Page 219 - Photoreactive Organic Thin Films

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% azobenzene moieties in the side chains have been deposited and studied
using circular dichroism, IR spectroscopy, X ray reflection, and surface plas-
mon resonance. The macroscopic structure of freshly prepared LBK films
consists of deformed polypeptide helixes arranged in layers about 2.0 nm
thick. 82
a-helical polypeptides have rigid rodlike backbones. Equipped with flexi-
ble side chains, the then-called "hairy rod"-like polymers can be fabricated
83 85
into LBK films, having exceptional quality and stability. " Azobenzene
chromophores can be dissolved in the side chain region of such poly(L-gluta-
45 49
mate)s having long alkyl chains. ' Miscibility between the azobenzene dye
and the side chain region is an issue, however, and this approach works well
only for very oleophilic dyes.
More versatile and stable systems are prepared by covalent attachment of
flexible alkyl chains and of the azobenzene dye to the polypeptide main
chain. Such copolymers 37 form stable monolayers, which can readily be
86
deposited on solid substrates to give LBK films of excellent optical quality.
The azobenzene moieties are not aggregated, can be isomerized by irradia-
tion, and show spectral properties very similar to those of a solution. These
characteristics indicate that the chromophores are embedded in a liquid-like
matrix. Being embedded in a very fluid matrix, however, the isomerization
does not produce significant changes in the structure of the LBK film. There
is a very slow structural relaxation upon irradiation that resembles the relax-
ation observed upon heating the LBK film over the "melting temperature" of
87
the side chains. It was concluded that the molecular motions resulting from
the repeated isomerization of the azobenzene moieties have the same effect
on the LBK film structure as the increased chain mobility does on heating.
The number of chromophores in the LBK film can be enhanced by
mixing the poly(glutamate) (37) with a low molecular weight azobenzene
dye. Both chromophores, the covalently bound and the admixed dye, can be
isomerized in LBK films by irradiation with UV light. The trans to cis isomer-
ization reduces significantly the interaction between the bound and the
admixed chromophores, and phase separation occurs. Upon repeated isomer-
ization cycles, the low molecular weight dye is expelled from the LBK film
and forms crystals on top of the film. This can be easily detected by polarized
49
light microscopy.
To enhance the chromophore density and to obtain stable films, poly-
glutamates that have azobenzene moieties in every side chain have been
88 89
synthesized (38, 39). ' In this case, the side chains have to be tailored to
keep the "hairy rod" character of the polymer. The length of the alkyi spacer
between the chromophore and the polymer backbone (n = 2, 3, 4, 6), as well
as the length and means of tethering of the alkyl tail to the chromophore,
13
have to be adjusted (-QH or -OC 10H 2i) so that the side chain region is in a
39
liquid crystalline state. The flexibility in the side chain region is necessary to
ensure the "hairy rod" character and thereby to gain high monolayer stability
and ensure that the monolayer can be transferred to solid substrates,
Furthermore, the flexibility in the side chain region facilitates photoinduced
trans to cis isomerization in LBK films with a high content of c/s-isomer
in the photostationary state. Particularly good results were obtained with