We compare the photoluminescence spectra of the low-temperature Mott insulator $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Cl ($T_{\mathrm{MIT}}=40$ K) with spectra of metallic $\kappa$-(BEDT–TTF)${}_2$Cu[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)${}_2$Cu[N(CN)${}_2$]Cl we observe a luminescence band at 1.95 eV due to the recombination of an exciton created by a highest occupied molecular orbital–lowest unoccupied molecular orbital optical excitation. This luminescence is quenched both in the high-temperature bad metal state of $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Cl and in metallic $\kappa$-(BEDT–TTF)${}_2$Cu[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.

• Received 7 March 2013

DOI:https://doi.org/10.1103/PhysRevB.88.115109