Picosecond solvation of electronically excited solutes in alcoholic solvents: non-debye behaviour of the time-dependent fluorescence shift related to h

Time-dependent fluorescence shifts (TDFS) of polar compounds in a series of alcoholic solvents have been studied on a pico-second time scale. A non-exponential TDFS kinetic behaviour is observed, particularly at short times (0<t< 50 ps). It cannot be represented by a sum of decreasing exponentials. The effects of solute-solvent interactions on the kinetics of TDFS have been analysed. It is shown that the non-standard kinetics is due to hydrogen bonding in the solvent, particularly to the presence of alcoholic hydrogen-bonded aggregates. Following electronic excitation of the polar solute reorganization of solvent aggregates controls the TDFS time development. Our results show that a simple Debye model is inadequate to explain such a process.     

Partial spin depolarization of muonium in ice

Muon spin precession signals arising from both muonium and a diamagnetic muon species have been studied in single crystal of ice over the temperature range 90–263 K. Conversion of initial signal amplitudes to fractions of muon polarization reveals that for temperatures above 200 K part of the original muon polarization is unaccounted for. Such a missing fraction is well known for liquid water. but was not found in earlier work on polycrystalline ice. Muonium signals in Polycrystalline ice were reanalyzed using a non-exponential decay function appropriate to the powder pattern spectrum. Smaller muonium fractions were found. consistent with those determined for single crystall, thus confirming the existence of the missing fraction. The origin of the missing fraction is discussed, and it is proposed that non-reactive spin exchange encounters between muonium and hydrogen atoms may be the cause.     

Excitation-emission-lifetime analysis of multicomponent systems—II. Synthetic model data

To determine the efficacy of three-dimensional principal component factor analysis (PCFA) for extracting non-exponential decay parameters from multicomponent data, we have constructed synthetic data matrices which mimic the possible outcomes of experiments in the nanosecond time domain with copper porphyrins. Our results demonstrate that PCFA is capable of determining non-exponential time decay in systems with two and three emitting species. The accuracy of the rate constants determined by this method is limited by the accuracy of the non-linear Marquardt algorithm that we have used for the final fits. Although extremely overlapped components have been resolved using this method, degeneracy in one of the dimensions is problematic.     

A study of electron transfer in Ru(dcbpy)2(NCS)2 sensitized nanocrystalline TiO2 and SnO2 films induced by red-wing excitation

Excited state dynamics and electron transfer from the Ru(dcbpy)2(NCS)2 (RuN3) sensitizer to semiconductor nanoparticles were studied using time-resolved femtosecond absorption spectroscopy. We found that excitation of the red wing of the absorption spectrum of the sensitizer populates the (3)MLCT state directly, both in solution and attached on semiconductor nanoparticle films. Electron injection is slowed down and becomes gradually less efficient as excitation moves towards red from the absorption maximum at 535 nm. At 675 nm the injection is non-exponential and characterized by 5, 30 and 180 ps time constants. The non-exponential electron injection observed is assigned to injection from a distribution of triplet states with energies below the semiconductor conduction band edge.     

Anomalous distance dependence of electron transfer across peptide bridges

The first investigation on the distance dependence of a dissociative electron transfer process across peptide bridges is reported. This study was carried out by using a series of donor-peptide-acceptor systems in which the donor is a phthalimido moiety, the peptide bridges are provided by alpha-aminoisobutyric acid (Aib) homooligomers, and the acceptor is a peroxide functional group. The intramolecular electron transfer from the electrogenerated phthalimido radical anion to the peroxide was studied in comparison with the thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular rate constants were determined in N,N-dimethylformamide by taking into account the corresponding intermolecular values. The experimental results point to an unusual non-exponential dependence of the intramolecular electron transfer rate on the number of bridge units. The same trend could be verified also by taking into account the actual donor-acceptor edge-to-edge distance. The peculiar distance dependence that was observed for the intramolecular electron transfer rate is attributed to the mediating effect of the intramolecular C=O...H-N hydrogen bonds.     

Non-exponentiality in electron transfer kinetics: Static versus dynamic disorder models

Non-exponential electron transfer kinetics in complex systems are often analyzed in terms of a quenched, static disorder model. In this work we present an alternative analysis in terms of a simple dynamic disorder model where the solvent is characterized by highly non-exponential dynamics. We consider both low and high barrier reactions. For the former, the main result is a simple analytical expression for the survival probability of the reactant. In this case, electron transfer, in the long time, is controlled by the solvent polarization relaxation—in agreement with the analyses of Rips and Jortner and of Nadler and Marcus. The short time dynamics is also non-exponential, but for different reasons. The high barrier reactions, on the other hand, show an interesting dynamic dependence on the electronic coupling element,V _el.     

Photophysics of unsaturated polymers. Styrene-phenylacetylene copolymer

The characteristics of fluorescence emission from a polymer containing polyene chromophores have been examined in solution and in the solid state. The shorter wavelength emissions have non-exponential decays. A mechanism of energy redistribution among the polyenes of different size is proposed.     

Laser studies on the phosphorescence decay of benzophenone in polymer matrices—I. Intensity dependence

Use of a pulsed nitrogen laser in the study of the non-exponential nature of the phosphorescence decay profile of benzophenone in poly(methyl methacrylate) led to the proposal that the mechanism involved a triplet-triplet annihilation process in which the polymeric matrix itself participated.     

Merocyanine ↔ spiropyran transformation in a polymer matrix: An example of a dispersive chemical reaction

The non-exponential time dependence of spiropyran ↔ merocyanine transformation in a co-polymer has been reinvestigated experimentally and analysed in terms of a dispersive first-order chemical reaction. Non-exponential dependence can be accounted for by a Gaussian distribution of activation energies whose width decreases with increasing temperature.     

