Photoinduced electron transfer, singlet charge-separated state

Immediately upon excitation of an IPCT band with a fs laser at 400 nm, transient absorption was observed for both salts in solutions with a peak at about 600 nm, characteristic of 4,4/-bipyridinium radical cations. Figure 20 shows the transient absorption spectra of PV2+(I )2 in methanol solution. A marked increase in the absorbance of the 4,4/-bipyridinium radical cations took place within 1 ps after excitation. 4,4/-Bipyridinium radical cations were thus formed in a fs time scale by the photoinduced electron transfer from a donor I- to an acceptor 4,4/-bipyridinium upon IPCT excitation [48], The time profiles of transient absorption at 600 nm are shown in Fig. 21 for (a) PV2+(I )2 in a film cast from DME and (b) PV2+(TFPB )2 in DME solutions. Both of them showed a very rapid rise in about 0.3 ps, which was almost the same as the time resolution of our fs Ti sapphire laser measurement system with a regenerative amplifier. Similar extremely rapid formation of 4,4/-bipyridinium radical cations was observed for PV2+(I )2 salts in methanol and dimethylsulfoxide solutions upon IPCT excitation, respectively. These results demonstrated that the charge separated 4,4/-bipyridinium radical cations were formed directly upon IPCT excitation because of the nature of IPCT absorption bands (that the electrons correlated with the IPCT band are transferred partially at the ground state and completely at the excited state). Such a situation is very different from usual photochromism which is caused by various changes of chemical bonds mainly via the excited singlet state. No transient absorption was observed for PV2+(I )2... 

Interporphyrin photoinduced electron transfer can of course be the basis of light-driven charge separation in more complex systems. For example, triad 42 is similar in structure to dyad 1, with the addition of a carotenoid moiety, which can serve as a secondary donor as in 35 [13]. Excitation of 42 in dichloromethane with a laser pulse at 590 nm creates two porphyrin first excited singlet states, C- Pzn-Pp and C-Pzn- Pp. As with 1, both states decay at least in part by photoinduced electron transfer to give an initial C-Pzn -Pp charge-separated state. This state can recombine to the ground state, but as with the other triads discussed above, electron donation from the carotene competes with this to yield a final C +-Pzn-Pp state with an overall quantum yield of 0.32 with 590-nm excitation. The lifetime of this final state is 240 ns. 

Excitation of either porphyrin subunit in this bisporphyrin results in electron transfer from zinc to gold porphyrins to form the respective %-radical ions. For the zinc porphyrin, both singlet and triplet excited states function as electron donors whereas for the gold porphyrin intersystemcrossing to the triplet manifold is quantitative and the triplet acts as an electron acceptor. Consequently, virtually every photon absorbed by the bisporphyrin results in charge separation and the compound provides a unique opportunity to compare singlet and triplet state photoinduced electron transfer processes. 

The photocurrent generation in the present system is initiated by photoinduced charge separation from the porphyrin excited singlet state (1H2P /H2P+ = -0.7 V vs. NHE) [78] in the dendrimer to C60 (C60/Cf>0 = -0.2 V vs. NHE) [78] in the porphyrin dendrimer-C60 complex rather than direct electron injection to conduction band of Sn02 (0 V vs. NHE) system [91] The reduced C60 injects electrons into the Sn02 nanocrystallites, whereas the oxidized porphyrin (H2P/H2P+ = 1.2 V vs. NHE) [78] undergoes electron-transfer reduction with iodide (I3 /I = 0.5 V vs. NHE) [78] in the electrolyte system [91]. 

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