Femtosecond Laser Spectroscopy and Computer Simulations of Barrierless Isomerization in Solution

The barrierless isomerization of 1,1-diethyl-4,4-cyanine in alcohol solutions has been studied with 75-fs laser pulses in transient absorption experiments. The measured dynamics are compared with Monte-Carlo simulations based on the BFO-theory (Bagchi, B.; Fleming, G.R.; Oxtoby, D.W.J. Chem. Phys. 1983. 78: 7375) for condensed-phase barrierless isomerizations, and the viscosity dependence of the long-time exponential decay is well reproduced with a coherent set of parameters. In the most viscous solvents, early-time (≲ 1 ps) kinetics are observed which are characteristic of damped wavepacket dynamics in the reaction coordinate.     

Density-dependent photochemical branching ratio in supercritical CO2: Photodissociation and isomerization of diiodomethane

The photodissociation of diiodomethane following excitation at 305 nm in supercritical CO₂ was investigated by femtosecond pump-probe absorption spectroscopy at pressures between 10 and 100 MPa. As in liquid solution, transient absorption signals in the wavelength range from 350 to 450 nm generally show an instantaneous peak, followed by a fast initial decay (200–300 fs) and a subsequent rise on a 10-ps timescale. The initial fast decay time is found to be linearly dependent on viscosity, suggesting that dissociative motion on the repulsive surface is damped by solvent friction. Both amplitude and formation rate of the rising component, which is assigned to formation of iso-diiodomethane within the solvent cage, increase with increasing pressure. Spectral narrowing of the transient absorption band indicates vibrational cooling of hot isomer by energy transfer to CO₂ in about 20–40 ps. Immediately after excitation, this band shows absorption anisotropy for about 3 ps. The anisotropy decay rate increases from 3·10¹¹ s⁻¹ to 2·10¹² s⁻¹ as the pressure is lowered from 80 to 10 MPa. It is tentatively assigned to rotational relaxation of “hot” CH₂I radicals generated in ultrafast photodissociation of the parent molecule. The observed density dependence of formation rate and relative yield of iso-diiodomethane are described in terms of a simple kinetic model.     

Radiative and nonradiative relaxation phenomena in hydrogen- and oxygen-terminated porous silicon

Abstract

Using time-resolved photoluminescence spectroscopy over a wide range of temperatures, we were able to probe both radiative and nonradiative relaxation processes in luminescent porous silicon. By comparing the photoluminescence decay times from freshly prepared and oxidized porous silicon, we show that radiative processes should be linked with quantum confinement in small Si nanocrystallites and are not affected by oxidation. In contrast, nonradiative relaxation processes are associated with the state of oxidation where slower relaxation times characterize hydrogen-terminated porous silicon. These results are in a good agreement with the extended vibron model for small Si nanocrystallites.

PACS

78.55.Mb; 78.67.Rb; 78.47.jd     

Glassy dynamics of hydrogen-bonded heteroditopic molecules

A self-complementary heteroditopic molecule composed of thymine and diamidopyridine end groups and a flexible aliphatic interconnecting chain has been synthesized. The glassy dynamics of this hydrogen-bonded supramolecule have been investigated by using dielectric and rheological measurements, in combination with infra-red spectroscopy and solid-state ¹³C NMR experiments. Decoupling of main dielectric relaxation from viscosity has been found in the vicinity of the glass transition and the temperature dependence of viscosity appears to be stronger than that of dielectric relaxation. The unusual dynamic decoupling phenomenon is ascribed to the chemical/dynamic heterogeneity and formation of hydrogen bonds in the supramolecules.     

Photochemie substituierter Cycloheptatriene. XV. Zur Charakterisierung der Elektronischen Übergänge in 1-Aryl-4-(p-dimethylaminophenyl)-cycloheptatrienen

Photochemistry of Substituted Cycloheptatrienes. XV. About the Characterization of the Electronic Transitions in 1-Aryl-4-(p-dimethylaminophenyl)-cycloheptatrienes The absorption spectra of the title compounds are discussed in dependence on polarity of the solvent and its ability to form hydrogen bonds. The results suggest a hybridization of the N-atom of the dimethylamino group between sp³ and sp² in the ground state. Contrary to this, the nitrogen has sp²-hybridization in the short wavelength (SE) fluorescing state. In a second fluorescence state the geometry of the seven-membered ring should change. Therefore the viscosity of the solvents influences both the position and the quantum yield of this long wavelength emission (LE). The influence of polarity and viscosity of the solvents is described by multiple regression. The temperature dependence of fluorescence quantum yields is discussed, as well.     

