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-cyano phenyl)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 indisting uishable absorption, emission, and excitation spectra. Since the sourc e of the differences must be related to how the homologues are packed in their unit cells, an attempt was made to correlate the emission spe ctra and molecular packing (from X-ray diffraction studies) using a de nsity of states exciton model. The model fails to explain satisfactori ly the data due to several factors, including a lack of inclusion of d ynamic factors and the necessity to assume that atomic coordinates of ground- and excited-state molecules are the same in the crystals. Alth ough exciton hopping to energy sinks (i.e., molecules in a unit cell w ith lowest excitation energies) may be very important, hopping to defe ct sites does not contribute significantly to the reported spectra.