Synthesis, photophysical properties, and density functional theory studies of phenothiazine festooned vinylcyclohexenyl-malononitrile

Abstract

A novel phenothiazine derivative conjugated with vinylcyclohexenyl-malononitrile (PTZ-CDN) was synthesized through the Knoevenagel reaction of 10-octyl-10H-phenothiazine-3,7-dicarbaldehyde with 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)-malononitrile and fully characterized. The UV-vis absorption spectra of PTZ-CDN in different solvents showed a λ_max band at 497-531 nm with a high molar extinction coefficient attributed to intramolecular charge transfer (ICT) with the characteristics of a π-π* transition. Increasing the solvent polarity resulted in a bathochromic shift of λ_max . The PTZ-CDN fluorescence emission spectra were more sensitive to increasing the solvent polarity than the absorption spectra; they displayed a blue shift of λ_em by 85 nm. To understand the behaviour of the PTZ-CDN derivative, Stokes' shift ( $\Delta \overline{\nu })$ with respect to the solvent polarity, Lippert-Mataga and linear solvation-energy relationship (LSER) models were applied in which the LSER showed better regression than the Lippert-Mataga plots (r² = 0.9627). Finally, the TD-density functional theory (DFT) electronic transition spectra in dioxane and dimethyl formamide (DMF) were calculated. The DFT data showed that λ_max resulted from the support of the highest occupied molecular orbital to the lowest unoccupied molecular orbital transition with 74% and 99% in dioxane and DMF, respectively.

Keywords: DFT studies; energy gap; fluorescence; phenothiazine; solvatochromism.