Tetracyanobutadiene- and Dicyanoquinodimethane-Benzothiadiazole Push–Pull Systems: The Role of Terminal Electron Donors in Governing Intramolecular Charge Transfer

Push–pull molecular systems incorporating strong electron acceptor(s) and donor(s) are highly sought-after candidates for the construction of high-performance organic photovoltaic, optoelectronic, and related photonic devices. Consequently, there is growing interest in designing such molecular systems to elucidate fundamental structure–activity–photodynamics relationships. In the present study, a series of strong electron acceptors, tetracyanobutadiene and/or dicyanoquinodimethane, linked to the bis(N,N-dimethylaminophenyl)-benzothiadiazole molecular frame is designed and synthesized. As a result of the strong push–pull effect, the optical response extends into the visible and near-IR regions. The number of electron-acceptor entities was varied to enhance intramolecular charge transfer dominance. Compared with earlier series that used either triphenylamine or phenothiazine terminal electron donors instead of dimethylaminophenyl entities, the current systems exhibit better intramolecular charge transfer characteristics. The redox properties of these multiredox entities bearing systems were evaluated by cyclic and differential pulse voltammetry, while the spectral features of the first oxidized and reduced species were obtained by spectroelectrochemical studies. The energy level diagram derived from free energy calculations showed the thermodynamic feasibility of excited-state charge transfer and separation in these push–pull systems. DFT calculations helped determine the geometry and electronic structures and identify the parts of the push–pull systems that play electron-pushing and electron-pulling roles. Femtosecond transient absorption spectral studies provided evidence of excited-state charge transfer and separation, while global target analysis of the transient data helped in securing kinetic information. Improved charge transfer and separation in push–pull systems carrying a single tetracyanobutadiene or dicyanoquinodimethane entity, rather than symmetrically positioned double entities, were observed. These studies also showed that, compared with tetracyanobutadiene-bearing systems, dicyanoquinodimethane-bearing systems exhibit facile reduction, extended absorption coverage due to intramolecular charge transfer, ultrafast charge transfer, and charge separation.