Search

Raman spectroscopy of organic molecular materials in the low-wavenumber region gives access to lattice vibrational modes and to the wealth of information on solid state properties that these can provide. In the field of organic electronics a useful application concerns the discrimination of the crystalline forms i. e. polymorphism of the semiconductor. The capability of characterizing and identifying the polymorphs of a compound is in fact the prerequisite for an exhaustive study of the charge transport characteristics which arise from the relationship between molecular, electronic, and crystal structures. Thus, the need is felt of a non-invasive, non-destructive tool such as Raman, which probes the crystal phase by detecting the lattice modes which are sensitive even to subtle variations of the packing. Here we review the contribution of the technique to the study of organic pigments displaying promising semiconducting properties and characterized by polymorphism both in their bulk and thin film phases.

In homogeneous solid-state reactions, the single-crystal nature of the starting material remains unchanged, and the system evolves seamlessly through a series of solid solutions of reactant and product. Among [2+2] photodimerizations of cinnamic acid derivatives in the solid state, those involving salts of the 4-aminocinnamic acid have been recognized to proceed homogeneously in a "single-crystal-to-single-crystal" fashion by X-ray diffraction techniques. Here the bromide salt of this compound class is taken as a model system in a Raman spectroscopy study at low wavelengths, to understand how such a mechanism defines the trend of the crystal lattice vibrations during the reaction. Vibrational mode calculations, based on dispersion corrected DFT simulations of the crystal lattices involved in the transformation, have assisted the interpretation of the experiments. Such an approach has allowed us to clarify the spectral signatures and to establish a correlation between the dynamics of the monomer and dimer systems in a process where chemical progress and crystal structural changes are demonstrated to occur simultaneously.

A novel and enhanced electrosynthesis protocol that allows the deposition of thin films of Co/Al and Co/Fe layered double hydroxides (LDHs) on different supports is hereby proposed. The approach is based on potentiodynamic cathodic reduction. All the films have been characterised by cyclic voltammetry, powder X-ray diffraction, scanning electron microscopy, and Raman and atomic emission spectroscopies. Moreover, LDHs electrosynthesised on carbonaceous materials have also been investigated by X-ray absorption spectroscopy to analyse their local metal structure. These substrates are particularly interesting for the plethora of LDH applications ranging from energy storage, sensing, and electrocatalysis, and are also of industrial importance, due to their low cost, ecocompatibility, and easy handling. In particular, the material was exploited for 5-(hydroxymethyl)furfural (HMF) electro-oxidation.

A family of salts of R-(+)-(3)-hydroxyquinuclidinium [QH]+, with SO42−, BPh4−, BF4− and PF6− counter-anions, have been prepared by the metathesis of [QH]Cl and metal salts of the corresponding anions. Solid solutions of formula [QH](PF6)x(BF4)1−x for x = 0.9, 0.8, 0.7 have also been obtained. The crystalline materials have been investigated by a combination of solid-state techniques, including variable temperature XRD, thermal analyses, multinuclear (11B, 13C, 15N, 19F, and 31P) solid-state NMR spectroscopy, variable temperature wideline 19F T1 relaxation measurements, and micro-Raman spectroscopy to investigate their thermal stability and phase transition behaviors. It has been shown that the salts [QH]PF6 and [QH]BF4 undergo an order–disorder solid–solid phase transition to plastic phases, whereas [QH]2SO4·H2O and [QH]BPh4 do not display any plastic phase transition. Doping [QH]BF4 into the [QH]PF6 lattice up to 30% results in the formation of a solid solution that is plastic in an expanded thermal range, thanks to a phenomenon that we describe here for the first time as “reordering frustration”.

We show that the development of highly accurate density functional theory calculations coupled to low-frequency Raman spectroscopy constitutes a valid method for polymorph characterization alternative/complementary to X-ray. The method is applied here to the temperature-induced, first-order phase transition of coronene, known for a long time, but has remained structurally uncharacterized due to crystal breaking during the process. The astonishing fidelity of the Raman calculated spectra to the experiments allows us to unambiguously identify the low-temperature phase with the β-coronene polymorph, recently reported as new and obtained in the presence of a magnetic field. We also suggest that additional measurements are needed to confirm that a magnetic field can actually drive the growth of a β-polymorph surviving indefinitely at ambient temperature.

