Ultrafast electronic-vibrational relaxation upon excitation of the singlet charge-transfer b (1)A' 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 addition, energies, characters, and molecular structures of the emitting states were calculated 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 a (3)A" state at longer times. The analysis of the data allows us to identify three spectral components. The first two are characterized by decay times tau 1 = 85-150 fs and tau 2 = 340-1200 fs, depending on L, and are identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b (3)A"), respectively. The third component corresponds to the long-lived phosphorescence from the lowest a (3)A" state. In addition, it is found that the fluorescence decay time (tau 1) corresponds to the intersystem crossing (ISC) time to the two emissive triplet states. tau 2 corresponds to internal conversion among triplet states. DFT results show that ISC involves electron exchange in orthogonal, largely Re-localized, molecular orbitals, whereby the total electron momentum is conserved. Surprisingly, the measured ISC rates scale inversely with the spin-orbit coupling constant of the ligand L, but we 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.

No abstract

Femto-to picosecond excited-state dynamics of the complexes [Re(L)(CO) 3 (N,N)] n (N,N = bpy, phen, 4,7dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1þ) were investigated using fluorescence up-conversion, transient absorption in the 650-285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (∼100 fs) -1 than in imidazole (∼150 fs) -1 complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV-vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N •f Re II ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy •-) transitions. For phen and dmp, the UV excited-state absorption occurs at ∼305 nm, originating from a series of mixed ππ* and Re f CO;N,N •-MLCT transitions. UV-vis transient absorption features exhibit small intensity-and band-shape changes occurring with several lifetimes in the 1-5 ps range, while TRIR bands show small intensity changes (e5 ps) and shifts (∼1 and 6-10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (g15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed.

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 (FlUC) and UV-vis transient absorption (TA) with approximately 100 fs time resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF(4) ionic liquid. The complexes show a very broad fluorescence band at 540-550 nm at zero time delay, which decays with 100-140 fs (depending on L) by intersystem crossing (ISC) to a pipi* intraligand ((3)IL) and a Re(L)(CO)(3) --> bpy charge-transfer ((3)CT) excited states. A second emission decay component (1.1-1.7 ps), apparent in the red part of the spectrum, is attributed to (3)IL --> (3)CT conversion, leaving phosphorescence from the lowest (3)CT 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 that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are due 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 (3)IL --> (3)CT conversion as well as electronic and vibrational relaxation, and a approximately 15 ps rise of only the 373 nm pipi*(bpy(*-)) band, which slows down to approximately 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 in other classes of metal-based molecular complexes.

