Probing the Nature of the Redox Products and Lowest Excited State of [(bpy)2Ru(μ-bptz)Ru(bpy)2]4+: A Resonance Raman Study

The resonance Raman and infrared spectra of the bridging ligand 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) have been studied. Ab initio calculations on bptz and the radical anion bptz·− are able to predict the frequencies and intensities of the vibrational spectra. This analysis is used to interpret the vibrational spectroscopy of the bridged binuclear complexes [(bpy)2Ru(μ-bptz)Ru(bpy)2]4+ (1) and [(PPh3)2Cu(μ-bptz)Cu(PPh3)2]2+ (2) and their redox products. The resonance Raman spectroscopy of 1 reveals the bichromophoric nature of the complex, with bpy and bptz ligand modes being enhanced selectively as a function of excitation wavelength. The UV/Vis spectrum of 1+ reveals a strong IVCT band suggesting the mixed-valent species is Class III in nature. This is consistent with findings for other metal moieties with the bptz ligand. The resonance Raman spectrum of 1+ is also consistent with a Class III formulation. The resonance Raman spectrum of 2− is used to model bptz·− and the ab initio calculations show some correlation with the observed spectrum. The excited-state resonance Raman spectra of 1, generated with 532 and 633 nm excitation, show spectral features that are not strongly characteristic of bptz·−. This appears to be a consequence of the delocalised nature of the molecular orbitals involved in the excited-state of 1.