calculation by Bene and co-workers, in a helicity and energy-transfer process
(hFRET), the helicity in fluorescence from a rotating donor dipole could be
preserved [29]. In the instance of 9/ThT/AO system, the helicity of luminescent
donor 9 even ThT may be sequentially transferred to acceptor AO and amplified
the g lum of AO. For experimental aspect, the L-9/ThT system exhibited an amplified
g lum value (1.89 Â 10
À2 ) when excited by circularly polarized light contrasted to
the g lum value excited by unpolarized light (g lum ¼ 3.3 Â 10
À3 ). Thus, the helicity
in the donor excited state could be transferred to the acceptor and subsequently
amplified the g lum was indirectly illustrated.
11.3.2.3 CPL from Chiral π-Conjugated Gelators
In supramolecular gel system, π-conjugated gelators also play an important role
because of their unique electronic and optical properties. Ajayaghosh’s group
has reported the design of oligo( p-phenylenevinylene) (OPV)-derived gelators
and their hierarchical self-assembly properties [30]. Furthermore, the CPL property
of a photosensitive π-conjugated gelator azobenzene-linked phenyleneethynylene
(10) was also been investigated in Fig. 11.10 [31]. As shown in Fig. 11.10a, the
assembly of 10 showed a positive CPL before photoisomerization. Interestingly,
the signal of CPL was inverted after irradiation. A possible mechanism for the
observed photoinduced CPL inversion resulted from helicity inversion was proposed
in Fig. 11.10b. The assembly of 10 groups in methylcyclohexane (MCH) shows
P-helical. After ultraviolet (UV) photoirradiation, the remaining E,E isomers
were low in the photostationary state and the excess homonuclear M-helical YY
aggregates will nucleate and control the self-assembly. Therefore, the photoinduced
CPL signal was reversed.
11.3.3 CPL from Achiral Luminophores
11.3.3.1 Symmetry Breaking Triggered CPL
In the above description, luminophores conjugated with chiral gelaton moieties are
required for the fabrication of CPL-active gels. Nevertheless, not only chiral gelators
but chiral supramolecular gels can be formed by completely achiral gelators through
assembly, thus provided the possibility for the exclusive achiral fluorophores
to fabricate CPL materials [32]. If an asymmetric environment is provided during
self-assembly, some achiral gelators can result spontaneous symmetry breaking to
form chiral supramolecular assemblies in this situation. As showed in Fig. 11.11,
CPL activities could be observed if an achiral ionic polymer 11 and Rhodamine B
were under mechanical stirring [33]. Moreover, the stir direction could determine
the CPL signal: a positive sign was resulted from counterclockwise (CCW) stirring,
while clockwise (CW) stirring induced a negative sign (Fig. 11.11a). In order to
reveal the origin of the CPL, CPL spectra from four different angles of the cuvette
11 Circularly Polarized Luminescence from Gelator Molecules: From Isolated. . .
261
(hFRET), the helicity in fluorescence from a rotating donor dipole could be
preserved [29]. In the instance of 9/ThT/AO system, the helicity of luminescent
donor 9 even ThT may be sequentially transferred to acceptor AO and amplified
the g lum of AO. For experimental aspect, the L-9/ThT system exhibited an amplified
g lum value (1.89 Â 10
À2 ) when excited by circularly polarized light contrasted to
the g lum value excited by unpolarized light (g lum ¼ 3.3 Â 10
À3 ). Thus, the helicity
in the donor excited state could be transferred to the acceptor and subsequently
amplified the g lum was indirectly illustrated.
11.3.2.3 CPL from Chiral π-Conjugated Gelators
In supramolecular gel system, π-conjugated gelators also play an important role
because of their unique electronic and optical properties. Ajayaghosh’s group
has reported the design of oligo( p-phenylenevinylene) (OPV)-derived gelators
and their hierarchical self-assembly properties [30]. Furthermore, the CPL property
of a photosensitive π-conjugated gelator azobenzene-linked phenyleneethynylene
(10) was also been investigated in Fig. 11.10 [31]. As shown in Fig. 11.10a, the
assembly of 10 showed a positive CPL before photoisomerization. Interestingly,
the signal of CPL was inverted after irradiation. A possible mechanism for the
observed photoinduced CPL inversion resulted from helicity inversion was proposed
in Fig. 11.10b. The assembly of 10 groups in methylcyclohexane (MCH) shows
P-helical. After ultraviolet (UV) photoirradiation, the remaining E,E isomers
were low in the photostationary state and the excess homonuclear M-helical YY
aggregates will nucleate and control the self-assembly. Therefore, the photoinduced
CPL signal was reversed.
11.3.3 CPL from Achiral Luminophores
11.3.3.1 Symmetry Breaking Triggered CPL
In the above description, luminophores conjugated with chiral gelaton moieties are
required for the fabrication of CPL-active gels. Nevertheless, not only chiral gelators
but chiral supramolecular gels can be formed by completely achiral gelators through
assembly, thus provided the possibility for the exclusive achiral fluorophores
to fabricate CPL materials [32]. If an asymmetric environment is provided during
self-assembly, some achiral gelators can result spontaneous symmetry breaking to
form chiral supramolecular assemblies in this situation. As showed in Fig. 11.11,
CPL activities could be observed if an achiral ionic polymer 11 and Rhodamine B
were under mechanical stirring [33]. Moreover, the stir direction could determine
the CPL signal: a positive sign was resulted from counterclockwise (CCW) stirring,
while clockwise (CW) stirring induced a negative sign (Fig. 11.11a). In order to
reveal the origin of the CPL, CPL spectra from four different angles of the cuvette
11 Circularly Polarized Luminescence from Gelator Molecules: From Isolated. . .
261