Stimuli-Responsive Lipid -Based Self-Assembled Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 267

(a) (b)

hv

hv

N N

N N

hv´, k B T

Figure 9.5 Azobenzene photoisomerization: (a) Reversible trans to cis isomerization
on exposure to an appropriate wavelength of UV light. (b) Diagrammatic representa-
tion of the disruption of a liposome through the activation of the azo moiety with UV
light and subsequent drug release. [Adapted from Morgan et al. (1995) with permission
from Elsevier.]

Azobenzene Aromatic azo compounds are a group of molecules whose
photochemistry is determined by an azobenzene moiety; an azo group -N N -

in conjugation with two phenyl substituents. Upon UV irradiation, the azo-
benzene moiety undergoes a considerable change in length with the distance
between the 4 and 4

=

carbons decreasing from 9.0 Å in the trans form to 5.5 Å
in the cis form (Natansohn and Rochon, 2002). Azobenzene photoisomers also
differ in polarity; the cis isomer is more hydrophilic than the trans. The N

′

N
isomerization between trans and cis azobenzene isomers is shown in panel (a)
in Fig. 9.5. The extent of trans to cis isomerization is dependent on strength
and time of irradiation, solvent effects, and is slightly affected by substituent
type (Knoll, 2004 ).
The main uses of azobenzene are as yellow/orange synthetic dyes; however,
molecules and surfactants containing an azobenzene moiety have been used
to impart control over membrane permeability in aggregates such as emul-
sions (Bufe and Wolff, 2009; Cicciarelli et al., 2007; Han and Hara, 2005;
Khoukh et al., 2005), Langmuir-Blodgett fi lms (Haruta et al., 2008; Kumar
et al., 2009b; Oki and Nagasaka, 2009; Sorensen et al., 2008), liposomes and
vesicular membranes (Eastoe et al., 2008; Kuiper and Engberts, 2004; Kuiper
et al., 2008; Lei and Hurst, 1999; Liu et al., 2005; Morgan et al., 1995; Song
et al., 1995; Zou et al., 2008), polymeric vesicles (Liu et al., 2008; Tong et al.,
2005), hydrogels (Zhao and Stoddart, 2009), silica materials (Tanaka et al.,
2007), and liquid crystals (Corvazier and Zhao, 1999; Yamamoto et al., 2006,
2009; Yi et al., 2009). The isomerization of the azo-moiety is suffi cient to
destabilize the surrounding area and, as a consequence, induce a phase trans-
formation or disruption of the bilayer as schematically demonstrated in panel
(b) of Figure 9.5.

=

Spiropyrans Upon exposure to UV light, spiropyrans undergo a heterocyclic
ring cleavage that results in the relatively hydrophilic, open cationic mero-
cyanine form. The spiropyran (the relatively hydrophobic closed form) is