Ferrocenyl-functionalized phenothiazine conjugates: structure–property relationship and electrochemical energy storage studies

Abstract

A set of mono-, di-, and tri-ferrocenyl phenothiazine conjugates 1–3 were synthesized via Buchwald–Hartwig cross-coupling and Pd-catalyzed Suzuki cross-coupling reactions. Phenothiazine served as the central core, and its various sites were explored for functionalization. In the mono-ferrocenyl conjugate 1, ferrocene was linked at the nitrogen atom of the phenothiazine core via a biphenyl spacer, while in the di- and tri-ferrocenyl conjugates, phenyl-linked ferrocene was incorporated at the 3rd and 7th positions of phenothiazine. The photophysical, electrochemical and thermal behavior of these conjugates was analyzed to investigate the effect of structural variations. The di- and tri-ferrocenyl conjugates 2 and 3 showed a bathochromic shift of 50–55 nm as compared to the monofunctionalized ferrocenyl conjugate 1. Electrochemical studies showed two reversible oxidation waves for all conjugates 1–3, attributed to ferrocene and phenothiazine units. Additionally, their electrochemical performance was evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements using a three-electrode configuration in H₂SO₄ electrolyte, where the conjugates were employed as working electrodes by dropcasting on graphite foil (GF). The 1/GF electrode exhibited predominant capacitive behavior with remarkable stability. The 2/GF electrode demonstrated diffusion-controlled redox activity typical of battery-type behavior. In contrast, the 3/GF electrode showed a combination of capacitive and diffusion-controlled processes, achieving the highest specific capacitance of 160.8 F g⁻¹ at 0.5 A g⁻¹ along with excellent cycling stability. Computational calculations were performed to optimize molecular structures and evaluate frontier molecular orbital energy levels. Single-crystal X-ray diffraction confirmed the structure of 1.