photophysical switching between protonation and deprotonation of [5]HeliBI was
investigated by
1 H NMR and steady-state spectroscopy (i.e., absorption and fluorescence spectra). We also confirmed the CPL signals of protonated [5]HeliBI (H
+
-
[5]HeliBI). This is the first example regarding red-colored CPL behaviors of
helicene derivatives (Fig. 5.5b) [45].
5.2.3 Tetrathia[9]helicene Derivatives for CPL
In this section, we briefly focus on the longer helicene derives. A series of tetrathia
[9]helicene derivatives such as quinoxaline (acceptor)-fused tetrathia[9]helicene
(donor) were synthesized to optimize the photophysical behaviors and CPL properties (Fig. 5.6a) [46]. In this work, the “push-pull” character was induced by introducing a quinoxaline onto the tetrathia[9]helicene unit, which was enhanced by
further introduction of an electron-donating Me 2 N unit or an electron-accepting NC
unit onto the quinoxaline unit (i.e., Me 2 N-QTTH and NC-QTTH in Fig. 5.6a). These
properties were successfully evaluated by electrochemical methods and DFT calculations [46]. Large enhancements in the fluorescence quantum yields (Φ FL ) were
accordingly obtained. Particularly, the maximum Φ FL of Me 2 N-QTTH attains 0.43
in benzene (NC-QTTH: Φ FL ¼ 0.30), which is more than 20 times higher than that of
a pristine tetrathia[9]helicene, i.e., TTH (Φ FL ¼ 0.02). These enhanced trends can be
quantitatively explained by kinetic parameters such as the rate constants of fluorescence and intersystem crossing (ISC) pathways. This result enabled to demonstrate
the excellent CPL behaviors. The anisotropy factor, g lum of NC-QTTH, was estimated to be 3.0 Â 10
À3 (Fig. 5.6a) [46].
On the other hand, tetrasulfone[9]helicene (PTSH) was newly synthesized by a
single-step oxidation reaction of tetrathia[9]helicene (PTTH) as shown in Fig. 5.6b
[47]. In electrochemical evaluation, the first reduction potential of PTSH was
positively shifted (ca. 1.0 V) as compared to that of PTTH due to the electronaccepting group: sulfone units. The electrochemical trends are identical with the
energy levels estimated by DFT methods and steady-state spectroscopy. Furthermore, a significant improvement of the Φ FL was confirmed. The Φ FL of PTSH is
0.27, which is approximately an order of magnitude greater than that of PTTH
(Φ FL ¼ 0.03). The improved Φ FL can be successfully discussed by the corresponding
kinetic comparison. The plausible reason is due to the increased energy difference
between the lowest singlet (S 1 ) and triplet (T 1 ) excited states (ΔE ST ). Finally,
efficient anisotropy factor (g lum ) of PTSH was also confirmed. The obtained g lum
value was calculated to be 8.3 Â 10
À4 (Fig. 5.6b) [47].
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
107
investigated by
1 H NMR and steady-state spectroscopy (i.e., absorption and fluorescence spectra). We also confirmed the CPL signals of protonated [5]HeliBI (H
+
-
[5]HeliBI). This is the first example regarding red-colored CPL behaviors of
helicene derivatives (Fig. 5.5b) [45].
5.2.3 Tetrathia[9]helicene Derivatives for CPL
In this section, we briefly focus on the longer helicene derives. A series of tetrathia
[9]helicene derivatives such as quinoxaline (acceptor)-fused tetrathia[9]helicene
(donor) were synthesized to optimize the photophysical behaviors and CPL properties (Fig. 5.6a) [46]. In this work, the “push-pull” character was induced by introducing a quinoxaline onto the tetrathia[9]helicene unit, which was enhanced by
further introduction of an electron-donating Me 2 N unit or an electron-accepting NC
unit onto the quinoxaline unit (i.e., Me 2 N-QTTH and NC-QTTH in Fig. 5.6a). These
properties were successfully evaluated by electrochemical methods and DFT calculations [46]. Large enhancements in the fluorescence quantum yields (Φ FL ) were
accordingly obtained. Particularly, the maximum Φ FL of Me 2 N-QTTH attains 0.43
in benzene (NC-QTTH: Φ FL ¼ 0.30), which is more than 20 times higher than that of
a pristine tetrathia[9]helicene, i.e., TTH (Φ FL ¼ 0.02). These enhanced trends can be
quantitatively explained by kinetic parameters such as the rate constants of fluorescence and intersystem crossing (ISC) pathways. This result enabled to demonstrate
the excellent CPL behaviors. The anisotropy factor, g lum of NC-QTTH, was estimated to be 3.0 Â 10
À3 (Fig. 5.6a) [46].
On the other hand, tetrasulfone[9]helicene (PTSH) was newly synthesized by a
single-step oxidation reaction of tetrathia[9]helicene (PTTH) as shown in Fig. 5.6b
[47]. In electrochemical evaluation, the first reduction potential of PTSH was
positively shifted (ca. 1.0 V) as compared to that of PTTH due to the electronaccepting group: sulfone units. The electrochemical trends are identical with the
energy levels estimated by DFT methods and steady-state spectroscopy. Furthermore, a significant improvement of the Φ FL was confirmed. The Φ FL of PTSH is
0.27, which is approximately an order of magnitude greater than that of PTTH
(Φ FL ¼ 0.03). The improved Φ FL can be successfully discussed by the corresponding
kinetic comparison. The plausible reason is due to the increased energy difference
between the lowest singlet (S 1 ) and triplet (T 1 ) excited states (ΔE ST ). Finally,
efficient anisotropy factor (g lum ) of PTSH was also confirmed. The obtained g lum
value was calculated to be 8.3 Â 10
À4 (Fig. 5.6b) [47].
5 Structural Control of Fluorescent Helicates for Improved Circularly Polarized. . .
107