Interactions In Molecular Crystals Research Articles

Relations between microscopic and macroscopic lowest-order optical nonlinearities of molecular crystals with one- or two-dimensional units

Efficiency of three-wave interactions in molecular crystals depends on the conjugation of the molecular unit, which in turn is a one- or two-dimensional property. This strong anisotropy reduces the number of non-negligible molecular lowest-order hyperpolarizability coefficients to four. The lowest-order macroscopic optical nonlinearity can be expressed, in the absence of significant intermolecular effects, as the tensorial sum of molecular hyperpolarizabilities. This analysis is applied to the 17 relevant noncentrosymmetric crystal point groups, generalizing a previous analysis of nonlinear-optical properties of methyl-(2,4-dinitrophenyl)-aminopropanoate crystals. In several cases, the molecular unit anisotropy is shown to impose structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from the crystal point symmetry only. In such cases, nonlinear-optics experiments can be used for testing molecular anisotropy and molecular orientations within the unit cell in the absence of significant nonlinearity arising from intermolecular coupling. Similar relations can be derived between electro-optic coefficients, but limited to the case of weak contributions of intermolecular vibration to the electro-optic effect. We investigate for each point group the possibility of inferring hyperpolarizability coefficients from macroscopic nonlinear measurements, a complementary approach to that based on theoretical molecular calculations or electric-field-induced second-harmonic generation in solution. In the case of highly anisotropic one-dimensional charge-transfer systems (exemplified by $p$-nitroaniline), for each point group and a given molecular hyperpolarizability, the optimal orientation of the charge transfer axis, leading to the highest phase-matchable coefficient, is given. It is shown that crystal point groups 1,2,$m$, and $\mathrm{mm}2$ correspond to the highest possible value of this coefficient, while other crystal symmetry is less favorable. These considerations are applied to four available efficient molecular crystals and used either as a check of molecular orientations in a case of low crystalline symmetry or to estimate otherwise unavailable molecular nonlinear coefficients.

An ENDOR study of long-range spin interactions in molecular crystals. II. The 5-thymyl radical in a single crystal of thymidine

ENDOR spectroscopy was used to study the 5-thymyl radical in a single crystal of thymidine. Besides the known methyl and methylene proton couplings, five additional proton couplings were identified. Only one of them is intramolecular (N(3)-H). The intermolecular proton couplings were assigned with the use of the McConnell-Strathdee theory of the dipolar spin interactions. It is found that, in contrast to some earlier conclusions, in the 5-thymyl radical the oxygen O(4) remains unprotonated.

Laser induced phonons: A probe of intermolecular interactions in molecular solids

A new type of density dependent spectroscopy is developed and applied to the study of acoustics phonons and excited state–phonon interactions in molecular crystals. It is shown that intermolecular separations in solids can be varied in a controlled manner along a selected crystalline direction and that the consequences of the molecular displacements can be observed optically in time-resolved transient grating, absorption, and fluorescence experiments. The technique involves the generation of specific acoustic waves (phonon modes of definite wave vector) by transient grating optical excitation of the sample. The resulting time-dependent density changes cause time-varying spectral shifts, the magnitude of which are dependent on the detailed nature of the anisotropic intermolecular interactions in the crystal, e.g., anisotropic van der Waals interactions in molecular crystals can be directly investigated. First a formalism is presented which permits the determination of the nature of the phonons produced (longitudinal, quasilongitudinal, or quasitransverse) by transient grating excitation in anisotropic crystals. Next, the effect of the transient grating generated phonons on experimental observables (absorption, fluorescence, and transient grating) due to anisotropic spectral shifts is obtained. Model calculations illustrate the results. Finally experimental data for the perylene pure crystal excimer system and the pentacene in p-terphenyl mixed system are presented. In the perylene system, the various types of phonons predicted by theory are observed and phonon dispersions are obtained for several crystal directions. In pentacene in p-terphenyl, the theoretically predicted phonon-induced spectral shifts are clearly observed.

