Far-Red Excitation Induced Electron Transfer in Bis Donor-AzaBODIPY Push-Pull Systems; Role of Nitrogenous Donors in Promoting Charge Separation.

Abstract

A far-red absorbing sensitizer, BF2-chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push-pull systemslinked with a series of nitrogenous electron donors, viz., N,N-dimethylaniline, triphenylamine, and phenothiazine via an acetylene linker. The structural integrity of the newly synthesized triads was established by spectroscopic, electrochemical, spectroelectrochemical, and DFT methods. Differential pulse voltammetry studies revealed different redox states and helped in the estimation of the energies of the charge-separated states. Further, spectroelectrochemical studies performed revealed diagnostic peaks of azaBODIPY•- in the visible and near-IR regions. Free-energy calculations revealed the charge separation from one of the covalently linked donors to the 1azaBODIPY* to yield Donor•+-azaBODIPY•- to be energetically favorable in a polar solvent, benzonitrile. The femtosecond pump-probe studies revealed the occurrence of excited charge transfer (CT) in nonpolar toluene while a complete charge separation (CS) for all three triads in polar benzonitrile. The CT/CS products populated the low-lying 3azaBODIPY* prior to returning to the ground state. Global target (GloTarAn) analysis of the transient data revealed the lifetime of the final charge-separated states (CSS) to be 195 ps for NND-derived, 50 ps for TPA-derived, and 85 ps for PTZ-derived triads in benzonitrile.