Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

The ultrafast vibrational-electronic relaxation upon excitation into the singlet (1)A(2u) (d sigma*-> p sigma) excited state of the d(8)-d(8) 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 (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

Optical and infrared spectroelectrochemical studies of CN-substituted bipyridyl complexes of Ruthenium(II)

Ruthenium(II) polypyridyl complexes [Ru(CNMe-bpy)x(bpy)3−x]2+ (CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl2,2′-bipyridine, bpy = 2,2′-bipyridine, and x =1−3, abbreviated as 12+, 22+, and 32+) undergo four (12+) orfive (22+ and 32+) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+ and 32+ being separated by 150−210 mV; reduction of the unsubstituted bpy ligand in 12+ and 22+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Mebpy ligand shifts the ν(CN) IR band by ca. −45 cm−1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30 show distinct ν(C N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•−, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1−, and 2− exhibit a prominent band attributable to the CN-Me-bpy•− moiety between 6000 and 7500 cm−1, whereas bpy•−-based absorption occurs between 4500 and 6000 cm−1; complexes 2+, 3+, and 30 also exhibit a band at ca. 3300 cm−1 due to a CN-Me-bpy•− → CN-Me-bpy interligand charge-transfer transition. In the UV−vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(CN) IR band is shifted by −38 cm−1 and 5−7-fold-enhanced relative to the ground state

Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl-Bipyridine Complexes

Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b1A' state of [Re(L)(CO)3(bpy)]n (L = Cl, Br, I, n = 0; L = 4-Et-pyridine, n = 1+) in acetonitrile was investigated using the femtosecond fluorescence up-conversion technique with polychromatic detection. In addn., energies, characters, and mol. structures of the emitting states were calcd. by TD-DFT. The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a3A" state at longer times. The anal. of the data allows us to identify three spectral components. The first two are characterized by decay times t1 = 85-150 fs and t2 = 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b3A"), resp. The third component corresponds to the long-lived phosphorescence from the lowest a3A" state. In addn., it is found that the fluorescence decay time (t1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. t2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, MOs, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling const. of the ligand L, but the authors find a clear correlation between the ISC times and the vibrational periods of the Re-L mode, suggesting that the latter may mediate the ISC in a strongly nonadiabatic regime

Ultrafast Excited-State Dynamics ef [Re(L)(CO)(3)(bpy)](n) Complexes: Involvement of the Solvent

Ultrafast excited-state dynamics of [Re(L)(CO)(3)(bpy)](n) (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2'-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FIUC) and UV - vis transient absorption (TA) with similar to 100 is time,resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF4 ionic liquid. The complexes show a very broad fluorescence band at 540-550 nm at zero time delay, which decays with 100-140 Is (depending on L) by intersystem crossing (ISC) to pi pi* intraligand ((IL)-I-3) and a Re(L)(CO)(3) -> bpy charge-transfer ((CT)-C-3) excited states. A second emission decay component (1.1-1.7 ps). apparent in the red part of the spectrum, is attributed to (IL)-I-3 -> (CT)-C-3 conversion, leaving phosphorescence from the lowest (CT)-C-3 state as the only emission signal at longer time delays. The triplet conversion is slower in DMF? than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing,2 that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are clue to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV - vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to (IL)-I-3 -> (CT)-C-3 conversion as well as electronic and vibrational relaxation, and a similar to 15 ps rise of only the 373 nm pi pi*(bpy(center dot-)) band which slows down to similar to 1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative other classes of metal-based molecular complexes