Elucidation of Anthracene Arrangement for Excimer Emission at Ambient Conditions

Solid-state excimer emission of anthracene and its derivatives is a rare case at ambient conditions. We have designed organic salts composed of 9,10-bis(4-aminophenyl)anthracene (BAPA) and mineral acids in order to regulate the anthracene arrangement for investigation of the plausible geometry. From fluorescence measurement of three BAPA salts (nitrate: salt 1, chloride: salt 2, phosphate: salt 3), emission colors were changed by the effect of the mineral acids on the crystal structure. Notably, salt 3 crystal exhibited a bluish-green color derived from excimer emission at ambient conditions. On the basis of X-ray crystallographic analysis, the excimer emission of the salt 3 crystal was attributed to a tilt slide type of anthracene geometry. In the geometry, pi-planes of the anthracene moieties partially overlapped next to each other, an angle between the pi-planes is 44 degrees, and the nearest C-C distance is 3.7 angstrom. Such molecular geometry of partial overlapping of the anthracene rings and slightly longer C-C distance than that of common active pi-pi interaction (3.4-3.5 angstrom) was constructed of OH center dot center dot center dot O hydrogen bonds among mineral acid ions. These results suggest that the hydrogen bonds among mineral acid ions lead to the proximity of BAPA, following the excimer emission.