Exploring the phase behavior of C8-BTBT-C8 at ambient and high temperatures: insights and challenges from molecular dynamics simulations.

C8-BTBT-C8 is one promising candidate for the development of high-performance electronic devices based on thin-film technologies. Its monoclinic polymorph has a well-established role in thin-film growth. Yet, quite little information is available about its dynamics on the molecular scale, and the structures of the mesophases which form at high temperature (about 100 K above ambient temperature). The present study is devoted to the analysis of such phases, with the ultimate goal of developing molecular models. Already at ambient temperature, our molecular dynamics simulations reveal a rich conformational behavior of the alkyl side chains, with gauche conformations as leading structural defects. Heating promotes the formation of a stacking faulted mesophase (380 K), and a smectic phase, at 385 K, upon side chain melting. Although more disordered, this phase bears several analogies with the smectic A phase, experimentally observed at 382.5 K. At higher temperatures, the increase in configurational disorder is brought by molecular diffusion and other phenomena, finally leading to an isotropic molten phase. Our in-depth analysis, complemented by hot-stage polarizing microscopy data, provides interesting insights into this material, highlighting the challenges associated with the modeling of soft semiconducting systems.

Surface Reconstructions in Organic Crystals: Simulations of the Effect of Temperature and Defectivity on Bulk and (001) Surfaces of 2,2':6',2″-Ternaphthalene.

2,2':6',2″-Ternaphthalene (NNN) is a novel, blue-emitting material, suitable for preparation of organic light-emitting diodes. Its crystal structure has been solved recently, but its thermal behavior and surface properties have not yet been explored, partly due to the difficulty in obtaining high quality crystals. In the present study we use classical molecular dynamics to investigate the thermal behavior of bulk and (001) surfaces of NNN. Our bulk simulations indicate the occurrence of a phase transition at about 600 K. The transition is facilitated by the presence of a free (001) surface, since a reconstruction leading to a very similar structure occurs around 550 K in our surface models. This holds for both ideal and defective surface models, containing a small number of vacancies (one or two missing molecules in the outermost layer). In all cases, the process is triggered by thermal motion and involves the reorientation of the molecules with respect to the (001) plane. Both the bulk and surface phases share the monoclinic space group P21/a with a herringbone disposition of molecules. These findings and their implications for the use of NNN in organic electronics are discussed.

Grazing-incidence X-ray diffraction study of rubrene epitaxial thin films.

The growth of organic semiconductors as thin films with good and controlled electrical performances is nowadays one of the main tasks in the field of organic semiconductor-based electronic devices. In particular it is often required to grow highly crystalline and precisely oriented thin films. Here, thanks to grazing-incidence X-ray diffraction measurements carried out at the ELETTRA synchrotron facility, it is shown that rubrene thin films deposited by organic molecular beam epitaxy on the surface of tetracene single crystals have the structure of the known orthorhombic polymorph, with the (2 0 0) plane parallel to the substrate surface. Moreover, the exact epitaxial relationship between the film and the substrate crystalline structures is determined, demonstrating the presence of a unique in-plane orientation of the overlayer.

NIR emitting ytterbium chelates for colourless luminescent solar concentrators.

A new oxyiminopyrazole-based ytterbium chelate enables NIR emission upon UV excitation in colorless single layer luminescent solar concentrators for building integrated photovoltaics.

Electroless silver plating of the surface of organic semiconductors.

The integration of nanoscale processes and devices demands fabrication routes involving rapid, cost-effective steps, preferably carried out under ambient conditions. The realization of the metal/organic semiconductor interface is one of the most demanding steps of device fabrication, since it requires mechanical and/or thermal treatments which increment costs and are often harmful in respect to the active layer. Here, we provide a microscopic analysis of a room temperature, electroless process aimed at the deposition of a nanostructured metallic silver layer with controlled coverage atop the surface of single crystals and thin films of organic semiconductors. This process relies on the reaction of aqueous AgF solutions with the nonwettable crystalline surface of donor-type organic semiconductors. It is observed that the formation of a uniform layer of silver nanoparticles can be accomplished within 20 min contact time. The electrical characterization of two-terminal devices performed before and after the aforementioned treatment shows that the metal deposition process is associated with a redox reaction causing the p-doping of the semiconductor.

