Page 90 - Photoreactive Organic Thin Films

3 PHOTO-ORIENTATION BY PHOTOISOMERIZATION £9

orientational distribution in a nonpolar manner (nonpolar orientational hole
burning), a feature that causes both Abs// and Abs ± to decrease upon
irradiation, and (2) for unknown reasons, the long molecular axis of the azo
chromophore fades from the exciting light polarization, and molecular
rotation takes place in the whole photoisomerization process (orientational
redistribution).
Additional progress has been made by studying photoisomerization in
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spectrally individualizable isomers. ' These studies are summarized in this
chapter. Briefly, in contrast to trans- and cis-DRl, trans- and cis-azobenzene
are spectrally distinguishable. It was also shown that not only the trans
isomer but also the cis isomer is oriented by photoisomerization, and the
orientation memory of the azobenzene molecule is preserved when it changes
shape from cis to trans during the cis—»trans thermal isomerization. In azo-
benzene derivatives, the QYs and the rate of the cis—>trans thermal
isomerization play a major part in the dynamics and efficiency of photo-
orientation. Photochromic spiropyrans (SP) and diarylethenes (DE) isornerize
by ring closing/opening, a feature that contrasts with azobenzene derivatives,
which isomerize by shape change. The sign of the apparent photo-orientation
of SP and DE depends on the photochemical transition band, i.e., on the
analysis wavelength. It will be shown that the B isorner of SP and DE exhibit
perpendicular transitions in the UV versus the visible transition bands. These
experimental findings will be discussed in detail and in succession, after the
introduction to the theory of photo-orientation.

3.3.2 Theory of Photo-Orientation
Inasmuch as the optical ordering of photoisomerizable molecules is being
intensively studied, its theoretical quantification is needed to bridge inde-
pendent studies in the areas of optics and photochemistry. Zimmerman et
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al. , and Fisher, and Rau et al, developed an optical pumping population
change based theoretical background for photoisomerization within a pure
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photochemical framework. Michl et al. performed intensive research on
infrared vibrations and ultraviolet and visible (UV-vis) electronic transitions
of molecules that are already oriented either by introduction into stretched
polymer films or after photo-orientation. The theory that coupled photoiso-
merization and photo-orientation processes was developed a few years ago,
and its mathematical foundation is based on the formalism of Legendre
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polynomials. ' However, further theoretical developments are still needed
to quantify coupled photoisomerization and photo-orientation processes.
The most important concept within the framework of photo-orientation
that needs to be clearly addressed is the polarization nature of the optical
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transition itself. In this section, I will present a model based on purely
polarized optical transitions; I will also present the related rigorous solutions
to the general equations of the theory of molecular optical orientation for
the full quantification of the coupled isomerization and optical ordering
processes. This theory can be used for any type of photoisomerizable
chromophore, and I will discuss it for both spectrally overlapping and
distinguishable isomers.