Dimethylaniline-Tetracyanobutadiene and Dimethylaniline-Extended-Tetracyanobutadiene Functionalized BODIPYs Witnessing Ultrafast Charge Transfer

A new series of donor–acceptor push–pull systems, BODIPYs 1–4, were designed and synthesized by the palladium-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions in good yields. To prepare BODIPY 3 and 4, BODIPY 2 was modified with strong electron acceptors, tetracyanoethylene and 7,7,8,8-tetracyanoquinodimethane, resulting in tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded-TCBD (DCNQ) functionalized BODIPY 3 and 4, respectively. The effects of electron donor N,N-dimethylaniline (NND) and acceptors TCBD and DCNQ on the photophysical and redox properties of the BODIPYs are explored. The push–pull BODIPYs 1–4 exhibit intramolecular charge transfer (ICT) bands at longer wavelengths due to strong D–A interactions. The redox properties of the BODIPYs 1–4 exhibit multiple redox waves due to redox-active NND, BODIPY, TCBD, and DCNQ moieties. The computational studies were performed at the B3LYP/6-31G (d,p) level to understand the molecular geometry and electronic structure of the push–pull BODIPYs. Incorporating a strong electron acceptor, DCNQ decreased the LUMO levels more than the TCBD unit. Femtosecond pump–probe studies were performed to witness the excited-state charge separation process in dichlorobenzene for the BODIPYs 1–4. Finally, the data were analyzed by global target analysis (GloTarAn) which revealed the lifetime of charge-separated states in the range of 45–160 ps in dichlorobenzene, signifying their potential use in energy harvesting and other optoelectronic applications.