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

A set of ferrocene-functionalized phenothiazine (PTZ) derivatives 1-3 was designed and synthesized via the Pd-catalyzed Buchwald-Hartwig cross-coupling reaction in good yields. 1-3 were treated with m-chloroperbenzoic acid (m-CPBA), which oxidizes the sulfur (S) atom in the thiazine ring to a sulfone, resulting in ferrocene substituted phenothiazine sulfone derivatives 4-6. The influence of the sulfur (S) oxidation state on the photophysical, redox, and spectroelectrochemical properties, and thermal stability of the phenothiazine and phenothiazine sulfone derivatives, containing the ferrocene unit at the para, meta, and ortho positions of the phenylene ring attached to the N-position of phenothiazine and phenothiazine sulfone, was examined. The photophysical behaviour indicates that the phenothiazine sulfone derivatives 4-6 exhibit a hypsochromic shift in the deep UV region of the UV-vis absorption spectrum as compared to 1-3. The ortho-ferrocene-functionalized phenothiazine derivative 1 exhibits greater thermal stability than its sulfone counterpart 4. The cyclic voltammetry studies on 1-6 show two oxidation waves. The redox behavior of 1-6 reveals that phenothiazine sulfone derivatives (4-6) exhibit shifts towards more positive oxidation potentials compared to their phenothiazine counterparts (1-3). Density functional theory (DFT) and TD-DFT calculations were performed on 1-6 to analyze the molecular geometry, frontier molecular orbitals, molecular electrostatic potentials (MEPs), and density of states (DOS). The spectroelectrochemical behavior of redox-active species of 1-3 shows a new and intense absorption band centred around 525 nm and three new red-shifted broad peaks centred at around 700, 790, and 895 nm in the NIR region. The molecular structures of 1-6 were confirmed by single crystal X-ray diffraction. Phenothiazine sulfone derivatives (4-6) exhibit intermolecular hydrogen bonding within their crystal lattice, contributing to stabilizing their solid-state architecture.