Search

Herein, it is shown for the first time that cocrystals can be grown by cosublimation with unprecedented morphology control using tailor made additives. Multicrystalline sea-urchin motifs and solids that look like candy-floss are transformed into block-like and needle-like single crystals using additives. peer-reviewed 2021-04-15

The quality of various approximation methods for modelling anisotropic displacement parameters (ADPs) for hydrogen atoms was investigated in a comparative study. A multipole refinement was performed against high-resolution single crystal X-ray data of 9-diphenylthiophosphoranylanthracene (SPAnH) and 9,10-bis-diphenylthiophosphoranylanthracene·toluene (SPAnPS). Hydrogen-atom parameters and structural properties derived from our collected neutron data sets were compared with those obtained from the SHADE-server, the software APD-Toolkit based on the invariom database, the results from Hirshfeld atom refinement conducted in the OLEX2 GUI (HARt), and the results of anisotropic hydrogen refinement within XD2016. Additionally, a free refinement of H-atom positions against X-ray data was performed with fixed ADPs from various methods. The resulting C—H bond distances were compared with distances from neutron diffraction experiments and the HARt results. Surprisingly, the refinement of anisotropic hydrogen displacement parameters against the X-ray data yielded the smallest deviations from the neutron values. However, the refinement of bond-directed quadrupole parameters turned out to be vital for the quality of the resulting ADPs. In both model structures, SHADE and, to a lesser extent, APD-Toolkit showed problems in dealing with atoms bonded to carbon atoms with refined Gram-Charlier parameters for anharmonic motion. The HARt method yields the most accurate C—H bond distances compared to neutron data results. Unconstrained refinement of hydrogen atom positions using ADPs derived from all other used approximation methods showed that even with well approximated hydrogen ADPs, the resulting distances were still significantly underestimated.

The polymorphic behaviour of a co-crystal composed of neutral suberic acid and 1,2-bis(4-pyridyl)ethane is reported. A multidisciplinary approach has been employed, using thermal analysis methods (differential scanning calorimetry and polarized light thermomicroscopy), spectroscopic methods (infrared spectroscopy) and X-ray single crystal and powder diffraction. Two new polymorphs, Land III; have been identified, and for one of them the crystal structure has been elucidated and compared to that of a known polymorph, form II, showing that the assembling of the molecules in chains via H-bonds is a common feature of both polymorphs. Polymorph II, obtained by crystallization from an aqueous or ethanol solution or by grinding, undergoes a solid-solid transition to polyniorph I, which melts at 180.5 degrees C. Upon cooling from the melt, polymorph I is obtained, and a solid-solid transformation also occurs, to poly morph III'. Polymorph III was obtained serendipitously in an attempt to prepare a Nd(III) complex. The relative stability of the three polymorphic forms was established. (C) 2017 Elsevier B.V. All rights reserved.

Using a symmetry adapted polaron transformation of the Holstein Hamiltonian, we study the interplay of electronic excitation-vibration couplings, resonance excitation transfer interactions, and temperature in the linear absorption spectra of molecular J-aggregates. Semi-analytical expressions for the spectra are derived and compared with results obtained from direct numerical diagonalization of the Hamiltonian in the two-particle basis set representation. At zero temperature, we show that our polaron transformation reproduces both the collective (exciton) and single-molecule (vibrational) optical response associated with the appropriate standard perturbation limits. Specifically, for the molecular dimer excellent agreement with the spectra from the two-particle approach for the entire range of model parameters is obtained. This is in marked contrast to commonly used polaron transformations. Upon increasing the temperature, the spectra show a transition from the collective to the individual molecular features, which results from the thermal destruction of the exciton coherence.

We review a series of photochemical reactions related to the renowned family of anthracene derivatives. We first classify the photodimerizations of 9-methyl-anthracene, 9-cyano-anthracene and 9-anthracene-carboxylic acid as topochemical, non-topochemical and reversible, respectively. We then introduce a chemical reaction where the lattice change involves two different molecular species. In the latter case, the compatibility of the reactive species occurs at the level of symmetry matching of the unit cells rather than via a correct registry of molecular orientations. We then describe the relationship between incoming photons and mechanical motion, a common occurrence in crystal-to-crystal photoreactions, which shows stunning effects when 9,10-dinitro-anthracene is transformed to anthraquinone. Finally, the kinetics of these systems is shortly discussed. By means of a micro-Raman technique, we are capable of monitoring, at the same time and on the very same spot, the molecular (intramolecular vibrations or internal motions) and the lattice (intermolecular vibrations or lattice phonons) transformations. It turns out that molecular changes start first, producing, in a following time, the symmetry modifications of the crystal lattice. The extent of the delay of the structural change may considerably vary depending upon the system studied.