Anharmonic interactions in molecular crystals. Two-phonon absorption in crystalline OCS

The use of atom—atom potentials for the evaluation of third order coupling coefficients in molecular crystals is discussed. Expressions of the two-phonon absorption coefficients for combination bands involving an internal and an external phonon are obtained. The method described is applied to the phonon side bands of crystalline OCS. A satisfactory agreement with experiments is obtained.

Strong and Weak Exciton-Phonon Coupling in Molecular Crystals

Abstract The coupling cases for exciton-phonon interaction in molecular crystals are analysed with the help of three quantities: (i) the exciton bandwidth, related to the exciton transfer rate between molecules, (ii) the dispersive interaction between excitons and phonons, related to the rate of excitation interchange between phonons and excitons, and (iii) the phonon frequencies, related to the rate at which the phonon motions, and lattice structure, respond to changed lattice forces produced by molecular excitations. Cases intermediate between extreme weak coupling (exciton transfer faster than both lattice relaxation and exciton-phonon interchange) and extreme strong coupling (excitation locked on one site by lattice relaxation) are distinguished, and their spectral characteristics discussed.

Absolute Raman intensities of lattice modes‐investigation of intermolecular interactions in molecular crystals

AbstractAn investigation has been carried out on the correlation between intermolecular interactions and absolute Raman intensities of lattice modes. The analysis was performed by comparing absolute Raman intensities (measured using crystalline powders) with it could be shown that, in geoerd, the oriented‐gas model is inadequate for the description of the properties of molecular crystals. Large discrepancies in the model could be revealed for modes with both librational and translational character. The discrepancies depend strongly on the intermolecular interactions.

Optical detection of multiple- quantum spin-phonon interactions in molecular crystals

The direct observation of multiple-phonon-induced spin transitions in the lowest excited triplet state of 1,2,4,5-tetrachlorobenzene (TCB) in zero field is reported. Measurements of the change in TCB phosphorescence intensity induced by one or two sources of applied acoustic energy were made as a function of power, frequency, and polarization of the acoustic waves. We present evidence that the observed effects involve simultaneous multiple-phonon absorptions by the excited-state spin system rather than the absorption of single phonons produced by anharmonic effects of the host lattice.

One particle CPA and exciton-phonon interactions in molecular crystals

The intersite correlation function of scattering potentials for the exciton-acoustic phonon interaction in molecular crystals is calculated. It shows a complicated dependnece on the intersite distance and the temperature. The existence of such cross correlation clearly points out that one has to go beyond the one particle CPA in order to treat properly the problem of exciton-delocalized phonon interactions.

ChemInform Abstract: ENDOR STUDY OF LONG‐RANGE SPIN INTERACTIONS IN MOLECULAR CRYSTALS PART 1, 1‐METHYL URACIL

Chemischer InformationsdienstVolume 5, Issue 45 Physical Organic Chemistry ChemInform Abstract: ENDOR STUDY OF LONG-RANGE SPIN INTERACTIONS IN MOLECULAR CRYSTALS PART 1, 1-METHYL URACIL J. N. HERAK, J. N. HERAKSearch for more papers by this authorC. A. MCDOWELL, C. A. MCDOWELLSearch for more papers by this author J. N. HERAK, J. N. HERAKSearch for more papers by this authorC. A. MCDOWELL, C. A. MCDOWELLSearch for more papers by this author First published: November 12, 1974 https://doi.org/10.1002/chin.197445079AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume5, Issue45November 12, 1974 RelatedInformation

Optically detected acoustic paramagnetic resonance in molecular crystals

The optically detected acoustic paramagnetic resonance of the metastable triplet state of 1,2,4,5-tetrachlorobenzene (TCB) in zero field is reported. The spin-phonon interaction in molecular crystals is discussed in terms of the direct process single phonon absorption by the paramagnetic spin system. t is estimated that in many molecular solids, sufficient acoustic power densities may be produced to observe acoustic paramagnetic resonance by optical detection methods.

