Near-IR absorbing 1,1,4,4-tetracyanobutadiene-functionalized phenothiazine sulfones

Abstract

Triphenylamine (TPA) substituted π-conjugated chromophores TPA1–TPA5 were designed and synthesized via Pd-catalysed Sonogashira cross-coupling followed by [2 + 2] cycloaddition–retroelectrocyclization (CA–RE) reactions. The effects of acceptor 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (DCNQ) units in the photophysical studies and the HOMO–LUMO energy levels of the phenothiazine sulfones TPA1–TPA5 were evaluated. The absorption spectra of chromophores TPA4 and TPA5 show a significant change due to the incorporation of DCNQ units, resulting in bathochromically shifted broad absorption in the NIR region. The photophysical studies revealed that DCNQ-based chromophores TPA4 and TPA5 have a better D–A interaction than the TCBD functionalized TPA2 and TPA3. Density functional theory calculations and electrochemical studies were performed to examine the molecular geometry and frontier energy levels of the sulfone-based chromophores. Systematic structural modification of the chromophore TPA1 modulated the electrochemical properties and successively tuned the energy gaps for TPA2–TPA5. The theoretically estimated HOMO–LUMO gaps for TPA1–TPA5 exhibit good agreement with the experimental data calculated from the electrochemical studies. The chromophore TPA1 exhibits solvatochromism and aggregation-induced emission (AIE) behavior owing to the emission in the solid state.