NIR-Absorbing TCBD and Expanded TCBD-Functionalized Ferrocene–Phenothiazine Conjugates

A set of TCBD- and expanded TCBD-functionalized donor–acceptor ferrocene conjugates Fc2–3 and Fc5–6 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling, followed by [2 + 2] cycloaddition–retroelectrocyclization reactions. Phenothiazine (PTZ) and ferrocene (Fc) were used as the terminal donor units. The incorporation of cyano-based strong acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the ferrocene and ferrocene–phenothiazine conjugates resulted in substantial variation in their photophysical, electrochemical, and thermal behavior. The electronic absorption spectra of the ferrocene derivatives with TCNE (Fc2, Fc5) and TCNQ (Fc3, Fc6) acceptors show bathochromically shifted absorption compared to Fc1 and Fc4 due to a significant intramolecular charge-transfer (ICT) transition. The electrochemical analysis of Fc2–3 and Fc5–6 reveals reduction waves at lower potentials due to the TCBD and DCNQ acceptors. The TCBD-functionalized ferrocene derivative Fc2 shows higher thermal stability compared to other ferrocene conjugates. DFT-optimized geometries and frontier orbital energies of Fc1–6 indicate that increasing the acceptor strength stabilizes the LUMO and reduces the HOMO–LUMO gap. The spectroelectrochemical analyses of Fc2–3 and Fc5–6 revealed substantial spectral changes during the reduction cycle in the NIR region, whereas Fc1 and Fc4 displayed changes during the oxidation cycle in the NIR region.