Electron Transfer. Shunichi Fukuzumi

3 Photosynthetic Reaction Center Models

The first candidates of artificial photosynthetic reaction centers are porphyrins, which have been involved in a number of important biological electron‐transfer systems including the primary photochemical reactions of chlorophylls in the photosynthetic reaction centers. Porphyrins contain an extensively conjugated two‐dimensional π‐system and are thereby suitable for efficient electron transfer, because the uptake or release of electrons results in minimal structural and solvation change upon electron transfer, resulting in a small λ value of electron transfer [30]. In addition, rich and extensive absorption features of porphyrinoid systems guarantee increased absorption cross sections and efficient use of the solar spectrum [30]. In contrast with the two‐dimensional porphyrin π‐system, fullerenes contain an extensively conjugated three‐dimensional π‐system [31]. Electron transfer to C₆₀ is highly efficient because of the minimal changes of structure and solvation associated with the electron‐transfer reduction [31–33]. Thus, a series of porphyrin–fullerene linked molecules have been designed and synthesized to mimic the charge‐separation processes in the photosynthetic reaction center, starting from a zinc porphyrin–C₆₀ dyad (ZnP–C₆₀), a ferrocene–zinc porphpyrin–C₆₀ triad (Fc–ZnP–C₆₀), a zinc porphyrin‐free base porphyrin–C₆₀ triad, up to a tetrad (Fc–ZnP–H₂P–C₆₀) [34–39].

The driving force dependence of the electron‐transfer rate constants (k_ET) of these dyad, triad, and tetrad molecules for CS and CR processes is shown in Figure 3.1, where log k_ET is plotted against the driving force (−ΔG_ET) [39]. The lines in Figure 3.1 represent the best fit to Eq. (2.1) (ZnP–C₆₀: λ = 0.66 eV, V = 3.9 cm⁻¹; Fc–ZnP–C₆₀, Fc–H₂P–C₆₀, and ZnP–H₂P–C₆₀: λ = 1.09 eV, V = 0.019 cm⁻¹; Fc–ZnP–H₂P–C₆₀: λ = 1.32 eV, V = 0.00017 cm⁻¹) [39]. The λ‐value increases, whereas the V value decreases with increasing edge‐to‐edge distance in the order of the dyad (R_ee = 11.9 Å), the triad (R_ee = 30.3 Å), and the tetrad (R_ee = 48.9 Å). Such an increase in the λ value with increasing distance results from an increase in the solvent reorganization energy (λ_s), because the λ_s value is known to increase with increasing distance between an electron donor and an acceptor as given by Eq. (3.1):

(3.1)

      where r₁ and r₂ are the radii of the donor and acceptor, r₁₂ is the donor–acceptor distance, ε is the dielectric constant, and n is the refractive index [6,40].

Figure 3.1 Driving force (−ΔG_ET) dependence of intramolecular electron‐transfer rate constants (k_ET) in ZnP–C₆₀ (CS [○]; CR [□]), Fc–ZnP–C₆₀ (•), Fc–H₂P–C₆₀ (▴), ZnP–H₂P–C₆₀ (▪), and Fc–ZnP–H₂P–C₆₀ (△).

      Source: Imahori et al. 2001 [39]. Reproduced with permission of American Chemical Society.

Figure 3.2 Edge‐to‐edge distance (R_ee) dependence of optimal electron‐transfer rate constants (log[k_ET(optimal)/s⁻¹]) in ZnP–C₆₀ dyad, Fc–ZnP–C₆₀ triad, Fc–H₂P–C₆₀ triad, ZnP–H₂P–C₆₀ triad, and Fc–ZnP–H₂P–C₆₀ tetrad. The line represents the best fit to Eq. (2.1) (β = 0.60 Å⁻¹, V₀ = 210 cm⁻¹).

In the case of the dyad, the CR rate in ZnP^·+–C₆₀^·− in the Marcus inverted region is much slower than the CS rate constants from both the singlet and triplet excited states in the Marcus normal region (Figure 3.1). This enables subsequent electron transfer from Fc to ZnP^·+ in the triad (Fc–ZnP^·+–C₆₀^·−) and from ZnP to H₂P^·+ in ZnP–H₂P^·+–C₆₀^·− to produce the final CS state, Fc⁺–ZnP–C₆₀^·− and ZnP^·+–H₂P–C₆₀^·−, in competition with the back electron transfer in the initial CS states. In the case of Fc⁺–ZnP–H₂P–C₆₀^·−, the charges are separated at a long distance (R_ee = 48.9 Å) [39]. The lifetime of the resulting CS state at such a long distance in a frozen benzonitrile (PhCN) has been determined as 0.38 seconds [39]. Similar tetrad and pentad compounds were reported to attain long‐lived CS states up to 1.7 seconds [40–42]. It should be noted that the CS lifetime is temperature independent, since the CR process is at the Marcus top region (Figure 3.1) [39]. This is the first example to achieve a CS lifetime that is comparable to that observed for the bacterial photosynthetic reaction center. However, such an extremely long CS lifetime could only be determined in frozen media, since in condensed media bimolecular back electron transfer between two Fc⁺–ZnP–H₂P–C₆₀^·− is much faster than the unimolecular CR process [39]. The maximum k_ET values (k_ETmax) of dyads, triads, and tetrads in the Marcus plot (Figure 3.2) are correlated with the edge‐to‐edge distance (R_ee), separating the radical ions, according to Eq. (3.2) [39]:

(3.2)

Скачать книгу