Mixed Naphthalene Crystals Research Articles

Scaling behavior of diffusion limited annihilation reactions on random media

We investigate numerically the kinetics of diffusion limited annihilation reactions in disordered binary square lattices where the reacting particles are constrained to diffuse on a concentration p of the lattice sites. We find that the asymptotic decay of the particle concentration in the percolative regime is of the form c(t,p)−cr(p)∝t−ds/2, where cr(p) is the concentration of residual particles. We recover well known results such as ds(p≳pc)=d=2 with logarithmic corrections, and ds(pc)=1.34±0.02. For p<pc we employ a scaling theory and collapse the data onto a universal form dc/dt=τ−(ds(pc)/2+1)f(t/τ), with τ being a characteristic diffusion time and f(t/τ) representing the crossover from a power law decay to a stretched exponential one. We relate the present results with the kinetics of the excitation reaction (triplet + triplet → singlet) on isotopic mixed crystals of naphthalene.

Non-Classical Reaction Kinetics: Experiments

ABSTRACTIt has been well established by theory and simulations that the reaction rates of diffusion-limited reactions can be affected by the spatial dimension in which they occur. The types of reactions A + B → C, A + A → A, and A + C→, C have been shown, theoretically and/or by simulation, to exhibit non-classical reaction kinetics in low and fractal dimensions. We present here experimental results from several 1D and fractal systems.An A+B → C type reaction was experimentally investigated in a long, thin capillary tube in which the reactants, A and B, are initially segregated. This initial segregation of reactants means that the net diffusion is along the length of the capillary only, making the system effectively 1D and allowing some of the properties of the resulting reaction front to be studied. The reaction rate of excitonic fusion, A + A →A, as well as trapping, A + C→ C, reactions were observed via phosphorescence(P) and delayed fluorescence(F) of naphthalene within the channels of Vycor glass, in isotopically mixed naphthalene crystals and in the isolated chains of dilute polymer blends. In these experiments, the non-classical kinetics are measured in terms of the heterogeneity exponent, h, from the equation: Rate ∼ F =; kt-hpn,, which gives the time dependence of the rate coefficient. Classically h =; 0, while h =; 1/2 in 1D, as well as in the fractal dimensions discussed here, for A + A → A as well as A + C →C type reactions.

SOLID‐STATE PHOTOCHEMISTRY OF INDOLES WITH NAPTHALENE AND PHENANTHRENE

Abstract— Solid‐state irradiation of mixed crystals prepared by meltinga 1:3 molar mixture of indole and phenanthrene followed by resolidifying the melt gave an adduct 2a in 13.5% yield. An adduct 2b was obtained by irradiation of the mixed crystals of 3‐methylindole and phenanthrene. Irradiation of the same mixtures of indoles and phenanthrene in solution gave no photoproduct. Irradiation of the mixed crystals of indole and naphthalene gave a similar photoadduct 3a, which was also formed in solution. The 1:1 mixed crystals of indole and phenanthrene and of indole and naphthalene were characterized by various spectroscopic methods. Among them powder X‐ray diffraction spectral analysis and differential scanning calorimetry revealed that the former mixed crystal is a simple mixture of microcrystals of indole and phenanthrene, while the latter forms a molecular compound.

Temperature-dependent vibrational relaxation in isotopically mixed molecular crystals by picosecond CARS

Temperature-dependent dephasing of 511, 766 and 1385 cm −1 modes in the mixed crystals of naphthalene and perdeuteronaphthalene at various isotopic levels have been studied by picosecond CARS in the region of 6–130 K. In general, adding the isotope shows a significant effect on the low-temperature decay time. However, a small effect on the temperature dependence is observed in the Arrhenius plots of ln[Δν( T)-Δν(6 K)] versus 1/ T for all three modes. This behavior is described by a large change of the thermally activated phonon density of states at low temperature, but by a small change as temperature increases.

