[49]. Additionally, the Φ FL of [n]carbohelicenes largely decreased with increasing
the number (n) of the benzene units (Φ FL ~0.02 for n ! 7) [31]. Thus, this hampers
the utilization of simple carbohelicene derivatives for CPL evaluation in the NIR
region. In contrast, chiroptical properties of dipyrromethenes have improved
recently, and the dimeric formations using supramolecular method were highly
promising [50, 51]. Accordingly, we can expect that coordination-driven chiral
structure composed of a pair of dipyrromethene chromophores are suitable for
enhanced CD and CPL behaviors.
Therefore, in this section, we discuss the synthesis and enhanced chiroptical
properties of a rare-earth- and precious-metal-free homoleptic zinc(II) helicate
arranged by a pair of achiral ligands (benzo[a]phenanthrene-fused dipyrromethene)
(Fig. 5.7) [52]. Selective configuration of the chiral helical structures is obtained by
the possibility of tetrahedral conformation composed of two unsymmetrical
bidentate ligands.
First, Zn(Phena-dpm) 2 was synthesized by the reported method [52]. In the
steady-state spectroscopic measurements, the absorption bands of Zn(Phena-dpm) 2
(Fig. 5.8a, spectrum a) were red-shifted and broadened as compared to that of the
reference compound: Phena-dpm-H (Fig. 5.8a, spectrum b). More importantly, the
(P,P)-/(M,M)Zn(Phena-dpm) 2
Helicate Formation
𨐽 Exciton Coupled CD
Intense Far-Red CPL
Zn 2+
Phena-dpm-H
Achiral Ligand
Fig. 5.7 A proposed chiral formation of a homoleptic zinc(II) helical complex composed by a pair
of achiral dipyrromethene ligands: Phena-dpm-H (reprinted with permission from Ref. [52] Copyright 2018 Wiley-VCH)
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
109
the number (n) of the benzene units (Φ FL ~0.02 for n ! 7) [31]. Thus, this hampers
the utilization of simple carbohelicene derivatives for CPL evaluation in the NIR
region. In contrast, chiroptical properties of dipyrromethenes have improved
recently, and the dimeric formations using supramolecular method were highly
promising [50, 51]. Accordingly, we can expect that coordination-driven chiral
structure composed of a pair of dipyrromethene chromophores are suitable for
enhanced CD and CPL behaviors.
Therefore, in this section, we discuss the synthesis and enhanced chiroptical
properties of a rare-earth- and precious-metal-free homoleptic zinc(II) helicate
arranged by a pair of achiral ligands (benzo[a]phenanthrene-fused dipyrromethene)
(Fig. 5.7) [52]. Selective configuration of the chiral helical structures is obtained by
the possibility of tetrahedral conformation composed of two unsymmetrical
bidentate ligands.
First, Zn(Phena-dpm) 2 was synthesized by the reported method [52]. In the
steady-state spectroscopic measurements, the absorption bands of Zn(Phena-dpm) 2
(Fig. 5.8a, spectrum a) were red-shifted and broadened as compared to that of the
reference compound: Phena-dpm-H (Fig. 5.8a, spectrum b). More importantly, the
(P,P)-/(M,M)Zn(Phena-dpm) 2
Helicate Formation
𨐽 Exciton Coupled CD
Intense Far-Red CPL
Zn 2+
Phena-dpm-H
Achiral Ligand
Fig. 5.7 A proposed chiral formation of a homoleptic zinc(II) helical complex composed by a pair
of achiral dipyrromethene ligands: Phena-dpm-H (reprinted with permission from Ref. [52] Copyright 2018 Wiley-VCH)
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
109