Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution.

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha-Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis-trans isomerizations, respectively.

Optical transient absorption experiments reveal the failure of formal kinetics in diffusion assisted electron transfer reactions.

The ultimate goal of chemical kinetics is to understand why a given reaction is fast or not. To this end it is necessary to count on robust and experimentally well tested theories. One of the difficulties, long recognized in the study of bimolecular reactions, is the role of the molecular displacement, i.e. diffusion. Nonetheless the field is still lacking a compelling amount of case studies contrasting physical models to experiments. By performing transient absorption experiments on the photo-induced electron transfer reaction between perylene and N,N-dimethylaniline in liquid solutions over many orders of magnitude in time, we try to understand the factors determining the kinetics and yields of the full photocycle. We present a method to overcome potential pitfalls in the extraction of the relevant quantities, the transient populations, from the experimental data due to the changes in band shapes and positions. The results are compared to simulations of two different theories: a reaction-diffusion approach based on the encounter theories, and a formal kinetic scheme. We conclude that while the former explains the observed trends in the kinetics with quencher concentration and viscosity exceptionally well, the latter fails. Moreover the analysis of the data with the assistance of encounter theory unveils effects that otherwise would pass unnoticed. This approach and its results exemplify the path to follow in other condensed media whenever diffusion is involved.

Excited-state dynamics of a molecular dyad with two orthogonally-oriented fluorophores.

The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin-orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state.

Influence of the hydrogen-bond interactions on the excited-state dynamics of a push-pull azobenzene dye: the case of Methyl Orange.

The excited-state dynamics of the push-pull azobenzene Methyl Orange (MO) were investigated in several solvents and water/glycerol mixtures using a combination of ultrafast time-resolved fluorescence and transient absorption in both the UV-visible and the IR regions, as well as quantum chemical calculations. Optical excitation of MO in its trans form results in the population of the S2 ππ* state and is followed by internal conversion to the S1 nπ* state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the trans ground state and isomerisation to the cis ground state. Finally, the cis form converts thermally to the trans form on a timescale ranging from less than 50 ms to several minutes. Significant differences depending on the hydrogen-bond donor strength of the solvents, quantified by the Kamlet Taft parameter α, were observed: compared to the other solvents, in highly protic solvents (α > 1), (i) the viscosity dependence of the S1 state lifetime is less pronounced, (ii) the S1 state lifetime is shorter by a factor of ≈1.5 for the same viscosity, (iii) the trans-to-cis photoisomerisation efficiency is smaller, and (iv) the thermal cis-to-trans isomerisation is faster by a factor of ≥103. These differences are explained in terms of hydrogen-bond interactions between the solvent and the azo nitrogen atoms of MO, which not only change the nature of the S1 state but also have an impact on the shape of ground- and excited-state potentials, and, thus, affect the deactivation pathways from the excited state.

Ultrafast photochemistry in liquids.

Ultrafast photochemical processes can occur in parallel with the relaxation of the optically populated excited state toward equilibrium. The latter involves both intra- and intermolecular modes, namely vibrational and solvent coordinates, and takes place on timescales ranging from a few tens of femtoseconds to up to hundreds of picoseconds, depending on the system. As a consequence, the reaction dynamics can substantially differ from those usually measured with slower photoinduced processes occurring from equilibrated excited states. For example, the decay of the excited-state population may become strongly nonexponential and depend on the excitation wavelength, contrary to the Kasha and Vavilov rules. In this article, we first give a brief account of our current understanding of vibrational and solvent relaxation processes. We then present an overview of important classes of ultrafast photochemical reactions, namely electron and proton transfer as well as isomerization, and illustrate with several examples how nonequilibrium effects can affect their dynamics.

Time-frequency methods for structural health monitoring.

Detection of early warning signals for the imminent failure of large and complex engineered structures is a daunting challenge with many open research questions. In this paper we report on novel ways to perform Structural Health Monitoring (SHM) of flood protection systems (levees, earthen dikes and concrete dams) using sensor data. We present a robust data-driven anomaly detection method that combines time-frequency feature extraction, using wavelet analysis and phase shift, with one-sided classification techniques to identify the onset of failure anomalies in real-time sensor measurements. The methodology has been successfully tested at three operational levees. We detected a dam leakage in the retaining dam (Germany) and "strange" behaviour of sensors installed in a Boston levee (UK) and a Rhine levee (Germany).

