The Effect of Steric Hindrance on Triplet-Triplet Energy Transfer Rates: A Predictive Model Based on Spin-Density-Bearing van der Waals Surfaces.

Triplet-triplet energy transfer (TTET) via the Dexter mechanism is central to many photochemical applications, yet a quantitative structure-kinetic relationship for bimolecular quenching rate constants (kq) is still lacking. Here, using two photosensitizers and a series of anthracene-based acceptors with tailored steric profiles, we systematically investigate how nonconjugated substituents regulate kq. The most significant results are when the triplet energy gap exceeds ∼0.2 eV, kq is governed solely by steric hindrance and the suppression originates from spin-density-free units that block donor-acceptor wave function overlap. We establish a quantitative model in which the effective collision probability η equals the product of the van der Waals surface fractions carrying triplet spin density (P) for both donor and acceptor, giving kq,calc = kdiffuse × η. Predicted rates match experimental values well and are further validated against literature systems.