Pure Molecular Crystals Research Articles

Modeling Step Velocities and Edge Surface Structures during Growth of Non-Centrosymmetric Crystals

Almost all molecules are non-centrosymmetric, which produces interaction anisotropy within a crystal lattice. This anisotropy generates multiple types of kink sites on each crystal step and repeating patterns of rows with different growth units from the perspective of the lattice interaction environment, even for pure molecular crystals. As a result, unstable edge rows may be generated that dissolve under conditions of crystal growth. A method to account for edge surface structures, considering such effects, is required to accurately model the step velocity, which is vital for a mechanistic description of crystal growth. We classify both thermodynamic and kinetic contributions to step row instability and develop expressions for kink densities and step velocities that capture these important non-centrosymmetric phenomena. To demonstrate the utility of our framework, we consider in depth the case of an alternating-row A–B step. Our mechanistic predictions compare favorably to kinetic Monte Carlo simulations...

Phenomenological model of spin crossover in molecular crystals as derived from atom–atom potentials

The method of atom-atom potentials, previously applied to the analysis of pure molecular crystals formed by either low-spin (LS) or high-spin (HS) forms (spin isomers) of Fe(II) coordination compounds (Sinitskiy et al., Phys. Chem. Chem. Phys., 2009, 11, 10983), is used to estimate the lattice enthalpies of mixed crystals containing different fractions of the spin isomers. The crystals under study were formed by LS and HS isomers of Fe(phen)(2)(NCS)(2) (phen = 1,10-phenanthroline), Fe(btz)(2)(NCS)(2) (btz = 5,5',6,6'-tetrahydro-4H,4'H-2,2'-bi-1,3-thiazine), and Fe(bpz)(2)(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2'-bipyridine). For the first time the phenomenological parameters Γ pertinent to the Slichter-Drickamer model (SDM) of several materials were independently derived from the microscopic model of the crystals with use of atom-atom potentials of intermolecular interaction. The accuracy of the SDM was checked against the numerical data on the enthalpies of mixed crystals. Fair semiquantitative agreement with the experimental dependence of the HS fraction on temperature was achieved with use of these values. Prediction of trends in Γ values as a function of chemical composition and geometry of the crystals is possible with the proposed approach, which opens a way to rational design of spin crossover materials with desired properties.

On the role of interphase strain in solid-solid phase transformations: Ice clathrates and hexagonal ice

Abstract The enthalpy and entropy of some solids do not rise to their ultimate values at their first-order phase transformation temperature T t O and further heating is required to complete the transformation. This means that a substantial amount of the low-temperature phase persists in a superheated state above T t O We postulate that in such cases the increase in the strain energy at the two-phase boundary in the bulk of a partially transformed solid opposes the transformation favoured by the decrease in the Gibbs energy, and that the phenomenon should be pronounced when the enthalpy of transformation of a material is small in relation to the interphase strain energy. The abrupt decrease in the slope of heat capacity and entropy at T > T t O for, firstly, the proton order-disorder transformations of KOH-doped ice clathrates, secondly, ice XI ↔ ice I transformation with both phases containing a small amount of various alkali-metal hydroxides and, thirdly, certain pure molecular crystals occurs when the g...

Picosecond Photon Echo and Coherent Raman Measurements on Molecular Solids in a High Pressure Diamond Anvil Cell

Abstract We report the first picosecond photon echo measurements on a molecular solid in a high pressure diamond anvil cell at low temperature. We also present time-resolved coherent Raman measurements of vibrational relaxation in pure molecular crystals as a function of pressure in a diamond anvil cell at low temperature. Since photon echo spectroscopy eliminates inhomogeneous dephasing, we utilize this technique to yield the homogeneous dephasing rate for high pressure samples where sample inhomogeneities and inhomogeneous pressure gradients may exist. Since the Fourier transform of the photon echo decay yields the absorption lineshape, these experiments provide a means of obtaining high resolution spectra under extreme high pressure conditions. In addition, pressure induced changes in vibrational relaxation rates observed by picosecond coherent Raman measurements are used as a sensitive probe of the intermolecular interactions that give rise to relaxation phenomena.

Determination of the exciton diffusion length by surface quenching experiments

The purpose of this study is to discuss how a previous experimental suggestion by Kenkre, which bypasses capture information in the end-to-end energy migration observations, can be extended in order to minimize the ambiguities in the determination of the exciton diffusion lengths in pure molecular crystals.

Raman spectroscopy of matrix-isolated hydrogen. II. Influence of defects on matrices

The hydrogen molecule is treated as a model substance for matrix-isolation effects. Raman investigations are made concerning the influence of the defect on the matrix material Ar, Kr, Xe, N2, and O2. The influence of the matrix on the hydrogen impurities is investigated in the preceding paper.A detailed comparison of the spectra of pure molecular crystals with the spectra of doped crystals (1% H2 or 1% D2) shows distinct changes in the Raman spectra of the matrices. Weak influences of the impurity on site symmetry and on the vibron–phonon and phonon–phonon coupling mechanisms of the anisotropic matrices N2 and O2 are obtained. An additional line, observed in the vibron–phonon structure of N2 and O2, is interpreted as a coupled resonant mode. Direct evidence of a resonant mode at 39 cm−1 is only obtained in the system containing 1% D2 in N2.

