Steric Control of Photocatalytic Electron Transfer: A Predictive Framework Using Sterimol Parameters

Steric effects strongly influence excited-state electron transfer, yet accurately predicting their impact remains challenging. In this study, we quantified the steric bulk of 11 axial substituents on a silicon phthalocyanine photocatalyst using Sterimol parameters, and fluorescence-quenching experiments were performed with N,N-diisopropylethylamine to evaluate the influence of steric effects on electron-transfer efficiency. Multiparameter linear regression analysis revealed a correlation between the Sterimol parameters and the Stern-Volmer quenching constant (K_SV), with distinct contributions from the Sterimol descriptors wB₁, wB₅, and wL. Among these descriptors, the minimum substituent width (wB₁) was identified as the dominant steric parameter. Leave-two-out cross-validation demonstrated reasonable predictive performance within the branching catalyst subset, highlighting the potential of this Sterimol-based model to estimate K_SV values for new structurally related silicon phthalocyanine photocatalysts. This work serves as a proof-of-concept for using Sterimol parameters to quantify steric effects in photocatalyst quenching and establish predictive structure-property relationships that may be extended to other photocatalyst classes.