Solvatochromic Study of 2‑(N,N‑dimethyl)-3-alkynyl-pyridine Fluorophores

A series of pyridine-based fluorophores2-(N,N-dimethyl)-3-alkynyl-pyridine (NNDAP), 4-[2-[2-(dimethylamino)-3-pyridyl]-ethynyl]-anisole (NNDAP-OMe), and 4-[2-[2-(dimethylamino)-3-pyridyl]-ethynyl]-benzonitrile (NNDAP-CN)were synthesized and their solvent-dependent photophysical properties were systematically investigated. Solvatochromic shifts in absorption and emission spectra were analyzed using the Bilot-Kawski, Lippert-Mataga, Bakhshiev, and Reichardt correlation methods to estimate ground- and first excited-state electric dipole moments. All compounds exhibit increased polarity in the excited state, with NNDAP-CN showing the largest change in dipole moment upon excitation. Solute-solvent interactions were also examined using the Kamlet-Taft and Catalán multiparameter linear solvation energy relationship models, revealing that solvent dipolarity/polarizability interactions influence absorption shifts, while emission shifts are influenced by a combination of solvent dipolarity/polarizability and solvent basicity. All fluorophores readily permeate HeLa cells and exhibit stable blue fluorescence localized primarily in the perinuclear cytoplasmic region, supporting their potential application as environment-sensitive fluorescent probes.