Delayed fluorescence ODMR as a probe of triplet-triplet fusion probability

A kinetic scheme is presented to rationalize recent observations of delayed fluorescence ODMR in isotopically mixed naphthalene crystals, in which the sign of the signal changes from negative to positive as the critical concentration for triplet-triplet transport among the perprotonated traps is approached. The effects of chopped optical excitation, prolonged microwave pumping, temperature and light intensity are considered. The observation of positive DF-ODMR in the high concentration case is used to obtain a lower limit for the ratio of the annihilation constantk 2 to the pair dissociation constantk −1.

Steady-state vs. pulsed excitation recombination kinetics: Simulations and experiments on pores, powders and random media

Abstract The non-classical kinetics of exciton recombination in restricted geometries provides the foundation for a new experimental technique of probing the exciton dynamics and the sample topology. The phosphorescence and delayed flourescence decays exhibit a dramatic dependence on the duration of the excitation. The comparison of pulsed and steady-state excitation provides information on the local topology of the sample and on the average hopping time of the exciton and the exciton diffusion length. This is possible because the distribution of the exciton population is non-Poissonian under steady-state excitation conditions. In addition, the pulse-created distribution also loses its Poissonian character with time. The experimental systems are: 1) isotopic mixed naphthalene crystals above and below percolation; 2) naphthalene crystalline powder; 3) naphthalene embedded into porous glass. Except for the mixed crystals above the percolation concentration, all samples exhibit non-classical effects. The interpretation is aided by Monte Carlo simulations.

Delayed fluorescence ODMR in isotopically mixed naphthalene crystals: Effect of critical concentration transport on signal polarity

Abstract Optical detection of magnetic resonance has been observed in the delayed fluorescence (DF) from crystals of perprotonated naphthalene (N-h) in perdeuterated naphthalene (N-d) at 1.7 K. Signals corresponding to N-h traps and translationally equivalent dimers have been observed in both phosphorescence and delayed fluorescence. While the phosphorescence ODMR transitions are positive (increase in light) the DF-ODMR is negative in a 2.7% N-H crystal, and positive in a 9.6% crystal. Delayed fluorescence originates only from xx , yy or zz triplet pairs. Microwave pumping of isolated triplet trap spins in the 2.7% crystal tends to reduce population differences and hence the rate of formation of radiative pairs. In the 9.6% crystal, microwave pumping of triplet pairs effectively “dark” xz , yz or xy pairs to radiative pairs, and enhances DF.

Steady-State vs. Pulsed Excitation Recombination Kinetics: Simulations and Experiments on Pores, Powders and Random Media

Abstract The non-classical kinetics of exciton recombination in restricted geometries provides the foundation for a new experimental technique of probing the exciton dynamics and the sample topology. The phosphorescence and delayed fluorescence decays exhibit a dramatic dependence on the duration of the excitation. The comparison of pulsed and steady-state excitation provides information on the local topology of the sample and on the average hopping time of the exciton and the exciton diffusion length. This is possible because the distribution of the exciton population is non-Poissonian under steady-state excitation conditions. In addition, the pulse-created distribution also loses its Poissonian character with time. The experimental systems are: 1) Isotopic mixed naphthalene crystals above and below percolation; 2) Naphthalene crystalline powder; 3) Naphthalene embedded into porous glass. Except for the mixed crystals above the percolation concentration, all samples exhibit the non-classical effects. The...

Anomalous diffusion by tunnelling on a square lattice: trapping and fusion of triplet excitons in mixed crystals of naphthalene

This paper presents numerical simulations of dispersive hopping transport by tunnelling between random centres on a square lattice. The results are compared with two approximations, the continuous-time random walk and anomalous diffusion by hopping on percolation clusters. The continuous-time random walk is qualitatively and semi-quantitatively adequate for high concentrations, short times and weak disorder, but diverges from the simulations at long times. Hopping on percolation clusters does not adequately describe the results. Diffusion is in general anomalous, r2(t) infinity tx, 0<x<1, where the upper and lower bounds correspond respectively to weak and strong disorder. The simulations show the dependence of the apparent luminescence decay of a disordered material on the source of excitation. The stretched exponential decay of donor phosphorescence and the algebraic decay of delayed fluorescence of isotopically mixed crystals of naphthalene at 1.6 K are studied as a practical illustration.

