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 (XPCs; 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 XPCs 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 XPCs 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 OPC. In contrast, both SPC and SePC 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 OPC 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 XPCs including PC could act as triplet photosensitizers and especially both SPC and SePC are predicted to be the highly efficient ones.