Page 70 - Photoreactive Organic Thin Films

ULTRAFAST DYNAMICS IN THE

EXCITED STATES OF AZO

COMPOUNDS

TAKAYOSHI KOBAYASHI
TAKASHI SAITO
Department of Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan

2.1 INTRODUCTION
2.2 EXPERIMENTAL SECTION
2.3 RESULTS AND DISCUSSION
2.3.1 IPA2N
2.3.2 DMAAB

ABSTRACT

The photoisomerization dynamics of two azo compounds, one with and the
other without intramolecular hydrogen bonding, were studied using pico-
second and femtosecond spectroscopy apparatus. Isomerization time
constants in l-phenylazo-2-hydroxynaphthalene (1PA2N, Sudan) with hydro-
gen bonding at room temperature were determined to be 14*3, 28±5, and
110±30 ps in solvents with viscosities of 0.729, 2.06, and 9.63 cP, respectively,
by using 6-ps pulsed laser. The results were discussed in terms of the relation
between the viscosity and the association of isomerization and tautomeriza-
tion in the relaxation mechanism. The real-time dynamics of molecular vibra-
tion in the excited state of taz«s-4-(dimethylamino)azobenzene (DMAAB,
methyl yellow) in solution was also studied by sub-5fs pump-probe spec-
1 2
troscopy, using visible pulsed laser developed by our group. ' It was found
for the first time that the vibration frequencies of N=N and C-N stretching
modes are modulated quasi-periodically. This is explained in terms of
1
coupling between the two modes via an ~80cmf torsion mode. More detailed
analysis indicated that there is a small frequency difference between the two
modulations. From the results, we can conclude that the reaction does not
proceed via either the pure rotation or pure inversion mechanism.

Pholoreactive Organic Thin films 40
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