4
Electron Donor-Acceptor Dyads

Multistep electron-transfer processes have been utilized to attain a long distance charge separation (CS), mimicking the natural photosynthetic reaction center (vide supra). However, a significant amount of energy is lost during the multistep electron-transfer processes to reach the final CS state [11-15]. In photosynthesis a two-step photoexcitation, the so-called "Z-scheme," is thereby required to recover the energy loss via the multistep electron-transfer processes and to gain strong oxidizing power to oxidize water as well as high reducing power to reduce NAD+ coenzyme [4]. The design and synthesis of molecular machinery mimicking such an elaborated "Z-scheme" in nature seems far beyond our capability, and even if it could be done, the synthetic cost would certainly preclude any type of practical application. Thus, it is highly desired to design simple molecular electron donor-acceptor dyads that are capable of fast CS but can retain slow charge recombination (CR). Theoretically, it is possible to obtain such an electron donor-acceptor dyad, because the CS lifetime increases with increasing driving force of electron transfer in the Marcus inverted region (vide supra). However, the driving force of electron transfer should be lower than the triplet excited state of one of the components of donor-acceptor dyads. Otherwise, the CS state would decay rapidly to the triplet excited state in the Marcus normal region rather than to the ground state in the Marcus inverted region [35].

A number of simple donor-acceptor dyads have been designed and synthesized to attain long-lived CS state, where the donor and acceptor molecules are linked with a short spacer to minimize the solvent reorganization energy [45-50]. Efficient photoinduced electron transfer occurs in a zinc imidazoporphyrin-C60 dyad (ZnImP-C60) with a short linkage to form the CS state (ZnImP·+-C60·-) with the rate constant of 1.4?×?1010?s-1 (Scheme 4.1) [45]. The CS state (1.34?eV) is lower in energy than both the triplet excited states of C60 (1.50?eV) and ZnImP (1.36?eV) [45]. The CS state, produced upon photoexcitation of ZnImP-C60, is detected by the transient absorption spectrum, which has absorption bands at 700 and 1040?nm due to ZnImP·+ and C60·-, respectively [45]. The CS state decays by back electron transfer to the ground state, obeying first-order kinetics with a rate constant of 3.9?×?103?s-1 (the lifetime is 260?µs) at 298?K [45]. At 278?K the lifetime of the CS state was determined as 310?µs, which is much longer than those of conventional donor-acceptor dyads with longer spacers [7-9].

Scheme 4.1 Formation of a long-lived CS state of a zinc imidazoporphyrin-C60 dyad (ZnImP-C60) with a short linkage (Ar = 3,5-But2C6H3).

Source: Kashiwagi et al. 2003 [45]. Reproduced with permission of American Chemical Society.

An electron donor-acceptor dyad linked with a short spacer containing Au(III) and Zn(II) porphyrins (ZnPQ-AuPQ+ in Scheme 4.2) also affords a long-lived electron-transfer state with a lifetime of 10?µs in nonpolar solvents such as cyclohexane [46]. The introduction of quinoxaline to the gold porphyrin results in a lowering of the electron-transfer state energy. In contrast to the case of neutral donor-acceptor dyads, the energy of the electron-transfer state (ZnPQ·+-AuPQ) becomes smaller in a less polar solvent, which is lower than the energies of the triplet excited states of ZnPQ (1.32?eV) and AuPQ+ (1.64?eV) [46]. Photoinduced electron transfer occurs from the singlet excited state of the ZnPQ (1ZnPQ*) to the metal center of the AuPQ+ moiety to produce ZnPQ·+-AuIIPQ. The observed long lifetime of ZnPQ·+-AuIIPQ results from a small reorganization energy for the metal-centered electron transfer of AuPQ+ in nonpolar solvents due to the small change in solvation upon electron transfer as compared with that in polar solvents [46]. In a polar solvent such as benzonitrile (PhCN), no CS state was observed, but instead only the triplet-triplet absorption due to 3ZnPQ*-AuPQ+ 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-But2C6H3).

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 ? value of electron transfer. Among many choromophores, acridinium ion is the best candidate for such a purpose, since the ? 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...