Solvent Polarity Dependent Photophysics of a Fixed-Distance,Symmetric Chlorophyll Dimer. A Model of the Special Pair in Photosynthetic Reaction Centers

The synthesis and properties of a fixed-distance symmetric bis-pyrochlorophyllide-a molecule are described. The molecule, 1 , consists of two methyl pyrochlorophyllide-a (MePChlide-a) moieties which share a common vinyl group at the 2-position of each macrocycle. The two chlorophylls are trans to one another across the vinyl linkage. The dihedral angle between the plane of each macrocycle and the vinyl linkage is about 50°. The Q_y absorption band of this dimer occurs at 689 nm and is essentially independent of solvent. The band is red-shifted by about 27 run relative to that of MePChlide-a. The fluorescence maximum of 1 occurs at 722 nm. This is a red-shift of 49 nm relative to methyl MePChlide-a. The circular dichroism spectrum of 1 shows strong, nonconservative bands in the red region. The fluorescence quantum yield, fluorescence lifetimes, and picosecond transient absorbance kinetics all exhibit a very strong dependence on solvent polarity. The fluorescence quantum yields of 1 in toluene, butyronitrile, and N, N-dimethyl formamide (DMF) are 0.18, 0.008, and 0.002, respectively. The bleach of the 689 nm band (following excitation with a 1.5-ps, 610-nm laser pulse) recovers with single exponential time constants of 2.51 ns, 100 ps, and 64 ps in toluene, butyronitrile, and DMF, respectively. Possible mechanisms for the polarity-dependent increase of the nonradiative decay rate of 1 are considered.     

Fabrication of an anionic polythiophene/layered double hydroxide ultrathin film showing red luminescence and reversible pH photoresponse

Luminescent ordered ultrathin films (UTFs) based on a sulfonated polythiophene (SPT) and Mg–Al‐layered double hydroxide (LDH) nanosheets have been fabricated by the layer‐by‐layer assembly method. UV‐visible absorption and fluorescence spectroscopy showed that there was a stepwise and regular growth of the films with increasing number of deposition cycles. XRD, AFM, and SEM showed that the films had a periodic layered structure with a period of ca. 3.0 nm, and that the thickness can be finely controlled within the range ca. 26–100 nm. The SPT/LDH UTFs show well‐defined polarized photoemission with an anisotropy of ca. 0.3, and they show a reversible luminescence response to changes in pH. Periodic density functional theoretical calculations gave a band energy of 1.85 eV for the SPT/LDH system and showed that the valence electrons of SPT can be confined in the energy wells formed by the LDH monolayers, which effectively inhibits both the nonradiative relaxation and the π–π stacking interaction of the polymer chromophores. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Cyclization dynamics of polymers: 7. Applications of the pyrene excimer technique to the internal dynamics of poly(dimethylsiloxane) chains

Poly(dimethylsiloxane) (PDMS) polymers end-capped on both ends with pyrene chromophores have been synthesized. Rate constants for end-to-end cyclization 〈 k₁ 〉 have been determined for dilute solutions of these polymers in toluene solution using a combination of fluorescence decay and steady-state fluorescence measurements. While precise values of the critical exponent for the chain length (N¯) dependence of 〈 k₁ 〉 are not yet available, these results are consistent with the N¯^{? 3/2} dependence predicted by Wilemski-Fixman theory. PDMS chains cyclize somewhat more than two times faster than polystyrene chains of the same length in solvents of similar solvating power and viscosity. These results provide strong support for similar predictions made several years ago by Perico and Cuniberti, who used intrinsic viscosity data to parametrize the Rouse-Zimm model for analysis of polymer cyclization dynamics.     

