NIR absorbing ferrocenyl phenothiazine based push–pull chromophores

A series of ferrocenyl (Fc) functionalized phenothiazine (PTZ) π–conjugated push–pull chromophores (PTZ 1–6), were designed and synthesized using Pd-catalyzed Sonogashira cross–coupling, followed by thermally activated [2 + 2] cycloaddition-electrocyclic ring-opening reaction. The effect of extended π–conjugation and electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD, on the photophysical, electrochemical, and spectroelectrochemical properties, as well as theoretical study, was examined for PTZ 1–6. The photophysical properties indicate that TCBD and DCNQ functionalized PTZ 3–6 exhibit a bathochromic shift in their UV–vis spectra as compared to the acetylene-bridged compounds PTZ1 and PTZ2. The electrochemical analysis revealed multiple oxidation potential waves attributed to the donor units (ferrocene and phenothiazine), as well as reduction waves corresponding to the TCBD and DCNQ acceptors, observed at a low potential range. The spectroelectrochemical analysis indicates that the redox species formed during the redox cycles showed absorption spectra in the NIR region (630–930 nm). The computational calculations suggest that the DCNQ moiety stabilizes LUMO energy levels more effectively than the TCBD unit.