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.