Forster energy transfer quenching

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

The fluorescence for adsorbed molecules was apparently quenched due to the reduced lifetime of the excited state in a molecule adsorbed at the metal surface as discussed earlier. Quenching due to the energy transfer to the metal can also be observed in molecules that are desorbed but reside in the electrolyte layer near the electrode surface. According to the so-called Forster energy transfer mechanism, observed in the membrane studies [35,40] and in Langmuir-Blodgett films [41], the change of the fluorescence intensity with separation of the fluorescent molecule from the quencher (metal) is described by the formula... 

Fig. 1.25 Orbital comparison of long- and short-range quenching of an excited state (D ) and a ground state acceptor (A) by a Forster energy transfer (long-range), b Dexter energy transfer (short-range) and c photoinduced electron transfer (short range)...

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