Electron Transfer. Shunichi Fukuzumi

was observed [46]. The absence of an observable CS state in PhCN is ascribed to the much slower photoinduced electron transfer due to the large reorganization energy as compared with that in nonpolar solvents allowing an efficient intersystem crossing process in the ZnPQ–AuPQ⁺ dyad to produce the triplet excited state ³ZnPQ^*–AuPQ⁺ [46].

Scheme 4.2 Formation of a long‐lived CS state of ZnPQ–AuPQ⁺ in nonpolar solvents (Ar = 3,5‐Bu^t₂C₆H₃).

      Source: Fukuzumi et al. 2003 [46]. Reproduced with permission of American Chemical Society.

Figure 4.1 Structure of a closely linked ZnCh–C₆₀ dyad.

      Source: Ohkubo et al. 2004 [47]. Reproduced with permission of John Wiley & Sons.

A closely linked zinc chlorin–fullerene dyad (ZnCh–C₆₀ in Figure 4.1) affords a longer CS lifetime as compared with other zinc chlorin–fullerene dyads with longer spacers [47–51]. A deoxygenated PhCN solution containing ZnCh–C₆₀ gives rise upon a 388 nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [47]. The decay rate constant was determined as 1.0 × 10¹¹ s^–1, which agrees with the value determined from the fluorescence lifetime measurements [47]. The decay of absorbance at 460 nm due to ¹ZnCh^* is accompanied by an increase in absorbance at 590 nm due to ZnCh^·+ [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 k_BET/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 (k_BET) 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