RESUMO
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 (CRkT â¼ 1 × 108 s-1 or slower) can be correlated to proton hyperfine interactions (also called the radical pair mechanism), but substantially faster rates (CRkT â¼ 1 × 109 up to 2.5 × 1010 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.
RESUMO
We report on experimental high-resolution spectroscopic studies in combination with ab initio computational studies that investigate the excited-state dynamics of methyl-4-hydroxycinnamate thioester and (5-hydroxyindan-(1E)-ylidene)acetic acid, derivatives of the photoactive yellow protein (PYP) chromophore. These studies aim to elucidate (a) how the thioester moiety influences the photophysics and photochemistry of the p-coumaric acid chromophore and (b) to what extent rotation of the single bond adjacent to the phenyl ring is involved in the decay dynamics of the electronically excited states. The experimental studies show that sulfur substitution leads to broad, unstructured excitation spectra that contrast sharply with the well-resolved spectra of compounds with an oxygen-based ester. Furthermore, internal conversion to the lower-lying nπ* state is absent. The absence of this decay channel is rationalized by quantum-chemical calculations that reveal that in the nπ* state of the thio compounds the molecule exhibits a large out-of-plane "kink" at the sulfur atom. Franck-Condon simulations of the excitation spectra of the V(ππ*) state highlight the activity of various vibrational modes in the neutral chromophore and indicate that upon sulfur substitution internal conversion to the ground state occurs at a significantly higher rate. The similarities observed in the excitation spectra and decay dynamics of the locked and unlocked compounds suggest that in the present experiments single-bond torsion does not show up prominently. The conclusion that for the isolated molecule double-bond torsion is dominating the excited-state dynamics is tentatively confirmed by the quantum-chemical calculations.
