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.