Synthesis of green light emitting fused pyrazolinopiperidines - photophysical and electrochemical studies

Abstract

We have synthesized the new, green light emitting pyrazolinopiperidines (PyP) derivatives with extended π-conjugation pathway by simple, two step reactions and are characterized using spectral and single crystal X-ray analysis. The electronic properties of PyP were studied using steady state, and time resolved spectral techniques, theoretical and electrochemical methods. PyP derivatives exhibit intense green fluorescence both in the solid and solution state with the quantum yield of 0.29 in acetonitrile. Importantly, PyP shows non-overlapping absorption and emission spectrum with the large Stokes shift value of ∼8000 cm⁻¹. Solvent polarity dependent photophysical properties and theoretical HOMO–LUMO calculation reveal that the singlet excited state possesses significant charge transfer character. The substitution of electron donating/withdrawing substituents show negligible or little influence on the photophysical properties except the trifluoromethyl and 2,4-dichloro phenyl substituent. The electrochemical first oxidation potentials were not influenced by the substituent; however the first reduction potential becomes sensitive towards nature and position of the aryl substituent. Further, photophysical properties of PyP become sensitive to the nature of substituent (ortho/para) positions. The enhanced non-radiative decay for the 2,4-dichlorophenyl substituted PyP as compared to 2- and 4-chlorophenyl substituents (by a factor of two and seven times, respectively) emphasize the necessity for multiple electron withdrawing aryl substituents to induce the measurable charge transfer interactions. The enhanced non-radiative rate constant of trifluoromethyl substituent by eight times than methyl substituent is due to the strong intramolecular charge transfer from pyrazoline to styryl moiety.