The crystal structure and polymorphism of the well-known aminoazobenzene, N,N-dimethyl-4-aminoazobenzene (DAB) are studied; a second polymorph of DAB reported and the relationship between the two polymorphs studied using differential scanning calorimetry and by evaluating their lattice energies and absorption spectra. Without significantly strong intermolecular interactions present in the two forms of DAB, a balance between conformational strain and a cumulative π-orbital overlapping effect results in the existence of the enantiotropically related polymorphs and their alternative packing arrangements. The UV/vis spectra of the two polymorphs are different, thus illustrating the significance of small structural changes on the colour of a material, which could behave differently under particular sensing conditions. DAB has been used primarily as an acid/base indicator solution and a dye molecule and the implications of the polymorphic relationships established here are discussed with respect to the range of applications this molecule finds as a sensor and for its incorporation into various functional films and devices.

Nonlinear optical (NLO) activity of the compound (2E)-2-(ethoxycarbonyl)-3-[(1-methoxy-l-oxo-3phenylpropan-2-yl)amino] prop-2-enoic add is investigated experimentally and theoretically using X-ray crystallography and quantum chemical calculations. The NLO activity is confirmed by both powder Second Harmonic Generation (SHG) experiment and first hyper polarizability calculation. The title compound displays 8 fold excess of SHG activity when compared with the standard compound KDP. The gas phase geometry optimization and vibrational frequencies calculations are performed using density functional theory (DFT) incorporated in B3LYP with 6-311G++ (d,p) basis set. The title compound crystallizes in non-centrosymmetric space group P21. Moreover, the crystal structure is primarily stabilized through intramolecular N-H center dot center dot center dot O and O-H center dot center dot center dot O hydrogen bonds and intermolecular C-H center dot center dot center dot O and C-H center dot center dot center dot pi interactions. These intermolecular interactions are analyzed and quantified using Hirshfeld surface analysis and PIXEL method. The detailed vibrational assignments are performed on the basis of the potential energy distributions (PED) of the vibrational modes. (C) 2015 Elsevier B.V. All rights reserved.

The new molecular salt [1H]Br (1 = 4-amino-cinnamic acid) was obtained in two polymorphic modifications, forms A and B. While mechanical stress induces transformation of form A into form B, an unexpected effect of UV irradiation is observed on form A, i.e., a single crystal to single crystal (SCSC) transformation to a third polymorph, form C. Form B, on the contrary, responds to UV irradiation by undergoing a stepwise SCSC [2 + 2] photodimerization to α-[12H2]Br2, via formation of a solid state solutions in the whole compositional range. All transformations were followed by a combination of solid-state techniques, i.e., single crystal and powder X-ray diffraction, FTIR, and FT-Raman, and by solution 1H NMR. All structural changes also resulted in variations in crystal morphology/texture, and these were monitored via SEM analysis.

