Enhanced Emission in Self-Assembled Phenyleneethynylene Derived π-Gelators

Optical properties of pi-systems are of great significance for a wide range of applications in materials and biology. Aggregation and self-assembly induced emission are one of such phenomena. Herein, the self-assembly induced modulation of the emission of p-phenyleneethynylene (PE) chromophores bearing linear achiral (1) or branched chiral (2) alkoxy chains is reported. Self-assembled structures of both 1 and 2 from n-decane exhibit enhanced emission with fluorescence quantum yield (phi(F)) values of 0.34 and 0.25, respectively, whereas these molecules are less-emissive in chloroform (phi(F) = 0.02). Transmission electron microscopy and fluorescence microscopy studies reveal the formation of entangled blue-emissive fibers for 1 and supercoiled helical blue-emissive fiber bundles for 2. At higher concentrations (8.8 x 10(-3) m for 1 and 23.6 x 10(-3) m for 2) in n-decane, intense blue-emitting gels are formed. Significant shift in the emission toward longer wavelength can be seen from solution state to aggregates to the gel state. The wide-angle X-ray scattering and fluorescence data indicate that the interdigitated lamellar assembly with weaker pi-stacking and the resultant restriction of rotation of the PE chromophores are responsible for the enhanced emission of the self-assembled gel state.