Engineering Cofacial Trizinc(II)porphyrin Trimers: Linking Molecular Architecture with Excited-State Dynamics

We report a rational design of donor–acceptor–donor (D–A–D) Zn(II)porphyrin trimers adopting two conformational architectures, ABA (“ closed ”) and ACA (“ open ”), in which electron-rich terminal porphyrins flank an electron-deficient central unit linked by a flexible spacer. Minor variations in peripheral substitution at the central porphyrin trigger substantial structural reorganization, leading to pronounced differences in electronic coupling and excited-state behavior. The “ closed ” ABA architecture enforces close interchromophoric proximity, resulting in strong excitonic coupling and perturbation of the Soret band. In contrast, the “ open ” ACA conformer exhibits weaker coupling and minimal spectral distortion. As a result, ACA displays fluorescence resembling a linear superposition of its constituent monomers and a relatively long excited-state lifetime (∼3 ns). By comparison, ABA deviates significantly from monomeric behavior, consistent with enhanced intramolecular charge-transfer (ICT) interactions that shorten the fluorescence lifetime (∼800 ps) and suppress the fluorescence quantum yield. The more efficient ICT in ABA facilitates triplet-state formation, resulting in substantially higher singlet oxygen quantum yield than ACA . Accordingly, ABA exhibits superior photocatalytic activity, achieving quantitative oxidation of triphenylphosphine, whereas ACA shows markedly lower efficiency. Collectively, these results demonstrate that conformational modulation of excitonic and charge-transfer interactions provides an effective strategy for selective photooxidation.