LITMIR.BIZ

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].