yield (Φ FL ¼ 0.23) and lifetime (τ FL ¼ 3.0 ns) [52]. Although a mirror-image
relation between absorption and fluorescence emission spectra was noted, a Stokes
shift of 1100 cm
À1 indicated appropriate the occurrence of excited-state relaxation
regarding the coordination-driven helicate (vide infra). Strong and exact mirror
image CPL signals were observed for (P,P)- and (M,M)-forms of Zn(Phena-dpm) 2
in the corresponding fluorescence spectrum (Fig. 5.8e). The signs of CPL agree with
those of CD at the lowest-energy band, which is in consistent with the general trends
for the ECD and CPL spectra, including those reported for exciton-coupled systems.
The g lum of the P,P form was assigned to be À0.022 at 660 nm (Fig. 5.8f). Moreover,
the g lum profiles expanded up to ca. 850 nm (i.e., far-red region), as shown in
Fig. 5.8f.
To further discuss the origin of the chiroptical response in Zn(Phena-dpm) 2 , the
analysis of relevant electronic transitions were performed by DFT calculations. A
similar way has been successfully employed for a few simple organic compounds
[41, 54]. Therefore, the optimized structures of Zn(Phena-dpm) 2 in the ground and
excited states were firstly optimized at the (TD)DFT-M062X/def2-QZV(GD3) level
of theory [55]. Based on these calculated geometries in the S 0 and S 1 states, the
relevant electronic (μ) and magnetic (m) transition dipole moments were evaluated.
Noted that these values are highly associated with the rotational strength R for the
corresponding CD and CPL spectra. In contrast with small structural change of
single-crystal and calculated structures in the ground state, the structural relaxation
should be substantial on photoexcitation, which subsequently have an effect on the
chiroptical response. Thus, the entire structural change and distortion around the zinc
(II) ion was somewhat decreased in the excited state because the dihedral and torsion
angles of the two dipyrromethenes were smaller (53.7 and 39.2
, respectively). More
importantly, the overlapped trend between the two phenanthrene ligands was considerably changed in the excited state (Fig. 5.9), whereas the interplanar distances
are approximately similar each other (3.40 vs. 3.25 Å). The structural change is
attributable to the exciplex formation because of sufficient π-overlap between two
aromatic groups, whereas this is generally hampered in the ground state considering
the electrostatic and/or Pauli repulsion [56]. Thus, a slightly smaller chiroptical
response was probably due to the S 1 -to-S 0 transition (R ¼ 770, for CPL) in contrast
with the reverse S 0 -to-S 1 transition (R ¼ 1013, for CD).
Associated with the Rosenfeld equation, R is the imaginary part in the scalar
product of μ and m of the relevant electronic transition (i.e., R ¼ |μ||m| cosθ). Among
the possible factors contributing to the large R values for the dissymmetry factors
g abs and g lum , the small angle θ between these two moments was found to play an
important role. Indeed, these factors for the S 0 -to-S 1 and S 1 -to-S 0 transitions in the
complex were estimated to be 0.94 (i.e., θ ¼ 20.6
) and 0.96 (i.e., θ ¼ 15.7
),
respectively, which is totally different from the much smaller value of 0.027
(θ ¼ 91.6
) for free Phena-dpm-H. Such a drastic improvement is established by
the C2-symmetrical alignment of two identical phenanthrene-fused helical units in
Zn(Phena-dpm) 2 . In addition, the increase trend in absolute |m| value was also
attributable to the dimeric structure of Zn(Phena-dpm) 2 . Briefly, a proper alignment
of two identical helical components via coordination-organized contributes to
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
111
relation between absorption and fluorescence emission spectra was noted, a Stokes
shift of 1100 cm
À1 indicated appropriate the occurrence of excited-state relaxation
regarding the coordination-driven helicate (vide infra). Strong and exact mirror
image CPL signals were observed for (P,P)- and (M,M)-forms of Zn(Phena-dpm) 2
in the corresponding fluorescence spectrum (Fig. 5.8e). The signs of CPL agree with
those of CD at the lowest-energy band, which is in consistent with the general trends
for the ECD and CPL spectra, including those reported for exciton-coupled systems.
The g lum of the P,P form was assigned to be À0.022 at 660 nm (Fig. 5.8f). Moreover,
the g lum profiles expanded up to ca. 850 nm (i.e., far-red region), as shown in
Fig. 5.8f.
To further discuss the origin of the chiroptical response in Zn(Phena-dpm) 2 , the
analysis of relevant electronic transitions were performed by DFT calculations. A
similar way has been successfully employed for a few simple organic compounds
[41, 54]. Therefore, the optimized structures of Zn(Phena-dpm) 2 in the ground and
excited states were firstly optimized at the (TD)DFT-M062X/def2-QZV(GD3) level
of theory [55]. Based on these calculated geometries in the S 0 and S 1 states, the
relevant electronic (μ) and magnetic (m) transition dipole moments were evaluated.
Noted that these values are highly associated with the rotational strength R for the
corresponding CD and CPL spectra. In contrast with small structural change of
single-crystal and calculated structures in the ground state, the structural relaxation
should be substantial on photoexcitation, which subsequently have an effect on the
chiroptical response. Thus, the entire structural change and distortion around the zinc
(II) ion was somewhat decreased in the excited state because the dihedral and torsion
angles of the two dipyrromethenes were smaller (53.7 and 39.2
, respectively). More
importantly, the overlapped trend between the two phenanthrene ligands was considerably changed in the excited state (Fig. 5.9), whereas the interplanar distances
are approximately similar each other (3.40 vs. 3.25 Å). The structural change is
attributable to the exciplex formation because of sufficient π-overlap between two
aromatic groups, whereas this is generally hampered in the ground state considering
the electrostatic and/or Pauli repulsion [56]. Thus, a slightly smaller chiroptical
response was probably due to the S 1 -to-S 0 transition (R ¼ 770, for CPL) in contrast
with the reverse S 0 -to-S 1 transition (R ¼ 1013, for CD).
Associated with the Rosenfeld equation, R is the imaginary part in the scalar
product of μ and m of the relevant electronic transition (i.e., R ¼ |μ||m| cosθ). Among
the possible factors contributing to the large R values for the dissymmetry factors
g abs and g lum , the small angle θ between these two moments was found to play an
important role. Indeed, these factors for the S 0 -to-S 1 and S 1 -to-S 0 transitions in the
complex were estimated to be 0.94 (i.e., θ ¼ 20.6
) and 0.96 (i.e., θ ¼ 15.7
),
respectively, which is totally different from the much smaller value of 0.027
(θ ¼ 91.6
) for free Phena-dpm-H. Such a drastic improvement is established by
the C2-symmetrical alignment of two identical phenanthrene-fused helical units in
Zn(Phena-dpm) 2 . In addition, the increase trend in absolute |m| value was also
attributable to the dimeric structure of Zn(Phena-dpm) 2 . Briefly, a proper alignment
of two identical helical components via coordination-organized contributes to
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
111