Self-assembly of gold nanoparticles on functional organic molecular crystals.

The utilization of metal nanoparticles (NPs) to fabricate metal electrodes under mild conditions is one of the most studied topic in recent years. In this work, colloidal Au NPs were deposited on two isostructural molecular crystals, namely 1,2,3,4-tetrafluoro-7-thiomethyl-acridine (MeSAcr) and 1,2,3,4-tetrafluoro-7-methoxy-acridine (MeOAcr), exposing S atoms and O atoms, respectively, at their largest crystal faces. The depositions were carried out mainly by drop casting under ambient conditions, increasing the contact time from 1 to 120 min, and the samples were then analyzed by atomic force microscopy (AFM) to evaluate the coverage. Thanks to the affinity between S and Au atoms, Au NPs are observed to adhere on the MeSAcr surface within 1-min contact time, whereas at least 1h is required to find NPs on the MeOAcr surface. NP adsorption is also affected by the substrate surface morphology; indeed, step edges represent preferential adsorption sites even in the absence of Au-S interaction. Experiments under different conditions were performed to maximize the coverage on MeSAcr, reaching values up to 13%. AFM equipped with fluid cell was also employed to simultaneously depositing and imaging NPs, achieving a better understanding of the adsorption mechanism.

Organic-organic epitaxy of incommensurate systems: quaterthiophene on potassium hydrogen phthalate single crystals.

Hot-wall epitaxy and molecular-beam epitaxy have been employed for growing quaterthiophene thin films on the (010) cleavage face of potassium hydrogen phthalate, and the results are compared in terms of film properties and growth mode. Even if there is no geometrical match between substrate and overlayer lattices, these films are epitaxially oriented. To investigate the physical rationale for this strong orientation effect, optical microscopy, atomic force microscopy, and X-ray diffraction are employed. A clear correlation between the morphology of the thin films and the crystallographic orientation is found. The results are also validated by surface potential calculations, which demonstrate the primary role played by the corrugation of the substrate surface.

Role of desorption in the growth process of molecular organic thin films.

Growth studies of ultrahigh vacuum deposited thin films are often carried out ex situ, assuming the total film mass reached at the end of the deposition is preserved in the subsequent stages of film preparation. Many kinetic models commonly adopted to analyze quantitatively the mechanism of growth take into account the role of the deposition rate of molecules on the substrate surface, their diffusion, and their possible desorption. Within this framework, a strong simplification (and approximation) of the model is achieved when considering a regime of complete condensation (i.e., neglecting the possibility of re-evaporation of the deposited molecules, both during the deposition and the postdeposition stages of growth). Here, we demonstrate that, for molecular materials of relatively small organic molecules physisorbed on inert surfaces, this phenomenon may strongly affect not only the surface dynamics during deposition but also the postdeposition stage of thin film preparation. Some examples showing clearly its effects on the surface of single crystals and the thin film phase are reported and discussed. Finally, a quantitative description of desorption is provided by comparing the prediction of thermodynamics for the quaterthiophene/silica system with the experimental observation of the growth dynamics of the film and the results of approximate kinetic models. The thermodynamic model employs the surface free energies of a quaterthiophene crystal, which are evaluated by molecular simulation using a newly developed force field.

[Re5 (µ-H)4 (CO)20 ]- and [Re5 (µ-H)5 (CO)20 ], Two Isolobal Analogues of Cyclopentane.

The first five-membered rings of metal atoms connected by M-M or M-H-M bonds only have been obtained by a Re2 +Re3 condensation in which a polyhydride acts as a bridging bidentate ligand toward a coordinatively unsaturated fragment (see scheme below). In spite of the octahedral coordination of the Re centers, the Re5 rings display conformations (twisted and envelope) comparable with those observed for organic five-membered rings of tetrahedral carbon atoms.

