Phenothiazine and phenothiazine-5,5-dioxide-based push–pull derivatives: synthesis, photophysical, electrochemical and computational studies

Abstract

A set of phenothiazine (PTZ) and phenothiazine-5,5-dioxide-based π-conjugated push–pull chromophores PTZ 1–6 were designed and synthesized by Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclic ring-opening reactions in good yields. PTZ 1–3 were reacted with 3-chloroperbenzoic acid to increase the oxidation state of the sulfur (S) atom in the thiazine ring, resulting in phenothiazine-5,5-dioxide-based PTZ 4–6 in good yields. The effect of strong electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD, as well as the oxidation state of sulfur (S), on their photophysical, thermal stability and redox properties were explored. The photophysical properties suggest that the phenothiazine-5,5-dioxide functionalized PTZ 4–6 show a hypsochromic shift in the UV-vis spectra compared to PTZ 1–3. PTZ 6 exhibits higher thermal stability than PTZ 1–5. The cyclic voltammetry experiments suggest that the TCBD and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD substituted phenothiazine and phenothiazine-5,5-dioxide derivatives show multiple reduction waves in the low potential range due to the presence of strong acceptor units. Theoretical calculation reveals that the introduction of the cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD moiety stabilizes the LUMO energy level more as compared to the TCBD moiety.