Determination of long-distance intramolecular triplet energy-transfer rates. Quantitative comparison with electron transfer

Abstract

Recent efforts in these laboratories have been directed toward understanding the factors governing long distance intramolecular electron transfer (ET).2 In the models chosen for study the electronic coupling between donor and acceptor is sufficiently weak to assure nonadiabatic reactions. It occurred to us that long distance triplet energy transfer by the Dexter mechanism3 (TT) of which there are several examples in the literature4 should exhibit features similar to nonadiabatic electron transfer because both reactions are governed by the same theory of radiationless transitions. We therefore have started a program aimed at finding quantitative similarities and differences in these two processes when studied on directly comparable systems. Also, with one exception,5 the absolute rates of intramolecular triplet energy transfer have never been measured directly in liquid solution. In this communication we report our first results and conclusions.One of the series studied in ET, 1, involves compounds in which a 4-biphenylyl group (D) is connected via a rigid spacer (Sp) with a 2-naphthyl group (A).28 The spacers used were cyclohexane 1: A = 2-naphthyl; D = 4-biphenylyl 2: A = 2-naphthyl; D = 4-benzophenoneyl A-Sp-D and decalin ring systems. Simply by replacing the 4-biphenylyl group with 4-benzophenonyl, the series can by converted to an almost ideal series 2 for triplet energy transfer. The spacers are

Triplet transport in thin films: fundamentals and applications

“…It has been found that the rate of triplet transfer is proportional to the product of the diffusivity of the electrons and holes in amorphous films, in organic crystals, and in solution, provided the molecular reorganization energy is taken into account. [44][45][46][47] Closs, Scholes, and others have shown that the rate of TET can be estimated from the product of the rate of electron transfer and hole transfer. 46 As shown in Fig.…”

The fragment spin difference scheme for triplet-triplet energy transfer coupling

“…[20][21][22] Dexter's exchange coupling offers a simple physical insight into the exponential distance dependence observed in TET rates. 23,24 The overlap contribution was often neglected in earlier works 19,20 since it was assumed to be small in its magnitude. Although the Dexter theory offers a simple route to calculate the TET coupling, early theoretical calculations using the Dexter exchange coupling alone did not agree with experimental results.…”

A Structural Basis for Electron Transfer in Bacterial Photosynthesis

“…In this regard, Closs et a/. 31 have shown a connection between the matrix elements of electron transfer and triplet energy transfer using a frontier molecular orbital approach. Consequently, another method of examining the matrix elements of electron transfer is by indirect inference from the rate constants for triplet energy transfer.…”

“…Ultimately both approaches will require incorporation of the precise structural details of the molecular complex in which electron or triplet transfer occurs. In this paper no rate constants are measured; instead, the spectroscopic properties of excited singlet states, excited triplet states and cation states are described that can be interpreted in terms of frontier molecular orbitals using an approach similar to that of Closs et al 31 for transfer rates.…”