Real-time observation of the formation of excited radical ions in bimolecular photoinduced charge separation: absence of the Marcus inverted region explained.

Unambiguous evidence for the formation of excited ions upon ultrafast bimolecular photoinduced charge separation is found using a combination of femtosecond time-resolved fluorescence up-conversion, infrared and visible transient absorption spectroscopy. The reaction pathways are tracked by monitoring the vibrational energy redistribution in the product after charge separation and subsequent charge recombination. For moderately exergonic reactions, both donor and acceptor are found to be vibrationally hot, pointing to an even redistribution of the energy dissipated upon charge separation and recombination in both reaction partners. For highly exergonic reactions, the donor is very hot, whereas the acceptor is mostly cold. The asymmetric energy redistribution is due to the formation of the donor cation in an electronic excited state upon charge separation, confirming one of the hypotheses for the absence of the Marcus inverted region in photoinduced bimolecular charge separation processes.

Acylgermanes: photoinitiators and sources for Ge-centered radicals. insights into their reactivity.

Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane 1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical G• is formed via the triplet state of parent 1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical G• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 10(8) and 3.2 ± 0.2 × 10(8) M(-1) s(-1), in toluene and acetonitrile, respectively. This is ~1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge-O bonds. LC-NMR analysis of photocured mixtures containing 1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales.

Photoinduced electron transfer reactions: from the elucidation of old problems in bulk solutions towards the exploration of interfaces.

The activities of our research group in the field of photoinduced electron transfer reactions are discussed and illustrated by several examples.

Ion-Pair Dynamics upon Photoinduced Electron Transfer Monitored by Pump-Pump-Probe Spectroscopy.

The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump-pump-probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe•-). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S1) + donor state were identified: one occurring in a few picoseconds from Pe•-*(D1) and one taking place within 100-200 fs from Pe•-*(D n>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.

Photometrics of ultrafast and fast broadband electronic transient absorption spectroscopy: State of the art.

The physical limits of the photometric resolution in broadband electronic transient absorption spectroscopy are discussed together with solutions for how to reach these limits in practice. In the first part, quantitative expressions for the noise contributions to the transient absorption signal are derived and experimentally tested. Experimental approaches described in the literature are discussed and compared on this basis. Guide-lines for designing a setup are established. In the second part, a method for obtaining nearly shot-noise limited kinetics with photometric resolution of the order of 100 µOD in overall measurement times of a few minutes from femtosecond to microsecond time scale is presented. The results are discussed in view of other experiments of step-scan type which are subject to a background or to correlated noise. Finally, detailed information is provided on how to obtain transient absorption spectra where counting statistics are the sole source of noise. A method for how to suppress outliers without introducing bias is discussed. An application example is given to demonstrate the achievable signal-to-noise level and the fast acquisition time.

Impact of thiopurine methyltransferase activity and 6-thioguanine nucleotide concentrations in patients with chronic inflammatory diseases.

As azathioprine is one of the standard immunosuppressive drugs used for treatment of patients with different chronic inflammatory diseases, the effect of the azathioprine metabolizing enzyme thiopurine methyltransferase (TPMT) activity on incidence of adverse events (AE) was examined. In addition, potential correlations between the concentration of the azathioprine metabolite 6-thioguanine nucleotide (6-TGN) in erythrocytes (RBC) and inflammatory disease activity as well as hematological AE were investigated. TPMT activities were investigated prospectively in 139 patients (35 male, 104 female) with chronic inflammatory diseases [systemic lupus erythematosus (SLE, 38), progressive systemic sclerosis (PSS, 13), Wegener's granulomatosis (4), rheumatoid arthritis (RA, 5), and other chronic inflammatory diseases (79)]. In addition, 6-TGN concentrations were investigated in a second cohort of 58 patients (17 patients with SLE, 5 with PSS, 5 with vasculitides, 4 with undifferentiated connective tissue diseases, 1 with dermatomyositis, 1 with Sjögren's syndrome, 1 with RA, 20 with Crohn's disease, and 4 with ulcerative colitis) prior to and during therapy with azathioprine. The distribution of activities of TPMT in 139 patients showed a normal Gaussian distribution in the Caucasian population. Within the group of 96 patients taking azathioprine, known azathioprine-related AE could be observed: minor AE (sickness, rash, and increase in cholestasis parameters) in 11 patients (11.4%), and severe AE (bone marrow toxicity) in 7 patients (7.3%). Below a "cutoff" value of 11.9 nmol/mL RBC x h of TPMT activity, AE were significantly more frequent. In the second cohort of patients, no significant correlations could be observed between 6-TGN concentrations and parameters of disease activity. Reduced activity of TPMT in patients with chronic inflammatory diseases requiring immunosuppressive therapy with azathioprine, especially below a distinct cutoff, appears to inherit a substantial risk for development of AE.