Films of the chemical precursor (t)Boc-quinacridone obtained by the spin-coating and bar-assisted meniscus shearing methods were subjected to thermal deprotection to recover the organic semiconductor quinacridone in its crystalline form. We found that the final crystal structure of the semiconductor on the Si/SiO2 substrate is in fact determined by the chemical precursor starting structure, which is in turn induced by the deposition method. Indeed, the samples prepared by spin coating display the precursor structure known from the literature, which transforms into the beta-quinacridone phase. The shearing technique instead yields highly homogeneous films composed of a novel (t)Boc-quinacridone polymorph, which acts as a trigger for the subsequent formation of a pure, well oriented alpha-quinacridone phase. Although this crystalline form is the least stable of the many quinacridone polymorphs, here it turns out to be selectively induced and stabilized. Finally, the organic field effect transistor charge mobility of the alpha-quinacridone films was measured., T. S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919). This work was supported by the MIUR-PRIN project “From natural to artificial light-harvesting systems: unveiling fundamental processes towards a bio-inspired materials design” (HARVEST) protocol 201795SBA3. This work was funded by DGI (Spain) with the project GENESIS PID2019-111682RB-I00, the Generalitat de Catalunya (2017-SGR-918), and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S). The authors thank Dr Raphael Pfattner (ICMAB-CSIC) for fruitful discussions.

A method for structure solution in thin films that combines grazing incidence X-ray diffraction data analysis and crystal structure prediction was presented in a recent work (Braun et al. Cryst. Growth Des.2019, DOI: 10.1021/acs.cgd.9b00857). Applied to phenytoin form II, which is only detected in films, the approach gave a very reasonable, but not fully confirmed, candidate structure with Z = 4 and Z′ = 2. In the present work, we demonstrate how, by calculating and measuring the crystal Raman spectrum in the low wavenumber energy region with the aim of validating the candidate structure, this can be further refined. In fact, we find it to correspond to a saddle point of the energy landscape of the system, from which a minimum of lower symmetry may be reached. With the new structure, with Z = 4 and Z′ = 2, we finally obtain an excellent agreement between experimental and calculated Raman spectra., Experimental low wavenumber Raman spectroscopy validates the presence of a non-centrosymmetric packing arrangement that is energetically very close to a centrosymmetric saddle point structure.

We propose the use of solid solutions for the modulation of the kinetic in [2+2] photocycloaddition. Solid solutions with general formula [1H]BrxCl1‒x (1 = 4-amino-cinnamic acid; 0 < x < 1) were prepared via mechanochemical methods, structurally characterized, and found to undergo a single-crystal-to-single-crystal [2+2] photodimerization. The effect of the solid solutions composition on the kinetics was successfully investigated via a combination of FTIR and Principal Component Analysis (PCA), and the results were compared with those obtained for the parent materials [1H]Cl and [1H]Br. All systems follow the same nearly first-order law, and the overall reaction kinetics is influenced by a subtle interplay of structural and electronic factors as well as other processes regulating the formation and lifetime of the reacting excimer.

The 3:2 Charge Transfer (CT) co-crystal (Perylene)3(TCNQF1)2 is grown by the Physical Vapor Transport (PVT) method, and characterized structurally and spectroscopically. Infrared analysis of the charge sensitive modes reveals a low degree of charge transfer (less than 0.1) between donor and acceptor molecules. The crystal is isostructural to the other 3:2 CT crystals formed by Perylene with TCNQF2 and TCNQF4, whereas such stoichiometry and packing is not known for the CT crystals with non-fluorinated TCNQ. The analysis of the isostructural family of 3:2 Perylene–TCNQFx (x = 1,2,4) co-crystal put in evidence the role of weak F…HC bonding in stabilizing this type of structure, The work has been supported by the local research funds of Bologna and Parma Universities.

