Page 210 - Photoreactive Organic Thin Films

6. PHOTOISOMERIZATION IN LANGMUIR-BLODGETT-KUHN STRUCTURES 189

r ^ ^ ^•i r
^ /" ^ *^ ^1 ^
<
Q. a. O. Q. Q.
2 s 5
Q o O o O gas
r
WrWQr* YBr*i!®?5 1 v3 f!?j water

FIG. 6.10 Schematic representation of the molecular packing arrangement in the mixed monolayer
of an ionic azobenzene amphiphile and an oppositely charged "spacer" amphiphiie (reproduced with
permission from reference 50 Copyright (1995) American Chemical Society).

azobenzene unit with an amphiphile having an oppositely charged head
group and alkyl chains shorter than the spacer (see Figure 6.10). Due to the
design of the system and the coulomb interaction of the head groups, which
ensure molecular mixing, the azobenzene moiety is located atop a layer of
closely packed alkyl chains. The photoisomerization of the azobenzene is
unrestricted and a first-order kinetics can be observed. The structure of the
LBK film is not changed by the photoisomerization, and no change in surface
pressure is observed upon irradiation. 50
Azobenzene amphiphiles can be isolated from each other and equipped
with sufficient free volume for reversible photoisomerization by inclusion
into amphiphilic (i-cyclodextrin. The inclusion complex can be transferred to
give LBK films in which the azobenzene moieties photoisomerize as they
would in solution (see Figure 6.II). 51

6.4.2 Adjusting the Structure of the Azobenzene Amphiphile

The free volume around the azobenzene that is necessary for an unrestricted
photoisomerization can be established by the introduction of bulky head
groups or additional alkyl chains in the azobenzene amphiphile. An example
of this approach has been published by Markava et al. The authors compared
azobenzene amphiphiles with one (14) and two (15) alkyl chains as well as
amphiphiles with small (16) and large (17) head groups. They found that
photoisomerization takes place in LBK films of 15 and 17, where the areal
requirements of the amphiphile are high, but not in the LBK films of 14 or
16, which can be packed more densely. 52