Electron Transfer. Shunichi Fukuzumi

back electron transfer at higher temperatures because of the larger activation energy of the former than that of the latter. Such a remarkable result has sparked a flurry of work by others in the field of artificial photosynthesis [61].

Benniston et al. claimed that the triplet excited state of the acridinium ion moiety (³Acr^+*–Mes) might be formed rather than the electron‐transfer state (Acr^·–Mes^·+) and that the energy of ³Acr^+*–Mes is lower than that of Acr^·–Mes^·+ [62]. They reported that the triplet excitation energy of Acr⁺–Mes was 1.96 eV based on the phosphorescence spectrum [62]. If this value were correct, the one‐electron oxidation potential (E_ox) of ³Acr^+*–Mes would be −0.08 V vs. saturated calomel electrode (SCE), which is determined from the one‐electron oxidation potential of the Mes moiety (1.88 V) [60] and the triplet excitation energy (1.96 V). In such a case, electron transfer from the triplet excited state of Acr⁺–Mes to N,N‐dihexylnaphthalenediimide (NIm: E_red = –0.46 V vs. SCE) would be energetically impossible judging from the positive free energy change of electron transfer (0.38 eV). However, the addition of NIm (1.0 × 10⁻³ M) to a PhCN solution of Acr⁺–Mes and laser photoexcitation results in the formation of NIm^·− as detected clearly by the well‐known absorption bands at 480 and 720 nm [63,64], accompanied by the decay of transient absorption at 510 nm due to the Acr^· moiety of the electron‐transfer (ET) state as shown in Figure 4.3a [65]. Similarly, the addition of aniline (3.0 × 10⁻⁵ M) to a PhCN solution of Acr⁺–Mes results in the formation of aniline radical cation (λ_max = 430 nm) [66], accompanied by decay of the Mes^·+ moiety at 500 nm as shown in Figure 4.3b [65]. The rate constant of formation of aniline radical cation was determined to be 5.6 × 10⁹ M⁻¹ s⁻¹, 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