Vibronic and magnetic coupling in the radiative deactivation of the lowest excited state of tris(2,2'-bipyridine)osmium(2+) doped into tris(2,2'-bipyridine)ruthenium(II) bis(hexafluorophosphate)

Abstract

Small amounts of [Os(bpy)₃]²⁺+ doped into single-crystal [Ru(bpy)₃](PF₆)₂ exhibit highly resolved MLCT spectra corresponding to the transitions between the ground state and the lowest excited states. The electronic origin as well as the vibronic satellites appear as sharp lines with half-widths of ca. 2 cm⁻¹. Three distinct spectroscopic sites are identified. For the lowest energy site the lowest excited state |I) is located at 14423 cm⁻¹ and the second excited state |II) lies 72 cm⁻¹ above |I). Due to the polarization properties of the origins both states are assigned to be doubly degenerate (E representations in the D₃ double group). The emission from |I) shoes a very weak origin line compared to the intense vibronic satellites which mostly correspond to IR-active vibrations. It is proposed that the vibronic intensitiy is induced by spin-vibronic and/or spin-orbit-vibronic coupling. The electronic state(s) supplying allowedness to the radiative decay from |I) are assigned to doubly degenerate E state(s) of singlet parentage. Further, magnetic fields induce a mixing of the wave functions of |I) and |II), which results in an intensity increase of the electronic origin of the perturbed state|I')(B) by a factor of about 1000. This is accompanied by the appearance of resonance-Raman vibrations, which display the properties of the (unperturbed) state |II). Thus, the vibronic coupling properties of the lowest excited state are tunable by magnetic fields. The intensities of the magnetic field induced lines increase with the square of the magnetic field strength, as anticipated by first-order perturbation theory. The highly resolved emission spectra did not allow to detect any progression. Thus, the nuclear equilibrium positions of the lowest excited states and the ground state should be very similar. This, as well as the occurence of doubly degenerate states, is not compatible with the model of localization of the excited electron on one particular bpy ligand.