MCM-SO3H catalyzed synthesis of environment-sensitive fluorophores incorporating pyrene moiety: Optimization, fluorescence emission and theoretical studies

Abstract

Six new highly fluorescent 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines were synthesized by reaction of various (E)-3-aryl-1-(pyren-1-yl)prop-2-en-1-ones with phenyl hydrazine in the presence of sulfonated mesoporous silica (MCM-SO3H) as efficient and eco-friendly acidic catalyst. The chemical structures of all synthesized compounds were illustrated on the basis of spectral data (IR, 1H NMR and 13C NMR). This was followed by photophysical properties-based absorption and emission studies of the target compounds in solutions of different solvent polarities. The microenvironment-sensitive fluorescent pyrazolines labeled with pyrene exhibited valuable fluorescence properties with emission in the range of 600–700 nm with a marked response to change in the environmental polarities. A significant and pronounced red shift was observed in the emission spectrum of 5-(aryl)-1-phenyl-3-(pyren-1yl)-2-pyrazolines Δλ ∼ 50 nm compared to the absorption spectrum Δλ ∼ 10 nm upon increasing the solvent polarity. This indicated the presence of higher dipole moment in the excited state than in the ground state and the transition involved are π-π* transition through the charge transfer phenomena. The fundamental understanding of solvatochromic properties were analyzed through Lippert-Mataga and Reichardts correlations in order to estimate the change in dipole moments (Δμ) which suggested the emissive state of designed fluorescent 2-pyrazoline derivatives is of strong ICT character.