Spectroscopic properties of [Ru(bpy)₂(bpz)]²⁺

Abstract

We report on the temperature and magnetic field dependence of the low-temperature emission and excitation spectra of [Ru(bpy)₂(bpz)]²⁺ doped into [Zn(bpy)₃](ClO₄)₂. We could identify, for the first time, the origins corresponding to the three lowest electronic states. They are assigned to be triplet sublevels of a Ru4d - (bpy)π* metal to ligand charge transfer (MLCT) excitation. The energies of these zero-phonon lines are 15804 cm⁻¹ (line I), 15822 cm⁻¹ (line II), 15899 cm⁻¹ (line III). These sharp electronic transitions (halfwidths ca. 2 cm⁻¹) are associated with broad emission bands, which result from superpositions of vibrational satellites and multi-phonon structures. In particular, it is shown that magnetic fields have a drastic influence on the lowest electronic transition (including the vibronic structure). This transition is strongly forbidden. Thus, at zero field the origin line I is only detected in emission (τ = 130 μs). However, application of magnetic fields of B >= 8 T leads to a growing in of the corresponding excitation line, being in resonance with the luminescence line. For B = 12 T, this lowest origin line exhibits a Zeeman red-shift of ca. 4 cm⁻¹. However, the associated broad emission maximum is blue-shifted by ca. 400 cm⁻¹, where the shift attains a saturation near B = 4 T. These results are explained by a field-induced mixing of the wavefunctions of the lowest excited states. The described effects result from the large differences in the vibronic radiative deactivation paths and the radiative rates of the involved states.