Random walks on an ultra-metric space: a model for transport by tunnel effect in a mixed crystal

Hopping transport by tunnel effect in a disordered solid often implies a wide spread of the hopping times. This is described here by an ultra-metric space in which the distance between two centres is defined as the longest hopping time on the shortest path between them. The authors derive numerically and analytically the mean and the second moment of the number of distinct sites visited as functions of time. These results are applied to the calculation of the decay of luminescence due to hopping and trapping in the much studied triplet state of isotopically mixed crystals of naphthalene. The numerical simulations are quite well described by the scaling theory of percolation.

Novel photochemical reaction of indole with naphthalene and phenanthrene in the solid state

Irradiation of mixed crystals of indole and naphthalene gave an addition product, 1-(1,4-dihydro-1-naphthyl)indole ( 1 ), while irradiation in solution gave another product in addition to 1 . Similar solid-state photoreaction of indole and phenanthrene gave 2 .

Resonant secondary emission and spectral diffusion in isotopically mixed naphthalene crystals

Resonant secondary radiation (zero-phonon lines) flourescence spectra of isotopic 3D aggregates of traps (NH 8) in a host crystal (ND 8), as well as transfer-induced fluorescence at trap concentrations between 0.01 and 10% and at temperatures between 1.6 and 10 K, are presented. Laser-excited (0–1) resonant secondary (zero-phonon) emission is used to resolve close-lying electronic resonances, corresponding to monomers, aggregates and to their host and trap satellite lines, in a quasi-continuum of 3D trap states. Such states are also calculated by numerically diagonalizing a mixed-crystal 3D hamiltonian. Measured and calculated delocalization indices are given for all trap species. In addition, low-temperature spectral diffusion of photoselected trap species is measured by plotting the attenuation of the zero-phonon lines. We give a simple discussion on the modulation of the resonant secondary radiation spectra by concentration, excitation wavelength and temperature. We show that low-temperature exciton diffusion, assited by acoustical phonon creation, occurs above a very low concentration threshold ( C > 0.05% for monomers at T = 1.6 K). Thermally activated transfer of the photoselected excitons is also plotted for temperatures up to 10 K above which the zero-phonon lines are quenched.

Effect of high pressure on critical concentration energy transport in isotopically mixed naphthalene crystals

High pressure has been observed to enhance significantly triplet energy transfer from naphthalene-h/sub 8/ traps to ..beta..-methylnaphthalene supertraps in a naphthalene-d/sub 8/ crystal near the critical trap concentration, as indicated by supertrap/trap phosphorescence ratios. This suggests the importance of long-range interactions in triplet transport in this crystal system. 16 references, 1 figure, 1 table.

Vibrational relaxation in isotopically disordered naphthalene crystals

Vibrational relaxation in low temperature pure and isotopically mixed crystals of naphthalene is studied by picosecond coherent Raman scattering. Studies of pure and mixed crystals of naphthalene with several deuteronaphthalenes have allowed the comparison of vibrational relaxation under conditions where the level energies, state densities and coupling matrix elements are different.

Triplet exciton transport in isotopic mixed naphthalene crystals. II. Master equation analysis

