Solvent-Controlled Switching between Electron Transfer and Type II Photosensitization in a Zinc Phthalocyanine–Flavonol Pair

The impact of solvent nature on intermolecular interactions within a pair of phthalocyanine and 3-hydroxyflavone (flavonol) derivatives was examined with the aim of activating the UV-absorbing flavonol chromophore by visible/NIR light. Unlike its metal-free analog, Zn-phthalocyanine was found to form a 1:1 complex with flavonol in nonpolar dichloromethane. This complex showed near-diffusion-limited quenching of phthalocyanine fluorescence, which, however, did not lead to flavonol emission or triplet reactivity. A thorough examination of high-level DLPNO-CCSD(T)/CBS and constrained DFT electronic structure calculations revealed that photoinduced electron transfer within the excited complex is thermodynamically feasible. Concurrently, the formation of carbon monoxide from flavonol was observed, facilitated by singlet oxygen, generated by a Type II sensitization via triplet-excited phthalocyanine. In dimethyl sulfoxide (DMSO), solvent ligation inhibited phthalocyanine/flavonol complexation, and a Type II photosensitization was almost exclusively responsible for the flavonol oxygenation. The suppression of this reaction pathway in degassed DMSO resulted in intermolecular quenching of the phthalocyanine triplet by the flavonol moiety. The Wigner sampling of the T1-S0 energy gaps indicates that intramolecular proton transfer can decrease the flavonol triplet energy sufficiently to enable a nonvertical proton-coupled energy-transfer pathway from phthalocyanine to flavonol. Although this pathway is energetically accessible, it is inefficient.