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Increasing the metal-to-ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo- and heteroleptic complexes [Fe(cpmp)2]2+ (12+) and [Fe(cpmp)(ddpd)]2+ (22+) with the tridentate ligands 6,2’’-carboxypyridyl-2,2’-methylamine-pyridyl-pyridine (cpmp) and N,N’-dimethyl-N,N’-di-pyridin-2-ylpyridine-2,6-diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal-ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six-membered chelate rings. The push-pull ligand cpmp provides intra-ligand and ligand-to-ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 12+ and 22+ were accessed by X-ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X-ray emission spectroscopy, static and time-resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations.

Complexes of RhI and IrI of the [M(COD)(NHC)X] type (where M=Rh or Ir, COD=1,5‐cyclooctadiene, NHC=N‐heterocyclic carbene, and X=halide) have recently shown promising cytotoxic activities against several cancer cell lines. Initial mechanism of action studies provided some knowledge about their interaction with DNA and proteins. However, information about their cellular localization remains scarce owing to luminescence quenching within this complex type. Herein, the synthesis of two rare examples of luminescent RhI and IrI [M(COD)(NHC)I] complexes with 1,8‐naphthalimide‐based emitting ligands is reported. All new complexes are comprehensively characterized, including with single‐crystal X‐ray structures. Steric crowding in one derivative leads to two distinct rotamers in solution, which apparently can be distinguished both by pronounced NMR shifts and by their respective spectral and temporal emission signatures. When the photophysical properties of these new complexes are exploited for cellular imaging in HT‐29 and PT‐45 cancer cell lines, it is demonstrated that the complexes accumulate predominantly in the endoplasmic reticulum, which is an entirely new finding and provides the first insight into the cellular localization of such IrI(NHC) complexes., Anticancer NHC complexes: RhI and IrI N‐heterocyclic carbene (NHC) complexes provide an auspicious class of potential new anticancer agents; however, cellular imaging with luminescent derivatives of these complex types is often hampered by luminescence quenching. Building on an analysis of the parameters for metal‐induced quenching processes, new examples of this complex class are designed with properties suitable for cellular imaging. Their main localization in the endoplasmic reticulum is not only an unprecedented finding but also represents the first ever insight into the cellular localization of Ir(NHC) complexes in the oxidation state +I.

The effects of biomolecular embedding on the photoinduced relaxation process of the DNA-minor-groove binder berenil, diminazene aceturate, are studied with quantum mechanics/molecular mechanics, QM/MM, calculations that employ the algebraic diagrammatic construction through second-order, ADC(2), for the quantum mechanical part and an atomistic polarizable embedding for the classical part. The lowest singlet excitation to the S1 state is a bright transition with a ππ* character and a perichromatic red shift, due to the interactions with the solvent and DNA. The excited-state relaxation pathway is a two-step mechanism, an N═N azo-bond stretch followed by a volume-conserving bicycle-pedal twist. The DNA confinement and the coupling to solvent molecules via hydrogen bonds lead, for the excited-state relaxation process, only to small deviations from the ideal bicycle-pedal relaxation. Because of its volume-conserving character, the S1 excited-state relaxation proceeds almost unhindered, even in a fully rigid minor-groove confinement. With a fully frozen DNA minor groove and solvent, the energy gap for deexcitation from S1 to the ground state increased to 2.0 eV compared to 0.16 eV in aqueous solution. When the relaxation of the first solvation shell is included, the relaxation process on the S1 potential energy surface proceeds to a region on the potential energy surface, where only a small gap to the ground-state potential energy surface remains, 0.43 eV. These results show that the solvent relaxation has a significant effect in controlling the energy gap between the ground and S1 electronically excited states, which explains the experimental observations of the fluorescence characteristics of berenil in DNA confinement.

Reaktionsschritte komplexer Reaktionen in flussiger Phase lassen sich entratseln, wenn die Losungsmittelumgebung variiert wird. Die Werkzeuge dazu sind Ultrakurzzeitexperimente und Simulationen, in diesem Fall an Diphenylcarben.

Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. A disclosure of the underlying reaction mechanisms is challenging due to the rapid nature of these processes and the intricate identification of how many solvent molecules are involved. Here combining broadband femtosecond transient absorption and quantum mechanics/molecular mechanics simulations, we show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding dynamics determine which reaction channels are accessible in binary solvent mixtures at room temperature. In-depth analysis of the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Our results show that adjacent solvent molecules take the role of key abettors rather than bystanders for the fate of the reactive intermediate., Photochemistry in solution often involves coexisting reaction channels that may comprise intermediates capturing a solvent molecule. Here, the authors show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour.

