Page 228 - Photoreactive Organic Thin Films

6 PHOTOISOMERIZATION IN LANGMUIR-BLODGETT-KUHN STRUCTURES 207

X 1.64 • fi ^ n r ,^_^ \
Q)
1.62 •
"i 1.60- \ v^=
n n v. ^^
2 1.58- * " """• •— zzL-zn^
a)
a: 1.56 •
n v '
•1 f^A .
initial CIS trans
FIG. 6.23 Refractive indices for poly(L-giutamate) 38 (n = 6) LBK films in the initial state before any
irradiation, in the cis-photostationary state after irradiation at 365 nm, and in the mins-photostationary
state after irradiation at 440 nm, determined by waveguide spectroscopy (data from reference 90).

structure upon the first irradiation with UV light enhances the effect of the
72 92
photoisomerization on the refractive index, ' (see Figure 6.22). Employing
waveguide spectroscopy in thicker LBK films, the refractive indices can be
90
determined. The values for LBK films of poly(L-glutamate) 38 are shown in
Figure 6.23.
The problem with the application of these changes in optical properties
for optical data storage is that the significant drop in reflectivity upon
the first UV irradiation is related to an irreversible structural change. So the
film can be used a as write-once medium only. Furthermore, the sensitivity
of the system is low, so writing the information takes a rather long time.
The changes in the optical properties in subsequent irradiation cycles are
reversible, but they are connected with the unstable c/s-isomer, which renders
the inscribed information volatile.
To circumvent this problem of the unstable ds-isomer as information
carrier, one can take an approach that combines photochemistry and electro-
chemistry. The ds-isomer can be reduced to a hydrazobenzene species with
substantially more anodic potential than the trans-form, and there is a large
difference in the adsorption spectra of £ra«s-azobenzene and the hydrazoben-
zene. The photochemically inscribed and electrochemically developed infor-
mation can be read out by monitoring this spectral change. Because both
optical writing and electrochemical reduction are necessary for the formation
of the hydrazobenzene, optical reading will not destroy the stored informa-
tion. Electrochemical oxidation will erase the information. 109 Another
approach for circumventing the use of the unstable ds-isomer in optical data
storage applications involves using photoinduced birefringence (see Section
6.5.2).
The structural changes within LBK films upon irradiation can cause
morphological changes, too. Irradiation of LBK films of azobenzene
amphiphile 43 results in an increase of the surface roughness, as shown by
105
AFM measurements. The roughness is most likely caused by a recrystalliza-
tion of the azobenzene amphiphiles in the irradiated area.

43