Mixed Ruthenium-Rhodium Carbonyl Cluster Complexes. Synthesis of the Anions [Ru(3)Rh(CO)(13)](-) and [RuRh(3)(CO)(12)](-) and Crystal Structures of Their [N(PPh(3))(2)](+) Salts(1).

The new anion [Ru(3)Rh(CO)(13)](-) (1) has been obtained by reaction of [Rh(CO)(4)](-) ([N(PPh(3))(2)](+) or [PPh(4)](+) salt) with Ru(3)(CO)(12); [RuRh(3)(CO)(12)](-) (2) has been derived by oxidative degradation of [RuRh(4)(CO)(12)](2-). Their salts, [N(PPh(3))(2)][Ru(3)Rh(CO)(13)] (I) and [N(PPh(3))(2)][RuRh(3)(CO)(12)] (II), were characterized by single-crystal X-ray diffraction. Data for I: space group P&onemacr;, a = 9.827(2) Å, b =14.911(2) Å, c = 18.735(3) Å, alpha = 110.53(1) degrees, beta = 99.79(2) degrees, gamma = 91.85(2) degrees, R1 = 0.0342, wR2 = 0.0926 for 7257 independent reflections with I > 3sigma(I). Data for II: space group P2(1)/c, a = 14.746(3) Å, b = 21.395(4) Å, c = 16.140(4) Å, beta = 102.95(2) degrees, R1 = 0.0582, wR2 = 0.1383 for 4381 independent reflections with I > 3sigma(I). Both anions have a tetrahedral metal frame; 1 has an idealized C(3) symmetry, with the Rh atom and its unique terminal carbonyl on the 3-fold axis; nine more carbonyls are terminally bonded to the three Ru atoms, while three bridge the Ru-Rh edges. In 2, which has an idealized C(s)() symmetry, three carbonyls are terminally bound on the Ru atom, and one, one, and two CO, respectively, on Rh1, Rh2, and Rh3; five more CO bridge all edges but the Ru-Rh3 edge. It is worthy of note that subtle details of the CO ligands stereochemistry allowed the correct labeling of metal centers, otherwise indistinguishable on the basis of good quality X-ray diffraction data only. Compound 1 reacts with PPh(3), yielding the rhodium-substituted [Ru(3)Rh(CO)(12)(PPh(3))](-) (1a) ((31)P NMR; 56.9 ppm (d), J(P-)(Rh) = 188 Hz). (13)C NMR spectrum of 1 is a doublet (207 ppm, J(C-Rh) = 17.5 Hz) consistently with a fluxional behavior with complete CO scrambling, from 295 down to 170 K.

Syntheses, Structures, and Reactivity of Polynuclear Pyrazolato Copper(I) Complexes, Including an ab-Initio XRPD Study of [Cu(dmnpz)](3) (Hdmnpz = 3,5-Dimethyl-4-nitropyrazole).

