Masking effect of the ICT features and thermodynamics of trans-4-(dimethylamino)-β-(l-naphthyl) styrene

Abstract

Trans-4-(dimethylamino)-β-(1-naphthyl) styrene (DMANS) exhibits a fair ability to undergo intramolecular charge transfer (ICT), which polarizes the molecule into rigid D +A − moieties. This causes the excited molecule to emit anomalous fluorescence; its quantum yield is a function of the rigidity, while its energy is dependent on the polarity. Glycerol affords the most suitable conditions assisting the molecular rigidity and polarity. Low-temperature measurements of DMANS fluorescence in glycerol reveal a drop in the emission half band width and a rise in its energy and intensity on lowering the temperature, with these dependencies being equilibrated near 273 K. This might demonstrate a state of thermodynamic equilibrium. Under ordinary conditions, ICT-induced fluorescence dominates, while emission from the locally excited state becomes increasingly dominant at lower temperatures, probably due to freezing dipole interactions. Protonation-induced masking effect of the lone pair electrons at the amino group quenches both the ICT absorption and fluorescence so that the polarized, rigid molecular structure of DMANS reverts to a more delocalized one. This could be the origin of the solid state phase transition induced in the protonated form. These proton-induced structural shifts are traced by IR and UV spectroscopy, both in absorption and emission as well as thermally by DTA.