2,6-Diisopropyl-4-nitrophenylphosphate (A), synthesized in two steps from 2,6-diisopropyl-4-nitrophenol, serves as a convenient starting point to prepare several N-functionalized monoaryl phosphates via 2,6-diisopropyl-4-aminophenylphosphate (adippH(2)) (1), which quite interestingly exhibits five different pseudopolymorphic structures 1a-1e. Several of these five polymorphic forms are inter-convertible to one another through single crystal to single crystal (SC-SC) transformation, aided by evacuation and solvent soaking protocols. Crystallization of crude 1 in methanol results in the isolation of the first polymorph [adippH(2)center dot CH3OH center dot 1/3H(2)O](3) (1a), while evacuation of 1a results in the loss of solvent molecules from the crystal lattice to yield a new polymorph [adippH(2)center dot 1/3H(2)O](3) (1b). Polymorph 1b can be converted back to 1a by soaking the single crystals of 1b in methanol. Similarly, soaking single crystals of 1a in DMF or in water yields two more new polymorphs [adippH(2)center dot 1/3CH(3)OH center dot 1/3DMF center dot 1/3H(2)O](3) (1c) and [adippH(2)center dot 1/2H(2)O](2) (1d), respectively. Polymorph 1c can once again be re-transformed to the original 1a by soaking its single crystals in methanol, whereas the SC-SC conversion of 1a to 1d is found to be irreversible. Polymorph 1e is obtained by crystallization of 1a in dry methanol. Interaction of adippH(2) 1 with triflic acid and 1,10-phenanthroline yields the corresponding salts 2 and 3, respectively. Acetylation of 1a results in the isolation of acetylamino derivative 4. Phosphate diesters 5 and 6 featuring both aryloxy and alkoxy substituents on the central phosphorus have also been synthesized by a sequential reaction starting from (4-NO2-2,6-iPr(2)C(6)H(2)O)P(O)Cl-2. All new compounds have been exhaustively characterized by analytical and spectroscopic methods apart from establishing their crystal structures by single crystal X-ray diffraction studies. Owing to the simultaneous presence of phosphate ester groups (featuring P-OH and P=O functionalities), para amino or nitro group on the aryl substituent, and solvent molecules such as methanol and water, the crystal structures of 1a-1e and 2-6 exhibit interesting supramolecular aggregation that is primarily aided by H-bond networks.

Screened Coulomb interaction in dielectrics is often used as an argument for a lower exciton binding energy and easier exciton dissociation in a high dielectric material. In this paper, we show that at length scales of excitons (10-20 angstrom), the screened Coulomb law is invalid and a microscopic (quantum chemical) description is necessary to describe the medium effect on exciton dissociation. The exciton dissociation energy decreases with increasing dielectric constant, albeit deviating from the inversely proportional relationship. The electron-hole interaction energy, approximated with a point charge model, is apparently not affected by the dielectric constant of the environment. (C) 2014 Elsevier B.V. All rights reserved.

In organic materials, coupling of electronic excitations to vibrational degrees of freedom results in polaronic excited states. Through numerical calculations, we demonstrate that the vibrational distortion field accompanying such a polaron scales as the product of the excitonic interaction field and the exciton coherence function. This scaling relation is derived analytically in the regime where excitonic interactions are weak, yet it is shown to remain valid for interaction strengths ranging up to physically relevant values. Moreover, it is not affected by the magnitude of exciton-vibrational coupling or the presence of disorder in the molecular transition energies, despite the dramatic changes observed in the excited state. An application to helical MOPV4 aggregates is presented, followed by a quantitative study of the vibrational distortion field when excitonic interactions are strong. Our findings allow for a straightforward interpretation of widely varying polaron profiles, thereby facilitating the characterization of organic excited states.

Potential ferroelectricity in charge-transfer organic materials is often masked by the intrinsic conductivity. Here, we report the compelling evidence of ferroelectricity in organic p-conjugated meta-nitroaniline (m-NA) crystals as shown by the local electromechanical measurements using the piezoresponse force microscopy (PFM) technique. m-NA is a charge-transfer molecular material with the exceptional optical non-linearity and perceptible conductivity along the crystallographic polar axis. While standard Sawyer-Tower measurements revealed an apparently lossydielectric hysteresis, The PFM switching spectroscopy indicated clear ferroelectric behaviour in this technologically important multifunctional material. Further study of the pyroelectric properties in m-NA crystals confirmed their high spontaneous polarization of 18 microC/cm2 at room temperature, comparable to the best known organic ferroelectrics., This work was supported by the European Regional Development Fund (ERDF) through Programa Operacional Factores de Competitividade (COMPETE: FCOMP-01-0124FEDER-014628, FCOMP-01-0124-FEDER-009457)and FCT Grant No. PTDC/CTMNAN/114269/2009), Pest-C/CTM/ LA0011/013, and by RFBR Grant No. 13-02-90925. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials (POCI-01-0145FEDER-007679, FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement, and by Government of the RF (Act 211, Agreement 02.A03.21.0006).

International audience; This review summarizes the current understanding of electrostatic phenomena in ordered and disordered organic semiconductors, outlines numerical schemes developed for quantitative evaluation of electrostatic and induction contributions to ionization potentials and electron affinities of organic molecules in a solid state, and illustrates two applications of these techniques: interpretation of photoelectron spectroscopy of thin films and energetics of heterointerfaces in organic solar cells.

