Substitution induced solvatochromism behaviour of blue emissive N-aryl pyrazoline derivatives and DFT studies

In the present investigation, a series of 5-[4-(benzyloxy)phenyl]-3-(thiophen-2-yl)-4,5-dihydro-1H-pyrazole derivatives (A-E) were successfully synthesized via the cyclization of (2E)-3-[4-(benzyloxy)phenyl]-1-(thiophen-2-yl)prop-2-en-1-one (1) with differently substituted phenyl hydrazine respectively. The substituents on the para position of phenyl ring were methyl (-CH₃), bromine (-Br) cyano (-CN) and nitro (-NO₂) groups, leading to the formation of distinct pyrazoline derivatives. This study highlights the influence of different substituents as well as solvents on the potential physicochemical properties of these synthesized pyrazoline derivatives. To investigate the absorption and emission behaviours of the pyrazoline derivatives in various polar and nonpolar solvents; the solvent-solute interactions were analyzed using the solvent polarity parameter approach and Lipperte-Mataga equation. In polar solvents the compounds A-D exhibited higher Stokes shifts than nonpolar solvents. So the change in excited state intermolecular interactions such as H-bonding brought changes in dipole moments. These derivatives were evaluated for their fluorescence quantum yield (QY) which exhibited fluorescence emission in blue region, as confirmed by CIE diagram. The QY was also effected by polarity of the solvents as compounds A-D gave least QY in methanol compared to BuOH. The solid state absorption and emission spectral analysis indicated red shift in the emission spectra than in solvents with highest for compound E (-NO₂) which could be accounted for J-aggregation in the solid state. These aggregation patterns changed in solution state. The compound E did not show any emission in the polar solvents due to quenching. Additionally, Density functional theory (DFT) calculations were conducted to evaluate the optimized theoretical molecular geometry and frontier molecular orbital properties of the five synthesized organic molecules indicated their semiconductor behaviour. These results provide an avenue to consider the pyrazoline molecules as excellent luminescent materials.