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

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.