Photoinduced Electron Transfer in Axially Coordinated Supramolecular Zinc Tetrapyrrole Bis(styryl)BODIPY Donor-Acceptor Conjugates

Photoinduced electron transfer (PET) in newly assembled dyads formed via metal-ligand axial coordination of phenylimidazole-functionalized bis(styryl)BODIPY (BODIPY(Im)₂) and zinc tetrapyrroles, that is, zinc tetratolylporphyrin (ZnP), zinc tetra-t-butyl phthalocyanine (ZnPc) and zinc tetra-t-butyl naphthalocyanine (ZnNc), in non-coordinating o-dichlorobenzene (DCB) is investigated using both steady-state and time-resolved transient absorption techniques. The structure of the BODIPY(Im)₂ was identified by using single crystal X-ray structural analysis. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral studies were in the range of ∼10⁴ M⁻¹ for the first zinc tetrapyrrole binding and suggested that the two imidazole entities of bis(styryl)BODIPY behave independently in the binding process. The energy level diagram established using spectral and electrochemical studies suggested PET to be thermodynamically unfavorable in the ZnP-bearing complex while for ZnPc- and ZnNc-bearing complexes such a process is possible when zinc tetrapyrrole is selectively excited. Consequently, occurrence of efficient PET in the latter two dyads was possible to establish from femtosecond transient absorption studies wherein the electron transfer products, that is, the radical cation of zinc tetrapyrrole and the radical anion of BODIPY(Im)₂, was possible to spectrally identify. From target analysis of the transient data, time constants of circa 3 ns for ZnPc^⋅+:BODIPY⋅⁻ and circa 0.5 ns for ZnNc⋅⁺:BODIPY⋅⁻ were obtained indicating persistence of the radical ion-pair to some extent. The electron acceptor property of bis(styryl)BODIPY in donor-acceptor conjugates is borne out from the present study.