Effect of Dimer Structure and Inhomogeneous Broadening of Energy Levels on the Action of Flavomononucleotide in Rigid Polyvinyl Alcohol Films

The results of time-resolved fluorescence measurements of flavin mononucleotide (FMN) in rigid polyvinyl alcohol films (PVA) demonstrate that fluorescence intensity decays are strongly accelerated in the presence of fluorescent dimers and nonradiative energy transfer processes. The fluorescence decay originating both from H and J dimer states of FMN was experimentally observed for the first time. The mean fluorescence lifetimes for FMN dimers were obtained: τfl = 2.66 ns (at λ_exc = 445 nm) and τfl = 2.02 (at λ_exc = 487 nm) at λ_obs = 600 nm and T = 253 K from H and J state of dimers, respectively. We show that inhomogeneous orientational broadening of energy levels (IOBEL) affects the shape of the fluorescence decay and leads to the dependence of the average monomer fluorescence lifetime on excitation wavelength. IOBEL affected the nonradiative energy transfer and indicated that different flavin positioning in the protein pocket could (1) change the spectroscopic properties of flavins due to the existence of “blue” and “red” fluorescence centers, and (2) diminish the effectiveness of energy transfer between FMN molecules.     

Electro-optic birefringence and scattering of concentrated solutions of ethylcellulose

Photon correlation spectroscopy (PCS) at small scattering vector (kR_G < 1) and the Kerr electro-optical effect of ethylcellulose-solvent system in the semi-dilute concentrated regime are reported. The relaxation is broader than to be expected for a single relaxation time and both the rise and the decay transients are well fitted by a stretched exponential expression of the form φ (t) = exp (–t/τ)^β. The decay and rise relaxation times, respectively, are about equal (〈τ_d〉 ≈ 〈τ_r〉) and the same is true for the broadening parameters of the decay and rise transients (β_r ≈ β_d). The β values were found to decrease with increasing concentration. The experimental results are in qualitative agreement with the theoretical prediction of Doi and Edwards but in quantitative disagreement; thus implying that idealised models are far from describing the true observation. The semi-empirical theory of Douglas and Hubbard aptly described the concentration exponents observed in this work for the zero shear viscosity and the whole chain relaxation data.     

Cooperative Effects in the Photoluminescence of (In,Ga)As/GaAs Quantum Dot Chain Structures

Multilayer In_0.4Ga_0.6As/GaAs quantum dot (QD) chain samples are investigated by means of cw and time-resolved photoluminescence (PL) spectroscopy in order to study the peculiarities of interdot coupling in such nanostructures. The temperature dependence of the PL has revealed details of the confinement. Non-thermal carrier distribution through in-chain, interdot wave function coupling is found. The peculiar dependences of the PL decay time on the excitation and detection energies are ascribed to the electronic interdot coupling and the long-range coupling through the radiation field. It is shown that the dependence of the PL decay time on the excitation wavelength is a result of the superradiance effect.     

Decay Lifetimes of Para-Substituted Polystyrene in Solid Films and in Dichloromethane

The fluorescence decay times of polystyrene, poly(4-bromostyrene), poly(4-chlorostyrene), poly(4-methylstyrene), poly(α-methylstyrene), poly(4-methoxystyrene), and poly(4-tert butylstyrene) were measured in solid films and in dichloromethane solution. A detailed analysis of the emission profile performed by nanosecond time resolved fluorescence spectroscopy confirmed the presence of monomer fluorescence as well as excimer fluorescence in both media. Monomer fluorescence decay times gave shorter times compared with that of excimer decay times in dichloromethane solution. Fluorescence from substituted polystyrene was mainly excimer fluorescence with shift of maximum emission to longer wavelength. The ratio of monomer to excimer contributions was found to be dependent on the emission wavelength, but was not affected by polymer concentration. Both monomer and excimer fluorescence lifetimes as well as excimer intensity increase with increasing emission wavelength. An accompanying decrease in monomer contribution is also observed is solution in comparison with that in solid films.     