Optically Detected Nuclear Quadrupole Resonance and Transferred Hyperfine Coupling via Guest–Host Interactions in Molecular Crystals

Optically Detected Nuclear Quadrupole Resonance and Transferred Hyperfine Coupling via Guest–Host Interactions in Molecular Crystals

Zero point oscillation energy effect on plastic deformation in solidified gases

AbstractAn investigation is made of the deformation of molecular crystals of methane and argon under the conditions of tension and stress relaxation. The Peierls barriers are shown to be the main obstacles for dislocation movement in these crystals in a wide low‐temperature range. The result obtained is in good agreement with data determined for hydrogen, deuterium, and ammonium and is discussed in terms of the peculiarities of intermolecular interactions in molecular crystals. In particular, it is shown, that the contribution of zeropoint oscillation energy to the bonding energy increases with decreasing temperature at which the transition from the Peierls mechanism to higher energy mechanism occurs.

On exciton-photon coupling and retarded interactions in molecular crystals

A dispersion relation valid near the exciton pole of the crystal Green operator is derived using perturbation theory. It is evaluated by two routes. The first involves only static interactions while the second introduces retarded lattice sums. The equivalence of the two relations is demonstrated for cubic crystals by calculating the retarded lattice sum as an integral. The divergence in the retarded sum investigated by Avery is isolated and shown to be the exciton-photon coupling constant divided by the free-photon dispersion factor. Some new results for the retarded interaction between molecules in the dipole approximation, and a new formula for the retarded dipole sum over a thick spherical shell are given.

Calculation of the potential energy of molecular interactions in crystals of organic compounds

Calculation of the potential energy of molecular interactions in crystals of organic compounds

Comments on the Influence of Intermediate Excitons and Exchange Forces on the Interaction in Molecular Crystals: The Crystal Structure of Chlorine

The reasons for the failure of the Lennard-Jones potential to predict the correct space group of solid chlorine and the molecular orientation within the unit cell are examined. In the analysis given, the contribution of charge-transfer states to the intermolecular binding energy in the ground state is calculated. It is found that these states account for about 25% of the intermolecular interaction, and tend to reinforce the dispersive—repulsive forces in the region of the observed molecular angle of tilt. However, in our approximation these forces fail to account for the observed stability of the orthorhombic structure relative to the cubic structure, the latter being predicted to be more stable if the interaction is a Lennard-Jones potential. The repulsive interactions are calculated using an approximation to the exchange energy. It is concluded that the failure of the Lennard-Jones potential to predict the correct angle of molecular tilt within the unit cell arises from the incorrect form of the repulsive potential. Further, with the binding energies calculated in this paper, the difference between the energies of the cubic and orthorhombic structures is much reduced from that predicted from calculations based on the use of the Lennard-Jones potential.

On multipole interaction methods in exciton theory

Three features of the multipole method of treating excited state molecular interactions in crystals are examined. The first is the error caused by treating the interactions as those of point multipoles even for nearest neighbour molecules with some overlapping of electron clouds. The second is the question of the convergence of the multipole series, and of the conditions under which terms higher than the dipole-dipole must be included. The third is the influence of the coupling of the static moments of one molecule with the transition moments between different excited states in another. It is concluded from a discussion of principles, supported by calculations of examples, that `conventional' methods using only dipole-dipole interactions are adequate when (i) the transitions are at least moderately intense and (ii) the dipole lattice sums are not made very small by special conditions of molecular orientation. When these conditions are not satisfied quadrupole and octupole moments must be included. Their effects on factor-group splittings may be many times greater than those of the dipole term; in intensity calculations where the changes occur only in second order, the dipole approximation remains more acceptable but higher moments are not, in general, negligible.

Interaction of Excitation Waves in a One-Dimensional Crystal

The problem of exciton-exciton interaction in molecular crystals is discussed in terms of the stationary two-quantum excited states of an idealized one-dimensional crystal. If only a single molecular excited state is considered, the wave functions and energy levels of the two-quantum states may be obtained directly from those of the corresponding one-quantum problem with the proviso that the two excitons have different wave numbers. If in addition, a second molecular excited state is included, there can exist states in which two excitons remain bound to each other as they propagate through the crystal.