Shape and size effects on the optical properties of piezochromic organic nanoparticles

Two oligo(phenylene vinylenes), namely, (E,E)-1,4-alkyloxy-2,5-bis[2-cyano-2-(4-(methylsulfanyl)phenyl)vinyl]benzene (1) and (E,E)-1,4-dioctyloxy-2,5-bis[2-(4-(methylsulfanyl)phenyl)vinyl]benzene (2) are synthesized. Both compounds are piezochromic fluorescent, and the alkyloxy substituents present in the compounds significantly influence their optical properties. Nanoparticles of various shapes ranging from spherical to clavate are prepared by the reprecipitation method. The fluorescent excitation and emission properties measured for the nanoparticles dispersed in water are compared with those measured for dilute solutions of the compound in THF. The nanoparticles exhibit a red shift in their fluorescence emission spectra with a decrease in efficiency as their size increases from 30 to 50 nm. In contrast, the molecular UV-Vis absorptions of the nanoparticles are blue-shifted in comparison to that of the solution spectra of the compound. The large n-octyloxy substituents at the phenylene core cause distortion of the aromatic rings from planarity and hinder π-stacking.