Study of polarisation ratios in mixed molecular crystals

The perturbation theory of Craig and Thirunamachandran is used to obtain the polarisation ratio (b/a) of guest transitions in an anthracene host crystal as a function of trap depth. Results from several substituted anthracenes as dopant are compared with the calculated curve. Substantial agreement is obtained over a wide range of trap depths confirming the applicability of the simple theory in this energy region. Degree of misorientation and possible deviation of transition moment from the short molecular axis (M) are briefly discussed.     

Energy transfer via guest excitons in isotopic mixed naphthalene crystals

The temperature dependence of the fluorescence spectra of aggregates in naphthalene-perdeuteronaphthalene mixed crystals has been investigated between 1.4 and 70 K and for concentrations up to 50% naphthalene. It is shown that the most abundant traps — the monomer guest molecules — transfer energy like a guest exciton band 48 cm^?1 below the host exciton band. With increasing temperature, the excitation energy is redistributed between the different aggregate traps by thermal activation into the monomer states. The energy transfer constant within the monomer exciton band is measured as a function of concentration. It is suggested that dipole-dipole interaction between the monomer guests is responsible for the energy transfer via guest excitons.     

Random-walk studies of excitation trapping in crystals

We investigate energy trapping in regular crystals of different dimensionalities, assuming a nearest-neighbor random walk of the excitation and quenching at first trap encounter. From numerical simulatons we determine the number of different sites visited, and exact and approximate survival probabilities; their behavior changes drastically with the lattice dimensionality.     

Charge-transfer exciton trapping at vacancies in molecular crystals: photoconductivity quenching,optical damage and detonation

Calculations are presented that show that vacancies can trap charge-transfer (CT) states in anthracene, acetanilide and hexahydro-1,3,5-trinintro-1,3,5-triazine (RDX). Such trapping provides a mechanism for photoconductivity quenching by geminate recombination, and for optical damage and detonation by concentrating optical or mechanical energy stored in CT states.     

Optical dephasing of molecular dimers in mixed crystals: Picosecond photon echo experiments

Picosecond photon echo experiments are used to examine optical dephasing of substitutional dimers and monomers of tetracene and pentacene in p-terphenyl host crystals. A comparison of experiments on tetracene and pentacene dimers permits the mechanism responsible for temperature-dependent optical dephasing to be determined. It is shown that excitation of librations rather than scattering between delocalized dimer states is the principal mechanism.     

Picosecond time scale optical coherence experiments in mixed molecular crystals: pentacene in naphthalene

Picosecond time scale high-power pulse optical coherence measurements including photon echo and the stimulated photon echo are obtained with a mode-locked dye laser synchronously pumped by a double Q-switched and mode-locked Nd: YAG laser. Effects on coherence arising from excitation with gaussian laser pulses rather than square pulses are examined. Preliminary echo decay measurements are reported.     

The kinetics of diffusion-controlled reactions in anisotropic systems. Exciton trapping, detrapping and heterofusion in molecular crystals

The kinetic equations describing diffusion controlled trapping and heterofusion of excitons in molecular crystals are presented. The equations are valid for anisotropic diffusion, and contain terms describing effects due to correlation of the traps and percolation. The dependence of heterofusion on homofusion and on trapping is shown.     

Excavations in molecular crystals

Single crystals built from porous molecular networks can react with agents that penetrate the crystals, cleave fragments from the network, and thereby increase the volume available for guests, all without loss of crystallinity.     

Singlet exciton trapping and heterofission in tetracene doped anthracene crystals

In tetracene doped anthracene, the magnetic field modulation of prompt tetracene fluorescence following excitation into the anthracene singlet manifold has been measured as a function of the magnetic field orientation and optical excitation energy. The results show that this modulation with low energy excitation is caused by singlet heterofission into one anthracene triplet exciton and one tetracene triplet. With higher excitation energies this modulation is due to both the singlet heterofission and also singlet homofission into a pair of anthracene triplet excitons. Heterofission occurs mainly from anthracene molecules next to a tetracene and competes with the singlet trapping. From the singlet trapping rate and from the magnetic modulation of tetracene prompt fluorescence the heterofission rate is estimated as ≈10^?11s^?1.     

