of monomer emission should be attributed to an energy transfer mechanism by the
increased absorbance in the range of 380–410 nm in 2c, which well overlaps with the
pyrene monomer emission (Figs. 8.7a and 8.8a). The same mechanism partly
explains the quenching of the excimer-like emission at 500 nm. Although the
spectral overlap of excimer-like band at around 500 nm with absorption in 2c is
not as prominent as that of monomer emission band, the quenching of excimer-like
emission was a little more significant than that of monomer emission (Fig. 8.9). This
result might be attributed to the appreciable contribution of geometrical change in 2c
as depicted in Fig. 8.4. The structural change in the arrangement of pyrene units was
supported by an upfield shift of amide-protons from 2o to 2c in the
1 H NMR study,
corresponding to the dissociation of intramolecular hydrogen bonding interactions.
8.4 Dynamic Switching of Hierarchical Chirality
in Photo-Responsive Dinuclear Complexes
Unlike most chiral organic fluorophores affording CPL activity with a limited |g lum |
value less than 0.05, large |g lum | values over 0.1 have been reported for the magnetic
dipole transition in europium(III) complexes [41–45]. Eu(III) complexes have also
been combined with 6π-based photochromic units to modulate the
photoluminescence intensity [46–48]. Photochromic reaction is expected to modulate the emission property of Eu(III) ion through electronic and geometrical changes
in the photochromic ligand. For the purpose of induction and modulation of chirality
in the Eu(III) coordination cores, the pyrene unit in 2o was replaced with a coordinating ligand [49]. Terpyridine (terpy) ligands were introduced at both ends of the
helical tetrathiazole scaffold, forming 3o with Eu(III) ions and β-diketonato ligands
(tta) (Fig. 8.10). The local coordination site Eu(terpy)(tta) 3 including asymmetric
tta ligands is considered to have intrinsic chirality with eight possible chiral ninecoordination structures. The incorporation of Eu(terpy)(tta) 3 sites in the chiral
helical structure is expected to bring the complex sites close together in a chiral
arrangement. This chiral arrangement should serve as a chiral perturbation to the
coordination sites and one specific chiral coordination structure is preferentially
formed among the eight possible ligand orientations, inducing optical activity in
3o. Thus the point chirality in the amino acid spacer is expected to be transferred
over hierarchy. The point chirality in the amino acid spacer was first introduced as a
primary structure of a foldamer 3, controlling the handedness in the helical conformation as a secondary structure. The handedness-controlled chiral helical conformation of 3 arranges the inherently chiral coordination cores in a chiral manner,
inducing the biased formation of one-handed chiral ligand orientation in the coordination sites.
3o exhibited bright red emission from Eu(III)-centered f-f transitions with an
apparent emission quantum yield (Φ lum ) of 0.20 (λ ex ¼ 360 nm) and an emission
lifetime of 0.55 ms in CDCl 3 . The photoluminescence spectrum (Fig. 8.11a) gave
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T. Nakashima and T. Kawai
increased absorbance in the range of 380–410 nm in 2c, which well overlaps with the
pyrene monomer emission (Figs. 8.7a and 8.8a). The same mechanism partly
explains the quenching of the excimer-like emission at 500 nm. Although the
spectral overlap of excimer-like band at around 500 nm with absorption in 2c is
not as prominent as that of monomer emission band, the quenching of excimer-like
emission was a little more significant than that of monomer emission (Fig. 8.9). This
result might be attributed to the appreciable contribution of geometrical change in 2c
as depicted in Fig. 8.4. The structural change in the arrangement of pyrene units was
supported by an upfield shift of amide-protons from 2o to 2c in the
1 H NMR study,
corresponding to the dissociation of intramolecular hydrogen bonding interactions.
8.4 Dynamic Switching of Hierarchical Chirality
in Photo-Responsive Dinuclear Complexes
Unlike most chiral organic fluorophores affording CPL activity with a limited |g lum |
value less than 0.05, large |g lum | values over 0.1 have been reported for the magnetic
dipole transition in europium(III) complexes [41–45]. Eu(III) complexes have also
been combined with 6π-based photochromic units to modulate the
photoluminescence intensity [46–48]. Photochromic reaction is expected to modulate the emission property of Eu(III) ion through electronic and geometrical changes
in the photochromic ligand. For the purpose of induction and modulation of chirality
in the Eu(III) coordination cores, the pyrene unit in 2o was replaced with a coordinating ligand [49]. Terpyridine (terpy) ligands were introduced at both ends of the
helical tetrathiazole scaffold, forming 3o with Eu(III) ions and β-diketonato ligands
(tta) (Fig. 8.10). The local coordination site Eu(terpy)(tta) 3 including asymmetric
tta ligands is considered to have intrinsic chirality with eight possible chiral ninecoordination structures. The incorporation of Eu(terpy)(tta) 3 sites in the chiral
helical structure is expected to bring the complex sites close together in a chiral
arrangement. This chiral arrangement should serve as a chiral perturbation to the
coordination sites and one specific chiral coordination structure is preferentially
formed among the eight possible ligand orientations, inducing optical activity in
3o. Thus the point chirality in the amino acid spacer is expected to be transferred
over hierarchy. The point chirality in the amino acid spacer was first introduced as a
primary structure of a foldamer 3, controlling the handedness in the helical conformation as a secondary structure. The handedness-controlled chiral helical conformation of 3 arranges the inherently chiral coordination cores in a chiral manner,
inducing the biased formation of one-handed chiral ligand orientation in the coordination sites.
3o exhibited bright red emission from Eu(III)-centered f-f transitions with an
apparent emission quantum yield (Φ lum ) of 0.20 (λ ex ¼ 360 nm) and an emission
lifetime of 0.55 ms in CDCl 3 . The photoluminescence spectrum (Fig. 8.11a) gave
184
T. Nakashima and T. Kawai