Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 47

VIS irradiation Solution state

100

99

98

UV irradiation Gel state

97

trans cis trans cis trans cis trans cis trans cis

Isomer type

Figure 1.45 Reversible gelation and de-gelation by irradiation of 71 . Reprinted with permis-
sion from Ref. [54]. Copyright 1994 American Chemical Society.

in 1-butanol at 25 ◦ C is shown in Figure 1.45
[54]. Thus, it is possible to effect enormous reversible rheological changes in these
samples simply by irradiating them with different wavelengths.

example of the reversible gelation of 71

RO N

N

O

71

Irreversible destruction of the fibrillar networks upon UV irradiation has been
reported as well. For example, irradiations of dodecane gels of cholesteryl 4-(2-
anthryloxy)butanoate ( which contains

a photo-active anthryl moiety, were found to lead to loss of the gel phase and
formation of 4 photodimers, HH/S, HH/A, HT/S, and HH/A (Figure 1.46), in
relative yields similar to those obtained upon irradiation of a toluene solution; the
HH/HT dimer ratios were near unity. Protracted irradiation of

CAB , 72 , see Section 1.2.3) [11], an LMOG

in its neat solid
phase did not lead to detectable dimerization, although irradiation of the liquid
crystalline phase [131] did (and in relative photodimer abundances similar to those
found from the gel phase). These results highlight the aforementioned fact that
many molecular gelators are polymorphous; the molecular packing in the neat
solid and even in gels with different liquid components need not be the same.
Ihara and coworkers have used l-glutamic acid-derived lipids (

72

73-75 )as LMOG s.

Adding a spiropyran probe ( methyl, butyl, or hexadecyl) and irra-

diating in the benzene gels created the very polar merocyanine forms (

76 , where R =

77 )which

closedupbacktothe 76 form in the dark (Figure 1.47) [132]. It was found