Recently, structural studies in the thin-film phase of 6,6′-dibromoindigo, the main component of the Tyrian purple dye, revealed the presence of surface-induced structures. This finding is important, as the system is a good candidate for applications in organic electronics, and knowledge of the film structure is needed to explain the physical properties of the conducting layer. In this work, combining the results of low-frequency Raman spectroscopy, X-ray diffraction, and accurate density-functional theory (DFT) calculations of the vibrational properties of the proposed surface-induced structures, we suggest that a newly detected 6,6′-dibromoindigo bulk crystal form is structurally closely related to the surface-induced modifications. Thus, 6,6′-dibromoindigo behavior shows similarities and differences at the same time, with respect to other investigated systems., We thank CINECA Supercomputing Center for providing computer time through the ISCRA scheme (project C—SCSCDFT). T.S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919). The authors thank Prof. Roland Resel for the fruitful discussions. This work has been funded by the PRIN project “From natural to artificial light-harvesting systems: unveiling fundamental processes toward a bio-inspired materials design” (HARVEST) protocol 201795SBA3.

A careful analysis of the vibrational spectra of two isostructural charge transfer systems with potential applications in optoelectronics, (perylene)3‐(TCNQF4)2, and (perylene)3‐(TCNQF2)2, highlights the presence of three strong infrared bands, polarized along the (perylene‐TCNQFx‐perylene) trimeric units present in the crystal. These bands are recognized as due to the Holstein, or electron–molecular vibration (e–mv) interaction. Use of appropriate modeling, based on the analysis of the frequency shifts due to the e–mv coupling, allows to extract the relevant Holstein coupling constants for the HOMO of the perylene molecule. The so‐obtained experimental values agree very well with density functional calculations, thus validating the results. The perylene relaxation energy, or small polaron binding energy, is about 70 meV, of the same order of magnitude as that of pentacene., The authors thank Prof. Enrico Da Como for useful discussions in the early stages of this work. The work was supported by the local research funds of Bologna and Parma Universities.

This work presents a Raman based approach for the rapid identification of the molecular conformation in a series of new 2,3-thienoimide capped quaterthiophenes, whose crystal structures were determined by synchrotron radiation X-ray powder diffraction. These systems display two conformational polymorphs, known as forms A and B, as a result of the anti–anti–anti and syn–anti–syn arrangements of the quaterthiophene cores. In a micro-Raman and computational study, the spectroscopic differences between the conformers were detected and proved to be suitable markers for polymorph identification. Thus, the synergic employment of diffraction and Raman spectroscopy techniques yields a full and reliable characterization of 2,3-thienoimide capped quaterthiophene compounds in their solid state.

Indigo [2,2′-bis(2,3-dihydro-3-oxoindolyliden)], a commonly used natural dye, has been shown to exhibit a highly promising semiconducting behavior, allowing for the realization of ambipolar devices. Nevertheless, up to date, it is still unclear which crystal structure is present in the thin films, a piece of information relevant for device applications. In this work, we address this issue by an in-depth characterization of the polymorphs of Indigo in the bulk and in drop-cast films. To do this, X-ray diffraction (XRD) and micro-Raman spectroscopy have been employed jointly, with the support of state-of-the-art density functional theory calculations in the solid state. Structural and spectroscopic characterizations have established that the two known A and B polymorphs grow as concomitant in the bulk under most of the experimental conditions adopted in this work. In the drop-cast films, XRD cannot unambiguously identify the structure, but Raman spectroscopy is effective in establishing that only the B form is present. The calculations augment the experiments, providing valuable insights into the relative thermodynamic stability of the two forms as a function of temperature. They also allow for a more comprehensive characterization of the Raman modes.

We revisit the polymorphism of paracetamol by means of a micro-Raman technique, which has proved to be a powerful tool for structure recognition. Distinct lattice phonon spectra clearly identified the pure phases. Confocality enabled us to detect phase mixing between form II and either I or III on a micrometric scale in the same crystallite. Following the most recent findings on surface-mediated structures, we also investigated spin-coated films grown on glass, gold, and polystyrene substrates, confirming the selectivity of these surfaces for the metastable form III, which shows an unprecedented stability over a time span of several months. A mechanism of its transformation to phase II, via a partially ordered intermediate state, is suggested by polarized Raman measurements.

International audience; Thin films of the natural semiconductor indigo have been grown on Si/SiO2 substrates by physical vapour deposition under ultra-high vacuum conditions, monitoring the influence of the deposition rate on structure, morphology and thickness. This was done to allow well-defined films to be made in a controlled way, suitable for the manufacture of organic devices and the investigation of charge transport phenomena. The films have been characterized by X-ray diffraction, atomic force microscopy, microRaman spectroscopy and transmission electron microscopy, finding that the growth at the solid interface of polymorph B, the only phase present in the films, is governed by the formation of strong multiple hydrogen-bonds between neighbouring molecules, which play a predominant role in the interface organization of the molecular assemblies.

