Vibrational relaxation and intersystem crossing of binuclear metal complexes in solution

The ultrafast vibrational-electronic relaxation upon excitation into the singlet ¹A_2u (dδ*→?pδ) excited state of the d⁸-d⁸ binuclear complex [Pt ₂(P₂O₅H₂)₄]^4- has been investigated in different solvents by femtosecond polychromatic fluorescence up-conversion and femtosecond broadband transient absorption (TA) spectroscopy. Both sets of data exhibit clear signatures of vibrational relaxation and wave packet oscillations of the Pt-Pt stretch vibration in the ¹A_2u state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing (ISC) into the ³A _2u state. The vibrational relaxation and ISC times exhibit a pronounced solvent dependence. We also extract from the TA measurements the spectral distribution of the wave packet at a given delay time, which reflects the distribution of Pt-Pt bond distances as a function of time, i.e., the structural dynamics of the system. We clearly establish the vibrational relaxation and coherence decay processes, and we demonstrate that PtPOP represents a clear example of a harmonic oscillator that does not comply with the optical Bloch description due to very efficient coherence transfer between vibronic levels. We conclude that a direct Pt-solvent energy dissipation channel accounts for the vibrational cooling in the singlet state. ISC from the ¹A_2u to the ³A_2u state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the ¹A_2u excited state. The particular structure, energetics, and symmetry of the molecule play a decisive role in determining the relatively slow rate of ISC, despite the large spin-orbit coupling strength of the Pt atoms.