Temperature-sensitive ICT complex and FT-IR spectral background of trans-4-(dimethylamino)-β-(1-naphthyl) styrene (DMANS) Original Research Article

trans-4-(Dimethylamino)-β-(1-naphthyl) styrene (DMANS) affords the structural prerequisites of intra-molecular charge transfer (ICT) that renders the molecule highly polarized of rigid D+–A− moieties. This accommodates fluorescence of a quantum yield rising with molecular rigidity. The D+–A− form is thermally unstable and undergoes a structural shift in the ground state into a less rigid form of higher mesomerism, especially on approaching the higher freedom molten phase. The frequency guide He–Ne laser beam stimulates such fluorescence when DMANS is measured in the solid state by FT-IR spectroscopy at room temperature. This results in a high spectral background that slightly obscures the vibrational spectrum, particularly at high frequencies. The spectral background drops with temperature exponentially with a relevant drop in the fluorescence quantum yield. The same result may be obtained upon masking the lone pair electrons, the origin of the ICT-complex and subsequent spectral features, by a proton. The excitation energy dissipates via a radiationless intra-molecular motion of rotation or twisting accommodated in the less rigid, more mesomeric form induced thermally or by protonation. The ICT and laser-induced fluorescence would then be quenched thermally as well as by protonation, which permits a spectral correlation for both the effects on the molecular structure and FT-IR spectral background.