On the role of spin correlation in the formation, decay, and detection of long-lived, intramolecular charge-transfer states

This review presents some of the efforts that have been made over the last decades to produce systems in which photo-excitation leads to one or more intramolecular electron transfer events ultimately resulting in a charge-transfer (CT) excited state with a relatively long lifetime. This process is generally considered as a mimic of natural photosynthesis and is not only of relevance in relation to solar energy conversion but also in relation to perspectives such as molecular information storage, molecular electronics, and molecular photonics. A long-lived CT state in general may be considered as a weakly coupled radical (ion)pair and in this review we focus especially on the consequences of the eventual electron spin correlation in that radical (ion)pair. If substantial spin–spin interaction is still present, such as in compact dyads, CT states can be assigned pure singlet or triplet configurations (1CT, 3CT) and as we demonstrate this configuration has significant influence on the CT lifetime because charge recombination from 3CT is spin forbidden. For small spin–spin interaction such as is typical for CT states in which the radical sites are further removed from each other – e.g., in triads, tetrads, etc., – rapid interconversion of 1CT and 3CT becomes possible especially via a hyperfine interaction (HFI) driven mechanism. This HFI driven mechanism is strongly influenced by external magnetic fields, which allows sensitive detection of the actual spin–spin interaction via magnetic field effects on the electron transfer kinetics, as well as via time-resolved EPR and field-dependent CIDNP. Examples of such studies on artificial multichromophoric electron transfer systems are presented and the results are discussed.