Fe 0 at -1.77 eV, which shifts 180 mV more negative than the corresponding reduction potential of compound 2. Excitation of the MLCT absorption of compound 1b leads to a strong luminescence at 740 nm with lifetime of 105 ns in CH 3 CN. However, the luminescence of dyad 1a was significantly quenched with efficiency of 92%. Under the same condition, the luminescence of 1b was slightly quenched by one equivalent of compound 2. The quenching efficiency of 1b was changed from 4% to 65% when the concentration of 2 was increased from 1 to 10 equivalents. According to Rehm-Weller equation, the photoinduced electron transfer from the excited osmium complex to Fe 2 S 2 moiety neither in intramolecular dyad 1a nor in intermolecular (1b＋2) system is feasible for the uphill free energy change. Consequently, the luminescence quenching in intramolecular dyad 1a and intermolecular (1b＋2) system was tentatively attributed to energy transfer from 3 MLCT of osmium complex to Fe 2 S 2 cluster.