Tailoring intersystem crossing in phosphorus corroles through axial chalcogenation: a detailed theoretical study

Intersystem crossing (ISC) of visible-light absorbing metal-free corrole macrocycles can be greatly tuned by means of suitable chemical functionalization. Axially chalcogenated phosphorus corrole derivatives (X&z.dbd;PCs; X = O, S, Se) are expected to show large spin-orbit coupling (SOC) via the heavy-atom effect and therefore a much improved ISC. Excited-state deactivation of X&z.dbd;PCs including PC is studied using time-dependent optimally tuned range-separated hybrid functionals combined with a polarizable continuum model with toluene as a dielectric medium to account for polar solvent effects. PC and all X&z.dbd;PCs are dynamically stable and also show favourable thermodynamic formation feasibility as confirmed by Gibbs free energy analysis. In spite of the relatively smaller contribution of P and X to the frontier molecular orbitals compared to the tetrapyrrolic ring, SOC is considerably improved due to the heavy-atom effect. While PC shows a one-order larger ISC rate of ∼10 7 s −1 than fluorescence, competitive fluorescence and ISC rates of ∼10 7 s −1 are found for O&z.dbd;PC. In contrast, both S&z.dbd;PC and Se&z.dbd;PC exhibit significantly larger ISC rates of ∼10 9 s −1 and ∼10 13 s −1 , respectively, with much smaller fluorescence rates of ∼10 7 s −1 . Importantly, the first report of anti-Kasha's emission in metal-free corroles is predicted for O&z.dbd;PC with a radiative rate of ∼10 9 s −1 . Furthermore, calculated phosphorescence and ISC rates from the near-degenerate lowest excited triplets to the ground-state suggest millisecond to microsecond triplet lifetimes, signalling towards long-lived excited triplet formation. Overall, all three X&z.dbd;PCs including PC could act as triplet photosensitizers and especially both S&z.dbd;PC and Se&z.dbd;PC are predicted to be the highly efficient ones. Remarkably high intersystem crossing rates are predicted in axially chalcogenated phosphorus corroles using polarization consistent time-dependent optimally-tuned range-separated hybrid coupled with the kinetic rate theory.