Intersystem crossing from singlet states of molecular dimers and monomers in mixed molecular crystals: picosecond stimulated photon echo experiments

An experimental study of the excited-state dynamics of pentacene dimers and monomers in p-terphenyl host crystals is presented. Picosecond stimulated photon echoes, picosecond photon echoes, and fluorescence lifetime measurements are used to study intersystem crossing and homogeneous dephasing of delocalized dimers and monomers at 1.8 K. It is found that the dimer states can have intersystem crossing rate constants which are orders of magnitude greater than the corresponding monomers. Three mechanisms are considered to explain the differences between dimer and monomer intersystem crossing. Fluorescence lifetime measurements and photon echo measurements demonstrate that the only source of homogeneous line broadening at 1.8 K is population relaxation. These measurements combined with the stimulated echo measurements show that differences in lifetimes exhibited by the various dimer and various monomer sites are due solely to differences in intersystem crossing rate constants.     

New excimers in molecular crystals

A method of forming new excimeric systems is presented. We use a host crystal with pair structure to induce the excimeric configuration on the guest molecules. The emission and decay times of anthracene derivatives substituted in 9 and 9.10 positions in pyrene are studied. Excimers are formed in all the mixed crystals considered, including dimethyl anthracene which does not form excimers in its pure crystal.     

Rotational effects in molecular crystals

The Mössbauer spectrum of ferric oxinate exhibits and asymmetric quadrupole doublet. The modification of the dynamic Mössbauer parameters (to the equal values for each line) following the irradiation, was explained by the internal oxine ligands rotation inside of the octahedral coordination.     

Polarization effects in molecular crystals

A simple model of a molecular crystal is used to resolve apparent inconsistencies between existing approaches applied in calculations of polarization energies on the one hand and charge-transfer integrals on the other.     

Non-linear line-narrowing spectroscopy in mixed organic crystals

We report the elimination of inhomogeneous broadening in mixed organic crystals with multiply resonant four-wave mixing methods. Line narrowing and the line shifts characteristic of site selective methods are observed for both coherent anti-Stokes Raman spectroscopy (CARS) and multiply enhanced non-parametric spectrosocopy (MENS). The site selective capability of CARS is in agreement with the model proposed by Ouellette and Denariez-Roberge.     

Raman scattering in mixed defect chalcopyrite crystals

We have investigated with Raman spectroscopy pseudoternary Zn_xCd_1?xGa₂S₄ and CdGa₂(S_xSe_1?x)₄ phases over the whole composition range. In Zn_xCd_1?xCd_1?xGa₂S₄ a smooth variation of the spectra and a generalized one-mode behaviour has been observed, except for the lowest frequency modes. In CdGa₂(S_xSe_1?x)₄ the anion substitution has a larger influence on the spectra. Peculiar two-mode behaviour is displayed by the strong A-symmetry breathing modes.     

Sonofragmentation of molecular crystals

Possible mechanisms for the breakage of molecular crystals under high-intensity ultrasound were investigated using acetylsalicylic acid (aspirin) crystals as a model compound for active pharmaceutical ingredients. Surprisingly, kinetics experiments ruled out particle-particle collisions as a viable mechanism for sonofragmentation. Two other possible mechanisms (particle-horn and particle-wall collisions) were dismissed on the basis of decoupling experiments. Direct particle-shock wave interactions are therefore indicated as the primary mechanism of sonofragmentation of molecular crystals.     

Examination of stabilty of molecular agglomerates in molecular crystals

We report an examination algorithm of stability of molecular aggregates based on the estimation of rigidity of intermolecular contacts in a crystal structure. The algorithm includes the intermolecular interaction energy calculation (in the atom-atom potential approximation) of a pair of molecules selected in the crystal structure. Further, the energy is minimized using a least-squares technique by varying the position and orientation of one of the molecules. The contact rigidity is quantitatively assessed by the minimized rms difference between the positions of the atoms in the original and optimized structures (Zorkii’s criterion). Every rigid contact revealed in the structure determines finite or infinite stable agglomerates. The paper presents the results of testing the computer program based on this algorithm with a number of real crystal structures previously determined by single crystal X-ray diffraction, and also the examples of the most common stable molecular agglomerates found with the aid of the program.     

Cooperative exciton spectra in molecular crystals

It is shown that the singlet exciton spectrum in the crystal is coupled with that of the two-particle excitations (singlet, triplet excitons, magnons, triplet exciton-magnon). Splitting on the excitation spectrum occurs when the composite energy of the two-particle excitations is close to that of the singlet exciton.