Synthesis, photophysical and theoretical studies of substituted ethyl 4-(3-(naphthalen-1-yl)-1-phenyl-4,5-dihydro-1H-pyrazol-5-yl)benzoate

A series of five new fluorescent dyes of naphthalene based 2-pyrazoline derivatives with different electron donating/withdrawing substituents at para position of N1 phenyl group were synthesized and their structures were elucidated by spectroscopic methods and elemental analysis. The absorption and fluorescence properties of the new fluorescent dyes were studied using steady state and time-resolved techniques. Comparison of the spectroscopic properties of the five derivatives with different electron donating and withdrawing groups in dichloromethane revealed little influence of substituents on absorption spectra. However, the fluorescence spectra of these molecules have been greatly affected. Time-resolved measurements were performed and two lifetimes were obtained. The mean lifetimes for the pyrazolines containing donor groups were found to be higher than those with withdrawing groups. For all compounds, the ground state geometry was optimized in gaseous phase and in dichloromethane using density functional theory (DFT) at B3LYP/6-31G(d) level of theory to obtain frontier molecular orbitals, energy gap and dipole moments. The optimized excited state geometries using TD-DFT at the same level of theory was used to predict absorption and emission energies. HOMO and LUMO energies were also calculated using cyclic voltammetry and were found to have good agreement with those obtained by theoretical calculations. The reorganizational energies calculated theoretically suggest that these compounds can act as good electron and hole transport materials.