Cocrystalline Phases of Umbelliferone with Syndiotactic Polystyrene: A Route to Achieve Optical Brightening Films

Sorption of a fluorescent coumarin (7-hydroxy coumarin, umbelliferone, UMB) in delta and epsilon nanoporous crystalline (NC) syndiotactic polystyrene (sPS) films is studied. The formation of delta and epsilon cocrystalline (CC) phases of sPS with UMB is proved by wide-angle X-ray diffraction (WAXD) and polarized Fourier transform infrared spectroscopy (FTIR) measurements. For delta-form films, UMB guest molecules are mainly isolated in the crystalline cavities, with their molecular plane preferentially oriented perpendicular to the crystalline phase chain axes. For epsilon-form films, on the contrary, UMB guest molecules are mainly included as hydrogen-bonded linear aggregates in the crystalline channels with their molecular planes preferentially parallel to the crystalline phase chain axes. Procedures of sorption/desorption of suitable volatile guests on these sPS/UMB CC films lead to relevant FTIR spectral changes in both carbonyl and hydroxyl stretching regions, which indicate the formation of UMB hydrogen-bonded aggregates. Procedures of volatile-guest sorption/desorption on CC epsilon films also lead to remarkable changes in polarized FTIR spectra, with loss or even inversion of the sign of dichroism for all guest peaks, which indicate that UMB molecules are expelled from the epsilon crystalline phase and give rise to hydrogen-bonded aggregates in the polymer amorphous phase, where they assume molecular plane orientation preferentially perpendicular to the film stretching direction. Our analysis suggests that these aggregates could correspond to columns of UMB molecules, with column axes being preferentially parallel to the film stretching direction. The formation of hydrogen-bonded UMB aggregates leads to a shift of the emission band from the near-UV region to the visible region, thus producing optical brightening of the sPS films.