Margarita Romero

Anisochronous Dynamics in a Crystalline Array of Steroidal Molecular Rotors: Evidence of Correlated Motion within 1D Helical Domains

We describe the solid-state dynamics of a molecular rotator (2) consisting of a p-phenylene rotor flanked by two ethynyl steroidal moieties that act as a stator. Single-crystal X-ray diffraction analysis of polymorph I revealed a packing motif containing 1D columns of nested rotors arranged in helical arrays (space group P3(2)) with the central phenylenes disordered over two sites related by an 85° rotation about their 1,4-axes. Unexpected line shapes in quadrupolar-echo (2)H NMR measurements between 155 and 296 K for the same polymorph with a deuterated phenylene isotopologue (2-d(4)) were simulated by trajectories involving fast (>10(8) s(-1)) 180° rotation (twofold flips) in each of the two rotationally disordered sites and slower exchange (2 × 10(4) to 1.5 × 10(6) s(-1)) between them. A negative activation entropy and a low enthalpic barrier for the slower 85° exchange are interpreted in terms of highly correlated processes within the 1D helical domains.

Synthesis and solid state characterization of molecular rotors with steroidal stators: ethisterone and norethisterone

In this article we describe the synthesis and dynamic behavior of two new molecular rotors with 1,4-diethynylphenylene rotators axially linked to two conformationally rigid steroidal norethisterone acetate or ethisterone frames. The resulting 1,4-bis(19-nor-17alpha-ethynyltestosterone-17beta-acetate)benzene (1) and 1,4-bis(17alpha-ethynyltestosterone)benzene (2) were fully characterized in solution and in the solid state, and the rotational dynamics of the central phenylene were explored with the help of (13)C NMR with cross polarization and magic angle spinning (CPMAS), and with quadrupolar echo variable temperature (VT) (2)H NMR in the case of 1. Splitting of signals from the aromatic ring on the (13)C CPMAS NMR and a broad quadrupolar spin echo (2)H spectrum of polycrystalline samples indicated that the rotation of the central aromatic ring in these compounds was limited at ambient temperature in the solid state. Variable temperature (2)H NMR experiments at 350 K in the case of 1-d(4) suggested a 2-fold rotational exchange with upper frequency limit of ca. 10 kHz. Single crystal X-ray analysis of this compound revealed that a crowded environment around the prospective phenylene rotator is responsible of the restricted rotation in the solid state.

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.

Synthesis and Characterization of Novel Amphiphilic Azo-Polymers Bearing Well-Defined Oligo(Ethylene Glycol) Spacers

Four novel azo-polymers bearing oligo(ethylene glycol) spacers were synthesized and characterized. Monomers were prepared by esterification of the RED-PEG dyes previously synthesized by us: N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(5-hydroxy-3-oxapentas-1-yl)amine (RED-PEG-2), N-methylN-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(8-hydroxy-3,6-dioxaoctas-1-yl)amine (RED-PEG-3), N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(11-hydroxy-3,6,9-trioxaundecas-1-yl)amine (RED-PEG-4) and N-methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(17-hydroxy-3,6,9,12,15-pentaoxaheptadecas-1-yl)amine (RED-PEG-6) with methacryloyl chloride. The obtained monomers (nPEGMAN series) were polymerized in the presence of AIBN in THF to give the corresponding azo-polymers (pnPEGMAN series). Also, Langmuir films were prepared with these polymers; the obtained monolayers exhibited good reversible behavior upon successive compression/expansion cycles.

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.