Charge separation photoinduced electron transfer, lifetime

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

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. 

Fig. 16.8 Charge recombination lifetimes in the compounds shown in the inset in dioxane solvent. (J. M. Warman, M. P. de Haas, J. W. Verhoeven, and M. N. Paddon-Row, Adv. Chem. Phys. 106, Electron transfer—from isolated molecules to bio-molecules, Part I, edited by J. JortnerandM. Bixon (Wiley, New York, 1999). The technique used is time-resolved microwave conductivity (TRMC), in which the change in dielectric response of a solution is monitored following photoinduced electron transfer—a charge separation process that changes the solute molecular dipole. The lifetimes shown as a function of bridge length (number of a-bonds separating the donor and acceptor sites in the compounds shown in the inset) are for the back electron transfer (charge recombination) process.

From ratios of quantum yield expressions (equation 5) for triads 1 and 2 and the rate constants calculated above, it is possible to compare the yields of key pathways in the two triads. Numerical evaluation of these ratios requires the assumption that the yield of step 7 is unity. This is reasonable as the lifetime of analogous charge-separated states in triads lacking proton transfer is at least 70 ns and proton transfer is subpicosecond in triad 1. Because the yield of photoinduced electron transfer (step I) is essentially unity, the yield of electron donation to the porphyrin radical cation by the carotenoid in 2, which is analo-... 

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