{Lambda-[Ru(bpy)(3)]Delta-[Os(bpy)(3)]}(PF6)(4) is the first representative of anew class of materials. The crystal structure is built up of homochiral layers of Lambda-[Ru(bpy)(3)](2+) that alternate with homochiral layers of Delta-[Os(bpy)(3)](2+). At low temperature, the excitation energy is transferred efficiently by radiationless processes to one single crystallographic site of Delta-[Os(bpy)(3)](2+). Due to energy accumulation at this site of lowest energy, the new compound exhibits the interesting property of self-site-selectivity. Thus, one obtains highly resolved one-site emission spectra of Delta-[Os(bpy)(3)](2+) for every excitation wavelength. With application of a magnetic field, e.g., of B = 12 T, the intensity of the lowest electronic 0-0 transition increases by several orders of magnitude and the vibrational satellite structure is changed from a vibronically induced (Herzberg-Teller) to a Franck-Condon induced structure. This is due to a magnetically induced coupling between lower lying triplet substates. Similar observations have not been reported yet for other compounds. (C) 2002 Elsevier Science B.V. All rights reserved.