Chemistry Reference
In-Depth Information
electronic states characterized by different absorption spectra [
181
]. There are many ways in which
the photo-responsiveness of polymers can manifest itself. One might observe changes in viscosity of
polymeric solutions, in contraction of polymeric chains, in sol-gel transitions, in electrical conduc-
tivity, or even in color changes as a result of irradiation with light of an appropriate wavelength.
Another interesting manifestation of photo-responsiveness in some special polymers is a change in
the permeability to gases in films. These changes can and are utilized in many ways. Thus, for
instance, structural changes due to isomerization are employed to align liquid crystals and photo-
conductivity is utilized in xerography. Over the last 2 or 3 decades, the photo-responsive materials
have grown in practical and scientific importance, because such materials are useful in many
applications.
10.6.1 Polymers for Harvesting the Sun's Energy
The goal of harvesting light energy has led to research in polymeric materials that could potentially
mimic photosynthesis or harvest the sun's heat. In such materials, the choice of chromophores is the
most critical variable. The location of the chromophores on the polymeric chains and the tacticity
of the polymers are also very important. Weber pointed out, for instance, that among a number
of chromophores attached to polymeric chains, naphthalene and carbazole form very stable
eximers
, while phenanthrene and diphenyl anthracene do not [
182
]. At present, many polymeric
materials are utilized in the vast areas of nonsilver-based imaging, information storage, remote
sensing, electroresponsive materials for displays, and others. Fox and Cozzens had to conclude,
however, that none compare in efficiency to naturally occurring photon-harvesting polymers for
photosynthesis [
183
].
10.6.1.1 Polymers with Norbornadiene Moieties
One approach to harvesting light energy is to utilize pendant groups that reversibly absorb light energy,
rearrange, and then release this absorbed energy as heat in a rearrangement back to the original structure.
To that end, research is going on in various laboratories to develop systematically derivatized polymer
arrays than can collect and convert light energy. Among these, photo-rearrangements from
norbornadiene to quadricyclane and back are of considerable interest, because photo-energy can be
stored as strain energy (about 96 kJ/mol) in a quadricyclane molecule and later recovered [
184
]
ultraviolet light
> 310 nm
250 nm
This photo-isomerization reaction is referred to as a
valence isomerization
. It is a reaction in which
electron reshuffling occurs and the nuclei move to make or break new
bonds. A number of
polymers were, therefore, prepared with the norbornandiene moieties either in the backbone or as
pendant groups. Among them are polyesters that were synthesized with donor-acceptor norbornadiene
residues in the main chain [
184
] by polyaddition of 5-(4-methoxyphenyl)-l,4,6,7,7-pentamethyl-2,5-
norbornadiene-2,3-dicarboxylic acid or 5,6-bis(4-methoxyphenyl)-7,7- dimethyl-2,5-norbornadiene-2,
p
and
s
