Localized states in Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl probed by photoluminescence

We present the luminescence spectra of a low-temperature Mott insulator (T$_c$=35 K) $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl and a metal with a superconductor transition $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Br in the temperature range between 300 and 10 K. In the Mott insulating state we observe an appearance of a luminescence at 1.95 eV, which corresponds to a LUMO-HOMO transition in a BEDT-TTF molecule. This luminescence is quenched both in the higher-temperature semiconducting state of $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Cl and metallic $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Br. To our knowledge, it is the first observation of luminescence driven by electronic correlations. This observation gives an evidence of the local character of BEDT-TTF energy levels in the Mott insulating state, in contrast to the band-character in the metallic and semiconducting states.