Design, synthesis, and redox properties of ferrocene functionalized phenothiazine and phenothiazine sulfone isomers

Abstract

A set of ferrocene functionalized phenothiazine (PTZ) derivatives PTZ 1–3 were designed and synthesized by the Pd-catalyzed Buchwald−Hartwig cross-coupling reaction in good yields. PTZ 1–3 were treated with m-chloroperbenzoic acid (m-CPBA), which oxidize the sulfur (S) atom in the thiazine ring to sulfone, resulting in the ferrocene substituted phenothiazine sulfone derivatives PTZ 4–6. The influence of sulfur (S) oxidation state on the photophysical, redox properties, and thermal stability of the phenothiazine and phenothiazine sulfone derivatives were studied. The photophysical behaviour indicates that the phenothiazine sulfone derivatives PTZ 4–6 exhibit a hypsochromic shift in the UV-vis absorption as compared to PTZ 1–3. The ferrocene functionalized phenothiazine derivatives PTZ 1–3 exhibit better thermal stability than sulfone derivatives PTZ 4–6. The cyclic voltammetry studies on PTZ 1–6 show two oxidation waves. The density functional theory (DFT) and TD-DFT calculations were performed on PTZ 1–6 to analyze the molecular geometry, frontier molecular orbitals, molecular electrostatic potentials (MEP), and density of states (DOS). The influence of the ferrocene unit on the para, meta, and ortho position of the phenyl ring attached to the N-position of phenothiazine was investigated by spectroelectrochemical studies. The spectroscopic behavior of redox-active species of PTZ 1–3 shows a new and intense absorption band centred around 525 nm and two new red-shifted broad peaks centred at 750 and 850 in the NIR region. The structure of ferrocene functionalized phenothiazine and phenothiazine sulfone derivatives PTZ 1–6 was confirmed by single crystal X-ray diffraction. The photophysical and electrochemical properties of PTZ 1–6 offer valuable insights and potential applications in the development of redox-active materials, with significant contributions to the advancement of electronic and photonic technologies.