Defects In Molecular Crystals Research Articles

Thermal Annealing of Singlet Fission Microcrystals Reveals the Benefits of Charge Transfer Couplings and Slip-Stacked Packing

Defects in molecular crystals, created by nonequilibrium crystal packing, can have significant influence over the energetics and dynamics of singlet fission. Though normally thought of as detrimental to the performance of organic semiconductors, in principle, defects could have crystal packings with better energetics and couplings for singlet fission than equilibrium crystal packing structures. Using two-dimensional electronic microscopy and X-ray diffraction, we monitor the population of nonequilibrium slip-stacked geometries in microcrystals of TIPS-pentacene before and after thermal annealing. We find that the presence of these slip-stacked structures enhances the singlet fission yield, as measured by triplet excited state absorption. The slip-stacked geometry matches no known polymorph of TIPS-pentacene, according to X-ray diffraction but indicates that slip-stacked molecules are present as tiny crystalline domains within the larger microcrystal. The data presented here provide evidence that slip-stacked geometries, present as defects in larger TIPS-pentacene microcrystals, enhance singlet fission yields.

Structural defects in molecular crystals based on heterospin copper complexes

Jumplike changes in the microhardness, sample dimensions, and parameters of the EPR spectrum were observed in molecular Cu(hfac)2LEt crystals undergoing a phase transition. Defects that appear upon plastic deformation (e.g., dislocations) and paramagnetic defects were revealed. The latter defects are likely breaks in polymer chains and can serve as spin marks for investigating the magnetic state of the crystal lattice.

Exciton and electron traps on structural defects in molecular crystals with dipolar molecules

The depth of exciton and electron traps arising due to the presence of structural defects in organic crystals consisting of dipole molecules were calculated. In the model studied the structural defect appears if a molecule is rotated by 180° along the long axis as compared to the bulk molecules. The contribution of the electrostatic interaction to the depth of the trap state on such a defect is studied in detail. For crystals consisting of molecules which have large dipole moment in ground state and adopt large values in excited states this contribution may be dominant. The calculations are carried out for triplet excitons and electrons in benzophenone crystal.

Exciton and Electron Traps on Structural Defects in Molecular Crystals with Dipolar Molecules

AbstractThe depths of exciton and electron traps arising due to the presence of structural defects in organic crystals consisting of dipolar molecules are calculated. In the model studied a structural defect appears if a molecule is rotated by 180° around the long axis as compared to the bulk molecules. The contribution of the electrostatic interaction to the depth of the trap state on such a defect is studied in detail. For crystals consisting of molecules which have a large dipole moment in the ground state and adopting large values in the excited state this contribution may be dominant. The calculations are carried out for triplet excitons and electrons in benzophenone crystal.

Computational approach to the study of extended defects in molecular crystals. Part 2.—Structural changes at planar faults—their importance in facilitating photodimerization and in governing stacking fault energies

Using an approach based on the pairwise evaluation of non-bonded interactions it has proved possible to estimate the extent of structural changes that occur at an extended planar fault (e.g., stacking fault or antiphase boundary) of a prescribed type in organic molecular crystals. In so doing it is also possible to estimate the magnitude of the extra energy that has to be invested into a crystal in order to accommodate such a fault. The calculations are illustrated by reference to a substituted anthracene, 1,8-dichloro-9-methylanthracene on which some experimental information relating to extended defects was already available. The way in which the fault energy is diminished as a result of allowing the molecules in the vicinity of the fault to “relax” and the lattice to expand is demonstrated. In particular there are indications that for a (100) fault plane the lowest energy is achieved by incorporating a translation vector of 1/10 [250] and a small degree of folding of the constituent molecules in and adjacent to the plane of the fault. Such structural relaxation gives rise to incipient dimer pairs in which contiguous molecules, oriented in trans registry and with the C9—C10′ distance in the range 4.5–4.7 Å are “preformed”. These facts help to explain the facile solid-state photodimerization of 1,8-dichloro-9-methylanthracene to yield the trans dimer, since the crystal structure of the perfect solid implies that this material should be light stable (the C9—C9′ or C10—C10′ distances being > 7 Å and contiguous molecules along 〈100〉 direction are in cis registry).

Study of extended defects in molecular crystals by the atom–atom potential method. 1. Slip planes in the anthracene crystal

The atom–atom potential method was used to calculate the relative facility of slip in various directions in the (001), (010), (100) and (20text-decoration:overline1) crystallographic planes in the anthracene crystal. A uniform slip of one semi-infinite half of the crystal against the second such half was considered. Three quantities were calculated as functions of slip : the lattice energy of the whole crystal, the energy of interaction between the two half-crystals, and the specific surface energies of various slip planes.The results of the calculations were compared with reported dislocation densities. It was found that the slip direction with the lower energy profiles correspond to the more frequently observed Burgers vector direction of the dislocations. Several fault structures of relatively low energies were found, suggesting possible mechanisms of dissociation of full dislocations into partials.The possible applicability of the procedure as an approach to the calculation of the strain energies and geometries of dislocations in molecular crystals was considered.

Real space crystallography and defects in molecular crystals

An image plane technique of crystallographic analysis (real space crystallography) is shown to be particularly useful in studying defects in thin molecular crystals. Contrast in electron images from line defects is enhanced at bend contours and details of the interaction between the strain fields of dislocations and the bend contours give an indication of the Burgers vector. The shift in bend contours at coherent planar boundaries separating two orientationally related structures (as occur in martensitic transformations) is found to be directly related to a variation in deviation (excitation) parameter as a result of a change in the position of a particular reciprocal lattice point perpendicular to the electron beam. Observations are consistent with a previously suggested model for such a transformation in 1∶8 dichloro-10-Me anthracene. In addition, dark-field images taken around a bend contour pole in anthracene provide evidence for the existence of forbidden reflections resulting from double diffraction in the space group P21/a.

Trapping States Formed by Structural Defects in Molecular Crystals

Abstract Results of calculations of the energetic distributions of trapping states in simple molecular crystals are given in the paper. A crystal with primitive regular unit cells has been taken into account, and three kinds of simple isolated linear defects have been considered: an edge dislocation, a linear array of vacancies, and a defect formed by a linear array of molecules larger than the host ones. Equilibrium positions of molecules in the vicinity of the defects have been calculated describing the intermolecular interactions by the Lennard-Jones type potential function. Local values of the polarization energy and hence depths and distributions of traps formed by molecules neighbouring to the defects have been calculated in the ion-dipole approximation. It has been shown that the formation of trapping states by thermal vibrations can be neglected as being a small effect.