The divinylphenylene-bridged diruthenium complexes (E,E)-[{(P i Pr 3) 2 (CO)ClRu} 2 (µ-HCdCHC 6 H 4 CHd CH-1,3)] (m-2) and (E,E)-[{(P i Pr 3) 2 (CO)ClRu} 2 (µ-HCdCHC 6 H 4 CHdCH-1,4)] (p-2) have been prepared and compared to their PPh 3-containing analogues m-1 and p-1. The higher electron density at the metal atoms increases the contribution of the metal end groups to the bridge-dominated occupied frontier orbitals and stabilizes the various oxidized forms with respect to those of m-1 and p-1. This has been confirmed and quantified electrochemically, because the two reversible oxidation waves were observed at considerably lower potentials than for the PPh 3 complexes. Owing to their greater stability, the one-and two-electronoxidized forms m-2 n+ and p-2 n+ of both complexes could be generated and spectroscopically characterized inside an optically transparent thin layer electrolysis cell. UV/vis/near-IR and ESR spectroelectrochemistry indicates that the oxidation processes are centered at the organic bridging ligand. σ-Bonded divinylphenylenes thus constitute an unusual class of "noninnocent" ligands for organometallic compounds. Electronic transitions observed for the mono-and dioxidized forms closely resemble those of donorsubstituted phenylenevinylene compounds, including oligo(phenylenevinylenes) (OPVs) and poly-(phenylenevinylene) (PPV) in the respective oxidation states. Strong ESR signals and nearly isotropic g tensors are observed for the monocations in fluid and frozen solutions. The metal contribution to the redox orbitals is illustrated by a shift of the CO stretching bands to notably higher energies upon stepwise oxidation. The shifts strongly exceed those observed for the PPh 3 containing, six-coordinated species (E,E)-[{(PPh 3) 2 (CO)Cl(L)Ru} 2 (µ-HCdCHC 6 H 4 CHdCH)] n+ (L) substituted pyridine). IR spectroelectrochemistry reveals the presence of two electronically different transition-metal moieties in m-2 + , while they resemble each other more closely in p-2 +. Differences in electronic coupling are illustrated by the charge distribution parameters calculated from the spectra. Bulk electrolysis experiments confirm the results from the in situ spectroelectrochemistry and the overall stoichiometry of the redox processes. Quantum-chemical calculations were performed in order to provide insight into the nature and composition of the frontier orbitals. The electronic transitions observed for the neutral forms were assigned by TD DFT. IR frequencies calculated for m-2 and p-2 in their various oxidation states retrace the experimental observations. They fail, however, in the case of m-2 + , where a symmetrical structure is calculated, as opposed to the distinctly asymmetric electron distribution observed by IR spectroscopy. Geometryoptimized structures were calculated for all accessible oxidation states. The structural changes following stepwise oxidation agree well with the experimental findings: e.g., a successive low-energy shift of the CdC stretching vibration of the bridge. The radical cation m-...

The chelate ligand dipyrido [3,2-a:2',3'-c] phenazine (dppz), its 11,12-dimethyl derivative dmdppz, and corresponding complexes with [Ru(bpy)2]2+ were studied in multiply reduced states by low-temperature cyclic voltammetry and UV/vis and EPR spectroscopy. The (dm)dppz ligands are reduced in two reversible steps, followed by a very moisture-sensitive third step. Highly resolved EPR and 'H-ENDOR spectra of the intermediate anion radicals were obtained and analyzed. The results are interpreted using a HMO/McLachlan perturbation approach of x spin populations and orbital energies. Three low-lying unoccupied x molecular orbitals can be identified as phenazinetype (bi, lowest) and as the ^(b,) and xfo) orbitals of the a-diimine moiety. Complexes with the N(4),N(3)-bound [Ru(bpy)2]2+ fragment show at least six reversible one-electron reduction steps in rigorously dried DMF at 200 K; the first four persistent reduced states were characterized by EPR and UV/vis spectroscopy. The EPR spectra of the first three reduced states of the complexes show a signal which proves the occupation of the phenazine-localized x* orbital of (dm)dppz by a single electron, the stepwise reduction of the bpy ligands resulting in temperaturedependent intensity loss of that EPR signal. The very basic quadruply reduced state exhibits EPR characteristics which are typical for Ru(II)-bound a-diimine anion radicals. All assignments are supported by UV/vis spectra and analyses of redox potential values. Because the very easily protonated higher reduced states are not sufficiently persistent for EPR and UV/vis characterization, further assignments could thus be based only on the analysis of redox potential values. The particular composite electronic structure of the complexes with differing redox and "optical" orbitals is related to their "light switch" behavior, i.e. to the absence of luminescence quenching in a nonaqueous environment.

