Antonín Vlček

Ligand-dependent excited state behaviour of Re(I) and Ru(II) carbonyl–diimine complexes

Abstract The relations between the structure and excited state properties of Re(E)(CO) 3 ( α -diimine) and Ru(E)(E′)(CO) 2 ( α -diimine) complexes (axial ligand E, E′=halide, alkyl, benzyl, metal fragment) are unravelled and discussed in detail. For example, it is shown how the increasing π -donor strength of an axial ligand, such as a halide changes the character of the lowest excited state from MLCT to LLCT. On the other hand, the presence of a covalently bound axial ligand in the coordination sphere introduces a σ π * lowest excited state that involves an excitation of an electron from the metal–ligand σ -bonding orbital to the π * orbital of the α -diiamine. While the orbital parentage of the lowest excited state—MLCT, LLCT, σ π * or IL—is mostly determined by the axial ligand(s), its detailed properties (energy, lifetime, reactivity, decay mechanism) are dependent on both the axial and diimine ligands. Depending on the molecular structure and the medium, the excited state behaviour of these complexes ranges from a strong, long-lived emission to a very fast photochemical homolysis of a metal–ligand bond. Photochemical and photophysical properties of the complexes with different types of the lowest excited state are explored and pertinent structural effects discussed. It is shown how the excited state properties of the Re and Ru carbonyl–diimine complexes can be controlled by a judicious choice of the axial and diimine ligands and by the medium. These relations can be employed to design new functional molecular photonic materials, e.g. sensitisers, luminophores, photocatalysts, radical photoinitiators, luminescent probes or sensors.

Tryptophan-Accelerated Electron Flow Through Proteins

Energy flow in biological structures often requires submillisecond charge transport over long molecular distances. Kinetics modeling suggests that charge-transfer rates can be greatly enhanced by multistep electron tunneling in which redox-active amino acid side chains act as intermediate donors or acceptors. We report transient optical and infrared spectroscopic experiments that quantify the extent to which an intervening tryptophan residue can facilitate electron transfer between distant metal redox centers in a mutant Pseudomonas aeruginosa azurin. CuI oxidation by a photoexcited ReI-diimine at position 124 on a histidine(124)-glycine(123)-tryptophan(122)-methionine(121) β strand occurs in a few nanoseconds, fully two orders of magnitude faster than documented for single-step electron tunneling at a 19 angstrom donor-acceptor distance.

Highlights of the spectroscopy, photochemistry and electrochemistry of [M(CO)4(α-diimine)] complexes, M=Cr, Mo, W

Abstract Tetracarbonyl-diimine complexes [M(CO) 4 (α-diimine)] (M=Cr, Mo, W; α-diimine=polypyridyl (bpy, phen), pyridine-2-carbaldehyde (R-PyCa) or 1,4-diaza-butadiene, (R-DAB)) have very interesting structural, spectroscopic, electrochemical and photochemical properties. Their comprehensive experimental and theoretical investigations have important implications for our understanding of the chemistry of organometallic complexes with noninnocent ligands. The most interesting physical and chemical aspects of [M(CO) 4 (α-diimine)] complexes, which have more general relevance, are highlighted and discussed.

The life and times of excited states of organometallic and coordination compounds

Abstract The photochemistry of transition metal organometallic and coordination compounds is discussed from the perspective of time domains, in which excited-state reactions occur. Ultrafast photochemical processes, which are competitive with nuclear motion, are distinguished from reactions of long-lived, thermally equilibrated, excited states. Many special photochemical features of transition metal complexes and organometallic compounds stem from the simultaneous presence of excited states of different localisations and orbital origins in the same chromophoric molecule, together with high state densities. A brief survey of recently studied systems and reactions shows the present level of understanding and highlights the scientific challenges and possible applications of photoprocesses occurring on different time scales. Factors that influence excited-state lifetimes are discussed and differences in the chemical properties of electronically excited and ground states demonstrated using the radical-like behavior of [Pt II 2 (P 2 O 5 H 2 ) 4 ] 4− and electron-transfer reactivity of [Ru(bpy) 3 ] 2+ as typical examples. It is shown that reactions of long-lived excited states can become ultrafast when a chromophore is inserted into a redox-active supramolecular assembly or attached to an electrode surface. This has important implications for light-energy conversion and manipulation of information at a molecular level. Optically prepared Franck–Condon excited states of many transition metal complexes have an ultrafast chemistry of their own. This includes relaxation to lower excited states, electron transfer, energy transfer or bond splitting. Typical examples are discussed with an emphasis on phenomena occurring only on very short time scales. A final outlook points to the most challenging questions in mechanistic inorganic photochemistry and to possible applications.

Ultrafast excited-state dynamics of rhenium(I) photosensitizers [Re(Cl)(CO)3(N,N)] and [Re(imidazole)(CO)3(N,N)]+: diimine effects.

Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)(3)(N,N)](n) (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) 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(•-) → 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 → 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 (≤5 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 (≥15 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.

Vibrational relaxation and intersystem crossing of binuclear metal complexes in solution.

