Page 232 - Photoreactive Organic Thin Films

6. PHOTOISOMERIZATION IN LANGMUIR-BLODGETT-KUHN STRUCTURES 211

the use of the difference in the UV spectra due to the trans to as isomeriza-
tion for read-out of the inscribed information. More sophisticated read-out
techniques, such as surface piasmon microscopy, have been developed to min-
imize the sensitivity problems of this approach. Another problem that arises
from trans to as photoisomerization in optical data storage is the transient
nature of the stored information because of the unstable os-isomer. It has
been suggested that the combination of photoisomerization and electrochemi-
109
cal reduction of the os-isomer could overcome this problem. The most
promising approach for optical data storage, however, is based on photo-
induced birefringence as a consequence of photorientation due to repeated
photoisomerization of the chromophores. LBK films have been used as
models for investigating the underlying mechanism of the photoreorientation,
but only spin-coating films have the potential to be used in technical
applications,
An example of tailoring the properties of photochromic moieties employ-
ing the LBK technique to obtain a material suitable for multifrequency
n
optical data storage has been proposed by Hibino et aL The authors investi-
gated a number of spirobenzopyrans showing J-aggregation that differed
based on their substitution and the matrix. By tailoring these parameters,
they created a layered system in which the dye for each layer has a very nar-
row adsorption band with a position slightly different from that in all other
layers. Up to ten different layers were deposited. The photoreaction shows a
nonlinear dependency on the light intensity. Therefore, it is possible to write
and nondestructively read at the same wavelength as long as the intensity of
the read laser is low.

6.6.2 Sensors
122
Thin organic films have frequently been suggested for sensor applications,
The aim is to find an appropriate combination of functional units in a
supramolecular assembly such that an external stimulus can be transferred

45
FIG. 6.27 Change in (A) absorbance at 356 nm and (B) conductivity of azobenzene amphiphile/
TCNQ complex 45 LBK film (adapted from reference 123 with permission).