Simulation of ultrafast non-exponential fluorescence decay induced by electron transfer in FMN binding protein

The ultrafast non-exponential fluorescence decay of FMN binding protein (FBP) was analyzed with three electron transfer (ET) theories, Marcus theory, Bixon and Jortner theory and Kakitani and Mataga theory. Center to center distances between electron acceptor, the excited isoalloxazine, and donors, Trp-32, Tyr-35 and Trp-106, in FBP were determined by molecular dynamic simulation. Electron transfer parameters containing in these theories were determined so as to fit the calculated decay with the observed decay, according to a non-linear least squares method. Introduction of electrostatic energies between isoalloxazine anion and other ionic groups and between the donor cations and other ionic groups in the protein into any ET theories improved the fitting. The non-exponential behavior in the fluorescence decay is considered to be ascribed to a fluctuation of the protein structure with long period.     

Rotational dynamics of solvated carbon dioxide studied by infrared, Raman, and time-resolved infrared spectroscopies and a molecular dynamics simulation

Rotational dynamics of solvated carbon dioxide (CO(2)) has been studied. The infrared absorption band of the antisymmetric stretch mode in acetonitrile is found to show a non-Lorentzian band shape, suggesting a non-exponential decay of the vibrational and/or rotational correlation functions. A combined method of a molecular dynamics (MD) simulation and a quantum chemical calculation well reproduces the observed band shape. The analysis suggests that the band broadening is almost purely rotational, while the contribution from the vibrational dephasing is negligibly small. The non-exponential rotational correlation decay can be explained by a simple rotor model simulation, which can treat large angle rotations of a relatively small molecule. A polarized Raman study of the symmetric stretch mode in acetonitrile gives a rotational bandwidth consistent with that obtained from the infrared analysis. A sub-picosecond time-resolved infrared absorption anisotropy measurement of the antisymmetric stretch mode in ethanol also gives a decay rate that is consistent with the observed rotational bandwidths.     

Incomplete vibrational relaxation of pyrene in its first excited singlet state in the vapour phase

The non-exponential fluorescence decay of pyrene is assigned to incomplete vibrational relaxation in the first excited singlet state. The increasing rate of this fluorescence decay with increasing vibrational energy is mainly attributed to an increase of the intersystem-crossing rate and to a smaller extent to a temperature dependence of the radiative rate constant.     

Current and spin density correlation functions for computer nitrogen

Current and spin density correlation functions are simulated for computer nitrogen using the algorithm of Tidesley and Streett. The functions are oscillatory and non-exponential even for the lowest values of momentum transfer κ. A semi-empirical interpretation on the basis of a three-variable Mori approximant is attempted.     

On the heterogeneity of fluorescence lifetime of room temperature ionic liquids: onset of a journey for exploring red emitting dyes

An excitation and emission wavelength dependent non-exponential fluorescence decay behaviour of room temperature ionic liquids (RTILs) has been noted. Average fluorescence lifetimes have been found to vary by a factor of three or more. Red emitting dyes dissolved in RTILs are found to follow hitherto unobserved single exponential fluorescence decay behaviour.     

Non-exponential picosecond fluorescence decay in isolated pentafluorobenzene and hexafluorobenzene

Two-component fluorescence decay has been observed in isolated pentafluorobenzene and hexafluorobenzene excited at 266 nm. From the temperature and pressure dependence of the decay constants an attempt has been made to separate sequence congestion effects from other possible causes of non-exponential behavior.     

Influence of triplet—triplet excitation transfer on the decay function of donor luminescence

The triplet—triplet electronic excitation transfer from benzophenone to naphthalene in rigid solutions has been investigated by measuring the non-exponential decay curve of benzophenone photphorescence using a laser pulse for excitation. The results of measurements were satisfactorily reproduced by Inokuti—Hirayama's formula based on Dexter's theory of exchange interaction.     

Repulsive potentials for Ne*(3P)—He and Ar*(3P)—He from differential scattering

Crossed beam measurements of differential scattering of metastable Ne and Ar by He are used to derive the repulsive parts of the respective interatomic potentials. The potentials are similar to one obtained for Li—He. They show non-exponential behavior at small r suggestive of a core-interaction effect.     

Photochemistry in polymer solids—4. On the intensity dependence of non-exponential decay of benzophenone phosphorescence in poly(methyl methacrylate)

Change in the intensity of the irradiation laser pulse did not influence the non-exponential decay profile of benzophenone phosphorescence in poly(methyl methacrylate) at 20°. This result is incompatible with the biphotonic triplet-triplet annihilation process, but supports the diffusion-controlled dynamic quenching mechanism including non-equilibrium √t term.     

The short time regime of segmental dynamics of glass-forming polymers

The short time regime (10⁻¹³ s ≤ t ≤ 10⁻¹¹ s) of the segmental dynamics of several glass-forming polymers with different chemical microstructure and glass-transition temperature has been investigated by means of time of flight (TOF) neutron scattering techniques. In this paper we present a summary of the main results obtained as well as the theoretical framework proposed by us to interpret these results. The main conclusion is that the segmental dynamics of glass-forming polymers display in the very short time range (t = 2 ps) a crossover from a simple exponential behaviour towards a non-exponential regime. The first exponential decay, which is controlled by C-C rotational barriers, can be understood as a trace of the behaviour of the system in absence of the effects (correlations, cooperativity, memory effects…) which characterise the dense supercooled liquid like state against the normal liquid state. The non-exponential regime at t > 2 ps corresponds to what is usually understood as α and β relaxations. Some implications of these results are also discussed.