Φ FL of [7]Hl-NAIQx is an order of magnitude greater that of pristine [7]
carbohelicene, i.e., [7]Hl, which is quite comparable to the previous related
systems of helical molecules [25, 32, 33]. Then, the quantum yields of ISC
(ΦISC) of these compounds were estimated by
1 O 2 phosphorescence utilizing
triplet-triplet energy transfer from these helicene derivatives to O 2 . Noted that
we assume that the quenching process from the singlet-excited state was negligible
[34–37]. Singlet oxygen generated from triplet states of these helicene derivatives
in oxygen-saturated toluene was confirmed by
1 O 2 phosphorescence at ~1270 nm.
Following the reported data of Φ ISC of fullerene (C 60 ) in solution [Φ ISC ¼ 0.96]
[38], C 60 was employed as the standard molecule. The Φ ISC of pristine [7]
carbohelicene (i.e., [7]Hl) was not reported, so far. Regarding the quantum yields
of internal conversion (Φ IC ), we can estimate the yields by subtracting Φ FL and
Φ ISC from the unity. The summarized quantum yields including Φ ISC and Φ IC are
shown in Table 5.1. It should be noted that the total sum of the above three
quantum yields is close to unity within the experimental error although measurement instruments were totally different. The Φ FL of [7]Hl-NAIQx (0.25) is much
larger than those of [7]Hl-Qx (Φ FL ¼ 0.05) and [7]Hl (Φ FL ¼ 0.02) regardless of
the corresponding low Φ ISC values.
Finally, the respective rate constants of these three processes such as fluorescence
emission (k FL ), intersystem crossing (k ISC ), and internal conversion (k IC ) were
estimated as shown in Table 5.1. It should be emphasized that the k FL of [7]HlNAIQx is much larger than that of [7]Hl. In particular, the k FL values such as
6.25 Â 10
7 s
À1 for [7]Hl-NAIQx and 3.01 Â 10
7 s
À1 for [7]Hl-Qx are one order
of magnitude greater than 1.45 Â 10
6 s
À1 for [7]Hl. According to the basic theory of
photochemistry [39], k FL is largely related to the molar extinction coefficients. The ε
values are therefore estimated from the 0–0 absorption bands (ε 0–0 ), as shown in
Table 5.1. The evaluated ε 0–0 values of [7]Hl-NAIQx and [7]Hl-Qx are significantly
larger as compared to [7]Hl, which coincides with the photophysical trends of the
k FL . Then, we also discussed the ISC processes such as the rate constant: k ISC and
quantum yield: Φ ISC . The estimated k ISC constants of [7]Hl-Qx and [7]Hl-NAIQx
are 5.78 Â 10
8 and 1.55 Â 10
8 s
À1 , respectively. In the same manner as k FL , these
rate constants are significantly larger than that of pristine [7]carbohelicene i.e., [7]Hl
(k ISC % 6.5 Â 10
7 s
À1 ). Generally, k ISC is highly associated with the energy gaps
between singlet- and triplet-excited states. To experimentally estimate the energy
differences of these derivatives, phosphorescence spectra were measured at low
temperature, as shown in Table 5.1. The obtained similar gaps of singlet- and
triplet-excited states in the range from ca. 0.7 to 0.8 eV presumably do not have
an effect on the k ISC rate constants. Regarding the enhanced fluorescence behaviors
of [7]Hl-NAIQx, we need to consider the following two contents from the above
discussions. The first content is the internal charge-transfer (ICT) emission associated with the asymmetrical dialkylquinoxaline (i.e., electron donor-electron acceptor
pair unit) [28]. The other point includes the planar aromatic structure between the
quinoxaline and phenanthrene units originated from alkylation. This enables to
decelerate the ISC pathway in comparison with the nonplanar structure. The excited
state of [7]Hl-NAIQx largely depends on the planar steric configuration but not the
102
T. Hasobe
carbohelicene, i.e., [7]Hl, which is quite comparable to the previous related
systems of helical molecules [25, 32, 33]. Then, the quantum yields of ISC
(ΦISC) of these compounds were estimated by
1 O 2 phosphorescence utilizing
triplet-triplet energy transfer from these helicene derivatives to O 2 . Noted that
we assume that the quenching process from the singlet-excited state was negligible
[34–37]. Singlet oxygen generated from triplet states of these helicene derivatives
in oxygen-saturated toluene was confirmed by
1 O 2 phosphorescence at ~1270 nm.
Following the reported data of Φ ISC of fullerene (C 60 ) in solution [Φ ISC ¼ 0.96]
[38], C 60 was employed as the standard molecule. The Φ ISC of pristine [7]
carbohelicene (i.e., [7]Hl) was not reported, so far. Regarding the quantum yields
of internal conversion (Φ IC ), we can estimate the yields by subtracting Φ FL and
Φ ISC from the unity. The summarized quantum yields including Φ ISC and Φ IC are
shown in Table 5.1. It should be noted that the total sum of the above three
quantum yields is close to unity within the experimental error although measurement instruments were totally different. The Φ FL of [7]Hl-NAIQx (0.25) is much
larger than those of [7]Hl-Qx (Φ FL ¼ 0.05) and [7]Hl (Φ FL ¼ 0.02) regardless of
the corresponding low Φ ISC values.
Finally, the respective rate constants of these three processes such as fluorescence
emission (k FL ), intersystem crossing (k ISC ), and internal conversion (k IC ) were
estimated as shown in Table 5.1. It should be emphasized that the k FL of [7]HlNAIQx is much larger than that of [7]Hl. In particular, the k FL values such as
6.25 Â 10
7 s
À1 for [7]Hl-NAIQx and 3.01 Â 10
7 s
À1 for [7]Hl-Qx are one order
of magnitude greater than 1.45 Â 10
6 s
À1 for [7]Hl. According to the basic theory of
photochemistry [39], k FL is largely related to the molar extinction coefficients. The ε
values are therefore estimated from the 0–0 absorption bands (ε 0–0 ), as shown in
Table 5.1. The evaluated ε 0–0 values of [7]Hl-NAIQx and [7]Hl-Qx are significantly
larger as compared to [7]Hl, which coincides with the photophysical trends of the
k FL . Then, we also discussed the ISC processes such as the rate constant: k ISC and
quantum yield: Φ ISC . The estimated k ISC constants of [7]Hl-Qx and [7]Hl-NAIQx
are 5.78 Â 10
8 and 1.55 Â 10
8 s
À1 , respectively. In the same manner as k FL , these
rate constants are significantly larger than that of pristine [7]carbohelicene i.e., [7]Hl
(k ISC % 6.5 Â 10
7 s
À1 ). Generally, k ISC is highly associated with the energy gaps
between singlet- and triplet-excited states. To experimentally estimate the energy
differences of these derivatives, phosphorescence spectra were measured at low
temperature, as shown in Table 5.1. The obtained similar gaps of singlet- and
triplet-excited states in the range from ca. 0.7 to 0.8 eV presumably do not have
an effect on the k ISC rate constants. Regarding the enhanced fluorescence behaviors
of [7]Hl-NAIQx, we need to consider the following two contents from the above
discussions. The first content is the internal charge-transfer (ICT) emission associated with the asymmetrical dialkylquinoxaline (i.e., electron donor-electron acceptor
pair unit) [28]. The other point includes the planar aromatic structure between the
quinoxaline and phenanthrene units originated from alkylation. This enables to
decelerate the ISC pathway in comparison with the nonplanar structure. The excited
state of [7]Hl-NAIQx largely depends on the planar steric configuration but not the
102
T. Hasobe