This chapter describes an experimental layout for time and spectrally resolved fluorescence measurements with femtosecond time resolution based on Kerr gating. The combination of data recorded using different Kerr media allows a temporal dynamic range from ~100 fs to several nanoseconds. Simultaneous analysis of multiple datasets is described.

We use pump-repump-probe transient absorption spectroscopy to investigate the role of higher-lying electronic states in the photochemistry of a molecular switch. Moreover, replacing the pump pulse by a pulse-shaper-generated phase-stable double pulse, triggered-exchange two-dimensional (TE2D) electronic spectroscopy is established in the visible regime.

We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 μm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.

We employ transient absorption from the deep-UV to the visible region and fluorescence upconversion to investigate the photoinduced excited-state intramolecular proton-transfer dynamics in a biologically relevant drug molecule, 2-acetylindan-1,3-dione. The molecule is a s-diketone which in the electronic ground state exists as exocyclic enol with an intramolecular H-bond. Upon electronic excitation at 300 nm, the first excited state of the exocyclic enol is initially populated, followed by ultrafast proton transfer (≈160 fs) to form the vibrationally hot endocyclic enol. Subsequently, solvent-induced vibrational relaxation takes place (≈10 ps) followed by decay (≈390 ps) to the corresponding ground state.

International audience; We report a theoretical study that elaborates the influence of the polarization state of both the pump and the probe pulse in ultrafast coherent vibrational ladder climbing experiments in the mid-infrared. Whereas a subensemble in a randomly oriented sample of molecules is excited by the pump pulse in this multiphoton process, further inhomogeneities such as the spatial profile of the laser beams, the longitudinal attenuation in the sample, and the probe beam polarization have to be taken into account. Analytical expressions for a density function describing the number of molecules that are exposed to an effective pump intensity are introduced, and the variation of the population distribution and the actual transient absorption signal in dependence on the polarization-state combinations for pump and probe pulse are discussed in detail. In simulations on the model system carboxy-hemoglobin, it is demonstrated that the polarization states play important roles both for exciting a certain population distribution and for actually observing it. In particular, it will be discussed under which conditions experimental data indicates a population inversion. Cop. 2011 American Chemical Society.

International audience; In heme-based sensor proteins, ligand binding to heme in a sensor domain induces conformational changes that eventually lead to changes in enzymatic activity of an associated catalytic domain. The bacterial oxygen sensor FixL is the best-studied example of these proteins and displays marked differences in dynamic behavior with respect to model globin proteins. We report a mid-IR study of the configuration and ultrafast dynamics of CO in the distal heme pocket site of the sensor PAS domain FixLH, employing a recently developed method that provides a unique combination of high spectral resolution and range and high sensitivity. Anisotropy measurements indicate that CO rotates toward the heme plane upon dissociation, as is the case in globins. Remarkably, CO bound to the heme iron is tilted by similar to 30 degrees with respect to the heme normal, which contrasts to the situation in myoglobin and in present FixLH-CO X-ray crystal structure models. This implies protein-environment-induced strain on the ligand, which is possibly at the origin of a very rapid docking-site population in a single conformation. Our observations likely explain the unusually low affinity of FixL for CO that is at the origin of the weak ligand discrimination between CO and O(2). Moreover, we observe orders of magnitude faster vibrational relaxation of dissociated CO in FixL than in globins, implying strong interactions of the ligand with the distal heme pocket environment. Finally, in the R220H FixLH mutant protein, where CO is H-bonded to a distal histidine, we demonstrate that the H-bond is maintained during photolysis. Comparison with extensively studied globin proteins unveils a surprisingly rich variety in both structural and dynamic properties of the interaction of a diatomic ligand with the ubiquitous b-type heme-proximal histidine system in different distal pockets.

International audience; We apply a Fourier filtering technique for the global removal of coherent contributions, like perturbed freeinduction decay, and noise, to experimental pump-probe spectra. A further filtering scheme gains access to spectra otherwise only recordable by scanning the probe's center frequency with adjustable spectral resolution. These methods cleanse pump-probe data and allow improved visualization and simpler analysis of the contained dynamics. We demonstrate these filters using visible pump/mid-infrared probe spectroscopy of ligand dissociation in carboxyhemoglobin. Cop. 2009 Optical Society of America.