Fluorescence properties of some polyurethanes with stilbene and pyrene rings
S. Gurluia, E. Buruianab, T. Buruianab, G. Stratc, M. Strata, V. Pohoataa
a Al. I. Cuza University of Iasi, Romania (sgurlui@uaic.ro)
bP. Poni Institut of Macromolcular Chemistry, Iasi, Romania
cGh. Asachi Technical University of Iasi, Romania
The main motivation for this work is to broaden the platform of photoreactive coatings through the preparation of new polyurethane ionomers with stilbene and pyrene pendants covalently attached to polymers to evaluate and eventually improve the sensitivity of the resulting materials. The incorporating of a trans-stilbene chromophore and pyrene chromophore in the polymeric chains proved to be a useful tool for developing of polymer properties such as LC behavior, NLO response, and electroluminescent effect or to investigate some biomembranes. Given the broad spectrum of properties of polyurethanes and implicitly their applications in various fields including biomedical area, a series of ionic or non-ionic polymers bearing stilbene groups anchored in the hard segments of the polyurethane backbone were prepared and characterized. Although it is well known that the photoisomerization of the trans stilbene to cis isomer controls the chain conformation in the polymer backbones and is accompanied of a loss of the fluorescence, there is not a complete theory capable to explain the relationships between photochemical/photophysical process and polymer properties. The microstructure determination of the photopolymers and the presence of micro/nanostructured aggregates in thin films were evidenced by fluorescent spectroscopy and AFM technique. Hydrophilic ionic polyurethanes with pyrene chromophore were prepared by a quaternization reaction, in which the amount of pyrene covalently attached to the polymeric backbone ranged from 1.14 to 19.82 mmol of fluorophore/100 g of polymer. The process of excimer formation between the pyrene molecules attached to the ionic polyurethane was investigated in dimethylformamide(DMF), tetrahydrofuran (THF), film, and THF/H2O to illustrate the expected differences in the polymer behavior compared with that of the starting pyrene derivative. The formation of aggregates or core-shell micelles was sustained by the fluorescence data, which indicated the existence of pyrene units in the ground state of the molecule, giving rise thus to an explanation for the high excimer-to-monomer intensity ratio. The fluorescence decay of pyrene polyurethanes in the presence of various concentrations of nitrobenzene used as a quencher was analyzed, too.