Phenothiazine-BODIPY-Fullerene Triads as Photosynthetic Reaction Center Models: Substitution and Solvent Polarity Effects on Photoinduced Charge Separation and Recombination

Novel photosynthetic reaction center model compounds of the type donor2-donor1-acceptor, composed of phenothiazine, BF2-chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X-ray structures of three of the phenothiazine-BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N-substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C-substituted analogue. The geometry and electronic structures were obtained by B3LYP/6-31G(dp) calculations (for H, B, N, and O) and B3LYP/6-31G(df) calculations (for S) in vacuum, followed by a single-point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO-1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy-level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ-BODIPYC+-C-60(-); subsequent hole shift resulted in PTZC+ -BODIPY-C-60(-) charge-separated species. The return of the charge-separated species was found to be solvent dependent. In nonpolar solvents the PTZC+-BODIPY-C-60(-) species populated the C-3(60)* prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The (BODIPY)-B-1* generated radical ion-pair in these triads persisted for few nanoseconds due to electron transfer/hole-shift mechanism.