Fig. 4. Schematic representation of thermodynamic viability of mechanistic steps.

a–l Mechanistic diagrams with ΔG of key steps denoted in comparison to corresponding thresholds for PET-RAFT polymerisation via OQP (a–d), RQP (e–h) and O-RQP (i–l), catalysed by ZnOEP (a, e, i), ZnTPP (b, f, j), TPP (c, g, k) and ZnTSP (d, h, l), respectively. Green is favourable, orange is less efficient and red indicates inertness. Coloured values are the ΔG of the corresponding process in kcal/mol, in conjunction with the threshold in grey (derived in the “Methods” section). Coloured percentage values are Φ_SET,GS-GS of the corresponding process. Bottom: Bar charts illustrating ΔG of the critical catalytic steps regarding the activation of OQP (OQP-I, m) and RQP (RQP-I, n and RQP-II, o) for ZnOEP, ZnTPP, TPP and ZnTSP on a comparative basis, with thermodynamic thresholds in kcal/mol (green dotted line: activation threshold; red dotted line: activation limit). Φ_ET,GS–GS: quantum yield of the single electron transfer process between two ground states, defined by Supplementary Eq. 1. OQP oxidative quenching pathway, RQP reductive quenching pathway, O-RQP oxygen-mediated reductive quenching pathway. ES–GS SET single electron transfer from an excited state to a ground state, GS–GS SET single electron transfer between two ground states, CT-assisted SET single electron transfer assisted by charge transfer in the donor–acceptor complex.