Low-Molecular Push–Pull Pyrazoline Derivatives: Solvatochromic Effect and Theoretical Insights into Dye-based Molecular Engineering

Intermolecular interactions between dyes and solvents are essential in understanding a number of phenomena. In this contribution, we focused on similarities and differences between two molecules belonging to the same pyrazoline derivatives group: the small and rigid- 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) molecule and the quite long and flexible one of (4-(2-(1-phenyl-4,5-dihydro-1H-pyrazol-3-yl)­vinyl)­benzylidene) malononitrile (PY-PhdiCN). We experimentally show and use theoretical calculations to prove that extension of π-connector significantly changes the electrostatic potential distribution, which results in delocalization of negative charge along the whole donor group and increases potential surface of interaction with solvent molecules giving large spectral shifts. We also show that pyrazoline derivatives are very effective and sensitive solvent indicators with long-time stability.