The influence of the relative molecular weight of natural rubber on its thermo-oxidative stability

The influence of relative molecular weight on rubber thermo-oxidation in air at 130 ± 1°C has been studied by means of infrared spectroscopy. The carbonyl group formation in single fractions of natural rubber has been measured. It was found that the development of kinetic curves ΔA_C?O = f(t) corresponds for both fractionated and unfractionated extracted samples. Kinetic curves of natural rubber formation are characterized by an induction period τ. Its magnitude is a function of molecular weight up to the value [η] = 0.2 m³/kg, and then stays invariant to changes in [η]. In the range of low values of the internal viscosity up to 0.2 m³/kg, the dependence τ_C?O = f[η] has an exponential character as a consequence of the statistical character of chains breaking down during the degradation. It has been proved that the reciprocal value dependence of the mean viscosity molecular weight of unfractionated acetone-extracted natural rubber on the length of induction period is linear. The change in double-bond amount during the oxidation is not reflected in the induction period determined by infrared spectroscopy.     

Small and large scale segmental motion in polymers: estimating cooperativity length by ordinary relaxation experiments

We derive a suitable expression for estimating the size of the cooperatively rearranging regions (CRRs) in supercooled polymer melts by fitting data worked out by ordinary relaxation experiments carried out in isothermal conditions. As an example, the average CRR size in poly(n‐butyl methacrylate) in proximity to the glass transition temperature is derived from a stress relaxation experiment performed by means of an atomic force microscopy setup. Good agreement is found with results in the literature derived from measurements of temperature fluctuations (the so‐called Donth method). The temperature dependence of the CRR size is explored for poly(butadiene); in this case the segmental relaxation function is derived through a novel method for the analysis of the efficiency with which free induction decay echoes are refocused in ¹H NMR experiments. It is found that the CRR size increases upon cooling. The results derived from the analysis of the NMR data are found to be in satisfactory agreement with those worked out from broadband dielectric spectroscopy data in the literature. © 2015 Society of Chemical Industry     

Effect of blending on the rheological properties of polystyrene

Experimental non-Newtonian viscosity, primary normal stress difference, complex viscosity, and shear stress relaxation were taken for highly fractionated polystyrene in Aroclor 1248 (a chlorinated biphenyl) as well as for blends differing in molecular weight and concentration. The data are described by three parameters: a zero-shear rate value, the slope of the log–log plots in the high shear rate region, and a time constant defined as the inverse of the shear rate at the intersection of the low and high shear rate asymptotes. For the functions measured, the low shear rate region is characterized by a dependence on M_w and the high shear rate region by a dependence on M_n. Implications to polymer processing are discussed.     

Photoluminescence study of β relaxation of polyethylene blends

Secondary β relaxation of polyethylene blends (50/50 HDPE/LDPE) were studied by photoluminescence of anthracene molecules dissolved in the polymer bulk. The temperature of the β relaxation has been determined as T_β = ?40°C by the dependence of the ratio of vibronic components of the fluorescence band on the temperature. The molecular mechanism of this relaxation has been discussed considering the possibility of the energy migration involving anthracene molecules in the singlet electronic excited and ground states. © 1993 John Wiley & Sons, Inc.     

Surface characterization of silanized glass fibers by labeling with environmentally sensitive fluorophores

Glass fibers have been treated with γ-aminopropyl-triethoxysilane (APES) through different silanizating procedures, which include APES aqueous solutions and APES vapor adsorption. Transmission Fourier transform IR (FTIR) measurements have been performed on the silanized samples to characterize the silanization reaction. Dansyl-sulfonamide conjugates have then been formed by reaction of dansyl chloride in dimethylformamide solution with the amine functionality's immobilized on the glass fiber surface. Steady-state and time-resolved fluorescence measurements have been performed on dansylated samples. A dependence of the fluorescence intensity and the wavelength of the maximum emission on the silanization procedure has been observed. Good fits of the fluorescence decays of dansyl labels are found when biexponential functions are used for deconvolution, whereas the decay of dansylamides in fluid solution is single exponential. A two-state model for the solid solvent relaxation seems to apply for this samples. Several surface structural changes produced by the different silanization methods have been proposed. FTIR results support the conclusions drawn from fluorescence measurements. © 1996 John Wiley & Sons, Inc.     

