[47]. This indicates that electron transfer from 1ZnCh* to C60 occurs rapidly to form the CS state, ZnCh·+–C60·–. The CS state decays via back electron transfer to the ground state rather than to the triplet excited state, because the CS state is lower in energy (1.26 eV) than the triplet excited states of both C60 (1.50 eV) and ZnCh (1.36–1.45 eV) [47]. The lifetime of the CS state is determined as 230 μs at 298 K. The large temperature dependence of the CS lifetime is observed and the lifetime of the CS state at 123 K becomes as long as 120 seconds [47].
Covalently and non‐covalently linked porphyrin–quinone dyads constitute one of the most extensively investigated photosynthetic reaction center models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photosynthetic electron transfer [52–54]. Unfortunately, the CR rates of the CS state of porphyrin–quinone dyads are also fast and the CS lifetimes are mostly on the order of picoseconds or subnanoseconds in solution [52–54]. In general, a three‐dimensional C60 is superior to a two‐dimensional quinone in terms of the smaller reorganization of electron transfer of C60 as compared with quinone (vide supra) to attain the long‐lived CS state [31–33,55]. When the geometry between a porphyrin ring and quinone is optimized by using hydrogen bonds, which can also control the redox potentials of quinones, however, a surprisingly long lifetime up to one microsecond has been attained [56]. In a series of ZnP–n–Q (n = 3, 6, 10) in Scheme 4.3, the hydrogen bond between two amide groups provides a structural scaffold to assemble the donor (ZnP) and the acceptor (Q) moiety, leading to attaining the long‐lived CS state [56].
Scheme 4.3 Zinc porphyrin–quinone linked dyads (ZnP–n–Q; n = 3, 6, 10) with hydrogen bonds.
Source: Okamoto and Fukuzumi 2005 [56]. Reproduced with permission of American Chemical Society.
As described above, the closely linked donor–acceptor dyads afford long‐lived CS states. As long as porphyrins and C60 are used as components of donor–acceptor dyads, however, the low lying triplet energies of porphyrins and C60 have precluded to attain the long‐lived CS states with a higher energy than the triplet energies [35]. In such a case, it is highly desired to find a chromophore that has a high triplet energy and a small λ value of electron transfer. Among many choromophores, acridinium ion is the best candidate for such a purpose, since the λ value for the electron self‐exchange between the acridinium ion and the corresponding one‐electron reduced radical (acridinyl radical) is the smallest (0.3 eV) among the redox‐active organic compounds [57]. Another important property of acridinium ion is a high triplet excited energy [58,59]. Thus, an electron donor moiety (mesityl group) is directly connected at the 9‐position of the acridinium ion to yield 9‐mesityl‐10‐methylacridinium ion (Acr+–Mes) [60], in which the solvent reorganization of electron transfer is minimized because of the short linkage between the donor and acceptor moieties. The X‐ray crystal structure of Acr+–Mes is shown in Figure 4.2a [60]. The dihedral angle made by aromatic ring planes is perpendicular and therefore there is no π conjugation between the donor and acceptor moieties. Indeed, the absorption and fluorescence spectra of Acr+–Mes are superpositions of the spectra of each component, i.e. mesitylene and 10‐methylacridinium ion. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals of Acr+–Mes calculated by a density functional theory (DFT) method with Gaussian 98 (B3LYP/6‐31G* basis set) are localized on mesitylene and acridinium moieties (Figure 4.2b,c), respectively [60]. The energy of the electron‐transfer state (Acr·–Mes·+) in PhCN is determined by the redox potentials of each component of Acr+–Mes as 2.37 eV [60].
Figure 4.2 (a) X‐ray crystal structure of Acr+–Mes. (b) HOMO and (c) LUMO orbitals calculated by DFT method with Gaussian 98 (B3LYP/6‐31G* basis set). (d) Plot of kBET/T vs. T−1.
Source: Fukuzumi et al. 2004 [60]. Reproduced with permission of American Chemical Society.
Photoirradiation of a deaerated PhCN solution of Acr+–Mes by a nanosecond laser light at 430 nm results in the formation of Acr·–Mes·+ with a quantum yield close to unity (98%) via photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the acridinium ion moiety (1Acr+*–Mes) [60]. The intramolecular back electron transfer from the Acr· moiety to the Mes·+ moiety in Acr·–Mes·+ was too slow to compete with the intermolecular transfer (kBET) in Figure 4.2d, agreeing with the Marcus equation in the deeply inverted region (Eq. (2.1)). The lifetime of the electron‐transfer state in frozen medium becomes longer with decreasing temperature to approach a virtually infinite value at 77 K [60]. However, the decay time profile of Acr·–Mes·+ in solution obeyed second‐order kinetics (NOT first‐order kinetics) [60]. This is the same as the case of Fc+–ZnP–H2P–C60·−, in which bimolecular back electron transfer predominates due to the slow intramolecular back electron transfer (vide supra) [39]. In contrast, the decay of Acr·–Mes·+ obeys first‐order kinetics in PhCN at high temperatures [60]. This indicates that the rate of the intramolecular back electron transfer of Acr·–Mes·+ becomes much faster than the rate of the intermolecular 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 (Eox) 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: Ered = –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 (λmax = 430 nm) [66], accompanied by decay of the Mes·+ moiety at 500 nm as shown in Figure
