Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

Abstract

Abstract:The ultrafast vibrational-electronic relaxation upon excitation into the singlet 1 A 2u (dσ*fpσ) excited state of the d 8 -d 8 binuclear complex [Pt 2 (P 2 O 5 H 2 ) 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 1 A 2u state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing (ISC) into the 3 A 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 1 A 2u to the 3 A 2u state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the 1 A 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.

Ultrafast Relaxation Dynamics of Osmium–Polypyridine Complexes in Solution

“…In fact, while Fe or Ru polypyridine complexes exhibited ISC times of <30 fs, 15,20 diplatinum complexes had ISC times three orders of magnitude longer. 38 An extreme case was also reported by Steffen et al for rhodium-cyclopentadienes, where the singlet fluorescence was dominant as a result of an inefficient ISC to the triplet state. 39 The picture that emerges from these observations is that the SOC constant is not the only important ingredient but that the density of states and the molecular structure need to be taken into account for a proper description of the ISC rates.…”

Ultrafast Wiggling and Jiggling: Ir2(1,8-diisocyanomenthane)42+

“…Looking ahead, TRIR has the potential to monitor interactions between electronically excited Ir(dimen) and substrates, including accompanying changes in electron density distributions and Ir oxidation states. Our finding of bridging-ligand vibrational response to Ir-Ir wave packet oscillations demonstrates that coherent effects in d 8_ d 8 complexes 1,3,6,9 extend well beyond the central dimetallic unit, thereby indicating that excited-state reactions (e.g., electron-, energy-, or atom transfer) could be coupled to intramolecular coherent motions triggered by ultrafast excitation.…”

“…The Lorentzian excited-state ν(C≡N) band shape (also found for 3 MLCT states of W(C≡N-Aryl) 6 complexes 11 ) is due mainly to homogeneous line broadening that in turn indicates uniformity in the interactions between the C≡N oscillator and solvent molecules over the ensemble of excited molecules.…”

“…1,2,3,4 Of particular interest is that the slow rates of excited singlet to triplet intersystem crossing (ISC) have allowed workers to detect and characterize the two spin states separately. 3,5,6 Many of these complexes also are redox-active, most especially as powerful reductants and oxidants in photoinduced electron-and atom transfer reactions. 1 II , and Au III complexes also exhibit rich photoluminescence and photochemistry.…”

“…2,5 Femtosecond laser excitation of these complexes creates a ν(M-M) wave packet whose coherent motion is manifested by oscillations of the stimulated emission (Pt, Ir) and excited-state absorption (Pt) that persist for more than 2 ps. 3 In addition to X-ray and optical spectroscopic studies, excited states of isocyanidebridged complexes are candidates for time-resolved IR (TRIR), which monitors the temporal evolution of features attributable to C≡N-stretching vibrations (ν(C≡N)). As the ν(C≡N) frequency and intensity depend strongly on both electronic and molecular structures, 11,12,13 TRIR spectroscopy undoubtedly will shed new light on d 8_ d 8 photophysics, most especially on the role of ligands in excited-state dynamics.…”