Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States.
J Phys Chem B. 2020 Dec 31;124(52):11901-11909. doi: 10.1021/acs.jpcb.0c09450. Epub 2020 Dec 21.
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-NAP of both tetracyanoethene (TCNE) and tetracyanoquinodimethane (TCNQ) and companion time-dependent density functional theory (TD-DFT) analysis of the bands observed. Specifically, DMAP-C-NAP (, = 1; = 2; DMAP = ,-dimethylaniline; NAP = -isopropyl-1,8-naphthalimide) react with TCNE to yield the tetracyanobutadiene (TCBD) derivatives ( and , respectively) and with TCNQ to yield the dicyanoquinodimethane (DCNQ) derivatives ( and , respectively). IR spectroelectrochemical studies showed the emergence/intensification of new CN stretches upon reductions. Ultraviolet-vis-NIR (UV-vis-NIR) spectroelectrochemical study of 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 to the reduced DCNQ moiety. UV-vis-NIR spectroelectrochemical study of , 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 and , 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 and elucidate the origin of the transient features observed spectroelectrochemically for the 1e and 2e reduced species.
[2+2]环加成（CA）和随后的反环化（RE）反应可用于构建非平面给体-受体生色团，这些生色团表现出非线性光学性质和分子内电荷转移跃迁。然而，CA-RE 衍生生色团的红外（IR）和可见-近红外（vis-NIR）光谱电化学响应很少被深入探索。本贡献报道了对 DMAP-C-NAP 与四氰乙烯（TCNE）和四氰醌二甲烷（TCNQ）的 CA-RE 加合物的全面 IR 和 vis-NIR 光谱电化学研究，并对观察到的带进行了伴随的时间相关密度泛函理论（TD-DFT）分析。具体来说，DMAP-C-NAP（，= 1；= 2；DMAP = ，-二甲苯胺；NAP = -异丙基-1,8-萘酰亚胺）与 TCNE 反应生成四氰丁二烯（TCBD）衍生物（和，分别），与 TCNQ 反应生成二氰醌二甲烷（DCNQ）衍生物（和，分别）。IR 光谱电化学研究表明，还原时会出现/增强新的 CN 伸展。对的紫外-可见-近红外（UV-vis-NIR）光谱电化学研究表明，中性物种中原有的电荷转移（CT）带部分漂白，并出现源于 NAP 到还原的 DCNQ 部分的新 CT 带。令人惊讶的是，对的 UV-vis-NIR 光谱电化学研究表明，还原时变化非常小。两种情况下的中红外吸收的 C≡C 和 C≡N 都观察到动态变化，表明二氰桥的不对称性增强和离子对的形成。DFT 和 TD-DFT 分析用于获得和的前线轨道的半定量图像，并阐明在 1e 和 2e 还原物种的光谱电化学中观察到的瞬态特征的起源。
