Expanding the Chemical Space of Tetracyanobuta-1,3-diene (TCBD) through a Cyano-Diels-Alder Reaction: Synthesis, Structure, and Physicochemical Properties of an Anthryl-fused-TCBD Derivative

Tetracyanobuta‐1,3‐diene (TCBD) is a powerful and versatile electron‐acceptor moiety widely used for the preparation of electroactive conjugates. While many reports addressing its electron‐accepting capability have appeared in the literature, significantly scarcer are those dealing with its chemical modification, a relevant topic which allows to broaden the chemical space of this interesting functional unit. Here, we report on the first example of a high‐yielding cyano‐Diels‐Alder (CDA) reaction between TCBD, that is, where a nitrile group acts as a dienophile, and an anthryl moiety, that is, acting as a diene. The resulting anthryl‐fused‐TCBD derivative, which structure was unambiguously identified by X‐ray diffraction, shows high thermal stability, remarkable electron‐accepting capability, and interesting electronic ground‐ and excited‐state features, as characterized by a thorough theoretical, electrochemical, and photophysical investigation. Moreover, a detailed kinetic analysis of the intramolecular CDA reaction transforming the anthryl‐TCBD‐based reactant into the anthryl‐fused‐TCBD product was carried out at different temperatures.

Photoinduced Charge Separation Prompted Intervalence Charge Transfer in a Bis(thienyl)diketopyrrolopyrrole Bridged Donor-TCBD Push-Pull System

Intervalence charge transfer (IVCT), a phenomenon observed in molecular systems comprised of two redox centers differing in oxidation states by one unit, is reported in a novel, newly synthesized, multi-modular donor-acceptor system comprised of central bis(thienyl)diketopyrrolopyrrole (TDPP) hosting two phenothiazine-tetracyanobutadiene (PTZ-TCBD) entities on the opposite sides. One-electron reduction of TCBD promoted electron exchange between the two TCBD resulting in IVCT transition in the near-infrared region. The stabilization energy, -ΔG_com and comproportionation equilibrium constant, K_com calculated from peak potentials of the split reduction waves were found to be 1.06×10⁴  J mol^-1 , and 72.3 M^-1 , respectively. Further, the IVCT transition was also witnessed during the process of thermodynamically feasible electron transfer upon excitation of the TDPP entity in the system, and served as a diagnostic marker to characterize the electron transfer product. Subsequent transient absorption spectral studies and data analysis by Global and Target analyses revealed occurrence of ultrafast charge separation (k_cs ≈10¹⁰  s^-1 ) owing to the close proximity and good communication between the entities of the multi-modular donor-acceptor system. The role of central TDPP in promoting IVCT is borne out from the present investigation.