Energy transfer and harvesting in [Ru_(1-x)Os_x(bpy)_3](PF_6)_2 and {Λ-[Ru(bpy)_3]Δ-[Os(bpy)_3]}(PF_6)_4

摘要

{Λ-[Ru(bpy)_3]Δ-[Os(bpy)_3]}(PF_6)_4 is the first representative of a new class of materials. The crystal structure is built up of homochiral layers of Λ-[Ru(bpy)_3]~(2+) that alternate with homochiral layers of Δ-[Os(bpy)_3]~(2+). In this new material, the excitation energy is transferred efficiently by radiationless processes to one single crystallographic site of Δ-[Os(bpy)_3]~(2+) (site A), where the excitation energy is harvested. We develop an understanding of: (1) the different steps of energy transfer, (2) the dominating mechanisms (Foerster and/or Dexter), and (3) the electronic states involved. Therefore, we study, typically at T = 1.3 K, properties of the low-lying excited states and of energy transfer processes occurring in rac-[Ru(bpy)_3](PF_6)_2, [Ru_(1-x)Os_x(bpy)_3](PF_6)_2 (with x = 0.01, 0.1, 0.2, and 1.0) and we compare the results to those obtained for {Λ-[Ru(bpy)_3]Δ-[Os(bpy)_3]}(PF_6)_4. It can be concluded for this latter compound that the different inter-layer and intra-layer steps of energy transfer to the lowest site of Δ-[Os(bpy)_3]~(2+) are dominated by efficient Dexter exchange processes. Due to energy accumulation at this lowest site, the new compound exhibits the interesting property of self-site-selectivity. This means, one obtains one-site emission spectra for every excitation wavelength from the UV to 693 nm. And since this lowest site of Δ-[Os(bpy)_3]~(2+) is well shielded from its environment, the spectra are highly resolved and thus reveal directly detailed properties of the low-lying electronic states and their vibronic coupling behavior. In particular, these properties depend strongly on an applied magnetic field. For example, with application of a field up to H = 12 T, the intensity of the lowest electronic 0-0 transition can be tuned in and increased by several orders of magnitude. Simultaneously, the vibrational satellite structure is completely altered. We observe changes from a vibronically induced (Herzberg-Teller) structure to a structure dominated by Franck-Condon satellites. This property of Δ-[Os(bpy)_3]~(2+) is due to a magnetically induced coupling between lower lying triplet substates. Similar observations have not yet been reported for other compounds.