Protein low-frequency vibrational modes are an important portion of a proteins' dynamical repertoire. Yet, it is notoriously difficult to isolate specific vibrational features in the spectra of proteins. Given an appropriately chosen model peptide, and using different experimental conditions, we can simplify the system and gain useful insights into the protein vibrational properties. Combining neutron scattering, depolarized light scattering, and molecular dynamics simulations, we analyse the low frequency vibrations of biological molecules, comparing the results from a small globular protein, lysozyme, and an amphiphilic peptide, NALMA, both in solution and in powder states. Lysozyme and NALMA present similar spectral features in the frequency range between 1 and 10 THz. With the aid of MD simulations, we assign the spectral features to methyl groups' librations (1-5 THz) and hindered torsions (5-10 THz) in NALMA. Our data also show that, while proteins display boson peak vibrations in both powder and solution forms, NALMA exhibits boson peak vibrations in powder form only. This provides insight into the nature of this feature, suggesting a connection of BP collective motions to a characteristic length scale of heterogeneities present in the system. These results provide context for the use of model peptide systems to study protein dynamics; demonstrating both their utility, and the great care that has to be used in extrapolating results observed in powder to solutions.

International audience; The quadratic (proportional to E-2) nonlinear optical (NLO) properties of pseudosymmetric organic cations are investigated. These chromophores belong to a family of highly polarizable streptocyanine dyes built in two steps by amination of their related carboxonium salts. They exhibit a nearly perfect optical transparency in the 200-1100 nm range, except around 700 nm, where a single but narrow band is present. Some of these dyes crystallize in noncentrosymmetric space groups, which, despite a symmetrical pi-electronic structure, leads to symmetry lowering and restores a push-pull character to the optical transition and hence a sizable molecular hyperpolarizability. One of these molecules is optimized for a solid-state NLO effect and exhibits an efficiency 120 times that of urea in second harmonic generation at 1.907 mu m.

Pentacene crystallizes in a layered structure with a herringbone arrangement within the layers. The electronic properties depend strongly on the stacking of the molecules within the layers (Haddon et al., 2002). We have synthesized four different polymorphs of pentacene, identified by their layer periodicity, d(001): 14.1, 14.4, 15.0 and 15.4 A. Single crystals commonly adopt the 14.1 A structure, whereas all four polymorphs can be synthesized in thin film form, depending on growth conditions. We have identified part of the unit cell parameters of these polymorphs by X-ray and electron diffraction. The 15.0 and 15.4 A polymorphs transform at elevated temperature to the 14.1 and 14.4 A polymorphs, respectively. Using SCLC measurements, we determined the mobility of the 14.1 A polymorph to be 0.2 cm^2/Vs at room temperature., 9 pages, including 6 figures

International audience; Benzoxaboroles are organoboron molecules which are gaining growing interest in different fields, notably for the development of new drugs. However, extensive characterization of these molecules in the solid state is still lacking. Here, questions related to the structure and spectroscopic signatures of crystalline benzoxaborole phases are thus addressed, using a combined experimental-computational approach. Two simple benzoxaboroles were studied: 1,3-dihydro-1-hydroxy-2,1-benzoxaborole (denoted as BBzx) and 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (also referred to as AN2690, a newly developed antifungal drug). First, the crystal structures of AN2690 and BBzx at room temperature are discussed, emphasizing the intermolecular interactions which play an important role in their formation. Then, results of IR and multinuclear (H-1, B-11, C-13 and F-19) solid state NMR characterization are presented, together with density functional theory (DFT) calculations which were carried out to assist in the interpretation of the spectra. Finally, the influence of polymorphism and anisotropic thermal expansion properties of the crystal structures on the NMR parameters of BBzx and AN2690 is discussed

A high-pressure polymorph of the organic semiconductor rubrene was obtained above 6.0 GPa by hydrostatic compression of the triclinic form. In the high-pressure phase, rubrene adopts an unexpected and previously unobserved conformation, which is ca. 70 kJ/mol less stable than the planar one observed in the ambient-pressure phase and is characterized by a unique "double twisting" of the tetracene core and "scissoring" of the lateral phenyl groups, which favor the formation of C-H center dot center dot center dot pi contacts. The evolution of the structure as a function of pressure is monitored and quantified by Hirshfeld surfaces analysis and calculations of lattice and intermolecular interaction energies. The isosymmetric single-crystal-to-single-crystal transition is fully reversible and is primarily driven by a reduction in molecular volume.

