Page 71 - Photoreactive Organic Thin Films

§0 TAKAYOSHI KO8AYASHIANDTAKASH! SA1TO

2.! INTRODUCTION

Trans-cis photoisomerization is one of the most important and widely investi-
gated unimolecular photochemical reactions. Molecular systems showing the
isomerization contain double bonds such as C=C, N=N, and C=N bonds.
Most representative examples of the molecules are stilbene, azobenzene, and
anile. There are many other molecular systems, including biological chro-
mophores. Photoisomerization triggers conformational changes in retinoid
3
proteins such as rhodopsin and bacteriorhodopsin, relevant to vision and
4
ATP synthesis, respectively. Although the spectra of both isomers in many
molecular systems are well known, the dynamics of these processes have not
been sufficiently studied because the rate of conversion from one isomer to
the other is quite often very fast and at room temperature, and in nonviscous
solvents, it frequently occurs in the picosecond range. Azobenzene and its
derivatives are known to form a group of molecules showing the trans-cis
photoisomerization. They are studied to gain basic understanding of the fun-
damental chemistry. These are also widely investigated for various photonics
applications, such as optical switches and high-density optical memory stor-
5 6
age devices. ' To determine the rate of ultrafast isomerization, it is important
to search for azo compounds that are best-suited to such applications.
In the first part of this article, we present a kinetic study on the isomer-
ization of l-phenylazo-2-hydrozynaphthalene (1PA2N) measured by picosec-
ond spectroscopy. The selection of this compound was based on its known
photochemical properties of cis-trans isomerization and tautomerization by
proton transfer. The phototautornerization of 1PA2N is evidenced by the fact
that in solution this compound is a mixture of the azo tautomer A and the
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hydrazo tautomer H, " as shown in Figure 2.1. It is thought that the inter-
nal hydrogen bond between the oxygen and a nitrogen atom is a strong
factor in the tautomerization process. In fact, all o-hydroxyazo compounds
exhibit an internal hydrogen bond between the oxygen and the nitrogen, and
the presence of tautomerization enforces the thought that this internal bond
facilitates tautomerization. In this article, we shall present the results of the
time constant measurement of the isomerization of l-phenylazo-2-naphthol
and correlate this rate with the viscosity of the solvent. We shall also discuss
the relaxation channel in the system in comparison with other hydrogen-
bonded systems of indigo and tioindigo.
There is another aspect of the dynamics of isomerization in azo com-
pounds, namely the rotation versus inversion mechanism. Since azobenzene
has lone-pair electrons on the nitrogen atoms, the n-ft* electronic transition is
observed in addition to the n-it* transition, which may result in a different
isomerization mechanism from that of stilbene. It has been proposed that
the photoisomerization mechanism of traws-azobenzene depends on the
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excitation wavelength. ' Figure 2.2A shows the schematics of a generally
accepted model. According to this model, isomerization proceeds with a rota-
tion of the phenyl ring(<I») around the N=N double bond after 7E-TC*(S 2) excita-
tion in the rotation mechanism. On the other hand, n-7t*(S 1) excitation
induces the in-plane bending of the phenyl ring through a configuration with
+
a straight -N =N~=<I> group (like an allene structure) in the inversion mecha-