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

A multi-modular donor-acceptor triad composed of zinc porphyrin, BF-chelated dipyrromethene (BODIPY), and C 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-C 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 BODIPY* to ZnP with a rate constant k = 1.7 × 10 s in toluene was observed in the case of the ZnP-BODIPY dyad. Interestingly, in the case of the ZnP-BODIPY-C triad, the selective excitation of ZnP resulted in electron transfer leading to the formation of the ZnP˙-BODIPY-C˙ 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-C* as the major product. The C* populated the low-lying C* 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.