Polarizabilities calculated for sexithiophene using Hartree-Fock and density-functional approaches with a 6-31G* basis set are used to calculate refractive indices and local electric field tensors in the alpha -sexithiophene crystal. The calculations under-estimate the (incomplete) experimental refractive indices but polarizabilities are devised that fit them. These predict that the unknown refractive index perpendicular to the crystal be plane is larger than those in the plane. The tilted molecular long axis produces sizeable off-diagonal local-field components in the crystal ac plane. This complicates the analysis of electro-absorption spectroscopy.

In this work, a method is described to extend the iterative Hirshfeld-I method, generally used for molecules, to periodic systems. The implementation makes use of precalculated pseudo-potential based charge density distributions, and it is shown that high quality results are obtained for both molecules and solids, such as ceria, diamond, and graphite. The use of such grids makes the implementation independent of the solid state or quantum chemical code used for studying the system. The extension described here allows for easy calculation of atomic charges and charge transfer in periodic and bulk systems., 11 pages, 4 Tables, 5 Figures, pre-referee draft only, much extended post referee version only available at publisher

We study the transport of collective excitations (Frenkel excitons) in systems with static disorder in the transition energies, not limiting ourselves to Gaussian transition energy distributions. Instead, we generalize this model to the wider class of Levy stable distributions, characterized by heavy tails. Phonon-assisted scattering of excitons, localized by the disorder, leads to thermally activated exciton motion. The time evolution of the second moment of the exciton distribution is shown to be sublinear, thus indicating that the exciton dynamics in such systems is not diffusive, but rather subdiffusive instead. The heavier the tail in the transition energy distribution is, the larger are the deviations from the diffusive regime. This from fluctuations of site energies larger than the exciton band width (outliers). We show that the occurrence of subdiffusive transport for heavy-tailed disorder distributions can be understood from the scattering rate distributions, which possess a (second) peak at zero scattering rate. (C) 2013 AIP Publishing LLC.

The triplet state relaxation dynamics in R6G doped in boric acid glass have been probed using luminescence and transient grating studies. The measured transient grating and luminescence decay signal profiles are explained by excited state relaxation mainly through triplet-triplet annihilation. The observed delayed emission is identified as P-type delayed fluorescence. Our findings support saturable absorption as the mechanism for optical nonlinearity.

Graphical tools based on Hirshfeld surfaces and two-dimensional fingerprint plots have been used to compare the different polymorphs of rubrene and the structures of rubrene substitution derivatives, deposited in the Cambridge Structural Database. Focusing on C center dot center dot center dot C contacts displayed both on the surfaces and on the fingerprint plots, some distinctive features related with the herringbone disposition of the molecules and their arrangement in pi-stacks have been observed. These features do not occur only for those rubrene molecules modified by the insertion of small functional groups but also in the case of bulky substituents. Although no particular trend was identified, this study should contribute to encourage the design of new improved rubrene derivatives that could exhibit the typical intermolecular contacts of orthorhombic rubrene, a feature that has been suggested to contribute to its excellent performance as a semiconducting material. As a consequence, the limited number of rubrene derivatives presently available should be increased, thus allowing a deeper analysis and a better understanding of the relationships between physical properties and crystal structures.

A model of vibronic coupling in a manifold of coupled Frenkel and charge transfer states is applied to evaluate the electric-field-induced shifts of lowest vibronic levels deriving from the lower Davydov component of the Frenkel exciton in sexithiophene. With respect to the isolated-molecule value, vibronic terms combined with the mixing between Frenkel and CT configurations amplify the field-induced shift by the factor of seven for the 0-0 line and by further 60% for the vibronic replica in the main progression-forming mode. Confirmation is found in the existing experimental literature. (C) 2012 Elsevier B. V. All rights reserved.