The ultrafast vibrational-electronic relaxation upon excitation into the singlet (1)A(2u) (dσ*→pσ) 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.

Mechanistic roles of metal-to-ligand charge-transfer excited states in organometallic photochemistry

Abstract Metal-to-ligand charge-transfer (MLCT) excitation of organometallic and coordination complexes has diverse chemical and physical consequences, depending on the molecular structure and the medium. It can promptly start an ultrafast ligand dissociation, populate a long-lived, charge-separated, 3MLCT state that shows typical reactivity of the oxidised metal atom or reduced ligand (electron transfer, associative substitution, oxidative addition), or lead to a long-lived 3MLCT state from which another, more reactive, excited state is populated thermally. Alternatively, a MLCT state may be unreactive with respect to the breaking or formation of metal–ligand bonds but engaged in electron- or energy-transfer reactions or just decaying to the ground state. The MLCT photochemistry is discussed in terms of the bonding properties and dynamics of the MLCT excited states which are, to some extent, dependent on interactions with other excited states of different orbital origins (LF, σπ*, IL). An examination of actual mechanistic roles of MLCT states in representative classes of photochemical reactions allows to outline more general relations between the molecular structure and photoreactivity of MLCT-active organometallic compounds.

Relaxation dynamics of Pseudomonas aeruginosa Re(I)(CO)3(alpha-diimine)(HisX)+ (X = 83, 107, 109, 124, 126)Cu(II) azurins.

Photoinduced relaxation processes of five structurally characterized Pseudomonas aeruginosa Re(I)(CO)(3)(alpha-diimine)(HisX) (X = 83, 107, 109, 124, 126)Cu(II) azurins have been investigated by time-resolved (ps-ns) IR spectroscopy and emission spectroscopy. Crystal structures reveal the presence of Re-azurin dimers and trimers that in two cases (X = 107, 124) involve van der Waals interactions between interdigitated diimine aromatic rings. Time-dependent emission anisotropy measurements confirm that the proteins aggregate in mM solutions (D(2)O, KP(i) buffer, pD = 7.1). Excited-state DFT calculations show that extensive charge redistribution in the Re(I)(CO)(3) --> diimine (3)MLCT state occurs: excitation of this (3)MLCT state triggers several relaxation processes in Re-azurins whose kinetics strongly depend on the location of the metallolabel on the protein surface. Relaxation is manifested by dynamic blue shifts of excited-state nu(CO) IR bands that occur with triexponential kinetics: intramolecular vibrational redistribution together with vibrational and solvent relaxation give rise to subps, approximately 2, and 8-20 ps components, while the approximately 10(2) ps kinetics are attributed to displacement (reorientation) of the Re(I)(CO)(3)(phen)(im) unit relative to the peptide chain, which optimizes Coulombic interactions of the Re(I) excited-state electron density with solvated peptide groups. Evidence also suggests that additional segmental movements of Re-bearing beta-strands occur without perturbing the reaction field or interactions with the peptide. Our work demonstrates that time-resolved IR spectroscopy and emission anisotropy of Re(I) carbonyl-diimine complexes are powerful probes of molecular dynamics at or around the surfaces of proteins and protein-protein interfacial regions.

Photochemistry and picosecond absorption spectra of aqueous suspensions of a polycrystalline titanium dioxide optically transparent in the visible spectrum

Abstract Optically transparent suspensions of (Degussa P25) titanium dioxide which mimic well the photochemical and photophysical properties of the bulk material were prepared by sequential centrifugation. Quantum yields of photocatalyzed oxidation of I − and of propanal were measured under monochromatic 313 nm irradiation and found to be very similar to those estimated for concentrated P25 slurries but higher than those measured in colloidal TiO 2 solutions. A unit limiting quantum yield for propanal photooxidation suggests very efficient utilization of photogenerated holes. Time-resolved absorption spectra were measured in the 50 ps-10 ns time interval following excitation by a 30 ps, 355 nm laser pulse and found to be definitely different from picosecond spectra observed earlier for TiO 2 colloids. It is proposed that the observed transient signal may arise from a photogenerated hole that undergoes thermalization and surface-trapping with a lifetime of approximately 1 ns.

Ultrafast Excited-State Processes in Re(I) Carbonyl-Diimine Complexes: From Excitation to Photochemistry

Rhenium(I) carbonyl-diimines are chemically robust and synthetically flexible photo- and redox active compounds that can be incorporated into supramolecular systems, polymers or biomolecules. They can be used as efficient and fast photosensitizers. In this chapter, we will follow excited-state evolution of ReI complexes from the instant of photon absorption through intersystem crossing, relaxation of “hot” triplet states, to their decay either to the ground state or to photoproducts. Out of many decay pathways, we concentrate on nonradiative decay following the energy gap law, excited-state electron- and energy transfer and ligand isomerization. Characterization of the lowest excited state by ultrafast IR spectroscopy and DFT calculations are discussed in detail. It is shown that excited-state properties are much influenced by mixing of Re(CO)3→diimine, L→diimine CT and intraligand ππ* excitations.