Page 225 - Photoreactive Organic Thin Films

204 HENN1NG HENZEL

FIG. 6.20 UV-vis spectra of LBK film of polymer 38 (80 layers, 20 mN/m), as a function of irradia-
tion time for irradiation with UV light (A, = 360 nm) (left) and for irradiation with visible light (X ==
440 nm) (right). (Reproduced with permission from reference 93).

UV-vis spectroscopy can be used to monitor these structural changes.
The spectra of a LBK film of polymer 38 (n = 2), recorded after different
irradiation times, are depicted in Figure 6.20. The decrease of the n-n* band
at 348 nm upon irradiation is not monotone, as would be expected for a
unimolecular reaction, but rather there is a transient increase of the n-n*
band absorption. The absorbance at the n-n* band (at 445 nm), which is
characteristic for the as-isomer, increases monotonically. This indicates that
the irradiation continuously produces c/s-isomer, even when the n-n* band
increases. Therefore, within the LBK film there must be additional processes,
apart from photoisomerization, that increase the number of detectable chro-
mophores faster than they are consumed by photoisomerization. From these
results, in combination with X ray investigations, it was concluded that there
29 93
is a structural change within the LBK film upon irradiation with UV light. '
Furthermore, the development of the absorption at the n-n* band indicates
that the restructuring of the LBK film does not start immediately upon
irradiation but rather that there is an induction period. Obviously, the
restructuring starts only after a critical number of chromophores has been
photoisomerized to the c/s-isomer. After completion of the rearrangement
within the LBK film, "normal" decrease of the n-n* band occurs. 93
Figure 6.21 allows a closer examination of the rearrangement by depict-
ing the absorbance at wavelengths that are characteristic for the aggregated
(323 nm) and the nonaggregated trans-isomeis (348 nm), and for the c/s-
isomer (445 nm). Furthermore, the wavelength of the n-n* band maximum
has been depicted as a function of irradiation time. The irradiation process can
be divided in three periods (see the dotted lines in Figure 6.21). The absorbance
for both the aggregated and nonaggregated chromophores decreases in the
first period, but for the aggregated chromophores, it decreases faster. In
the second period, the absorbance increases. The uneven decrease of the
absorbance at 348 nm and 323 nm results in a shift of the peak maximum for
the n-n* band from 323 nm to 340 nm in the "decrease" period and indicates