molecules show the peculiar chiroptical properties such as circular dichroism
(CD) with large anisotropy factor (g-value).
In contrast, the chirality derived from luminescence compounds and materials
induce circularly polarized luminescence (CPL), i.e., the differential emission intensity of right- and left-circularly polarized light [15–24]. However, the fluorescence
quantum yields (Φ FL ) of helicenes are generally extremely small because of the
acceleration of the intersystem crossing (ISC) processes (the ISC quantum yield
ΦISC: $0.9). Therefore, synthetic strategies for improvements of Φ FL are definitely
required [21, 25–27]. To optimize the excited-state dynamics of π-conjugated
molecules, one of the useful ways is substitution of electron-accepting groups
(e.g., maleimide groups) on the peripheral positions. Moreover, the supramolecular
dimer with the helical chirality is also interesting for improvement of CPL
properties [24].
Based on the above concepts, we discuss the structural and photophysical control
of fluorescent helicene derivatives and related homoleptic zinc(II) helicates for
improved circularly polarized luminescence properties. The details on the structural
and photophysical control as well as CPL properties are presented.
5.2 Results and Discussion
5.2.1 [7]Carbohelicene Derivatives Fused by Asymmetric
1,2-Dialkyl-Substituted Quinoxaline
Kang et al. previously demonstrated that the imine unit of quinoxalines reduced by
alkyl/aryllithiums results in a significant change of steric hindrance on the
quinoxaline unit [28]. This is derived from the presence of two dissimilar and
asymmetric conditions of nitrogen environments in a quinoxaline unit (one is an
electron-donating sp
3 site and the other is an electron-accepting sp
2 site). Such a
structural change is expected to control the optical and electronic properties. Based
on the above points, a new compound, 1,2-dialkylquinoxaline-substituted [7]
carbohelicene (denoted as [7]Hl-NAIQx) by introduction of two alkyl chain groups
onto a quinoxaline unit, was designed and synthesized (Fig. 5.1) [21].
Fig. 5.1 Structures of [7]carbohelicene in this work
100
T. Hasobe
(CD) with large anisotropy factor (g-value).
In contrast, the chirality derived from luminescence compounds and materials
induce circularly polarized luminescence (CPL), i.e., the differential emission intensity of right- and left-circularly polarized light [15–24]. However, the fluorescence
quantum yields (Φ FL ) of helicenes are generally extremely small because of the
acceleration of the intersystem crossing (ISC) processes (the ISC quantum yield
ΦISC: $0.9). Therefore, synthetic strategies for improvements of Φ FL are definitely
required [21, 25–27]. To optimize the excited-state dynamics of π-conjugated
molecules, one of the useful ways is substitution of electron-accepting groups
(e.g., maleimide groups) on the peripheral positions. Moreover, the supramolecular
dimer with the helical chirality is also interesting for improvement of CPL
properties [24].
Based on the above concepts, we discuss the structural and photophysical control
of fluorescent helicene derivatives and related homoleptic zinc(II) helicates for
improved circularly polarized luminescence properties. The details on the structural
and photophysical control as well as CPL properties are presented.
5.2 Results and Discussion
5.2.1 [7]Carbohelicene Derivatives Fused by Asymmetric
1,2-Dialkyl-Substituted Quinoxaline
Kang et al. previously demonstrated that the imine unit of quinoxalines reduced by
alkyl/aryllithiums results in a significant change of steric hindrance on the
quinoxaline unit [28]. This is derived from the presence of two dissimilar and
asymmetric conditions of nitrogen environments in a quinoxaline unit (one is an
electron-donating sp
3 site and the other is an electron-accepting sp
2 site). Such a
structural change is expected to control the optical and electronic properties. Based
on the above points, a new compound, 1,2-dialkylquinoxaline-substituted [7]
carbohelicene (denoted as [7]Hl-NAIQx) by introduction of two alkyl chain groups
onto a quinoxaline unit, was designed and synthesized (Fig. 5.1) [21].
Fig. 5.1 Structures of [7]carbohelicene in this work
100
T. Hasobe