Photophysics of Anionic Bis(4H-imidazolato)CuI Complexes

In this paper, the photophysical behavior of four panchromatically absorbing, homoleptic bis(4H-imidazolato)Cu^I complexes, with a systematic variation in the electron-withdrawing properties of the imidazolate ligand, were studied by wavelength-dependent time-resolved femtosecond transient absorption spectroscopy. Excitation at 400, 480, and 630 nm populates metal-to-ligand charge transfer, intraligand charge transfer, and mixed-character singlet states. The pump wavelength-dependent transient absorption data were analyzed by a recently established 2D correlation approach. Data analysis revealed that all excitation conditions yield similar excited-state dynamics. Key to the excited-state relaxation is fast, sub-picosecond pseudo-Jahn-Teller distortion, which is accompanied by the relocalization of electron density onto a single ligand from the initially delocalized state at Franck-Condon geometry. Subsequent intersystem crossing to the triplet manifold is followed by a sub-100 ps decay to the ground state. The fast, nonradiative decay is rationalized by the low triplet-state energy as found by DFT calculations, which suggest perspective treatment at the strong coupling limit of the energy gap law.

Ligand‐Induced Donor State Destabilisation – A New Route to Panchromatically Absorbing Cu(I) Complexes

The intense absorption of light to covering a large part of the visible spectrum is highly desirable for solar energy conversion schemes. To this end, we have developed novel anionic bis(4H‐imidazolato)Cu(I) complexes (cuprates), which feature intense, panchromatic light absorption properties throughout the visible spectrum and into the NIR region with extinction coefficients up to 28,000 M⁻¹ cm⁻¹. Steady‐state absorption, (spectro)electrochemical and theoretical investigations reveal low energy (Vis to NIR) metal‐to‐ligand charge‐transfer absorption bands, which are a consequence of destabilized copper‐based donor states. These high‐lying copper‐based states are induced by the σ‐donation of the chelating anionic ligands, which also feature low energy acceptor states. The optical properties are reflected in very low, copper‐based oxidation potentials and three ligand‐based reduction events. These electronic features reveal a new route to panchromatically absorbing Cu(I) complexes.

Excited-State Switching in Rhenium(I) Bipyridyl Complexes with Donor-Donor and Donor-Acceptor Substituents

The optical properties of two Re(CO)₃(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA₂) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA₂ the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH₂Cl₂ to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin-orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states.

Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 μs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing ³MLCT and ³ILCT contributions across the series.

An artificial photosynthetic system for photoaccumulation of two electrons on a fused dipyridophenazine (dppz)-pyridoquinolinone ligand

Increasing the efficiency of molecular artificial photosynthetic systems is mandatory for the construction of functional devices for solar fuel production. Decoupling the light-induced charge separation steps from the catalytic process is a promising strategy, which can be achieved thanks to the introduction of suitable electron relay units performing charge accumulation. We report here on a novel ruthenium tris-diimine complex able to temporarily store two electrons on a fused dipyridophenazine-pyridoquinolinone π-extended ligand upon visible-light irradiation in the presence of a sacrificial electron donor. Full characterization of this compound and of its singly and doubly reduced derivatives thanks to resonance Raman, EPR and (TD)DFT studies allowed us to localize the two electron-storage sites and to relate charge photoaccumulation with proton-coupled electron transfer processes.

Dramatic Alteration of 3ILCT Lifetimes Using Ancillary Ligands in [Re(L)(CO)3(phen-TPA)] n+ Complexes: An Integrated Spectroscopic and Theoretical Study

The ground and excited state photophysical properties of a series of fac-[Re(L)(CO)₃(α-diimine)] ⁿ⁺ complexes, where L = Br^-, Cl^-, 4-dimethylaminopyridine (dmap) and pyridine (py) have been extensively studied utilizing numerous electronic and vibrational spectroscopic techniques in conjunction with a suite of quantum chemical methods. The α-diimine ligand consists of 1,10-phenanthroline with the highly electron donating triphenylamine (TPA) appended in the 5 position. This gives rise to intraligand charge transfer (ILCT) states lying lower in energy than the conventional metal-to-ligand charge transfer (MLCT) state, the energies of which are red and blue-shifted, respectively, as the ancillary ligand, L becomes more electron withdrawing. The emitting state is ³ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR²), time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Systematic modulation of the ancillary ligand causes unanticipated variation in the ³ILCT lifetime by 2 orders of magnitude, ranging from 6.0 μs for L = Br^- to 27 ns for L = py, without altering the nature of the excited state formed or the relative order of the other CT states present. Temperature dependent lifetime measurements and quantum chemical calculations provide no clear indication of close lying deactivating states, MO switching, contributions from a halide-to-ligand charge transfer (XLCT) state or dramatic changes in spin-orbit coupling. It appears that the influence of the ancillary ligand on the excited state lifetime could be explained in terms of energy gap law, in which there is a correlation between ln( k_nr) and E_em with a slope of -21.4 eV^-1 for the ³ILCT emission.

Unusually Short-Lived Solvent-Dependent Excited State in a Half-Sandwich Ru(II) Complex Induced by Low-Lying 3MC States

A ruthenium complex with a half-sandwich geometry ([(p-cymene)Ru(Cl)(curcuminoid)]) was synthesized, characterized, and investigated regarding its ultrafast photophysics. These photophysical investigations of the complex revealed a weak and short-lived emission from the initially populated ¹MLCT state and solvent-dependent photoinduced dynamics, where the secondarily populated ³MC state is stabilized by nonpolar solvents. Overall the decay of the ³dd-MC state to the ground state is completed within picoseconds. This short excited-state lifetime is in stark contrast to the typically observed long-lived ³MLCT states with lifetimes of nanoseconds or microseconds in unstrained, octahedral ruthenium complexes but is in good agreement with the findings for distorted octahedral complexes. This is pointing to the half-sandwich geometry as a new and easy approach to study these otherwise often concealed dd states.