Composite exciton‐phonon states in molecular crystals

AbstractIt is known that the diagonalization of the exciton‐phonon Hamiltonian gives rise to the composite exciton‐phonon energy states. Within the single phonon approximation, two different diagonalized Hamiltonians are obtained for 0–0 and 0–1 phonon transitions in pure molecular crystals. The resulting secular equations are solved, and analytical expressions for the energy eigenvalues corresponding to the two transitions are derived. The thermal broadening due to the composite exciton‐phonon states interaction is also calculated at low temperatures, and the results agree with experiments.

Investigation of the appropriateness of sensitized luminescence to determine exciton motion parameters in pure molecular crystals.

A wealth of experimental data has been collected over the years regarding the sensitized luminescence of molecular crystals. Indeed, such observations have played a primary role in attempts to characterize the dynamical aspects of exciton transport in these materials. Nonetheless, as has been noted previously, serious questions of interpretation remain concerning the relationship between primary experimental observables and microscopic parameters which govern exciton transport and capture. In the past these questions have led to an uncertainty, not always acknowledged, in the values of exciton diffusion constants reported in the literature. On the other hand, careful analysis of some recent experiments suggests that this uncertainty is much larger than is commonly assumed, and calls into question the utility of sensitized luminescence experiments as a quantitative probe of exciton transport. In this paper we carefully review these questions and show how they arise in a quantitative analysis of time-dependent sensitized luminescence in naphthalene, anthracene, and 1,2,4,5-tetrachlorobenzene crystals. This work, in conjunction with similar analyses of exciton-annihilation and transient-grating experiments, leads us to conclude that sensitized luminescence in its present form is useful as a probe of the capture process but not of exciton motion in pure crystals unless it is aided by independent additional experiments. We propose and discuss such experiments which might assist in disentangling effects arising from the capture process from those associated with exciton motion.

Excited state dynamics in pure molecular crystals: perylene and the excimer problem

Perylene excited state dynamics are examined using steady-state and time-resolved fluorescence spectroscopy as well as picosecond probe pulse and picosecond transient grating experiments. Relaxation of the initially prepared state into the excimer state is observed. The gerade Davydov components are approximately located. Measurements of excited state mobility at 298 K and 1.7 K are reported.

Phosphorescence and Triplet←Singlet Absorption Spectra of Benzophenone Crystal at 4.2°K

The phosphorescence spectrum of benzophenone crystal at 4.2° has been measured to determine whether or not pure molecular crystals phosphoresce. The comparison of its vibrational structure with the infrared and Raman spectra leads to the conclusion that the phosphorescence really comes from benzophenone, not from some chemical impurities. Furthermore, the triplet←singlet absorption has been observed with the benzophenone crystal at 4.2°K for the purpose of comparison with the emission spectrum. The 0–0 band of the phosphorescence has been found to be lower in energy by 81 cm−1 than that of the triplet←singlet absorption. The origin of the gap between the absorption and the emission is discussed in an effort to understand the mechanism of the phosphorescence in the crystalline state. Finally, the oscillator strength of triplet←singlet absorption is theoretically evaluated to be 1.74 × 10−7, in agreement with the experimentally estimated value of 3.2 × 10−7.

Vibrational Properties of Excitons in Molecular Crystals

A general theory is developed for the interaction of exctions with molecular vibrations in pure and impure molecular crystals. A study is made of a model Hamiltonian which describes the interaction of Frenkel excitons with harmonic oscillators, with particular attention paid to the case in which exciton bandwidth is fairly narrow. Equations of motion for molecular excitons are obtained in the form of Dyson equations using the method of double-time Green's function. By solving the Dyson equations, it is shown that the vibrational energies of excitons in pure and impure molecular crystals, which can be classified into zero-phonon and one-phonon energies, are very different for different combinations of the energies of molecular vibrations and the exciton bandwidth. For example, a series of vibronic exciton bands is obtained for a pure crystal when the energies of phonons are larger than the exciton bandwidth while for an impure crystal, under certain circumstances, molecular vibrations can induce a delocalization of exciton impurity states in the opposite case. A study is then made of the role of phonons in the exciton energy transfer in a pure molecular crystal. It is shown that the effect of exciton—phonon interactions is to reduce the exciton bandwidth from free exciton value while there can exist a phonon-assisted energy transfer with emission or absorption of a single phonon, which has a long-range oscillatory interaction form similar to the Ruderman—Kittel interaction. A brief discussion is also given of the transfer of energy associated with an exciton impurity state to host molecules.

Three-dimensional refinement of the structure of β-succinic acid

The crystal structure of β-succinic acid has been refined by three-dimensional Fourier methods, followed by differential refinements with series-termination corrections and anisotropic temperature factors. The central C—C bond at 1·533 Å is shown not to differ significantly from the length of the single bond in diamond, but the next bond connecting the chain to the carboxyl group, C—COOH, is 1·485 ± 0·013 Å and the contraction here is highly significant. The C—O distances are 1·252 and 1·322 Å, with standard deviations of about 0·012 Å. The main feature of the thermal motion is that the movement in the direction of the hydrogen-bonded molecular columns is very much less than in any other direction in the crystal, and also much less than in pure molecular crystals like benzene or naphthalene. In other directions where van der Waals contacts only exist the movement is greater. There is a molecular angular oscillation of perhaps 9° r.m.s. amplitude about an axis close to the axis of the molecular columns. A difference synthesis in the plane of the oxygen atoms is given which shows the hydrogen atom responsible for hydrogen bonding.