investigating the complexation behaviors of other receptor-ligand binding systems
and stimulates the mechanistic study in molecular detail.
Dong et al. reported the use of cavitand to control stilbazole ST1, a TICT
fluorophore, in supramolecular microcrystals [63] with temperature-controlled tunability. Stilbazoles lack emissivity which is a crystalline state due to aggregationinduced quenching. This disadvantage was overcome by inclusion chemistry, which
afforded dual advantage of aggregate prevention and conformational control. The
authors report broadband tunable microlasers built by incorporating a highly polarized organic intramolecular charge-transfer (ICT) compound with β-CD, which
resulted in a 1:1 inclusion complex. As the LE and TICT emission occur at different
wavelengths, controlling the planar vs. nonplanar conformations of the fluorophore
in excited states would afford ability to control visible color emitted by designed
material (Fig. 19). It was deduced for stilbazolium fluorophores (ST-1) that at lower
temperatures (À60
C), the lack of energy to overcome rotational barrier prevents
geometric transition from planar to nonplanar conformation resulting in exclusive
emission from the lower-wavelength LE state at 550 nm. Whereas at higher temperatures (120
C), the TICT state is accessed more easily leading to mixed
(LE + TICT) emission at a higher wavelength (670 nm). The spatial confinement
of the ICT dye, imposed using β-CD, facilitated generation of an optimized energy
level system that favors controlled population distribution between the LE and TICT
states. As a result, the authors realized a wide tuning of lasing wavelengths in the
organic supramolecular microcrystals based on temperature-controlled population
transfer from the LE to TICT state. The results provided a useful demonstration of
rational design of miniaturized lasers with tunable optical performances.
Schoder et al. reported a series of easily accessible bipyridinium fluorophores,
whose emission is quenched by a TICT mechanism [64]. The donor-acceptor
arrangement for TICT emission based on the bispyridyl-biphenyl duo has not been
explored before (Fig. 20). Based on experimental and theoretical studies, absorption
bands were attributed to HOMO-LUMO transitions, with the HOMO being centered
on the biphenyl and the LUMO on the bipyridinium parts. Photoexcitation of BP in
homogeneous media does not result in emission from LE state due to quenching
mechanism from the TICT structure (through a metastable state, MSS), which is
easily accessible at room temperature given its low rotational barrier (c.a. 15 kJ/mol).
Therefore, it was reasoned that encapsulation of the fluorophores in macrocyclic
cavitands capable of enforcing co-planarity by destabilizing TICT geometry should
enhance the LE emission.
BP-Et was investigated for its luminescence in three different hosts (CB7, CB8,
and β-CD) which offer a unique insight into the effect of bonding and steric
supramolecular variations on the excited-state dynamics. Encapsulation of BP in a
CB7 host gave a 1:1 complex exhibiting a moderate emission increase (Fig. 21, blue
spectrum) due to destabilization of the TICT state inside the relatively less polar
cucurbituril cavity. On the other hand, CB8 formed both 1:2 and 2:2 complexes with
a much stronger fluorescence enhancement which is observed in 2:2 complexes. This
was attributed to the additional conformational restriction of rotations around the
aryl/aryl bonds (Fig. 21, purple spectrum) due to reduced cavity volume from
Photophysicochemical Processes Directed Within Nano-Containers
341
and stimulates the mechanistic study in molecular detail.
Dong et al. reported the use of cavitand to control stilbazole ST1, a TICT
fluorophore, in supramolecular microcrystals [63] with temperature-controlled tunability. Stilbazoles lack emissivity which is a crystalline state due to aggregationinduced quenching. This disadvantage was overcome by inclusion chemistry, which
afforded dual advantage of aggregate prevention and conformational control. The
authors report broadband tunable microlasers built by incorporating a highly polarized organic intramolecular charge-transfer (ICT) compound with β-CD, which
resulted in a 1:1 inclusion complex. As the LE and TICT emission occur at different
wavelengths, controlling the planar vs. nonplanar conformations of the fluorophore
in excited states would afford ability to control visible color emitted by designed
material (Fig. 19). It was deduced for stilbazolium fluorophores (ST-1) that at lower
temperatures (À60
C), the lack of energy to overcome rotational barrier prevents
geometric transition from planar to nonplanar conformation resulting in exclusive
emission from the lower-wavelength LE state at 550 nm. Whereas at higher temperatures (120
C), the TICT state is accessed more easily leading to mixed
(LE + TICT) emission at a higher wavelength (670 nm). The spatial confinement
of the ICT dye, imposed using β-CD, facilitated generation of an optimized energy
level system that favors controlled population distribution between the LE and TICT
states. As a result, the authors realized a wide tuning of lasing wavelengths in the
organic supramolecular microcrystals based on temperature-controlled population
transfer from the LE to TICT state. The results provided a useful demonstration of
rational design of miniaturized lasers with tunable optical performances.
Schoder et al. reported a series of easily accessible bipyridinium fluorophores,
whose emission is quenched by a TICT mechanism [64]. The donor-acceptor
arrangement for TICT emission based on the bispyridyl-biphenyl duo has not been
explored before (Fig. 20). Based on experimental and theoretical studies, absorption
bands were attributed to HOMO-LUMO transitions, with the HOMO being centered
on the biphenyl and the LUMO on the bipyridinium parts. Photoexcitation of BP in
homogeneous media does not result in emission from LE state due to quenching
mechanism from the TICT structure (through a metastable state, MSS), which is
easily accessible at room temperature given its low rotational barrier (c.a. 15 kJ/mol).
Therefore, it was reasoned that encapsulation of the fluorophores in macrocyclic
cavitands capable of enforcing co-planarity by destabilizing TICT geometry should
enhance the LE emission.
BP-Et was investigated for its luminescence in three different hosts (CB7, CB8,
and β-CD) which offer a unique insight into the effect of bonding and steric
supramolecular variations on the excited-state dynamics. Encapsulation of BP in a
CB7 host gave a 1:1 complex exhibiting a moderate emission increase (Fig. 21, blue
spectrum) due to destabilization of the TICT state inside the relatively less polar
cucurbituril cavity. On the other hand, CB8 formed both 1:2 and 2:2 complexes with
a much stronger fluorescence enhancement which is observed in 2:2 complexes. This
was attributed to the additional conformational restriction of rotations around the
aryl/aryl bonds (Fig. 21, purple spectrum) due to reduced cavity volume from
Photophysicochemical Processes Directed Within Nano-Containers
341