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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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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 ∼10⁷ s^-1 than fluorescence, competitive fluorescence and ISC rates of ∼10⁷ s^-1 are found for OPC. In contrast, both SPC and SePC exhibit significantly larger ISC rates of ∼10⁹ s^-1 and ∼10¹³ s^-1, respectively, with much smaller fluorescence rates of ∼10⁷ s^-1. Importantly, the first report of anti-Kasha's emission in metal-free corroles is predicted for OPC with a radiative rate of ∼10⁹ 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PCs including PC could act as triplet photosensitizers and especially both SPC and SePC are predicted to be the highly efficient ones.