The single crystals of helicene derivatives were arranged using vapor diffusion
method. The crystal structures of rac-[7]Hl-Qx and rac-[7]Hl-NAIQx are shown in
Fig. 5.2. [7]Hl-Qx possesses a nonplanar formation between the two phenanthrenes
(Fig. 5.2a). The C3–C5–C5’–C13 torsion angle between the alkyl-quinoxaline and
phenanthrene is calculated to be 159
. On the other hand, [7]Hl-NAIQx has a helical
conformation, which is formed by the two phenanthrenes and a planar structure
between the alkyl-quinoxaline and phenanthrene units (Fig. 5.2b). The torsion angle
between the alkyl-quinoxaline and phenanthrene units (C39–C29–C2–C21) is determined to be 179
. This trend is significantly different from that of [7]Hl-Qx as
discussed above. In contrast, the torsion angle of the two phenanthrenes (C21–C29–
C2–C10) in [7]Hl-NAIQx (56
) is larger than that of C13–C5–C5’–C8 torsion in [7]
Hl-Qx (44
). The differential angles may directly have an effect on the delocalized
π-electrons in the entire molecules, which results in the enhancement of fluorescence
emission properties (vide infra) [21].
To quantitatively discuss the photophysical properties of these carbohelicene
derivatives, fluorescence lifetimes were measured. Fluorescence lifetimes (τ FL )
were successfully estimated from monoexponential fitting analysis. The τ FL values
of [7]Hl-Qx (1.66 ns) and [7]Hl-NAIQx (4.00 ns) are calculated, respectively.
These are shorter as compared to the reference molecule: [7]Hl (13.8 ns). Then,
fluorescence quantum yields (Φ FL ) of these [7]carbohelicene derivatives were
evaluated. As compared to the Φ FL value of [7]Hl (Φ FL ¼ 0.02), those of [7]HlQx: Φ FL ¼ 0.05 and [7]Hl-NAIQx: Φ FL ¼ 0.25 largely increased. In particular, the
Fig. 5.2 ORTEP diagrams. (a) rac-[7]Hl-Qx (proton units were omitted for clarity, ellipsoids set at
50% probability) and (b) rac-[7]Hl-NAIQx (proton units were omitted for clarity, ellipsoids set at
50% probability) (reprinted with permission from Ref. [21] Copyright 2015 American Chemical
Society)
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
101
method. The crystal structures of rac-[7]Hl-Qx and rac-[7]Hl-NAIQx are shown in
Fig. 5.2. [7]Hl-Qx possesses a nonplanar formation between the two phenanthrenes
(Fig. 5.2a). The C3–C5–C5’–C13 torsion angle between the alkyl-quinoxaline and
phenanthrene is calculated to be 159
. On the other hand, [7]Hl-NAIQx has a helical
conformation, which is formed by the two phenanthrenes and a planar structure
between the alkyl-quinoxaline and phenanthrene units (Fig. 5.2b). The torsion angle
between the alkyl-quinoxaline and phenanthrene units (C39–C29–C2–C21) is determined to be 179
. This trend is significantly different from that of [7]Hl-Qx as
discussed above. In contrast, the torsion angle of the two phenanthrenes (C21–C29–
C2–C10) in [7]Hl-NAIQx (56
) is larger than that of C13–C5–C5’–C8 torsion in [7]
Hl-Qx (44
). The differential angles may directly have an effect on the delocalized
π-electrons in the entire molecules, which results in the enhancement of fluorescence
emission properties (vide infra) [21].
To quantitatively discuss the photophysical properties of these carbohelicene
derivatives, fluorescence lifetimes were measured. Fluorescence lifetimes (τ FL )
were successfully estimated from monoexponential fitting analysis. The τ FL values
of [7]Hl-Qx (1.66 ns) and [7]Hl-NAIQx (4.00 ns) are calculated, respectively.
These are shorter as compared to the reference molecule: [7]Hl (13.8 ns). Then,
fluorescence quantum yields (Φ FL ) of these [7]carbohelicene derivatives were
evaluated. As compared to the Φ FL value of [7]Hl (Φ FL ¼ 0.02), those of [7]HlQx: Φ FL ¼ 0.05 and [7]Hl-NAIQx: Φ FL ¼ 0.25 largely increased. In particular, the
Fig. 5.2 ORTEP diagrams. (a) rac-[7]Hl-Qx (proton units were omitted for clarity, ellipsoids set at
50% probability) and (b) rac-[7]Hl-NAIQx (proton units were omitted for clarity, ellipsoids set at
50% probability) (reprinted with permission from Ref. [21] Copyright 2015 American Chemical
Society)
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
101