The photodimerization of anthracene derivatives constitutes a model system for intermolecular [4 + 4] cycloadditions. In this paper we deal with the elusive 9-anthracene–carboxylic case and study the crystal state of the head-to-tail dimer, obtained by the reaction of the monomer in various solvents both in its unary and solvated forms, by X-ray diffraction and confocal Raman microscopy in the lattice phonon region. A number of solvates have been identified, and their structures have been solved and here presented. The 9-anthracene–carboxylic acid dimer appears to be an exemplary case of a molecular crystal easily prone to host solvent molecules in the interstices of the framework generated by homomolecular hydrogen bonds. Alternatively, the hydrogen bonds between solvent molecules and the carboxylic group may establish supramolecular structures of closely packed architectures. Raman microscopy has also allowed us to investigate the short-lived dimer, which is produced in the crystal-to-crystal photoreaction triggered by the irradiation of the monomer single crystal.

Thioindigo (2-(3-Oxo-1-benzothiophen-2(3H)-ylidene)-1-benzothiophen-3(2H)-one) is a synthetic dye related to the natural compound Indigo. Notwithstanding the interest aroused recently by its employment as a functional material in a number of applications, a satisfactory characterization of its solid state is still missing. In this work, we study the occurrence of the two thioindigo α and β polymorphs under various growth conditions, and find that their structural similarity implies they often coexist. However, whereas polymorph β is certainly predominant in the bulk phase, polymorph α grows preferentially on substrates, turning out to be the surface stabilized phase in highly homogeneous and ordered films obtained by the bar‐assisted meniscus shearing method (BAMS). DFT calculations support the experimental findings, aiding in the polymorph spectroscopic identification and to the interpretation of the order in the films of polymorph α., We thank CINECA Supercomputing Center for providing computer time through the ISCRA scheme (project C – HP10CA2TWT). The authors thank the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), the DGI (Spain) project, FANCY CTQ2016-80030-R, the Generalitat de Catalunya (2017-SGR-918) and the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa‘ Programme for Centres of Excellence in R&D (SEV- 2015–0496). We acknowledge the Elettra Synchrotron Trieste for beamtime allocation and thank Luisa Barba for assistance in using beamline XRD1. T.S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919).

We report on the solution growth of the two known structures of Perylene-F2TCNQ charge transfer complexes. The transformation accompanied by a marked morphological change from needle 1:1 to platelet 3:2 crystal structure is observed in the mother liquor. Lattice phonon Raman spectroscopy is used for an easy structure identification of the different morphologies before and after the process. X-ray and lattice phonons spectra of reference samples obtained by physical vapor transport is used to identify the two complexes. A fully spectroscopic analysis of the intramolecular Raman modes is presented to estimate the degree of ionicity, which is found to agree with the value previously reported.

Directed crystal growth often requires considerable experimental effort to achieve epitaxial control. In this work a simple solution step is shown to yield crystals of two different molecules simultaneously within a thin film and with defined epitaxial relations to each other. This was achieved by dissolving simultaneously carbamazepine (CBZ), a drug molecule, and iminostilbene (ISB), its precursor, in a single solution. When the sample is prepared using slow solvent evaporation rates, both molecules grow into needle-shaped crystals. The ISB crystals contact the substrate surface and extend for hundreds of μm. CBZ tends to minimize the contact with the substrate and assembles on top or is ledge-directed. In both cases a defined inclination of CBZ to the underlying ISB crystals is seen with optical microscopy showing a 36° inclination of the elongated crystals. In a similar way to pole figure measurements, rotating grazing incidence X-ray diffraction allows the determination of crystallographic properties and epitaxial relation, i.e. how the unit cells of both molecules align with respect to each other in terms of contact plane and azimuthal orientation. A point-on-line coincidence was identified, which can be related to the herringbone packing in the ISB crystals. Furthermore, changing the solute content as well as the processing times/kinetics, the appearance and the quality of crystal growth changes, but with the epitaxial relations remaining unaffected; crystals grow more frequent on top when processed slowly and when processed fast, growth takes place more often at facet inclined 90° from the top one., The authors want to thank the NAWI Graz for support. Elettra synchrotron is acknowledged for providing beamtime at the XRD1 beamline. The authors want to thank Luisa Barba and Nicola Demitri for excellent support.

