The influence of crystal packing on the solid state fluorescence behavior of alkyloxy substituted phenyleneethynylenes

The study reports the solid state photophysical properties of a series of alkyloxy-substituted oligo(phenyleneethynylene) s, methoxy to hexyloxy, supported by a detailed analysis of molecular packing obtained from single crystal X-ray diffraction data. While the emission peaks are highly red shifted (by as much as 114 nm) in the solid state, all molecules exhibit similar absorption and emission in dilute solutions. The red shift is maximum in ethoxy and minimum in methoxy, while other crystalline films exhibit intermediate values. In the crystal structures, the spacing between the molecular pairs forming the J-aggregates is varied between 3.48 to 4.65 angstrom, with no systematic dependence on the chain length. The red shifted emission maximum is found to vary linearly with the spacing between the interacting molecules in the J-aggregate. Thus, the emission in the solid state is determined by the extent of dipolar coupling between the molecules, the alkyl chain length influencing the properties only indirectly.