Cu(i) vs. Ru(ii) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system

The charge transfer behavior of heteroleptic Cu(i) photosensitizers was investigated by spectroelectrochemistry and compared to their structurally related Ru(ii) complexes.

Synthesis of three series of ruthenium tris-diimine complexes containing acridine-based π-extended ligands using an efficient “chemistry on the complex” approach

Novel multi-step chemistry on the complex strategy.

Highly entangled ground States in tripartite qubit systems

We investigate the creation of highly entangled ground states in a system of three exchange-coupled qubits arranged in a ring geometry. Suitable magnetic field configurations yielding approximate Greenberger-Horne-Zeilinger and exact W ground states are identified. The entanglement in the system is studied at finite temperature in terms of the mixed-state tangle tau. By generalizing a conjugate gradient optimization algorithm originally developed to evaluate the entanglement of formation, we demonstrate that tau can be calculated efficiently and with high precision. We identify the parameter regime for which the equilibrium entanglement of the tripartite system reaches its maximum.

Lipopolysaccharide/endotoxin induces IL-18 via CD14 in human peripheral blood mononuclear cells in vitro

IL-18 shares activities with IL-12 in generating T-helper 1 cells and cytokine response. It mediates LPS/endotoxin lethality by IL-12 independent interferon-gamma synthesis and it induces bacteria-related organ failure. As peripheral blood mononuclear cells (PBMC) are potent producers of IL-18, we studied the regulation of IL-18 upon exposure to LPS and Staphylococcus aureus (SAC) in vitro. Freshly isolated PBMC constitutively expressed IL-18 mRNA. After unstimulated preincubation for 48 h, however, IL-18 transcripts were nearly not detectable by RT-PCR, but inducible by LPS or SAC (P<0.01). Both LPS and SAC were potent stimuli of IL-18 protein secretion (P<0.01). LPS-mediated IL-18 gene expression and secretion was CD14-dependent and significantly inhibited by co-incubation of PBMC with neutralizing CD14 antibody (P<0.01). We conclude that LPS-driven IL-18 is dependent on the expression of costimulatory factors and that IL-18 inhibition might attenuate IL-18-related toxic effects.

Ammonia-induced swelling of rat cerebral cortical slices: implications for the pathogenesis of brain edema in acute hepatic failure

The pathogenesis of brain edema in fulminant hepatic failure is incompletely understood. Our previous studies in models of this disease suggest the presence of a cytotoxic mechanism; as cortical astrocytes appeared predominantly swollen, we hypothesized that ammonia, metabolized to glutamine solely within this cell, could play a role in brain water accumulation. We determined ammonia levels in different brain regions of rats after hepatic devascularization, a model previously shown to exhibit brain edema. Concentrations of 2.5 mM were observed in the edematous cerebral cortex. We then added several concentrations of ammonium chloride to the first cortical brain slice, a preparation used to study cytotoxic brain edema. At a final bath concentration of ammonia of 5 and 10 mM, swelling could be detected: a decrease in the space of distribution of inulin was seen at the 10 mM concentration, suggesting intracellular water accumulation. Neuropathologically, astrocytes appeared involved even at subswelling doses of ammonia. Octanoic acid, at a 10 mM concentration, also resulted in demonstrable swelling. Ammonia, at concentrations in the incubation bath that approach the levels seen in an in vivo model of brain edema, results in water accumulation of cortical brain slices. Toxins implicated in the pathogenesis of hepatic encephalopathy, such as ammonia and octanoic acid, may, result in brain water accumulation.

Effect of body temperature on brain edema and encephalopathy in the rat after hepatic devascularization

Brain edema is a fatal complication of fulminant hepatic failure and its pathogenesis remains unclear. To determine its presence in a model of ischemic hepatic failure, rats were subjected to a portacaval anastomosis followed by hepatic artery ligation. Brain water was measured using the sensitive gravimetric method. Preliminary studies revealed marked hypothermia in devascularized animals kept at room temperature (26.9 degrees +/- 2.8 degrees C). An additional group of devascularized rats was kept in an incubator. As expected for hypothermia, such animals had a lower arterial pressure and heart rate; the duration of encephalopathy was markedly prolonged. Water content of the cortical gray matter was only increased in normothermic devascularized rats: 80.14% +/- 0.31%, normal; 80.06% +/- 0.22%, portacaval shunt only; 80.42% +/- 0.26%, devascularized at room temperature; 81.29% +/- 0.38%, devascularized at controlled temperature (p less than 0.001). Such differences could not be detected using the dry-weight technique in whole cerebral hemispheres. Astrocyte changes in the cortical gray matter were noted in both edematous and nonedematous devascularized groups, coupled with the presence of vesicles containing horseradish peroxidase in the endothelial capillary cell. This suggests that in this model, brain edema may be due to both a cytotoxic mechanism and changes in the permeability of the blood-brain barrier. Future studies with this widely used model will require strict control of temperature to allow interpretation of experimental results. A therapeutic role for hypothermia in the management of brain edema deserves further attention.