Statistical methods are used to derive an expression for the average X-ray diffraction intensity, as a function of (sin theta)/lambda, of crystals with an incommensurate one-dimensional modulation. Displacive and density modulations are considered, as well as a combination of these two. The atomic modulation functions are given by truncated Fourier series that may contain higher-order harmonics. The resulting expression for the average X-ray diffraction intensity is valid for main reflections and low-order satellite reflections. The modulation of individual atoms is taken into account by the introduction of overall modulation amplitudes. The accuracy of this expression for the average X-ray diffraction intensity is illustrated by comparison with model structures. A definition is presented for normalized structure factors of crystals with an incommensurate one-dimensional modulation that can be used in direct-methods procedures for solving the phase problem in X-ray crystallography. A numerical fitting procedure is described that can extract a scale factor, an overall temperature parameter and overall modulation amplitudes from experimental reflection intensities.

A comprehensive theory of linear vibronic coupling in a coupled manifold of Frenkel and charge-transfer states in an infinite molecular crystal is presented and applied for sexithiophene. The approach, valid in the intermediate-coupling regime, includes up to three-particle terms of the Philpott expansion, with the vibronic wavefunctions represented in the Lang-Firsov basis. As a stringent test, the scheme is used to reproduce the complete set of available sexithiophene absorption and electroabsorption spectra within a unified theoretical framework. The input is based primarily on independent calculations and to some extent on independent experiments, with explicit fitting contained within the limits set by the estimated inherent errors of a priori parameter estimates. Reasonably good quantitative agreement with experimental spectra is achieved. The results resolve some existing interpretational ambiguities and expose some peculiarities of electric field effect on vibronic eigenstates of Frenkel parentage, highlighting the role of charge-transfer interactions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3597607]

Solid-state reactions of copper(I) iodide with 1,4-diazabicyclo[2.2.2]octane (DABCO) and piperazine in different conditions (kneading, vapour digestion, thermal treatment) resulted in the formation of four new luminescent systems, which have been characterized via X-ray diffraction and photo-luminescence spectroscopy. At room temperature all compounds exhibit a bright emission in the solid state that markedly shifts bathochromically on cooling to 77 K. The luminescence of the almost isostructural 2D-coordination polymers [Cu(2)I(2)(L)(2)](infinity) (L = DABCO, piperazine) exhibits an unusually long lifetime (125 and 53 us, respectively), which becomes even longer (609 and 420 us, respectively) at 77 K.

The role of dispersive interactions on the structure, energetic and vibrational features of brucite [Mg(OH)(2)], portlandite [Ca(OH)(2)] and kaolinite [Al2Si2O5(OH)(4)] layered materials has been addressed for the first time. Dispersion contribution is included with a -C-6/R-6 empirical correction to the B3LYP functional (B3LYP-D* recipe) which has recently been employed to study molecular crystals. To decrease the spurious effect of the basis set superposition error, Gaussian basis sets of triple-zeta plus polarization functions were adopted. Comparing B3LYP and B3LYP-D* results shows the latter to provide significant improvement as far as structure and energetic data are concerned. For the treated systems the cell parameter controlling the inter-layer distance, usually overestimated by the pure B3LYP, is in good agreement with experiment. The inter-layer interaction energy is dramatically increased by the dispersion contribution which, for kaolinite, fully justifies the request of strong Lewis basic molecules needed to swell the material. In general, B3LYP harmonic frequencies are scarcely changed by the dispersive correction albeit some modes sensitive to the inter-layer separation may be significantly perturbed at B3LYP-D*. The present results are encouraging although fine tuning of the proposed empirical correction for inorganic systems might be needed

Nonlinear optical susceptibilities provide a convenient means of relating macroscopic optical measurements to microscopic models. The susceptibilities are most useful when the radiation field and the material degrees of freedom are weakly coupled. In the opposite case, the dynamics is interpreted in terms of combined radiation-matter modes (polaritons) and susceptibilities are usually not used. In this review we analyze both situations from a unified dynamical framework based on equations of motion. The present formalism is also particularly suitable for the calculation of optical nonlinearities in nanostructures with restricted geometries. The transient grating and its frequency-domain analogue (degenerate four-wave mixing) are used to illustrate the formalism in both the strong and the weak radiation-matter coupling limit.

The UV-visible optical spectra of 1,2,3,4-tetrafluoro-7-(N,N)dimethyl-amino-acridine single crystals are reported. The results are discussed on the basis of the molecular transitions and crystal packing in the framework of the theory of molecular excitons under a fluctuating potential field due to dynamic disorder. A strong local geometry distortion is demonstrated by applying the Urbach rule to the absorption tails, which is the amplitude of the local potential fluctuation being larger than the intermolecular transfer energy. The lineshape and linewidth of the emission band and its temperature dependence give further evidence of exciton self-trapping. © 2007 American Institute of Physics.

