Symmetric and Asymmetric Push-Pull Conjugates: Significance of Pull Group Strength on Charge Transfer and Separation

Abstract

The effect of acceptor strength on excited-state charge transfer (CT) and charge separation (CS) in central phenothiazine (PTZ)-derived symmetric 1 (PTZ-(TCBD-TPA)2) and asymmetric 2 (PTZ-(TCBD/DCNQ-TPA)2) push-pull conjugates, in which triphenylamine (TPA) acts as end capping and 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-ylidene-expanded TCBD (DCNQ) act as electron acceptor units, is reported. Due to strong push-pull effects, intramolecular CT was observed in the ground state, extending the absorption into the near-infrared region. Electrochemical, spectroelectrochemical, and computational studies coupled with energy-level calculations predicted both 1 and 2 to be efficient candidates for ultrafast CT. Subsequent femtosecond transient absorption studies along with global target analysis, performed in both polar and nonpolar solvents, confirmed such processes in which the CS was efficient in asymmetric 2, having both TCBD and DCNQ acceptors in polar benzonitrile, while in toluene, only CT was witnessed. This work highlights the significance of the number and strength of electron acceptor entities and the role of solvent polarity in multimodular push-pull systems to achieve ultrafast CS. © 2021 American Chemical Society. All rights reserved.