Ideality of pressure‐sensitive paint. II. Effect of annealing on the temperature dependence of the luminescence

This article reports the effect of annealing on a pressure‐sensitive paint (PSP) consisting of platinum tetra(pentafluorophenyl)porphine (PtTFPP) in a fluoroacrylic polymer called FIB (Puklin, E.; Carlson, W. B.; Gouin, S.; Costin, C.; Green, E.; Ponomarev, S.; Tanji, H.; Gouterman, M. J Appl Polym Sci 2000, 77, 2795). Samples annealed at 150°C, 75°C, annealed by a heat gun, and dried at room temperature are compared to nonannealed samples. Temperature dependences of luminescence intensity and lifetime are studied as a function and pressure and temperature and fit with Arrhenius and Stern–Volmer equations. We find that heating above T_g is more important than drying at room temperature in lowering the temperature dependence and obtaining ideal PSP, ideal meaning independent effect of pressure and temperature on luminescence properties of the paint. Ideality is achieved by lowering the activation energy for oxygen diffusion, presumably by relaxation of the polymer network. It is shown that ideal behavior occurs only over a limited temperature region. This range is more useful for PtTFPP in FIB than for ruthenium bathophenanthroline in polydimethylsiloxane (PDMS), another common PSP formulation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2805–2814, 2000     

Relationship between melt viscosity and dielectric relaxation time for a series of epoxide oligomers

Melt viscosity has been investigated for a series of bisphenol-A type epoxide oligomers with different weight-average mol wts (M?_w), ranging from 388 to 2640. The temperature dependence of the melt viscosity is described by the Williams–Landel–Ferry (WLF) equation. The melt viscosity η is correlated with both the direct current (dc) conductivity σ and the dielectric relaxation time τ. The two relationships between these three properties, σ·η^κ = const (0.63 ≦ κ ≦ 1.12) and η/τ^? = const (0.73 ≦ ? ≦ 1.06), are experimentally derived. Both exponents, κ and ?, depend on the M?_w of the oligomer. The lower M?_w oligomer has the larger value of κ. The κ value is close to unity for the low M?_w oligomer, which agrees with Walden's rule, σ·.η = const, applicable to most low mol wt liquids. The ? value is near unity for the epoxide oligomer with higher M?_w than 2000, which means that the melt viscosity is proportional to the dielectric relaxation time. The low M?_w oligomer (M?_w < 2000), on the other hand, has a smaller value of ? below unity. The result indicates that the melt viscosity is not proportional to the dielectric relaxation time for the low M?_w epoxide oligomer, whose dielectric α-relaxation is not governed by the Debye equation. © 1993 John Wiley & Sons, Inc.     

First international symposium on polymer electrolytes

We review the effects of temperature on conductance and viscosity in the liquid state of vitreous ionic conductors and show how differences may be best understood by comparison of relaxation times for electrical and mechanical stresses acting on liquid or glassy states of the material. This leads to the definition of a conductivity/viscosity mode decoupling index, useful as a figure of merit for the solid electrolyte. In applying the same approach to polymer electrolyte systems a problem is encountered due to the molecular weight dependence of the viscosity. This is resolved by deriving a ‘monomer’ shear relaxation time for the polymer electrolyte solution and showing that this quantity corresponds closely with the ‘local’ mechanical relaxation time obtained from light scattering studies (which is a molecular weight independent quantity for pure polypropylene oxide and other polymers of low T_g). Comparison of the electrical relaxation times of the polymer solution with the ‘local’ (or ‘matrix’) relaxation times then shows that the relationship found for superionic glass-forming systems is inverted in the case of polymer electrolytes. The latter have fractional decoupling indexes which may be interpreted in terms of serial coupling phenomena; i.e. ions must first decouple from their partner ions (in ion pairs), or from their intramolecular solvation states, before they can contribute to conductivity relaxation which is itself coupled to the local polymer matrix relaxation. An extreme case is illustrated using the weak electrolyte, lithium acetate, for which the decoupling index is ～6 × 10^?5.