Design, synthesis, and redox properties of donor–π–donor ferrocenyl functionalized phenothiazine derivatives

Abstract

A set of ferrocenyl functionalized phenothiazine derivatives Fc1–4 were designed and synthesized via Pd-catalyzed Buchwald–Hartwig and Suzuki cross-coupling reactions. Phenothiazine was used as a central donor and ferrocenyl as a terminal donor unit. Herein we examine the impact of variation of substituents at the N-position of phenothiazine, including H, phenyl, phenyl-linked ferrocenyl, and biphenyl-linked ferrocenyl units on the photophysical, electrochemical, and thermal behaviors. The electronic absorption spectra reveal that the N–H and phenyl-substituted phenothiazine derivatives Fc1 and Fc2 show red-shifted absorption as compared to the tri-ferrocenyl substituted phenothiazine derivatives Fc3 and Fc4. The electrochemical studies reveal that Fc1 shows two quasi reversible and one reversible oxidation wave, whereas Fc2–4 exhibits two reversible oxidation waves. The spectroelectrochemical analysis of Fc1–4 revealed significant NIR spectral changes during the second oxidation cycle. DFT calculations were performed to optimize the molecular geometry and evaluate the frontier energy levels of Fc1–4. The structure of ferrocenyl functionalized phenothiazine derivatives Fc2 and Fc3 was confirmed by a single-crystal X-ray diffraction study. Fc2 and Fc3 crystallize as red needle-shaped crystals exhibiting zig-zag 3D crystal packing arrangements. This study highlights the photophysical, electrochemical, and redox characteristics of ferrocenyl-functionalized phenothiazine derivatives, making them potential candidates for optoelectronic applications.