Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 20

(a) (c)

OH

O

HO
HO

O

O

OH

19

(b)

500 nm

282 nm

Solution at 60 ° C

150

Fast-cooled gel
Slow-cooled gel

(d)

100

50

0

− 50

250 nm − 100

286 nm

300 350 400 450

250

Wavelength/nm

at 20 ◦ C. SEM micrograph of the slow-cooled
(c) and fast-cooled (d) gel of 5 mg mL − 1 19
in 40/60 (v/v) H 2 O/1,4-dioxane. Reprinted
with permission from Ref. [78g]. Copyright
2010 American Chemical Society.

Figure 1.17 (a) Sugar-appended terphenyl
gelator 19 . (b) CD spectra of a solution/sol
of 2 mg mL − 1 19 in H 2 O/1,4-dioxane
(40/60 v/v) at 60 ◦ C( ), the fast-cooled gel
state ( • ), and the slow-cooled gel state ( )

Gelation-induced helix formation can be achieved and strong CD signals can
be observed even when the chiral centers are far from a chromophore. Oligo( p -
phenylenevinylene) (OPV) functionalized with two chiral side groups and four
dodecyl side groups [79] ( , Figure 1.18a) was found to gelate alkanes such as

dodecane, heptane, and cyclohexane (CGC

20

10 −3 M). Although a chloroform = 6.4 ×

10 −4 M was CD inactive, suggesting a
lack of aggregation (Figure 1.18b inset), a strong exciton-coupled CD spectrum
was observed in dodecane at the same concentration, indicating the transfer of
molecular chiral information to the self-assemblies in a helical sense (Figure 1.18b
inset). In sharp contrast, compound

solution of 20 at a concentration of 5.3 ×

21 , with six chiral side groups, did not form a

gel in any of the solvents examined.
The shape of the CD spectrum changed with concentration (Figure 1.18b),
exhibiting two transitions: (i) a Cotton effect with zero crossing at 440 nm be-
low 3 × 10 −5 M and (ii) a zero crossing at the absorption maximum (400 nm)

above 3 10 −5 M where a true exciton-coupled CD spectrum is observed. Based

upon additional data from AFM (Figure 1.18c), this unusual phenomenon was
interpreted to arise from two hierarchical supramolecular assemblies that in-
volve the formation of left-handed chiral aggregates (in the low concentration
regime) and secondary assembly to coiled-coil ropes (in the high concentration
regime).

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