INFLUENCE OF ALKYL CHAIN-LENGTH ON THE FLUORESCENCE OF CRYSTALLINE 4-ALKYL-N-(4-CYANOPHENYL)PIPERIDINES

The solid-state emission spectra of a series of six 4-alkyl-N-(4-cyanophenyl)piperidines (I (n = 4-9)) are reported. They provide a rich and unexpected diversity even though each of the emission spectra from I contains the same lumophoric group that leads in solution to indistinguishable absorption, emission, and excitation spectra. Since the source of the differences must be related to how the homologues are packed in their unit cells, an attempt was made to correlate the emission spectra and molecular packing (from X-ray diffraction studies) using a density of states exciton model. The model fails to explain satisfactorily the data due to several factors, including a lack of inclusion of dynamic factors and the necessity to assume that atomic coordinates of ground- and excited-state molecules are the same in the crystals. Although exciton hopping to energy sinks (i.e., molecules in a unit cell with lowest excitation energies) may be very important, hopping to defect sites does not contribute significantly to the reported spectra.