Lifetimes of charge-transfer states in molecular crystals

A simple microscopic model is proposed to described the transition rates to, from and within the manifold of charge-transfer states. The results suggest possible reasons for the existense of long-lived CT states in molecular crystals.     

Hypochromic and hyperchromic effects on the vibronic structure of molecular crystal spectra. The dimer model

It is shown on model calculations that crystal-field mixing of Frenkel excition states in molecular crystals lead not only to a redistribution of their relative intensities but also to changes in the vibrational structure of weak exciton bands.     

THE SINGLE CRYSTAL PHOSPHORESCENCE OF ADENINE AT 77 K

Abstract— The low temperature phosphorescence spectra of large single crystals of adenine salts (adenine sulphate, adenine hemisulphate monohydrate, adenine hydrobromide hemihydrate) were compared with the spectra of polyadenylic acid in polydlcoholic glasses compiled from literature. The results show that adenine single crystals may serve as a convenient molecular system for modeling energy levels of polyadenylic acid and energy transfer between adenine molecules, at least in the case of triplet states. The main advantage of such a model system is that their periodic molecular arrangement and the site of protonation are exactly known from the X-ray diffraction studies.     

Study of molecular orientational states of ferroelectric liquid crystals in a surface stabilized geometry

The molecular orientational states of homogeneously aligned, helix unwinding, chiral smectic C liquid crystals placed in a thin cell (surface-stabilized ferroelectric liquid crystals [SSFLC]) were studied. They were classified by the optical viewing conditions and the relationship between the directions of the chevron layer structure and the surface pretilt. The molecular orientational models of the states were considered and illustrated with regard to the experimental results. The models of molecular orientation give us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Furthermore, the effect of the surface pretilt angle on the orientational and optical properties of SSFLCs is discussed.     

Charge transport in high mobility molecular semiconductors: classical models and new theories

The theories developed since the fifties to describe charge transport in molecular crystals proved to be inadequate for the most promising classes of high mobility molecular semiconductors identified in the recent years, including for example pentacene and rubrene. After reviewing at an elementary level the classical theories, which still provide the language for the understanding of charge transport in these systems, this tutorial review outlines the recent experimental and computational evidence that prompted the development of new theories of charge transport in molecular crystals. A critical discussion will illustrate how very rarely it is possible to assume a charge hopping mechanism for high mobility organic crystals at any temperature. Recent models based on the effect of non-local electron-phonon coupling, dynamic disorder, coexistence of localized and delocalized states are reviewed. Additionally, a few more recent avenues of theoretical investigation, including the study of defect states, are discussed.     

GPC analysis of degraded annealed single crystals

Annealed polyethylene single crystals have been degraded with fuming nitric acid, and the molecular weight distribution of the fragments determined by using a gel-permeation chromatograph. Peaks due to chain folding were observed in these distributions as for unannealed single crystals. The peaks moved to lower molecular weight with increasing degradation time. Comparison of the lowest molecular weight peak length after a given degradation time with the low-angle x-ray periodicity before degradation gave information about a disordered surface layer. The thickness of this layer at early states of degradation was dependent solely on annealing temperature, though changes in the layer must have occurred with annealing time, since there was an increase in reaction rate with annealing time. At higher degradation states, the thickness of the layer was dependent solely on the original low-angle periodicity. This has been related to the depth at which some folds are buried beneath the lamellar surfaces. The relevance of these observations to the structure of annealed single crystals is discussed.     

Interaction of excitons with charge carriers in organic solids: charged excitonic complexes

The coupled states of molecular excitons with charge carriers are considered. In centrosymmetric crystals the attraction of excitons to charge carriers arises mainly from the increase of molecular polarizability on electronic excitation, so that the exciton energy decreases in the electric field of the charge. In noncentrosymmetric crystals, the main contribution to the energy of exciton-charge interaction comes from the change of the static dipole moment of the molecule on excitation. Some physical consequences and possible experimental observations of the above-mentioned coupled states are discussed.     

Metal-halide molecular clusters: from fundamentals to model interactions

Computer simulation studies of molten salts and disordered ionic solids employing phenomenological ionic models have proved useful in helping to understand the structure of these systems and their structure-dependent properties. A minimal requirement on such models is that they should give a reasonable account of cohesion in crystals and in molecules and small clusters of these compounds, and in turn the analysis of cohesive and vibrational properties of ionic systems in solid and molecular states has helped to determine useful model interactions. For an introduction to these topics the author first reports from the work carried out in the 1980s with W. Andreoni and G. Galli on what is learnt in Hartree-Fock and configuration–interaction calculations on the neutral and ionized monomer and dimer of NaCl. Then the author recalls how a deformation-dipole model was built for the cohesion of the neutral alkali halide monomers by combining a classical multipolar expansion with a quantum overlap expansion, and how this model relates to the theory of the cohesion and the vibrational spectrum of the alkali halide crystals. Finally, the author illustrates some applications to molecules and microclusters of polyvalent metal halides, from work carried out with Z. Akdeniz and coworkers. Transferability of model parameters for the halogen ions between different compounds and different aggregation states is crucial for these applications, and is achieved in the deformation-dipole model.     