Layered double hydroxides (LDHs) have been widely studied for their plethora of fascinating features and applications. The potentiostatic electrodeposition of LDHs has been extensively applied in the literature as a fast and direct method to substitute classical chemical routes. However, the electrochemical approach does not usually allow for a fine control of the M-II/M-III ratio in the synthesized material. By employing a recently proposed potentiodynamic method, LDH films of controlled composition are herein prepared with good reproducibility, using different ratios of the trivalent (Fe or Al) to bivalent (Co) cations in the electrolytic solution. All the obtained materials are shown to be effective oxygen evolution reaction (OER) catalysts, and are thoroughly characterized by a multi-technique approach, including FE-SEM, XRD, Raman, AES and a wide range of electrochemical procedures.

Thin films of Al, and Co or Ni based Layered Double Hydroxides (LDH) have been electrochemically deposited on four different supports (Grafoil, Toray Carbon Paper, Carbon Cloth and Nickel Foam) through a potentiostatic or a potentiodynamic approach. The obtained films have been fully characterized to compare their properties in dependence on the different deposition techniques and substrates. Finally, the Grafoil support modified with a Ni /Al-LDH has been employed as cathode in combination with a Grafoil anode modified with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) also electrosynthesized, in order to develop an all-binder-free hybrid supercapacitor as a proof of concept to demonstrate the applicability in the field of energy storage.

The classic photodimerization reaction of 9-cyanoanthracene in the solid state has been studied by means of Raman microscopy, analyzing the time evolution both of the molecular and of the lattice phonon spectra from the reactant to the product. This has allowed us to verify that the formation of the dimer crystal structure always takes place with some delay with respect to the onset of the chemical reaction, as already observed in another crystal to crystal photoreaction.1 This evidence points to a phenomenon of reconstructive phase transition.2 The kinetic analysis of the spectroscopic data confirms the previous hypothesis of an autocatalytic mechanism for the process.

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics.

Raman microscopy in the lattice phonon region coupled with X-ray diffraction have been used to study the polymorphism in crystals and microcrystals of the organic semiconductor 9,10-diphenylanthracene (DPA) obtained by various methods. While solution grown specimens all display the well-known monoclinic structure widely reported in the literature, by varying the growth conditions two more polymorphs have been obtained, either from the melt or by sublimation. By injecting water as a nonsolvent in a DPA solution, one of the two new polymorphs was predominantly obtained in the shape of microribbons. Lattice energy calculations allow us to assess the relative thermodynamic stability of the polymorphs and verify that the energies of the different phases are very sensitive to the details of the molecular geometry adopted in the solid state. The mobility channels of DPA polymorphs are shortly investigated.

SCOPUS: ar.j, info:eu-repo/semantics/published

This work assesses the reliability of different van der Waals (vdW) methods to describe lattice vibrations of molecular crystals in the framework of density functional theory (DFT). To accomplish this task, calculated and experimental lattice phonon Raman spectra of a pool of organic molecular crystals are compared. We show that the many-body dispersion (MBD@rsSCS) van der Waals method of Ambrosetti et al. and the pairwise method of Grimme et al. (D3-BJ) outperform the other tested approaches (i.e., the D2 method of Grimme, the TS method of Tkatchenko and Scheffler, and the nonlocal functional vdW-DF-optPBE of Klimeš et al.). For the worse-performing approaches the results could not even be fixed by the introduction of scaling parameters, as commonly used for high-energy intramolecular vibrations. Interestingly, when using the experimentally determined unit cell parameters, DFT calculations using the PBE functional without corrections for long-range vdW interactions provide spectra of similar accuracy as the MBD@rsSCS and D3-BJ simulations.

