Photoinduced energy and charge transfer in a bis(triphenylamine)–BODIPY–C60 artificial photosynthetic system

Abstract

Triphenylamines (TPAs), boron dipyrromethenes (BODIPYs) and fullerenes C₆₀ are excellent building blocks for the design of artificial photosynthetic systems. In the present work, we report the synthesis, characterization and detailed photophysical studies of a novel (TPA)₂–BODIPY–C₆₀ tetrad in polar and nonpolar solvents. The absorption spectrum of this compound covered virtually the entire visible region (350–700 nm) and could be interpreted as a superposition of the spectra of individual components. Upon TPA-part excitation, a fast and very efficient excitation energy transfer (EET) delivers the excitation to the BODIPY moiety resulting in complete quenching of the TPA first excited singlet state as well as the appearance of the BODIPY fluorescence. The efficiency of EET process was estimated to be 1. Direct or indirect (via EET) excitation of the BODIPY-part of the tetrad is followed by photoinduced charge transfer to the charge-separated state BODIPY˙⁺–C₆₀˙⁻ irrespective of the solvent used. In polar N,N-dimethylformamide (DMF) charge recombination occurs directly to the ground state with the charge recombination rate, k_CR, slower than 10⁸ s⁻¹, whereas in nonpolar toluene (TOL) a small energy gap between the charge-separated state and first excited singlet state of the BODIPY moiety facilitates the back charge transfer process. The latter results in the appearance of thermally activated delayed fluorescence. The rate of charge separation was found to be ca. 2 times faster in TOL than in DMF.