Acceptor-Dependent Intervalence Charge Transfer and Separation Dynamics in Broad-Band-Capturing Push–Pull Chromophores

Abstract

Organic donor–acceptor (D–A)-based electrochromic systems exhibiting intervalence charge transfer (IVCT) under applied potential have been a rarity even after their increased development toward artificial photosynthesis. In this article, we have made efforts to address this challenge by designing and synthesizing a new set of D–A chromophores comprising phenothiazine (PTZ) donors and tetracyanobutadiene (TCBD) or dicyanoquinodimethane (DCNQ) acceptors following the Corey–Fuchs reaction via Evano’s condition and [2 + 2] cycloaddition retroelectrocyclic ring-opening reactions in concert. Electronic communication leading to electron exchange between the adjacent donors demonstrating a broad IVCT band in the NIR region upon one-electron oxidation has been reported for the first time in this class of systems, which is again found to be significantly influenced by the acceptors in controlling electronic coupling of the redox-active centers. Though the TCBD-based symmetrical molecule shows IVCT, the same is significantly suppressed after DCNQ incorporations owing to its more robust acceptor nature, which, in turn, inhibits electron exchange between the donors. The calculated electronic coupling element (H_IV), stabilization energy (−ΔG_com), and comproportionation equilibrium constant (K_com) for the formation of radical cations susceptible to IVCT are also found to be strongly impacted by adjacent acceptor entities. On the contrary, DCNQ, however, facilitates intramolecular photoexcited charge transfer (CT) dynamics over TCBD-based D–A chromophores, leading to low-lying broad absorptions extendable even up to the ∼1200 nm region. Theoretical calculations coupled with electrochemical data identify the moieties involved in CT and subsequent charge separation (CS) transitions and further access the thermodynamic feasibility of these photoinduced processes, while femtosecond pump–probe measurement traces their associated spectral and kinetic information upon excitation in the ultrafast time domain. Relatively faster CT and successive CS dynamics of DCNQ-based molecules, along with wide band absorptions, not only unveil their promising utility in photovoltaics but also make the current D–A chromophores an emerging variant of acceptor-controlled organic electrochromatic systems for color-switching optoelectronics under variable operating voltages.