which is close to the diffusion rate constant in PhCN [60]. Thus, the photogenerated state of Acr+–Mes has both the reducing and oxidizing abilities to reduce NIm and to oxidize aniline, respectively. Only the electron‐transfer state (Acr·–Mes·+) has such a dual ability, which has now been well confirmed for electron‐transfer oxidation of many electron donors with Acr·–Mes·+ and electron‐transfer reduction of many electron acceptors such as hexyl viologen, p‐benzoquinone, and Selectfluor (fluorinating reagent) with Acr·–Mes·+ [67–70]. However, this conclusion is contradictory to the reported triplet energy (1.96 eV), which is lower in energy than the ET state [62]. This contradiction comes from an acridine impurity, which may be left in the preparation of by Benniston et al. who synthesized the compound via methylation of the corresponding acridine [62]. The yield of acridinium ion is about 50–70% after reflux at high temperature for a few days [62]. In such a case, acridine may remain as an impurity even after purification of the acridinium ion by recrystallization. When Acr+–Mes was prepared by the Grignard reaction of 10‐methyl‐9(10H)acridone with 2‐mesitylmagnesium bromide, there was no acridine [60]. Thus, Acr+–Mes without acridine afforded no phosphorescence spectrum in both deaerated glassy 2‐MeTHF and ethanol at 77 K. It is well known that acridine derivatives exhibit phosphorescence at 15650–15850 cm−1 [71]. It was confirmed that the phosphorescence maximum of 9‐phenylacridine in glassy 2‐MeTHF at 77 K afforded the same spectrum reported by Benniston et al. [62] Thus, the reported low triplet energy of Acr+–Mes, which contradicts our results on the long‐lived electron‐transfer state, results from the acridine impurity contained in Acr+–Mes used by Benniston et al. who also reported that photoirradiation of a PhCN solution of Acr+–Mes results in the formation of the acridinyl radical (Acr·–Mes) [62]. They implied that this stable radical species could be mistaken as a long‐lived electron‐transfer state [62]. When PhCN is purified, however, no change in the absorption spectrum is observed [60,65]. The formation of Acr·–Mes results from electron transfer from a donor impurity contained in unpurified PhCN (e.g. aniline) to the Mes·+ moiety of Acr·–Mes·+ as indicated in Figure 4.3b. Even an extremely small amount (5.0 × 10−5 M) of aniline is enough to react with Acr·–Mes·+ to produce Acr·–Mes, which is stable due to the bulky Mes substituent, because the lifetime of Acr·–Mes·+ is long enough to react with such a small concentration of an electron donor. It should be noted that no net photochemical reaction occurs without a donor impurity because the long‐lived Acr·–Mes·+ decays via bimolecular back electron transfer to the ground state [60,65]. Thus, misleading effects of impurities indeed result from the long‐lived electron‐transfer state, which has both oxidizing and reducing abilities.
Figure 4.3 Transient absorption spectra of Acr+–Mes (5.0 × 10−5 M) in deaerated MeCN at 298 K taken at 2 and 20 μs after laser excitation at 430 nm in the presence of (a) N,N‐dihexylnaphthalenediimide (1.0 × 10−3 M) or (b) aniline (3.0 × 10−5 M). Inset: Time profiles of the absorbance decay at 510 nm and the rise at 720 nm and (b) the decay at 500 nm and the rise at 430 nm.
Source: Fukuzumi and coworkers 2005 [65]. Reproduced with permission of Royal Society of Chemistry.
In contrast to the photoirradiation of a purified PhCN solution of Acr+–Mes at 298 K, which results in no change in the absorption spectrum (Figure 4.4a), when the photoirradiation of the same solution was performed at low temperatures (213–243 K) with a 1000 W high‐pressure mercury lamp through the UV light cutting filter (>390 nm) and the sample was cooled to 77 K, the color of the frozen sample at 77 K was clearly changed as shown in the inset of Figure 4.4b. When a glassy 2‐methyltetrahydrofuran (2‐MeTHF) is employed for the photoirradiation of Acr+–Mes at low temperature, the resulting glassy solution measured at 77 K affords the absorption spectrum due to the electron‐transfer state, which consists of the absorption bands of the Acr· moiety (500 nm) and the Mes·+ moiety (470 nm) as shown in Figure 4.4b. No decay of the absorption due to the electron‐transfer state in Figure 4.2b was observed until liquid nitrogen ran out [65].
The long lifetime of the ET state of Acr+–Mes has allowed observing the structural change in the Acr+–Mes(ClO4−) crystal upon photoinduced ET directly by using laser pump and X‐ray probe crystallographic analysis (Figure 4.5) [72]. Upon photoexcitation of the crystal of Acr+–Mes(ClO4−), the N‐methyl group of the Acr+ moiety was bent and its bending angle was 10.3(16)° when the N‐methyl carbon moved 0.27(4) Å away from the mean plane of the ring as shown in Figure 4.5 [72]. This bending is caused by the photoinduced electron transfer from the Mes moiety to the Acr+ moiety to produce Acr·–Mes·+, because the sp2 carbon of the N‐methyl group of Acr+ is changed to the sp3 carbon in the one‐electron reduced state (Acr·) [72]. The bending of the N‐methyl group by photoexcitation was accompanied by the rotation and movement of the ClO4− by the electrostatic interaction with the Mes·+ moiety (Figure 4.5) [72]. Thus, the observed bending of the N‐methyl group and the movement of ClO4− provide strong evidence for the generation of the ET state of Acr+–Mes upon photoexcitation. In contrast to the case of Acr+–Mes, no geometrical difference was observed upon photoexcitation of Acr+–Ph, which does not afford the ET state [72].
Figure 4.4 (a) UV–vis spectral change in the steady‐state photolysis of a deaerated PhCN solution of Acr+–Mes (3.3 × 10–5 M). Spectra were recorded at 90‐second interval. (b) UV–vis absorption spectra obtained by photoirradiation with high‐pressure mercury lamp of deaerated 2‐MeTHF glasses of Acr+–Mes at 77 K. Inset: picture images of frozen PhCN solutions of Acr+–Mes before and after photoirradiation at low temperatures and taken at 77 K.
Figure 4.5 (a) Diagram of the reaction cavity: (left) diagram around the N‐methyl group, with numbers indicating the volumes of the divided cavity formed by the dotted line; (right) drawing around ClO4−. (b) Cooperative photoinduced geometrical changes. The dashed line indicates the suggested Mes·+⋯ClO4− electrostatic interaction.
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