Making triplets from photo-generated charges: observations, mechanisms and theory

Triplet formation by charge recombination is a phenomenon that is encountered in many fields of the photo-sciences and can be a detrimental unwanted side effect, but can also be exploited as a useful triplet generation method, for instance in photodynamic therapy. In this Perspective we describe the various aspects that play a role in the decay of charge separated states into local triplet states. The observations and structures of a selection of (pre-2015) molecular electron donor-acceptor systems in which triplet formation by charge recombination occurs are reported. An overview is given of some more recent systems consisting of BODIPY dimers, and BODIPYs attached to various electron-donor units displaying this same triplet formation process. A selection of polymer-fullerene blends in which triplet formation by (non-geminate) charge recombination has been observed, is presented. Furthermore, in-depth information regarding the mechanistic aspects of triplet formation by charge recombination is given on spin dephasing, through hyperfine interactions, as well as on spin-orbit coupling occurring simultaneously with charge recombination. The limits and constraints of these factors and their role in intersystem crossing are discussed. A pictorial view of the two mechanisms is given and this is correlated to aspects of the selection rules for triplet formation, the so-called El-Sayed rules. It is shown that the timescale of triplet formation by charge recombination is indicative for the mechanism that is responsible for the process. The relatively slow rates ( CR k T ∼ 1 × 10 8 s −1 or slower) can be correlated to proton hyperfine interactions (also called the radical pair mechanism), but substantially faster rates ( CR k T ∼ 1 × 10 9 up to 2.5 × 10 10 s −1 or faster) have to be correlated to spin-orbit coupling effects. Several examples of molecular systems showing such fast rates are available and their electron donor and acceptor orbitals display an orthogonal relationship with respect to each other. This orientation of (the nodal planes of) the π-orbitals of the donor and acceptor units is correlated to the mechanisms in photodynamic agents and photovoltaic blends. Photo-excitation of electron donor-acceptor systems can lead to the generation of a charge separated state (CT). Sometimes the charge recombination occurs mainly to the local triplet excited state (T 1 ). How does the spin flip?