Accelerated Intramolecular Charge Transfer in Tetracyanobutadiene- and Expanded Tetracyanobutadiene-Incorporated Asymmetric Triphenylamine–Quinoxaline Push–Pull Conjugates

Abstract

The excited-state properties of an asymmetric triphenylamine− quinoxaline push−pull system wherein triphenylamine and quinoxaline take up the roles of an electron donor and acceptor, respectively, are initially investigated. Further, in order to improve the push−pull effect, powerful electron acceptors, viz., 1,1,4,4tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded tetracyanobutadiene (also known as expanded-TCBD or exTCBD), have been introduced into the triphenylamine−quinoxaline molecular framework using a catalyst-free [2 + 2] cycloaddition−retroelectrocyclization reaction. The presence of these electron acceptors caused strong ground-state polarization extending the absorption well into the near-IR region accompanied by strong fluorescence quenching due to intramolecular charge transfer (CT). Systematic studies were performed using a suite of spectral, electrochemical, computational, and pump−probe spectroscopic techniques to unravel the intramolecular CT mechanism and to probe the role of TCBD and exTCBD in promoting excited-state CT and separation events. Faster CT in exTCBD-derived compared to that in TCBD-derived push−pull systems has been witnessed in polar benzonitrile.