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, π-planes of the anthracene moieties partially overlapped next to each other, an angle between the π-planes is 44°, and the nearest C–C distance is 3.7 Å. Such molecular geometry of partial overlapping of the anthracene rings and slightly longer C–C distance than that of common active π–π interaction (3.4–3.5 Å) was constructed of OH···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.