show that
decay kinetics of BSE is nearly linear and the
process is rather slow (Fig. 4). But, for BSH, the
Fig. 1. Solution emission spectral changes for BSE with irradi-
ation (in chloroform, 5×10−5 M; uex=276 nm). From a to l,
each irradiation time=100 s; total, 1100 s.