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

A far-red absorbing sensitizer, BF2-chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push-pull systems linked with different nitrogenous electron donors, viz., N,N-dimethylaniline (NND), triphenylamine (TPA), and phenothiazine (PTZ) via an acetylene linker. The structural integrity of the newly synthesized push-pull systems was established by spectroscopic, electrochemical, spectroelectrochemical, and DFT computational methods. Cyclic and 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 in a thin-layer optical cell revealed diagnostic peaks of azaBODIPY(center dot-) in the visible and near-IR regions. Free-energy calculations revealed the charge separation from one of the covalently linked donors to the (1)azaBODIPY* to yield Donor(center dot+)-azaBODIPY(center dot-) to be energetically favorable in a polar solvent, benzonitrile, and the frontier orbitals generated on the optimized structures helped in assessing such a conclusion. Consequently, the steady-state emission studies revealed quenching of the azaBODIPY fluorescence in all of the investigated push-pull systems in benzonitrile and to a lesser extent in mildly polar dichlorobenzene, and nonpolar toluene. 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 push-pull systems in polar benzonitrile. The CT/CS products populated the low-lying (3)azaBODIPY* 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 push-pull systems in benzonitrile.

Automated summary

A series of push-pull systems were synthesized by using a far-red absorbing sensitizer, BF2-chelated azadipyrromethane (azaBODIPY), as an electron acceptor and different nitrogenous electron donors linked via an acetylene linker. The structural integrity of the synthesized compounds was confirmed by various spectroscopic, electrochemical, spectroelectrochemical, and computational methods. The results revealed different redox states, energies of charge-separated states, and diagnostic peaks of azaBODIPY(center dot-) in the visible and near-IR regions. The steady-state emission studies showed quenching of azaBODIPY fluorescence in polar and nonpolar solvents, while femtosecond pump-probe studies demonstrated excited charge transfer (CT) and complete charge separation (CS) in nonpolar and polar solvents, respectively. GloTarAn analysis of transient data revealed the lifetime of the final charge-separated states to be dependent on the push-pull systems and solvent.