The experimental data on triplet exciton transport in isotopically mixed crystals of naphthalene/perdeuteronaphthalene (paper I) are contrasted with singlet exciton transport in the same samples (20%–100%) and analyzed in terms of incoherent hopping models. The master equation approach is emphasized and extended. We modify the conventional continuum master equations via a physically plausible cutoff of the high-frequency transfer rates. This results in an experimentally acceptable functional form (transport linear with high power of concentration) and nearest-neighbor transfer time (100 ps). We also developed a lattice master equation (numerically soluble), using an experimentally tested exciton superexchange formula. The somewhat surprising result is that the lattice master equations do not fit the experimental functional form. The success of the continuum models and the failure of the lattice models are attributed to the latter’s neglect of the spread in transfer rates for a given intersite distance. We claim that clusterization as well as diagonal homogeneous and/or inhomogeneous disorder cause the above spread. On the other hand, these energy mismatches are small with respect to the thermal energy, in contrast to the singlet exciton transport case, where, due to larger energy mismatches, a percolation-like critical concentration is observed. Thus for the given concentration and temperature regimes, the triplet exciton transport is diffusive while the singlet exciton transport is percolative. Lower temperatures and/or concentrations are required for percolative triplet energy transport in these systems.

Triplet exciton transport in isotopic mixed naphthalene crystals. I. Kinetic analysis of trapping and fusion

Time-resolved triplet exciton transport in binary mixed crystals has been studied for C10H8 in C10D8 (20%–100%) at liquid helium temperatures. The delayed fluorescence decays are exponential and range from milliseconds to nanoseconds as Cg goes from 0.2 to 1.0. The analysis is based on the presence of a BMN supertrap in large amounts (10−3) and on the fact that supertrapping of the guest excitons dominates the kinetics while heterofusion (triplet guest-triplet supertrap annihilation) dominates over the homofusion (guest–guest annihilation) and was used to monitor the kinetics. We also investigated in detail photodetrapping and the relative efficiencies of the various fusion channels. An analysis of the transport data is given in paper II.

Phosphorescence spectra in mixed naphthalene crystals: a transition from site to crystal states

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhosphorescence spectra in mixed naphthalene crystals: a transition from site to crystal statesJ. P. Lemaistre, A. Blumen, F. Dupuy, P. Pee, R. Brown, and P. KottisCite this: J. Phys. Chem. 1984, 88, 20, 4655–4660Publication Date (Print):September 1, 1984Publication History Published online1 May 2002Published inissue 1 September 1984https://doi.org/10.1021/j150664a043RIGHTS & PERMISSIONSArticle Views43Altmetric-Citations5LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (601 KB) Get e-Alerts

Exciton transport in disordered crystals: velocity correlation functions

The two-particle coherent-potential approximation is used to calculate Frenkel-exciton group-velocity correlation functions for substitutionally disordered crystals. Most of the results can be described by a relaxation-time approximation, provided that k-dependent and complex relaxation times are allowed; however, some evidence for long-time tails, associated with the finite frequency range of the scattering potentials, is found. The probable accuracy of the approximation and its relationships with localization and kinetic theory are discussed, as is its relevance to experimental systems (triplet excitons in isotopic mixed naphthalene and anthracene crystals).

Triplet States in Isotopically Mixed Naphthalene Crystals: High Resolution Optical Spectroscopy

The triplet 0.0 transitions (T1↔ S0) of guest and host in various isotopically mixed naphthalene crystals, with guest states below and above the host exciton band, are investigated applying both laser excitation spectroscopy and phosphorescence emission spectroscopy. Guest monomer and aggregate spectra are analyzed at helium temperatures (1.6-4.2 K) with regard to line positions, splittings, polarizations and intensities. Shift and broadening of the host Davydov components are studied as a function of the guest concentration (at 1.6 K). In particular, the analysis provides precise values for the individual isotopic shifts U0 and the exciton band structure parameters including the critical isotopic shift. Site splitting is spectrally resolved for several monomers and also for the AB-dimer of 1,4-Nd2 in Nd8

Time and temperature dependences of the delayed fluorescence in isotopically mixed naphthalene crystals

Etude de la fluorescence retardee dans un cristal de naphtalene deuterie de 1,6 a 30 K pour des concentrations isotopiques comprises entre 0,6% et 15%. La fluorescence retardee est independante de la temperature de 1,6 K a 5 K. A des temperatures plus elevees, on observe une forte augmentation de la fluorescence et une forte extinction. Discussion relative aux processus radiatifs et non radiatifs en fonction de la topologie des sites et de l'excitation laser