Rafael Arcos-Ramos

Molecular materials for switchable nonlinear optics in the solid state, based on ruthenium-nitrosyl complexes

The promising class of (polypyridine-ruthenium)-nitrosyl complexes capable of high yield Ru–NO/Ru–ON isomerization is targeted as a potential molecular device for the achievement of complete NLO switches in the solid state. A computational investigation conducted at the PBE0/6-31+G** DFT level for benchmark systems of general formula [R-terpyridine-RuIICl2(NO)](PF6) (R being a substituent with various donating or withdrawing capabilities) leads to the suggestion that an isomerization could produce a convincing NLO switch (large value of the βON/βOFF ratio) for R substituents of weak donating capabilities. Four new molecules were obtained in order to test the synthetic feasibility of this class of materials with R = 4′-p-bromophenyl, 4′-p-methoxyphenyl, 4′-p-diethylaminophenyl, and 4′-p-nitrophenyl. The different cis-(Cl,Cl) and trans-(Cl,Cl) isomers can be separated by HPLC, and identified by NMR and X-ray crystallographic studies.

Crystalline arrays of molecular rotors with TIPS-trityl and phenolic-trityl stators using phenylene, 1,2-difluorophenylene and pyridine rotators

In this work, we describe the synthesis and solid-state characterization of a series of molecular rotors with tri-isopropylsilyloxy-substituted (TIPS) trityl stators axially linked to 1,4-diethynylphenylene, 3,6-diethynyl-1,2-difluorophenylene and 2,5-diethynylpyridine rotators to produce 1,4-bis[(3,3-diphenyl-3-(3′-(tri-isopropylsilyloxy)-phenyl)-prop-1-yn-1-yl)]benzene (1), 1,4-bis[(3,3-diphenyl-3-(3′-(tri-isopropylsilyloxy)-phenyl)-prop-1-yn-1-yl)]-2,3-difluorobenzene (2) and 2,5-bis[(3,3-diphenyl-3-(3′-tri-isopropylsilyloxy)-phenyl)-prop-1-yn-1-yl)]pyridine (3). The subsequent removal of the TIPS protecting group led to their corresponding hydroxyl-substituted trityl derivatives (4) and (5). TIPS- and HO-substituted stators are involved in different inter- and intramolecular interactions (hydrogen bonding, phenyl embraces, C–H–π interactions) that give rise to isomorphic packing motifs that constrained the rotational dynamics in the solid-state to the slow exchange regime.

Synthesis and Evaluation of Molecular Rotors with Large and Bulky tert-Butyldiphenylsilyloxy-Substituted Trityl Stators

The search for voluminous stators that may accommodate large rotator units and speed rotational dynamics in the solid state led us to investigate a simple and efficient method for the synthesis of molecular rotors with tert-butyldiphenylsilyl-protected (TBDPS) triphenylmethyl stators. Additionally, solid state characterization of these systems with two-, four-, and six-TBDPS groups provided us with a description of their crystallinity and thermal stability. Among them, molecular rotor 7c with the largest and most symmetric stator resulting from six peripheral silyl groups showed the best tendency to crystallize, and the study of its isotopologue 7c-d(4) by solid state (2)H NMR revealed a 2-fold motion of the 1,4-diethynylphenylene-d(4) rotator in the kHz regime.

The amide bridge in donor–acceptor systems: delocalization depends on push–pull stress

Transmission of electronic information through amide bonds may be, under appropriate conditions, effectively achieved. In this work, a family of explicitly designed donor–(amide bridge)–acceptor architectures was synthesized. NMR studies and UV-vis absorption solvatochromism support that cross-conjugation leads to measurable polarization across push–pull, amide-bridged molecules. Computational analysis of structural parameters and frontier molecular orbitals shows the contribution of an additional, dienoid amide canonical structure to intramolecular electron delocalization, as the electron donor–acceptor strength of the substituents increases. Within the context of nonlinear optics and molecular materials, computational comparison between amide-bridged molecules and those containing typical linkers shows that there is a compromise between nonlinear optical response, ease of synthesis and chemical inertness, making the systems studied herein interesting alternatives for such applications.

