Page 77 - Photoreactive Organic Thin Films

5 £ TAKAYOSHI KOBAYASHi AND TAKASH1 SA1TO

Therefore, the 530-nm excited HT cannot relax via the AT form, but it must
necessarily remain within the HC manifold. As in the case of stilbene, we
believe that for 1PA2N there is an intermediate I that might have a pyramidal
19
form different from the perpendicular form of the stilbene intermediate,
Relaxation from the HT form takes place via the I intermediate, with the rate
being dependent upon the viscosity of the solvent. In methylcyclohexane, the
time constant is found to be about 20 ps, and therefore fast enough to
quench the fluorescence. In more viscous solvents, such as mixed methyl-
cyclohexane-cyclohexanol, the decay process becomes slower and the proba-
bility of fluorescence increases.
In the case of stilbene two mechanisms of cis-trans isomerization have
been proposed. One suggests that the isomerization proceeds via the singlet,
and the other invokes a triplet-state intermediate. There are cases, however,
where only one mechanism is operative, such as with azonaphthol, where cis-
trans photoisomerization is thought to take place via the singlet mechanism
23
only. Our present study on 1PA2N is consistent with the singlet mechanisms
in contrast to indigo and 6,6'-dimethoxyindigo, which do not show photo-
chromism. Their singlet lifetimes are ~50 ps for 6,6'-dirnethoxyindigo and
150 ps for indigo. This highly efficient internal conversion to the ground
24
state provides a strong reason for the photostability of indigo and 6,6'-
dimethoxyindigo. Thus, it can be argued that the excited singlet states of
these indigos are quenched by fast internal conversion enhanced by hydrogen
bonding, and that therefore they do not exhibit luminescence or isomeriza-
24
tion. On the other hand, l-phenylazo-2-naphthol is hydrogen-bonded, but
it shows fluorescence at low temperature. Therefore, we propose that the
very fast decay from the singlet state of 1PA2N hydrazone proceeds via a
trans-cis isomerization path.

23.2 DMAAB

2.3.2.1 Absorption Spectrum
DMAAB sample of DMSO solution was prepared in a 0.1 mm-thick
handmade cell with a microscope cover-glass plate and a slide-glass plate as
front and back windows, respectively, to prevent the reduction of the time
resolution due to pulse broadening by the group-velocity dispersion. Figure
2.3 shows the absorption spectrum of the DMAAB sample solution (0.01 and
3
0.2mol dm"" ). A band with a peak around 420nm in the spectrum is assigned
to a strongly allowed n-n* transition. The substitution of azobenzene by
25 26
p-arnino groups is known to shift the n-n* band to longer wavelengths '
because of the electron-donative nature of the group. The tail extending to
650nm is mainly due to the weak n-n* absorption, the wavelength of which
27
is less dependent on substituents than that of the n-n* transition. This
absorption band is not clearly seen because of the intense n-n* band existing
close to the n-n* transition.
The absorption spectral shape does not show any concentration depend-
3
ence between 0.01 and 0.2mol dm" , indicating that there is no aggregate
3
formation. The sample of 0.2mol dm" was used mainly for the femtosecond