Page 129 - Photoreactive Organic Thin Films

I Qg ZOUHEIR SEKKAT AND WOLFGANG KNOLL

ABSTRACT

We review our work on photoisomerization and photo-orientation in films of
polymer by focusing on the influence of the chromophores' environment on
induced molecular movement. We compare photoisomerization of azobenzene
derivatives in supramolecular assemblies, i.e., Langmuir-Blodgett-Kuhn (LBK)
multilayers, molecularly thin self-assembled monolayers (SAMs), and amorphous
spin-cast films. In azo-silane SAMs, photoisomerization and photo-orientation
occur in molecularly thin layers much as they do in bulk spin-cast films, and
photoisomerization modulates reversibly the optical thickness of 9 A thin
layers. In LBK multilayers of azo-polyglutamates, the polymers' side-chain
structure influences the stability of the layers* stacking, and the azobenzene
molecule is orientationally trapped and isomerizes between a highly oriented
and a bend configuration, thereby controlling the film's optical order. Highly
organized LBK structures impede the orientational freedom required for the
chromophores' photo-(re)orientation. In amorphous spin-cast azo-polymer
films, molecules are initially randomly distributed without intermolecular
interaction, and photoisomerization and photo-orientation depend on the
polymer structure and the free volume. Near-pure photo-orientation occurs in
an azo-polyurethane polymer, and photo-orientation is observed 325°C below
Tg of a rigid azo-polyimide polymer containing no flexible connector or
tether. Isomerization is slowed by rigid embedding into rigid backbones, and
high-pressure application reduces the polymers' free volume and suppresses
the chromophores' photoisomerization.

4.1 INTRODUCTION

Organic materials that incorporate photosensitive molecular units are macro-
scopically photoresponsive, and their structural and/or optical properties can
1 2
be manipulated by light. * Photo-induced molecular structural change of the
photochromic units into polymers leads to interesting macroscopic properties,
such as changes in phase transition, viscosity, solubility, wettability, elasticity,
3
and so on. The molecular geometrical change that occurs in the trans«-»cis
photoisomerization process may lead to a loss of the initial orientation of the
4 9
molecules after an isomerization cycle, and anisotropy can be induced. " There
are numerous photochemical reactions that can lead to photochromism, 10
among which the trans<-x:is photoisomerization of azobenzenes is the cleanest
11
photo-reaction known to date. In azo dye doped polymeric films, where the
mobility of the guest molecules is still appreciable, photoisomerization leads
4 5
to reversible polarization holography. ' In azo dye functionalized polymeric
films, where the mobility of the azo chromophores is greatly reduced, photo-
isomerization creates a permanent alignment, which leads to writing erasing
6 9 12 19
optical memory " or to permanent second-order nonlinear-optical effects ""
20
or cubic optical nonlinearities. This chapter addresses the effect of the
chromophore microcogent environment, including intermolecular interaction,
free volume, and polymer structural effects on the photoisomerization and
photo-orientation of azobenzene derivatives in polymeric thin films.