Page 76 - Photoreactive Organic Thin Films

2. ULTRAFAST DYNAMICS IN THE EXCITED STATES OF AZO COMPOUNDS 5 5

configuration, forming an angle Q 0. The rings subsequently rotate to a new
equilibrium angle 0, and nonradiative deactivation of the excited state now
2
depends upon (0-0 0) , following the generalized Hook's model.
19
It was suggested by Gegiou et al. that the photoisomerization of
azobenzene probably proceeds via a mechanism different than the cis-trans
isomerization of stilbene. Such an isomerization process in azobenzene could
involve a pyramidal inversion of a nitrogen atom, in contrast to stilbene and
its derivatives, where rotation about the central double bond is required.
Based on the data, the AT—»AC isomerization mechanism may be similar to
that of the triphenylmethane dyes, whose ground state structure is known to
resemble a three-dimensional, propeller-shaped D 3 structure with the phenyl
rings rotated 32 degrees from the central plane.
The viscosity dependence of the internal-conversion time constant has
been measured previously for two kinds of triphenylmnethane dyes: crystal
20
21
violet and malachite green. The recovery time constant of the ground-state
depletion of crystal violet and the \le point for complete recovery were
1/3
8
2/3
reported to obey an 7j relationship rather than the 7j relationship that we
13
observed in 1PA2N. The reason for the jf dependence is not apparent to us.
We have also found transient absorption in the 545-nrn region for 1PA2N
dissolved in each of the three solvents discussed. Because of the low absorp-
tion cross-section of the intermediate at this wavelength, it was very difficult
to investigate the decay constant dependence on concentration and excitation
energy. The results indicate, however, that the same relaxation processes are
present in the 545-nm intermediate as were observed at 480 nm at the same
concentration. On the basis of this data, we can assign the cross section to
the following order: <r(HT, S 0, 480 nm) > tr (HT, S 1? 480 nm) > cr (I, 480 nm),
where I is the intermediate and S 0 and Sj are the ground and lowest-excited
singlet states, respectively.

2.3.1.5 Relaxation Mechanism
22
It has been stated that o-hydroxy derivatives are the only aromatic azo
compounds exhibiting luminescence. The reason for this phenomenon may be
the tautomeric phenylhydrazones, which exist in at least these few com-
14
pounds. Gabor et al. showed that among o- and p-hydroxyphenylazoben-
zenes and o-hydroxyazobenzene, only l-phenylazo-2-hydrozy-naphthalene,
2-phenylzao-o-hydrozynaphthanele, and diphenylhydrazone o-naphtho-
quinone emit any appreciable luminescence—i.e., above an estimated quan-
tum yield of 0.001. The fluorescence spectra are practically mirror images of
the absorption spectra in their respective hydrazone forms; they are also both
found independent of the wavelength of excitation from 360 to 546 nm.
However, the approximate quantum yields were ~0.05 and decreased sharply
14
at higher temperatures. These experimental results (Gabor et al. indicate
that the A form does not emit fluorescence. Yet after excitation with
360-480-nm light, this molecule relaxes to the hydrazone form via a proton
transfer process, and this hydrazone form emits at temperatures higher than
—180°C, most probably because of the more efficient trans-cis isomerization.
14
Experimental data obtained by Gabor et al. have provided evidence
that the lowest excited state of the H form is lower than that of the A form.