Analysis of the optical absorption spectra of diradical oligomers in diacetylene crystals

We report on electronic and vibrational properties of the diradical short-chain reaction intermediates of the low-temperature polymerization reaction in diacetylene single crystals. The positions, shapes and intensities of the zero-phonon lines and of vibronic lines of the polarized optical absorption series are analyzed. Theoretical model calculations concerning the vibrational and excitonic states in the molecular aggregates are performed and applied to the experiments.     

The phonon spectrum of molecular crystals from exciton sidebands

The phonon sidebands on the excitons observed in certain molecular crystals may be represented by a weighted density of states. It is here shown qualitatively and by reference to typical experiments that this function does not, in general, resemble the true density of states but often may be predicted from data on the Brillouin zone centre translational modes.     

Practical quantum mechanics-based fragment methods for predicting molecular crystal properties

Significant advances in fragment-based electronic structure methods have created a real alternative to force-field and density functional techniques in condensed-phase problems such as molecular crystals. This perspective article highlights some of the important challenges in modeling molecular crystals and discusses techniques for addressing them. First, we survey recent developments in fragment-based methods for molecular crystals. Second, we use examples from our own recent research on a fragment-based QM/MM method, the hybrid many-body interaction (HMBI) model, to analyze the physical requirements for a practical and effective molecular crystal model chemistry. We demonstrate that it is possible to predict molecular crystal lattice energies to within a couple kJ mol(-1) and lattice parameters to within a few percent in small-molecule crystals. Fragment methods provide a systematically improvable approach to making predictions in the condensed phase, which is critical to making robust predictions regarding the subtle energy differences found in molecular crystals.     

Infrared intensities of lattice vibrations in molecular crystals

From the dynamic multipoles model an expression is derived for the dipole moment derivative governing the intensity of infrared absorption by lattice modes in molecular crystals. The result depends on non-local susceptibilities which take proper account of the local electric field in a way consistent with dielectric theory. It is shown that the non-local rsponse follows naturally from microscopic lattice dynamical theory. It arises from dipolar coupling and is intimately connected with the delocalized exciton states produced by the same mechanism. Intensities calculated for the iodine crystal are inproved by including the local field, but a point quadrupole field proves too anisotropic to yield the measured intensity ratio. The treatment shows that infrared intensities can be used to obtain unique effective molecular polarizabilities.     

A unified theory for charge-carrier transport in organic crystals

To characterize the crossover from bandlike transport to hopping transport in molecular crystals, we study a microscopic model that treats electron-phonon interactions explicitly. A finite-temperature variational method combining Merrifield's transformation with Bogoliubov's theorem is developed to obtain the optimal basis for an interacting electron-phonon system, which is then used to calculate the bandlike and hopping mobilities for charge carriers. Our calculations on the one dimensional (1D) Holstein model at T=0 K and finite temperatures show that the variational basis gives results that compared favorably to other analytical methods. We also study the structures of polaron states at a broad range of parameters including different temperatures. Furthermore, we calculate the bandlike and hopping mobilities of the 1D Holstein model in different parameters and show that our theory predicts universal power-law decay at low temperatures and an almost temperature independent behavior at higher temperatures, in agreement with experimental observations. In addition, we show that as the temperature increases, hopping transport can become dominant even before the polaron state changes its character. Thus, our result indicates that the self-trapping transition studied in conventional polaron theories does not necessarily correspond to the bandlike to hopping transition in the transport properties in organic molecular crystals. Finally, a comparison of our 1D results with experiments on ultrapure naphthalene crystals suggests that the theory can describe the charge-carrier mobilities quantitatively across the whole experimental temperature range.     

System of metal electronegativities calculated from the force constants of the bonds

It is proposed to use a unified one-parameter equation which relates the force constants in molecules and crystals to the electronegativities of atoms. The results obtained for all currently studied compounds depend little on the ligand type, which made it possible to construct a complete electronegativity scale for elements in molecular and crystalline states equivalent in known cases to the thermochemical Pauling scale. An attention is paid to the chemical bond characteristics in some molecules and crystals of b subgroups.     

