Electronic energy trapping in a crystal due,to a dopant of higher excitation energy than the host

An excimer emitting crystal (9-cyanoanthracene) doped with a guest molecule (9-methoxyanthracene) having its first singlet level ca. 2000 cm^?1 above the host singlet exciton band exhibits efficient energy trapping as demonstrated by host sensitized, red-shifted emission and hetero-photodimerization. It is considered that the trapping is due to exciplex formation between host and guest molecules.