The structural and electronic properties of four L-amino acids alanine, leucine, isoleucine, and valine have been investigated using density functional theory sDFTd and the generalized gradient approximation. Within the crystals, it is found that the constituent molecules adopt zwitterionic configurations, in agreement with experimental work. Lattice constants are found to be in good agreement with experimentally determined values, although certain discrepancies do exist due to the description of van derWaals interactions.We find that these materials possess wide DFT band gaps in the region of 5 eV, with electrons highly localized to the constituent molecules. It is found that the main mechanisms behind crystal formation are dipolar interactions and hydrogen bonding of a primarily electrostatic character, in agreement with current biochemical understanding of these systems. The electronic structure suggests that the amine and carboxy functional groups are dominant in determining band structure.

Exciton diffusion has been studied in 5-25-nm-thick films of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides by intentional doping with quenchers using steady-state as well as time-resolved fluorescence spectroscopy. The fluorescence spectra of the films are very similar to those of solutions, indicating emission from localized exciton states. From the dependence of the fluorescence quenching on the quencher concentration and fluorescence lifetime measurements, the exciton diffusion can be concluded to be quasi-one-dimensional with an exciton diffusion length of 9 +/- 3 nm and an intrastack energy-transfer rate constant of 10(11)-10(12) s(-1). From fluorescence anisotropy decay measurements, we conclude that neighboring stacks aggregate in a herringbone structure, forming ordered domains that are randomly oriented in the substrate plane. These measurements indicate an interstack energy-transfer rate constant of (7 +/- 2) x 10(10) s(-1).

The prototype compound for the neutral-ionic phase transition, namely TTF-CA, is theoretically investigated by first-principles density functional theory calculations. The study is based on three neutron diffraction structures collected at 40, 90 and 300 K (Le Cointe et al., Phys. Rev. B 51, 3374 (1995)). By means of a topological analysis of the total charge densities, we provide a very precise picture of intra and inter-chain interactions. Moreover, our calculations reveal that the thermal lattice contraction reduces the indirect band gap of this organic semi-conductor in the neutral phase, and nearly closes it in the vicinity of the transition temperature. A possible mechanism of the neutral-ionic phase transition is discussed. The charge transfer from TTF to CA is also derived by using three different technics., Comment: 11 pages, 9 figures, 7 tables

International audience; Infrared spectroscopy of vibrational excitons is a good method to study the second order structural phase transitions which are observed in some molecular crystals. Crystalline phenothiazine (C/sub 12/H/sub 9/NS) undergoes such a transition at about T/sub c/=250 K from a high-temperature orthorhombic phase with a space group Pbnm (D/sub 2h//sup 16/) to a low-temperature monoclinic phase with a space group P2/sub 1//n (C/sub 2h//sup 5/). This transition is studied by infrared spectroscopy of two bands at 426 cm/sup 1/ (A" mode) and at 925 cm/sup -1/ (A' mode). The splitting of the exciton components of these bands is studied as a function of temperature and can be described by a (T/sub c/-T)/sup beta / law with a critical exponent beta of an approximate value of 0.45. This result agrees with Landau's theory of second order phase transitions.

We report on the synthesis and optical characterization of hydrazone derivatives developed for second-order nonlinear optics. Electric-field-induced second harmonic generation (EFISH) as well as semiempirical quantum-chemical computations have been carried out to investigate and explain the nonlinear optical properties of these molecules. We obtained values of up to 220 x 10(-40) m(4)/V for the molecular hyperpolarizability beta at lambda = 1907 nm. The investigation of a series of 39 hydrazone derivatives with the second harmonic generation powder test according to Kurtz and Ferry revealed that an extraordinarily large fraction of these hydrazones show efficient second-order nonlinear activity. Among them, 4-(dimethylamino)-benzaldehyde-4-nitrophenylhydrazone (DANPH) and 4-(1-azacycloheptyl)-benzaldehyde-4-nitrophenylhydrazone (ACNPH) were selected for more detailed investigations. Their crystalline structures are presented, and the role played by hydrogen bonds in their molecular packing is emphasized. Finally, preliminary linear and nonlinear optical data of DANPH crystals are reported. [References: 42]