Optical and electrochemical properties of ethynylaniline derivatives of phenothiazine, phenothiazine-5-oxide and phenothiazine-5,5-dioxide

Abstract

Three phenothiazine (PTZ) derivatives with varying degrees of sulfur oxidation states were synthesized as strong electron donors. The thioether, sulfoxide and sulfone PTZ-derivatives exhibited irreversible oxidation at 0.19 V, 0.29 V and 0.31 V versus ferrocene, respectively. Each PTZ derivative was emissive with lifetimes of 1.7 ns, 0.5 ns, and 0.5 ns and absolute quantum yields of 0.32, 0.23 and 0.23 for the thioether, sulfoxide and sulfone derivatives, respectively. Furthermore, these PTZ derivatives showed very large Stokes shifts ranging from 5600 cm⁻¹ to 2800 cm⁻¹. Calculations using DFT and TD-DFT methods resulted in an optimized ground state and the excited state geometries of the PTZ derivatives that compared favourably to experimental optical and electrochemical data. DFT calculations revealed that these butterfly shaped derivatives flatten upon excitation and this effect is greatest for the thioether PTZ derivative, resulting in the large Stokes shift. These potent electron donor systems also displayed electrochromic behaviour upon oxidation, which was attributed to a delocalized cation over the phenothiazine core and the appended ethynyl anilines. The electrochemically oxidized species had a wide absorption profile spanning from 300 nm to past 800 nm.