  We compare the photoluminescence spectra of the low-temperature Mott
insulator \etcl (T_{MIT}=40 K) with spectra of metallic
\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br, which is superconducting below T_c=11.8 K, in
the temperature range between 300 and 20 K. In the Mott insulating state of
\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl we observe a luminescence band at 1.95 eV due
to the recombination of an exciton created by a HOMO-LUMO optical excitation.
This luminescence is quenched both in the high-temperature bad metal state of
\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl and in metallic
\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br. The observation of the luminescence of an
exciton provides evidence for the local character of excitations in the Mott
insulating state.