The reaction of [Cu(CH(3)CN)(4)](BF(4)) with 3,5-dimethyl-4-nitropyrazole (Hdmnpz) in the presence of triethylamine yields the new copper(I) complexes [Cu(dmnpz)](3) (1) and (Et(3)NH)(2)[Cu(4)(dmnpz)(6)] (2), depending on the experimental conditions. The reactivity of 1 and 2 toward neutral ligands such as triphenylphosphine, cyclohexyl isocyanide (RNC), and carbon monoxide has been investigated. In particular, both complexes readily react with RNC, giving the dinuclear complexes [Cu(dmnpz)(RNC)](2) (4) and [Cu(dmnpz)(RNC)(2)](2) (5), depending on the copper/RNC ratio, and with PPh(3), affording the dimeric derivative [Cu(dmnpz)(PPh(3))](2) (6). The crystal and molecular structure of 1 has been determined ab initio using X-ray powder diffraction data from conventional laboratory equipment. Crystals of 1 are monoclinic, C2/c, a = 20.057(2) Å, b = 13.816(2) Å, c = 7.883(1) Å, beta = 95.912(4) degrees; R(F) and R(wp) 0.067 and 0.039, respectively, for Rietveld refinement on 3900 data points collected in the range 17 < 2theta < 95 degrees (Cu Kalpha radiation). Crystals of 1 contain planar trimers, with the copper atoms bridged by exo-bidentate ligands and short intermolecular Cu.Cu contacts (3.329(7) Å). For complexes 2 and 4-6, single-crystal X-ray diffraction studies have been performed. Crystals of 2 are monoclinic, P2(1)/c, a = 11.026(1) Å, b = 13.456(2) Å, c = 18.668(5) Å, beta = 92.91(2) degrees, Z = 2. The ionic packing of 2 contains tetranuclear complexes, with the copper atoms connected by six dmnpz ligands. Crystals of 4 are triclinic, P&onemacr;, a = 7.418(1) Å, b = 9.780(1) Å, c = 11.177(3) Å, alpha = 109.61(2); beta = 101.34(3) degrees, gamma = 105.82(1) degrees, Z = 2. Crystals of 5 are triclinic, P&onemacr;, a = 9.506(4) Å, b = 9.957(2) Å, c = 11.658(4) Å, alpha = 86.73(2) degrees, beta = 79.54(2) degrees, gamma = 82.47(4) degrees, Z = 1. Crystals of 6 are triclinic, P&onemacr;, a = 11.379(2) Å, b = 13.592(2) Å, c = 14.409(3) Å, alpha = 81.22(1) degrees, beta = 85.21(1) degrees, gamma = 87.55(1) degrees, Z = 2. 4, 5, and 6 are binuclear complexes bearing one RNC, two RNC, and one triphenylphosphine ligands on each copper atom, respectively. The steric requirement of the dmnpz ligands, in the presence of RNC or PPh(3), forces the inner [Cu(2)(dmnpz)(2)] core of the three complexes into markedly different conformations.

Silver(I) Pyrazolates. Synthesis and X-ray and (31)P-NMR Characterization of Triphenylphosphine Complexes and Their Reactivity toward Heterocumulenes.

Silver(I) pyrazolate was reacted with triphenylphosphine, leading to the dinuclear [Ag(2)(pz)(2)(PPh(3))(2)] and [Ag(2)(pz)(2) (PPh(3))(3)] complexes (Hpz = pyrazole), which were extensively characterized by (31)P-NMR methods and single-crystal X-ray analyses. Crystals of [Ag(2)(pz)(2)(PPh(3))(2)] are triclinic, space group P&onemacr;, with a = 9.567(2) Å, b = 11.440(2) Å, c = 10.073(2) Å, alpha = 93.59(2) degrees, beta = 64.01(2) degrees, gamma = 107.24(2) degrees, Z = 1, and D(calc) = 1.539 g.cm(-)(3); final R = 0.028 for 2532 independent reflections having F > 4sigma(F). They contain centrosymmetric dimers with trigonally coordinated silver atoms, which are 3.870(1) Å apart. Crystals of [Ag(2)(pz)(2) (PPh(3))(3)] are triclinic, space group P&onemacr;, with a = 9.752(2) Å, b = 14.163(2) Å, c = 20.450(2) Å, alpha = 101.84(1) degrees, beta = 99.83(2) degrees, gamma = 100.68(2) degrees, Z = 2, and D(calc) = 1.424 g.cm(-)(3); final R = 0.025 for 7075 independent reflections having F > 4sigma(F). Each molecule contains two inequivalent silver ions, bearing one and two phosphines, respectively, and a Ag(&mgr;-pz)(2)Ag ring with a rare twisted conformation. Both complexes readily react with heterocumulenes such as CS(2), COS, CO(2), and RNCO, affording new derivatives, the nature of which is dependent on the experimental conditions. Single-crystal X-ray analysis of [Ag(pz-CS(2))(PPh(3))(2)] has shown the unexpected S,S coordination of the pyrazolecarbodithioate fragment. Its crystals are triclinic, space group P&onemacr;, with a = 10.329(4) Å, b = 13.082(2) Å, c = 14.284(3) Å, alpha = 86.71(2) degrees, beta = 75.14(2) degrees, gamma = 74.79(2) degrees, Z = 2, and D(calc) = 1.431 g.cm(-)(3); final R = 0.028 for 5179 independent reflections having F > 4sigma(F).