Design and synthesis of triphenylamine-malonitrile derivatives as solvatochromic fluorescent dyes

A series of intramolecular charge transfer (ICT) molecules with multiple dicyanovinyl substitutes on triphenylamine were synthesized in good yields by Knoevenagel condensation of corresponding triphenylamine aldehydes and malonodinitrile. Photophysical property of these triphenylamine malonitrile dyes were studied, along with their intriguing solvatochromic behavior. The fluorescence emission of most dyes was highly sensitive to solvent polarity, yielding blue to yellow and even red fluorescence in different organic solvents. Both the fluorescence and the Stokes shifts were linearly dependent on the orientation polarizability (Δf) and empirical polarity parameter ET(30) according to Reichardt–Dimroth equation. Further analysis suggested that the solvatochromic behavior of these dyes was caused by dipole moment change upon excitation from ground to excited state.