Explicit spectral response of the geometrical isomers of a bio-active pyrazoline derivative encapsulated in β-cyclodextrin nanocavity: a photophysical and quantum chemical analysis.

The existence of two geometrical isomers (cis- and trans-) of a biologically significant pyrazoline derivative [5-(-1'-(4-bromo-phenyl)-3a',7a'-hexahydro-1'H-indazol-3'-yl)-3-methyl-1-phenyl-1H-pyrazole-4-carbonitrile] (PZ) has been established using a combined theoretical and experimental investigation. Solvatochromic analysis of PZ revealed the existence of said cis- and trans- isomers. The unique solvatochromic response of the PZ isomers and their preferential encapsulation within β-cyclodextrin (β-CD) nanocavity clearly shows the difference in the behavioral nature of the isomers of PZ in homogeneous and heterogeneous medium. Solvent polarity, time-resolved study, and anisotropy results also reinforce in favor of the existence of the isomers. To evaluate the actual orientation of cis and trans-PZ, the ground and excited state geometry of these isomers were optimized by the DFT/LanL2DZ and CIS/LanL2DZ methods, respectively. The experimentally observed results and the theoretically calculated results are found to be in close agreement.