壳聚糖在二氯乙酸中的溶致液晶性

制备了不同脱乙酰度和分子量在１－３ ～１５－１ ×１０５ 范围内的壳聚糖样品,并用傅里叶变换红外光谱法（ＦＴＩＲ）和粘度法进行了表征．借助偏光显微镜研究了不同壳聚糖样品在二氯乙酸中的液晶相转变和液晶状态．实验结果表明,分子量在１０５ ～１０６ 范围内的增大使壳聚糖形成液晶相的临界浓度（ Ｃ＊ ） 略有降低;脱乙酰度在７０ ～９０ ％ 范围内,对Ｃ＊ 的影响基本可以忽略．临界浓度的实验值与根据Ｋｈｏｋｈｌｏｖ Ｓｅｍｅｎｏｖ Ｏｄｉｊｋ 理论预示的值比较一致,说明蠕虫链模型可以很好地描述壳聚糖分子链在二氯乙酸中的溶致液晶行为．     

Uncovering the Intramolecular Emission and Tuning the Nonlinear Optical Properties of Organic Materials by Cocrystallization

The spectroscopic and photophysical properties of organic materials in the solid‐state are widely accepted as a result of their molecular packing structure and intermolecular interactions, such as J‐ and H‐aggregation, charge‐transfer (CT), excimer and exciplex. However, in this work, we show that Spe‐F₄DIB cocrystals (SFCs) surprisingly retain the energy levels of photoluminescence (PL) states of Spe crystals, despite a significantly altered molecular packing structure after cocrystallization. In comparison, Npe‐F₄DIB cocrystals (NFCs) with new spectroscopic states display different spectra and photophysical behaviors as compared with those of individual component crystals. These may be related to the molecular configuration in crystals, and we propose Spe as an “intramolecular emissive” material, thus providing a new viewpoint on light‐emitting species of organic chromophores. Moreover, the nonlinear optical (NLO) properties of Npe and Spe are firstly demonstrated and modulated by cocrystallization. The established “molecule‐packing‐property” relationship helps to rationally control the optical properties of organic materials through cocrystallization.     

形成晶体条纹的一个界面反应-扩散模型

基于晶体生长动力学的非平衡非线性特性和耗散结构的概念,提出了一个非理想的界面反应一扩散模型,其中界面动力学引起的晶面多重态现象和扩散过程的耦合可导致滞后现象。它可以解释某些晶体中有序条纹的形成。     

Influence of polarization fluctuations on the electronic structure of molecular solids

Fluctuations in the intermolecular polarization energies of charge states in molecular solids can lead to broad (ΔE ≈ 0.5 eV) distributions of localized states, especially in polymers. Such fluctuations are caused by defects (e.g. surfaces) and thermal vibrations in molecular crystals and also by variations in the local structure in polymers. The resulting energy distributions yield natural interpretations of such diverse observations on the broadening of the photoemission spectra of molecular solids and the contact charge exchange spectra of polymers.     

Gel permeation chromatographic studies of the degradation of polyethylene with fuming nitric acid. II. Bulk polyethylene

Preliminary results are reported on the use of gel permeation chromatography in morphological studies of bulk polymers and fibres. Several samples of bulk isotropic and drawn polyethylene were analyzed by gel permeation chromatography following nitric acid oxidation. In all cases suitable samples showed two peaks in the molecular weight distribution, suggesting a qualitative similarity with results for single crystals. It is concluded that the present data are consistent with chain folding in bulk polymers, both in the isotropic and oriented states, with a less degree of regularity than exists in single crystals.     

Detection of magnetic field effects by confocal microscopy

Certain pairs of paramagnetic species generated under conservation of total spin angular momentum are known to undergo magnetosensitive processes. Two prominent examples of systems exhibiting these so-called magnetic field effects (MFEs) are photogenerated radical pairs created from either singlet or triplet molecular precursors, and pairs of triplet states generated by singlet fission. Here, we showcase confocal microscopy as a powerful technique for the investigation of such phenomena. We first characterise the instrument by studying the field-sensitive chemistry of two systems in solution: radical pairs formed in a cryptochrome protein and the flavin mononucleotide/hen egg-white lysozyme model system. We then extend these studies to single crystals. Firstly, we report temporally and spatially resolved MFEs in flavin-doped lysozyme single crystals. Anisotropic magnetic field effects are then reported in tetracene single crystals. Finally, we discuss the future applications of confocal microscopy for the study of magnetosensitive processes with a particular focus on the cryptochrome-based chemical compass believed to lie at the heart of animal magnetoreception.

Confocal microscopy is showcased as a powerful technique for the measurement of spatiotemporally-resolved magnetic field effects in both solutions and single crystals.