SOLID-STATE FLUORESCENCE OF AROMATIC DICARBOXAMIDES - DEPENDENCE UPONCRYSTAL PACKING

The fluorescence of six secondary arenedicarboxamides has been investi gated in single crystals, powders, melts, and ethanol solution. Single crystals of four rodlike arenedicarboxamides which pack in one-dimens ional tapes display dual emission attributed to excited-state monomers and dimers. These tapes have a constant 5-Angstrom separation between the long axes of neighboring arenes but differ in the arene-arene dih edral angle. The frequency shift between monomer and dimer fluorescenc e and the intensity and vibronic structure of the dimer emission are d ependent upon the ground-state geometry. Single crystals of two naphth alenedicarboxamides which pack in two-dimensional sheets and have no c lose contacts between neighboring arenes within the same sheet display only monomer emission. Melted and resolidified samples display broad structureless emission attributed to fluorescence from a mixture of ex cited monomers and dimers with different ground-state geometries.