We report a combined experimental and theoretical investigation on the Raman spectra of the polymorphs α, β, γ, and δ of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), in the region of the intramolecular modes. The interpretation of the polarized spectra is supported by ab-initio calculations for the isolated molecules and by lattice dynamics calculations for the crystals. The calculations reproduce the peculiar, and surprisingly large, differences among the spectra of the various polymorphs. The phenyl groups of 1,1,4,4-tetraphenyl-1,3-butadiene may arrange themselves around the butadiene skeleton in 2 stable conformers, which have either inversion (Ci) or 2-fold (C2) symmetry and therefore exhibit intramolecular vibrations with quite different Raman selection rules and spectra. The compound forms 4 crystalline polymorphs (α, β, γ, and δ) with different combinations of Ci and C2 conformers, and correspondingly different intramolecular spectra. The theoretical calculations provide a quantitative analysis of the various spectra.

We report on the investigation of the influence of the molecular packing and film morphology on the field-effect charge mobility in 2,3-thienoimide-based oligothiophenes semiconductors (Cn-NT4N). Organic field-effect transistors are realized by implementing both vacuum and solution methods in order to control the solid-state phase of the active layer. Thermal sublimation in a high vacuum chamber and supersonic molecular beam deposition were used as vacuum-based fabrication approaches for preparing thin films, while lithographically controlled wetting was used, as a solution-deposition technique, for the fabrication of the microstructured films. Thermal sublimation leads to thin films with a phase packing showing ambipolar behaviour, while supersonic molecular beam deposition enables, by varying the deposition rate, the formation of two different crystal phases, showing ambipolar and unipolar field-effect behaviours. On the other hand, lithographically controlled wetting enables the formation of Cn-NT4N microstructured active layers and their implementation in field-effect transistors.

Confocal Raman microscopy in the lattice phonon region has been used to study the polymorphism of the organic semiconductor 5,6,11,12-tetraphenyl-tetracene (rubrene). Following literature guidelines, crystals of rubrene have been prepared using a number of solution growth and vapour deposition methods, obtaining samples of different morphologies which could be related to the various polymorphs of this compound. The technique has enabled us an easy and non invasive identification of the three known polymorphs and of their phase homogeneity with a lateral spatial resolution below 1 μm.

We present a Raman investigation of polymorphism in 1,1,4,4-tetraphenyl-butadiene (TPB), a well-known blue luminescent molecule, which retains its emissive properties in the solid state. The use of low-wavenumber (10–150 cm−1) Raman microscopy, very sensitive to the crystal packing, allows us to single and pick up four different polymorphs. X-ray analysis shows that the TPB molecules assume different conformations in the various polymorphs, a fact revealed also by the Raman spectra in the region of intra-molecular vibrations. Lattice dynamics calculations yield a preliminary assignment of the low-wavenumber Raman spectra and provide information of the relative stability of the phases. Copyright © 2013 John Wiley & Sons, Ltd.

We present a working example of phase recognition by lattice phonon Raman spectroscopy, a technique which has proved to be a powerful tool for identifying new polymorphs. The spectra show that a newly resolved triclinic structure of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF) is one of its four polymorphic modifications, labeled the δ-phase, which was previously identified only from spectroscopic data. The now available XRD data allow for the computation of the minimum potential energy structure. Calculated density and energy of the triclinic polymorph are found to be very close to the previously identified β polymorph. These similarities account for the often encountered co-existence of the β and δ crystal phases.

A multiscale investigation of N,N-bis(n-octyl)-x:y, dicyanoperylene-3,4:9,10- bis(dicarboximide), PDI8-CN2, shows the same molecular arrangement in the bulk and in thin films sublimated on SiO2/Si wafers. Non-conventional powder diffraction methods and theoretical calculations concur to provide a coherent picture of the crystalline structure. X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses of films of different thickness depos- ited at different substrate temperatures indicate the existence of two temper- ature-dependent deposition regimes: a low-temperature (room temperature) regime and a high-temperature (80­120 °C) one, each characterized by dif- ferent growth mechanisms. These mechanisms eventually result in different morphological and structural features of the films, which appear to be highly correlated with the trend of the electrical parameters that are measured in PDI8-CN2-based field-effect transistors.

Absorption, photoluminescence, and Raman scattering measurements have been performed on a fourfold alkoxy-substituted phthalocyanine in solution and in the discotic liquid crystal phase. As already known for this class of compounds, the features of the electronic spectra in the condensed state are determined by the spatial closeness and arrangement of the systems of the phthalocyanine cores, with the occurrence of collectively excited states and excimer-like emission. Quantitative information about the alignment of the molecules which compose the liquid crystal phase has been obtained from the analysis of polarized Raman spectra of homeotropically aligned samples.