Broadband ultraviolet-visible transient absorption spectroscopy in the nanosecond to microsecond time domain with sub-nanosecond time resolution.

A combination of sub-nanosecond photoexcitation and femtosecond supercontinuum probing is used to extend femtosecond transient absorption spectroscopy into the nanosecond to microsecond time domain. Employing a passively Q-switched frequency tripled Nd:YAG laser and determining the jitter of the time delay between excitation and probe pulses with a high resolution time delay counter on a single-shot basis leads to a time resolution of 350 ps in picosecond excitation mode. The time overlap of almost an order of magnitude between fs and sub-ns excitation mode permits to extend ultrafast transient absorption (TA) experiments seamlessly into time ranges traditionally covered by laser flash photolysis. The broadband detection scheme eases the identification of intermediate reaction products which may remain undetected in single-wavelength detection flash photolysis arrangements. Single-shot referencing of the supercontinuum probe with two identical spectrometer/CCD arrangements yields an excellent signal-to-noise ratio for the so far investigated chromophores in short to moderate accumulation times.

Comparison of universal approximators incorporating partial monotonicity by structure.

Neural networks applied in control loops and safety-critical domains have to meet more requirements than just the overall best function approximation. On the one hand, a small approximation error is required; on the other hand, the smoothness and the monotonicity of selected input-output relations have to be guaranteed. Otherwise, the stability of most of the control laws is lost. In this article we compare two neural network-based approaches incorporating partial monotonicity by structure, namely the Monotonic Multi-Layer Perceptron (MONMLP) network and the Monotonic MIN-MAX (MONMM) network. We show the universal approximation capabilities of both types of network for partially monotone functions. On a number of datasets, we investigate the advantages and disadvantages of these approaches related to approximation performance, training of the model and convergence.

Ultrafast excited state dynamics of the perylene radical cation generated upon bimolecular photoinduced electron transfer reaction.

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids.

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.

Photoinduced bimolecular electron transfer investigated by femtosecond time-resolved infrared spectroscopy.

Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.

Induction of cytokines and adhesion molecules in stable hemodialysis patients: is there an effect of membrane material?

BACKGROUND/AIMS: The aim of this study was to examine the effects of a biocompatible and a nonbiocompatible hemodialysis membrane on the secretion of cytokines and their specific antagonists as well as on the expression of adhesion molecules. METHODS: A crossover study using cuprophane (CU) and polysulfone (PS) dialyzers and measurements of serum tumor necrosis factor (TNF)-alpha, soluble TNF receptors, interleukin (IL)-6, IL-6 receptor, IL-2 receptor, IL-1 receptor antagonist and the adhesion molecules intercellular adhesion molecule 1, endothelial leukocyte adhesion molecule and vascular cell adhesion molecule was performed in 19 hemodialysis patients. 44 healthy volunteers served as controls. RESULTS: All of the measured cytokines and cytokine antagonists as well as all adhesion molecules were significantly elevated in hemodialysis patients compared to controls. There was a significant increase in TNF-alpha during a dialysis session with a CU dialyzer, but only a moderate increase using a PS dialyzer. None of the other cytokines and adhesion molecules were changed during a dialysis session. We were also able to show an upregulation of adhesion molecules in dialysis patients. CONCLUSION: Our study clearly demonstrates that levels of inflammatory cytokines as well as their antagonists and adhesion molecules are elevated in patients on hemodialysis therapy. Baseline values before the start of a dialysis session did not show any differences with regard to the usage of CU or PS dialysis membranes. However, CU dialyzers led to a significantly greater stimulation of TNF-alpha during the dialysis session in comparison to PS membranes, suggesting a higher degree of bioincompatibility.