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

Abstract

A new series of donor−acceptor push−pull systems, BODIPYs 1−4, were designed and synthesized by the palladium-catalyzed Sonogashira crosscoupling 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.