The results of the fifth blind test of crystal structure prediction, which show important success with more challenging large and flexible molecules, are presented and discussed., Following on from the success of the previous crystal structure prediction blind tests (CSP1999, CSP2001, CSP2004 and CSP2007), a fifth such collaborative project (CSP2010) was organized at the Cambridge Crystallographic Data Centre. A range of methodologies was used by the participating groups in order to evaluate the ability of the current computational methods to predict the crystal structures of the six organic molecules chosen as targets for this blind test. The first four targets, two rigid molecules, one semi-flexible molecule and a 1:1 salt, matched the criteria for the targets from CSP2007, while the last two targets belonged to two new challenging categories – a larger, much more flexible molecule and a hydrate with more than one polymorph. Each group submitted three predictions for each target it attempted. There was at least one successful prediction for each target, and two groups were able to successfully predict the structure of the large flexible molecule as their first place submission. The results show that while not as many groups successfully predicted the structures of the three smallest molecules as in CSP2007, there is now evidence that methodologies such as dispersion-corrected density functional theory (DFT-D) are able to reliably do so. The results also highlight the many challenges posed by more complex systems and show that there are still issues to be overcome.

We report a combined spectroscopic and structural study of polymorphism of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) blue luminescent crystals. We have identified two polymorphs, which can be grown by crystallization of the vapor. The first, α-TPB, has the monoclinic noncentrosymmetric structure (P2 1, two molecules per unit cell), whose lattice parameters have already been reported in the literature. The second, β-TPB, is a new polymorph, monoclinic centrosymmetric (P21/c, four molecules per unit cell). The previously reported triclinic structure, γ-TPB, has not been identified in samples grown by the physical vapor transport method. The effects of the different molecular configuration and packing of α- and β-TPB on Raman scattering, IR, and UV-vis optical properties are discussed.

Using sexithiophene as a benchmark compound, we present a very effective strategy for searching the potential energy minima of a crystalline material, described in terms of rigid molecules with Coulombic and atom-atom interactions. The strategy involves uniform sampling of the many-body energy hypersurface, mechanical identification of all constraints deriving from the crystallographic symmetry, and a "sight-resight" method, originally introduced in wildlife ecology, for assessing the completeness of the search. Thousands of distinct potential energy minima, with a surprising variety of structural arrangements, are identified for sexithiophene. Despite the large number of competing minima, the system presents a small number of deep minima, with very different structures and not particularly congested in energy or density. The two deepest minima correspond to the structures of the two known experimental polymorphs, which are satisfactorily described.

We present the temperature evolution of polarised Raman and IR reflectivity spectra of Tetrathiafulvalene–Chloranil (TTF–CA) in the lattice phonon region (10–200 cm −1 ). Quasi Harmonic Lattice Dynamics (QHLD) calculations prove to be essential in discussing the assignment of the phonons and their evolution across the temperature induced neutral–ionic phase transition.

We report a high-pressure optical study of pentacene by Raman, absorption, and reflectance spectroscopy. Two modifications, denoted C and H, were used as starting material. Raman spectra indicate a sluggish phase transition of the polymorph C to the H-phase with a very low onset pressure of 0.2 GPa. The H-phase remains stable up to at least 11 GPa. The vibrational properties of the H phase in this pressure range were modeled by lattice dynamics calculations. The optical absorption edge of pentacene exhibits a initial red shift of about —80meV/GPa. At pressures near 10 GPa the ππ* reflection bands disappears gradually, indicating a loss of aromaticity due to the formation of saturated C—C bonds.

We have recently studied two solid phases of bulk pentacene (polymorphs H and C) by means of lattice phonon Raman spectroscopy. The assignment, previously based on lattice dynamics calculations alone, is now verified by X-ray diffraction measurements, conclusively confirming the existence of both polymorphs. Furthermore, Raman phonon spectra indicate a pressure-induced phase transition where the polymorph C (lower density phase) transforms to the H form (higher density phase). The onset pressure for the phase transition is only 0.2 GPa. The phase change is irreversible.

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.