1H and 13C NMR characteristics of synthetic derivatives of steroid sapogenins Part III. 16β,23:23,26-Diepoxy side chains

The full assignments of the (1)H and (13)C NMR signals of steroids bearing the 16beta,23:23,26-diepoxy side chain are provided. Differentiation of the diasterotopic H-26 pair was achieved with the aid of NOESY experiments. The main substituent and steric effects associated with this moiety and their influence on the chemical shifts of the neighboring atoms are discussed.

Synthesis and characterization of dissymmetric molecular rotors based on 1,4-diethynylphenylene rotators and steroidal/trityl type stators

In this paper, we report the synthesis of dissymmetric molecular rotors with phenylene rotators and steroidal/trityl stators. Using a one-pot methodology, it was possible to prepare dissymmetric molecular rotors in moderate yields as established by standard characterization techniques in solution. In the solid state, the derivatives were highly amorphous, as established by X-ray powder diffraction precluding further studies on the dynamic behavior.Graphical abstract

Synthesis, NMR and crystal characterization of dimeric terephthalates derived from epimeric 4,5-seco-cholest-3-yn-5-ols

Two dimeric steroidal terephthalates derived from epimeric 4,5-seco-cholest-3-yn-5-ols were prepared starting from cholesterol in a five-step synthetic sequence. X-ray crystallography shows that the obtained compounds display novel supramolecular networks in the solid state in which the facial hydrophobicity of the steroidal skeletons plays an important role. Unambiguous NMR characterization of the obtained dimers is also provided.

Engineering organic semiconducting solids. Multicomponent access to crystalline 3-(4-aryl-1,2,3-triazolyl)coumarins

Crystalline 3-(4-aryl-1,2,3-triazol-1-yl)coumarins (ATCs) were prepared from commercial materials using a four-component methodology as a key step. In the present work, a feasible and environmentally friendly route to the title compounds was developed through the reaction between salicylaldehydes, ethyl bromoacetate, phenylacetylenes and sodium azide under mild conditions, with short reaction times and a simple workup. Crystalline solids are readily accessed from the featured products via solution processing and their arrays in the solid state were elucidated through SXRD; these molecules display a periodic overlap of π-systems, which facilitates carrier transport in organic electronic devices. Semiconductor band gaps for the obtained solids were derived through plane-wave DFT and compared with reference systems known to display superior performance in organic electronics. Thus ATCs represent attractive systems for research and implementation in molecular materials, a task which will be facilitated by the concise route herein described.

Crystalline arrays of side chain modified bile acids derivatives. Two novel self-assemblies based on π-π and belly-to-belly interactions.

Crystalline derivatives of side chain modified bile acids were efficiently prepared from the naturally occurring steroids by palladium-catalyzed cross coupling reaction as a key step. The solvent-free crystalline bile acids derivatives 2b-e are readily accessed by slow evaporation from selected solvents. A variety of steroidal scaffolds were found and elucidated by SXRD studies. The crystal packing of the title compounds are dominated by hydrogen-bonding interactions established between differently positioned acetyl protecting groups, which in the case of 2b and 2e take advantage of the facial amphiphilicity producing two novel steroidal supramolecular self-assemblies combining π-π and strong facial interactions. Thus, these crystalline arrays of side chain modified bile acids represent promising scaffolds for research and implementation in biomolecular materials or inclusion phenomena.

Synthesis of steroidal molecular compasses: exploration of the controlled assembly of solid organic materials

In this work, five steroid-flanked derivatives of p-nitroaniline were studied to assess the influence of these rigid, chiral frameworks on the solid-state patterning of a dipolar dye, striving to drive the self-assembly of noncentrosymmetric crystals, which is of interest in the development of organic materials for nonlinear optics and piezoelectricity. The featured compounds were prepared using Sonogashira cross-coupling reaction with 17α-ethynyl steroids as a single key step. The structures of these new steroidal derivatives were established using one and two dimensional NMR 1H and 13C experiments. Single X-ray diffraction analysis unequivocally confirmed the presence of the steroidal molecular compasses 8 and 11, whose potential applicability for organic materials is preliminarily assessed via computational modeling, finding candidates for nonlinear optics, organic electronic materials and molecular recognition.