Page 191 - Photoreactive Organic Thin Films

I 70 MIKHAIL V. KOZLOVSKY, LEV M. BLINOV, AND WOLFGANG HAASE

The two phase states of the same copolymer at the same temperature can
be distinguished not only by their optical properties but also by X ray scatter-
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ing and dielectric relaxation measurements. It is also worth noticing that
the phase behavior of the polymers can be switched not only by nonfiltered
white light of low intensity (e.g., from a desk lamp with power density 11
2
W/cm within the wavelength range 350 to 2000 tim), but also selectively by
2
blue-green light (400 < X < 500 nm) with power density as low as 0.3 W/cm .
That wavelength range corresponds to band C in the absorption spectra
(Figure 5.8), and the same light causes the light-induced birefringence (Figure
5.11).
The LCPT effect originates undoubtedly from a change in the proportion
of cis- and trans-isomers of azobenzene chromophores in the polymer film. It
is still not clear, however, which of those isomers is more favorable for the
formation of the IsoSm * structure and which prefers the straight Sm A type
of packing. We found it reasonable to first suggest, that illumination within
the n-JT* band suppresses a's-isomers completely and that the TGB A* struc-
ture cannot transform to the Sm A phase under illumination because of the
excess of ^raws-configured azobenzene fragments, which would possess higher
helical twisting power and "frustrate" the uniform smectic structure. That
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suggestion can be supported with our early observations that the rod-like
trans-isomer of a chiral low molar mass dye induces higher spontaneous
polarization in the Sm C matrix than the banana-shaped a's-form does. Also,
a -3-um-thick SK8 film cast from UV-irradiated solution (the ds-film) evi-
dently forms Srn A phase, in contrast to the mostly trans-film cast from virgin
solution.
On the other hand, as mentioned in Section 5.3, the trans-isomei also
absorbs within band C, especially in the condensed state of the copolymer, so
that illumination with blue-green light creates not a pure trans-film but a new
steady-state proportion of both isomers. Moreover, UV irradiation of the
Sm A film of SK8 has been reported to transform it back to the IsoSm*
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phase, Therefore, the t/s-isomers seem to be preferred for the formation of
the TGB A* phase.
Whatever the explanation, the LCPT effect has been used successfully for
photorecording purposes (a primitive setup for LCPT recording is shown in
Figure 5.27). Actually, if a copolymer film is being cooled slowly from above
the transition point and illuminated simultaneously through a mask, the
Sm A phase forms in a shadow area under the mask. It scatters incident light
strongly and appears white. At the same time, the cooling under illumination
ends up with the short-pitch TGB A* (IsoSm*) phase. In other words,
the written image is realized as a negative Sm A image on the IsoSm*
background.
There are, then, three possibilities for reading out the recorded image.
First, the image can be viewed in the scattering mode, if observed in the
daylight or in standard illumination; it appears then as a white graph on the
colorless background. This is illustrated in Figure 5.28A, where the name of
one of this chapter's authors is recorded. The image has already been kept for
three years at ambient temperature and shows no visible changes. Second, the
image can be read out in the transmission (dia) mode when looking through