Page 74 - Photoreactive Organic Thin Films

2, ULTRAFAST DYNAMICS IN THE EXCITED STATES OF AZO COMPOUNDS 5 3

H, having its strong absorption bands of n-n* transition in the region
450-550 mm. This strong n-n* absorption band overlaps the weak n-7t*
band of A. In the present experiments, the excitation wavelength is 530 nm.
It is safely assumed that only the H form is selectively excited. From the
absorption spectrum of 1PA2N in methylcyclohexane and mixed solutions of
methyl-cyclohexane-cyclohexanol, it can be clearly seen that when changing
from nonpolar methylcyclohexane solution to more polar mixed solvents, the
absorbance in the region 480-600 nm increases relative to that in the region
of 350-460 nm. This indicates that the concentration ratio of H to A increases
with the polarity of the solvent. The samples were placed in a 2-mm thick
cell, and the absorbance was adjusted and maintained between 0.25 and
0.30. Conditions that were other than these are specifically pointed out.

23.1.2 Absorbance Changes and Decay Kinetics
The absorbance changes, AA, in the spectrum of 1PA2N dissolved in neat
methylcyclohexane induced by 6-ps pulses, indicate that the kinetics of the
recovery of the bleached band do not follow a simple monotonic rate but
rather are composed of two time constants.
The effect of pulse excitation energy was studied on the 480-nm bleach-
ing kinetics of 1PA2N dissolved in pure methylcyclohexane and a 5:2 mixture
of methylcyclohexane-cyclohexanol. For the methylcyclohexane solution, we
observed that, when the excitation energy was increased by a factor of 2 from
2
70 to 140 mj/cm , the decay time constant of the short component was found
to decrease from 14±3 to 9±1 ps. This decrease is caused by a self-damping
17
process similarly observed for DODCI. The short time constant was also
observed to decrease from 28±5 to 22±8 ps for the 5:2 mixed solvent when
2
the excitation energy was increased from 70 to 140 mj/cm . In addition, we
found that when the concentration of the sample was increased by a factor of
1.7, i.e., absorbance at 530 nm is increased from 0.3 to 0.5, the decay time
constant of the short component is again decreased from 28±5 to 22±8 ps. To
avoid nonlinear effects such as self-damping and saturation, AA vs. concen-
tration and vs. energy were plotted for 1PA2N in various solutions. It was
2
found that slight saturation occurs at higher excitation density {-30 mj/cm ).
Hence, only the data obtained in the linear region are presented and dis-
cussed in the following sections.

2.3.1.3 Effect of Solvent Viscosity on the Decay Time Constant
The time dependence of AA after 6-ps pulse excitation of 1PA2N in
methylcyclohexane was monitored at 480 nm, which is near the maximum of
the visible absorption band. A negative absorbance change, i.e., bleaching,
was observed at 480 nm. This observation is in agreement with the work of
14
Gabor et al., showed that the absorbance of the H tautomer at 480 nm is
much higher than that of the A tautomer. The bleaching we observed was
indicated by a decrease of the H form of 1PA2N. The recovery kinetics
2
induced upon the A form by a 10-mJ/cm pulse were found to be governed by
two distinct rates, one with a lifetime of more than 1 ns and the other a fast
component with a time constant of 14±3 ps. The short lifetime component
was found to decrease to 9±1 ps by increasing the excitation pulse density