PET-Driven Fluorescence Modulation in Halochromic Styryl Hemicyanine Dyes Targeting DNA Minor Groove

A new series of styryl hemicyanine dyes featuring substituted N-phenylpiperazine end groups was synthesized using an environmentally friendly procedure. The photophysical properties of the dyes were systematically investigated in organic solvents of varying polarity and when bound to DNA, using a combination of spectroscopic techniques. The dyes show strong negative solvatochromism and exhibit fluorescence quenching upon DNA binding. The dyes are definitely halochromic, exhibiting pronounced fluorescent acidochromism, accompanied by a photoinduced electron transfer (PET) effect. Titration with acid of the dye-DNA complexes restores fluorescence, indicating suppression of the PET and, at the same time, rigidizing of the chemical structure. UV/VIS and fluorescence titration, circular dichroism spectroscopy, and molecular docking methods were used to investigate the interaction mode between the dyes and DNA. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) quantum chemical calculations were employed in deciphering the observed spectroscopic behavior and PET-related effects. The obtained results suggest the dyes' potential as pH-responsive fluorescent probes for nucleic acid environments.