Spectroelectrochemical and Computational Analysis of a Series of Cycloaddition–Retroelectrocyclization-Derived Donor–Acceptor Chromophores

Abstract

The [2+2] cyclcoaddition (CA) and subsequent retroelectrocyclization (RE) reactions are useful in constructing nonplanar donor-acceptor chromophores that exhibit nonlinear optical properties and intramolecular charge-transfer transitions. However, both the infrared (IR) and visible-near IR (vis-NIR) spectroelectrochemical responses of CA-RE-derived chromophores are rarely explored in depth. Reported in this contribution is a comprehensive IR and vis-NIR spectroelectrochemical study of the CA-RE adducts of DMAP-C_2<i>n</i>-NAP^iPr of both tetracyanoethene (TCNE) and tetracyanoquinodimethane (TCNQ) and companion time-dependent density functional theory (TD-DFT) analysis of the bands observed. Specifically, DMAP-C_2<i>n</i>-NAP^iPr (<b>1a</b>, <i>n</i> = 1; <b>1b</b> <i>n</i> = 2; DMAP = <i>N</i>,<i>N</i>-dimethylaniline; NAP^iPr = <i>N</i>-isopropyl-1,8-naphthalimide) react with TCNE to yield the tetracyanobutadiene (TCBD) derivatives (<b>2a</b> and <b>2b</b>, respectively) and with TCNQ to yield the dicyanoquinodimethane (DCNQ) derivatives (<b>3a</b> and <b>3b</b>, respectively). IR spectroelectrochemical studies showed the emergence/intensification of new CN stretches upon reductions. Ultraviolet-vis-NIR (UV-vis-NIR) spectroelectrochemical study of <b>3</b> revealed a partial bleach of the charge-transfer (CT) bands, originally appearing in the neutral species, and the emergence of new CT bands originating from NAP^iPr to the reduced DCNQ moiety. UV-vis-NIR spectroelectrochemical study of <b>2</b>, surprisingly, indicated a very minimal change upon reductions. Dynamic changes were observed in the mid-IR absorption for C≡C and C≡N for both <b>2</b> and <b>3</b>, indicative of enhanced asymmetry and the formation of ion pairs on the dicyano bridge. DFT and TD-DFT analyses were used to obtain the semi-quantitative pictures of the frontier orbitals of <b>1-3</b> and elucidate the origin of the transient features observed spectroelectrochemically for the 1e^- and 2e^- reduced species.