Page 75 - Photoreactive Organic Thin Films

54 TAKAYOSHI KOBAYASHI AND TAKASHS SAiTO

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from 10 to 20 mj/cm . Further experiments were performed on the same
molecules dissolved in a mixture of solvents with the purpose of examining
the effect of viscosity on the rate of isomerization of 1PA2N as revealed by
the kinetics of the recovery of the 480-nm band after excitation with a non-
saturating 5-ps, 530-nm pulse.
We present here only the kinetics of the 480-nm band, which is represen-
tative of the transient kinetics. We also make the obvious and trivial assump-
tion that a fraction of the excited singlet state population remains there
during the relaxation process. The factor of 10 changes in the viscosity of
the solvent were achieved by using methylcyclohexane (17=0.729 cP) and
methylcyclohexane-cyclohexanol 5:2 and 5:9 mixtures (with respective vis-
cosities of 2.09 and 9.63 cP).
For every solution, we found that the recovery of the bleaching 480-nm
band follows biphasic kinetics composed of two exponential functions.
The time constants correspond to the 5:9 methylcyclohexane-cyclohexanol
2
(77=9.63 cP) solution, excited with a 10-mJ/cm pulse. The long-lifetime
components have more than 1 ns time constant, while the short component
exhibits a time constant, of 107±24 ps. We believe that the variation in the
480-nm recovery time constant observed with the two solutions, 14±4 ps vs.
107±20 ps, which were excited and monitored with identical pulses, is caused
by the difference in viscosity.
This proposal regarding the dependence of the short decay time com-
ponent on viscosity is supported by the fact that the 530-nm pulse excites
preferentially the H, hydrazone, form of the 1PA2N molecule and specifically
the trans isomer of the H form (HT). In addition, the cis form is known to be
unstable, quite possibly because of the steric hindrance between the naphtha-
lene and benzene components of the 1PA2N, and therefore of the very low
ground state population.
If we assume that the HT form is excited mainly with the 530-nm pulse,
there are three possible candidates for the mechanism that governs the decay
rate of the fast component: (1) intersystem crossing from the excited singlet
state of HT to HT triplet; (2) trans-cis isomerization; and (3) internal conver-
sion to the ground state. The first possibility is excluded, based on the
absence of a reasonable yield of phosphorescence, even at low tempera-
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tures. Possibility 3 is not very plausible because of the existence of the
additional long component and the strong dependence on the viscosity of the
solvent. Trans-cis isomerization is the most reasonable possibility.
2.3.1,4 Trans-cis Isomerization
The assignment of the observed fast kinetic rate to trans-cis isomerization
is strongly supported by our experimental data, which show that the time
/3
constant is related to the viscosity by approximately irf . The Forster and
18 m
Hoffman model was developed originally to explain the Q= rj relation-
ship where Q is the fluorescence quantum yield and 17 is the viscosity of
the medium for triphenylmethane dyes. In addition, it was predicted that
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the fluorescence lifetime, r, should follow a similar relationship: r = c rj .
According to this model, absorption of light produces a vertically excited
Franck-Condon state with the phenyl rings still at a ground state equilibrium