The ReI(CO)3(4,7-dimethyl-1,10-phenanthroline)(histidine-124)(tryptophan-122) complex, denoted ReI(dmp)(W122), of Pseudomonas aeruginosa azurin behaves as a single photoactive unit that triggers very fast electron transfer (ET) from a distant (2 nm) CuI center in the protein. Analysis of time-resolved (ps-μs) IR spectroscopic and kinetics data collected on ReI(dmp)(W122)AzM (M = ZnII CuII, CuI; Az = azurin) and position-122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants together with excited-state DFT/TDDFT calculations and X-ray structural characterization reveal the character, energetics, and dynamics of the relevant electronic states of the ReI(dmp)(W122) unit and a cascade of photoinduced ET and relaxation steps in the corresponding Re-azurins. Optical population of ReI(imidazole-H124)(CO)3→dmp 1CT states is followed by ~110 fs intersystem crossing and ~600 ps structural relaxation to a 3CT state whose IR spectrum indicates a mixed ReI(CO)3,A→dmp/π→π*(dmp) character for aromatic amino acids A122 (A = W, Y, F) and ReI(CO)3→dmp MLCT for ReI(dmp)(K122)AzCuII. In a few ns, the 3CT state of ReI(dmp)(W122)AzM establishes an equilibrium with the ReI(dmp•−)(W122•+)AzM charge-separated state, 3CS, whereas the 3CT state of the other Y, F, and K122 proteins decays to the ground state. In addition to this main pathway, 3CS is populated by fs and ps W(indole)→ReII ET from 1CT and the initially “hot” 3CT states, respectively. The 3CS state undergoes a tens-of-ns dmp•−→W122•+ ET recombination leading to the ground state or, in the case of the CuI azurin, competitively fast (~30 ns over 1.12 nm) CuI→W•+ ET producing ReI(dmp•−)(W122)AzCuII. The overall photoinduced CuI→Re(dmp) ET through ReI(dmp)(W122)AzCuI occurs over a 2 nm distance in <50 ns after excitation, the intervening fast 3CT-3CS equilibrium being the principal accelerating factor. No reaction was observed for the three Y, F, and K122 analogues. Although the presence of Re(dmp)(W122)AzCuII oligomers in solution was documented by mass spectrometry and phosphorescence anisotropy, kinetics data do not indicate any significant interference from intermolecular ET steps. The ground-state dmp-indole ππ interaction together with well-matched W/W•+ and excited-state ReII(CO)3(dmp•−)/ReI(CO)3(dmp•−) potentials, that result in very rapid electron interchange and 3CT - 3CS energetic proximity, are the main factors responsible for the unique ET behavior of ReI(dmp)(W122)-containing azurins.

We herein describe a systematic account of mononuclear ruthenium vinyl complexes L-{Ru}-CH=CH-R where the phosphine ligands at the (PR'3)2Ru(CO)Cl={Ru} moiety, the coordination number at the metal (L = 4-ethylisonicotinate or a vacant coordination site) and the substituent R (R = nbutyl, phenyl, 1-pyrenyl) have been varied. Structures of the enynyl complex Ru(CO)Cl(PPh3)2(eta1:eta2-nBuHC=CHCCnBu), which results from the coupling of the hexenyl ligand of complex 1a with another molecule of 1-hexyne, of the hexenyl complexes (nBuCH=CH)Ru(CO)Cl(PiPr3)2 (1c) and (nBuCH=CH)Ru(CO)Cl(PPh3)2(NC5H4COOEt-4) (1b), and of the pyrenyl complexes (1-Pyr-CH=CH)Ru(CO)Cl(PiPr3)2 (3c) and (1-Pyr-CH=CH)Ru(CO)Cl(PPh3)3 (3a-P) have been established by X-ray crystallography. All vinyl complexes undergo a one-electron oxidation at fairly low potentials and a second oxidation at more positive potentials. Anodic half-wave or peak potentials show a progressive shift to lower values as pi-conjugation within the vinyl ligand increases. Carbonyl band shifts of the metal-bonded CO ligand upon monooxidation are significantly smaller than is expected of a metal-centered oxidation process and are further diminished as the vinyl CH=CH entity is incorporated into a more extended pi-system. ESR spectra of the electrogenerated radical cations display negligible g-value anisotropies and small deviations of the average g-value from that of the free electron. The vinyl ligands thus strongly contribute to or even dominate the anodic oxidation processes. This renders them a class of truly "non-innocent" ligands in organometallic ruthenium chemistry. Experimental findings are fully supported by quantum chemical calculations: The contribution of the vinyl ligand to the HOMO increases from 46% (Ru-vinyl delocalized) to 84% (vinyl dominated) as R changes from nbutyl to 1-pyrenyl.