Competition between Photoinduced Electron Transfer and Heterolytic Cleavage on Boron in B-Phenoxy Substituted BODIPY Photocages

Abstract

The photochemical reactivity of B-phenoxy-substituted BODIPY derivatives was studied to evaluate their suitability as photocleavable protecting groups (photocages) for phenol release. The phenoxy unit was modified with strong (CHO), neutral (CH2OH), or very weak (CH2OAc) electron-withdrawing groups (EWG) at the para-position. Upon irradiation in methanol, all compounds gave photomethanolysis products with quantum yields Φr = 0.01–0.03, independent of the substituent type. Computational analysis using time-dependent density functional theory indicated competing heterolytic and homolytic cleavage pathways of the B-phenoxy bond. Homolytic cleavage proceeds via a dark charge-transfer state accessed through an S2/S1 conical intersection, which forms more readily with the weak EWG. In contrast, the derivative bearing the strong EWG remains longer in the bright intra-BODIPY excited state, favoring heterolytic cleavage. In this case, the entire photosubstitution process, finalized by the proton transfer (PT) from CH3OH to the phenoxy, occurs on the S1 surface. Conversely, the homolytic cleavage tends to be nonproductive and leads to recombination to the starting material and energy dissipation. Overall, the BODIPY systems exhibiting photoelimination from boron show promise as photocages for phenols with Φr higher than from the meso-methyl group. The obtained mechanistic insights provide valuable guidance for designing improved photocleavable systems for biological applications.