Controlling electron and energy transfer paths by selective excitation in a zinc porphyrin–BODIPY–C60 multi-modular triad

Abstract

A multi-modular donor–acceptor triad composed of zinc porphyrin, BF₂-chelated dipyrromethene (BODIPY), and C₆₀ was newly synthesized, with the BODIPY entity at the central position. Using absorbance and emission spectral, electrochemical redox, and computational optimization results, energy level diagrams for the ZnP–BODIPY dyad and ZnP–BODIPY–C₆₀ triad were constructed to envision the different photochemical events upon selective excitation of the BODIPY and ZnP entities. By transient absorption spectral studies covering a wide femtosecond-to-millisecond time scale, evidence for the different photochemical events and their kinetic information was secured. Efficient singlet–singlet energy transfer from ¹BODIPY* to ZnP with a rate constant k_ENT = 1.7 × 10¹⁰ s⁻¹ in toluene was observed in the case of the ZnP–BODIPY dyad. Interestingly, in the case of the ZnP–BODIPY–C₆₀ triad, the selective excitation of ZnP resulted in electron transfer leading to the formation of the ZnP˙⁺–BODIPY–C₆₀˙⁻ charge-separated state. Owing to the distal separation of the radical cation and radical anion species (edge-to-edge distance of 18.7 Å), the radical ion-pair persisted for microseconds. By contrast, the selective excitation of BODIPY resulted in an ultrafast energy transfer to yield ZnP–BODIPY–¹C₆₀* as the major product. The ¹C₆₀* populated the low-lying ³C₆₀* via intersystem crossing prior to returning to the ground state. The present study successfully demonstrates the importance of supramolecular geometry and selection of excitation wavelength in regulating the different photoprocesses.