[en] The existence of Y states in excimer-forming molecular crystals is addressed through a simple extension of an earlier model of the formation of excimers as occurring through the relaxation of a nonlinear oscillator. These Y states, which do not appear in solution, and have been observed in only some crystals, are intermediate states with partial excimeric character. Such states are shown to arise naturally in our model as a simple consequence of additional interactions that exist in the crystal, but not in solution, and to be associated with an abrupt (rather than gradual) transition as the strength of those interactions is varied

[en] The linear chain in molecular crystal with local structural defect (dislocation) has been considered. By applying a particular theoretical approach the energies of localized excitations appearing in the system are determined and the dependence of these energies on the perturbation parameters is discussed. The theoretical approach is formulated in such a way to reduce the problem to the analysis of an ideal molecular chain with some extra off-diagonal terms in the Hamiltonian. These extra terms are considered as perturbation caused by the nonideality of the molecular chain. The results of the present study provide the first step towards the quantitative understanding of the effects of structural disorder on exciton dynamics, energy transfer processes and absorption line shapes in quasi-one-dimensional molecular crystals. (author). 4 refs, 2 figs

[en] Experiments for measurement of time taken to reach thermal equilibrium in Spodumene crystals - 2mm to 5,4mm thick - in the temperature range 100⁰ to 250⁰C are described. The measurements indicate a linear relationship between time and thickenes for heating as well as for cooling. Difference in thermal equilibrium time for heating and for cooling is about of 20 seconds. (author)

[en] Relationship between molecular freedom of amyloidogenic protein and its self-assembly into amyloid fibrils has been evaluated with α-synuclein, an intrinsically unfolded protein related to Parkinson's disease, by restricting its structural plasticity through an end-to-end disulfide bond formation between two newly introduced cysteine residues on the N- and C-termini. Although the resulting circular form of α-synuclein exhibited an impaired fibrillation propensity, the restriction did not completely block the protein's interactive core since co-incubation with wild-type α-synuclein dramatically facilitated the fibrillation by producing distinctive forms of amyloid fibrils. The suppressed fibrillation propensity was instantly restored as the structural restriction was unleashed with β-mercaptoethanol. Conformational flexibility of the accreting amyloidogenic protein to pre-existing seeds has been demonstrated to be critical for fibrillar extension process by exerting structural adjustment to a complementary structure for the assembly

[en] Energy transfer rates for the coupled coherent and incoherent motion of excitons in molecular crystals are derived starting from the Haken-Strobl model. It is shown that the situation discussed recently by Kenkre and Knox on the basis of generalized master equations as well as the limits of fast (Perrin) and slow (Foerster) energy transfer are regained as limiting cases. (orig.)

[en] Starting from the Haken-Strobl-model for the coupled coherent and incoherent motion of Frenkel excitons, a master equation is derived describing the quasi-incoherent motion of these excitations. In contrast to previous derivations no use is made of special symmetries. Thererfore this equation may also describe the energy transfer in materials with non-periodic structure, which play a role in biological systems. The master equation is solved for crystals with one and two molecules in the unit cell, and explicit expressions are given for the case of nearest neighbour interaction too. Furtheron, asymptotic forms of the solutions are discussed. (orig.)

No abstract available

[en] New possible structures for the compounds formed by intercalation of an alkali metal into a molecular crystal formed by polycyclic aromatic hydrocarbon (PAH) molecules have been theoretically searched for. Among them, a crystalline framework in which PAH molecules show a slipped parallel geometry provides an interesting alternative to the usual herringbone arrangement shown by pristine crystal structures of PAHs. While these different crystalline structures are energetically unfavorable for potassium alloying, they are energetically preferred at the highest cesium load (3:1 stoichiometry). The most remarkable feature of the proposed structure is the avoidance of the magnetic instability that leads to insulating phases of herringbone crystals when a 3:1 proportion of metal-Picene is reached. After the evaluation of the corresponding energies of formation, a robust metallic scenario is found for Cs₃Pentacene, making it possible the observation of superconductivity. (paper)

[en] A theorectical study of incoherent electronic energy transfer in an impurity band of substitutionally disordered molecular crystals is presented. It is assumed that the exciton is well described randomly distributed impurity sites is due to resonance or phonon-assisted energy transfer. In the case of phonon-assisted transfer we shall confine ourselves to transfer processes which do not depend on the energy mismatch between two sites. The time evolution of the microscopic probability of excitation a lattice site, n. p /su n/ t), is then described by a master equation. Due to the randomness of the impurity system, the transition from the microscopic description of an individual configuration to a macroscopic average probability of excitation, P(r,t) involves an averaging process over all possible configurations. The way this averaging process is performed, determines various kinds of models

[en] Second harmonic generation is observed in phenanthrene crystals. The experimental set is used allowed the simultaneous detection of two proton excitation (TPE) and second harmonic generation. (SHG)