Self-Assembly in Ultrahigh Vacuum: Growth of Organic Thin Films with a Stable In-Plane Directional Order

Biphasic dose-dependent G0/G1 and G2/M cell cycle arrest by synthetic 2,3-arylpyridylindole derivatives in A549 lung cancer cells

A collection of 2,3-arylpyridylindole derivatives were synthesized via the Larock heteroannulation and evaluated for their in vitro cytotoxic activity against A549 human lung cancer cells. Two derivatives expressed good cytotoxicity with IC 50 values of 1.18±0.25 μM and 0.87±0.10 μM and inhibited tubulin polymerization in vitro , with molecular docking studies suggesting the binding modes of the compounds in the colchicine binding site. Both derivatives have biphasic cell cycle arrest effects depending on their concentrations. At a lower concentration (0.5 μM), the two compounds induced G0/G1 cell cycle arrest by activating the JNK/p53/p21 pathway. At a higher concentration (2.0 μM), the two derivatives arrested the cell cycle at the G2/M phase via Akt signaling and inhibition of tubulin polymerization. Additional cytotoxic mechanisms of the two compounds involved the decreased expression of Bcl-2 and Mcl-1 antiapoptotic proteins through inhibition of the STAT3 and Akt signaling pathways.

Self-assembly of ambidentate pyridyl-carboxylate ligands with octahedral ruthenium metal centers: self-selection for a single-linkage isomer and anticancer-potency studies

The synthesis of six new [2+2] metallarectangles through the coordination-driven self-assembly of octahedral Ru(II)-based acceptors with ambidentate pyridyl-carboxylate donors is described. These molecular rectangles are fully characterized by (1)H NMR spectroscopy, high-resolution electrospray mass spectrometry, and single-crystal X-ray diffraction. In each case, despite the possible formation of multiple isomers, based on the relative orientation of the pyridyl and carboxylate groups (head-to-head versus head-to-tail), evidence for the formation of a single preferred ensemble (head-to-tail) was found in the (1)H NMR spectra. Furthermore, the cytotoxicities of all of the rectangles were established against A549 (lung), AGS (gastric), HCT-15 (colon), and SK hep 1 (liver) human cancer cell lines. The cytotoxicities of rectangles that contained the 5,8-dihydroxy-1,4-naphthaquinonato bridging moiety between the Ru centers (9-11) were particularly high against AGS cancer cells, with IC50 values that were comparable to that of reference drug cisplatin.

Self-assembled trimer structures highlight the competitive roles of intermolecular and adsorbate-substrate interactions: PVBA trimer on Pd(111)

We observed the orientation of 4-trans-2-(pyrid-4-yl-vinyl)benzoic acid (PVBA) trimers on Pd(111) using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV). The image showed three different types of trimers, one of which does not follow predicted dimer orientations. This type of trimer displays 10° rotations of each molecule in clockwise or counterclockwise directions. Calculations of adsorbate-substrate energy and hydrogen bonding energy revealed that the rotations are a result of competition between adsorbate-adsorbate and adsorbate-substrate interactions.

Self-assembly from the gas-phase: design and implementation of small-molecule chromophore precursors with large nonlinear optical responses

Efficiently organizing molecular nonlinear optical (NLO) chromophores having large first-order hyperpolarizabilities (beta) into acentric microstructures for electro-optic (EO) applications represents a significant materials synthesis and processing challenge, in part due to interchromophore dipolar interactions that promote centrosymmetric organization. Here we report the computational modeling, synthesis, and characterization of a series of eight heteroaromatic organic chromophores, designed to self-organize from the vapor phase via directed hydrogen-bond networks, into acentric thin films. Introduction of alpha,omega-donor-acceptor hydrogen-bonding substituents along the molecular long axes tunes properties such as hyperpolarizability, volatility, thermal stability, film-forming properties, and macroscopic NLO response (chi((2))). DFT-level molecular modeling, INDO/S optical property analysis, and sum-overstates computation indicate that molecular-core fluorination and hydrogen-bond donor incorporation can increase beta(vec) up to 40x versus that of typical fluorine-free chromophores. Furthermore, inclusion of sterically induced biphenyl conjugative decoupling between chromophore pi-donor substituents and the hydrogen-bonding donor sites increases beta by approximately 50%. Experimental thin-film second harmonic generation (SHG) spectroscopy confirms these trends in calculated responses, with chi((2)) increasing 7.5x upon chromophore core fluorination and 15x with hydrogen-bonding donor substitution, thereby achieving macroscopic responses as high as 302 pm/V at omega(o) = 1064 nm. In addition to response trends, cluster calculations also reveal linear additivity in beta(vec) with catenation for all benzoic acid-containing chromophores up to longitudinally aligned trimers. Linear scaling of SHG response with film thickness is observed for benzoic acid-containing chromophores up to 1.0 microm film thickness.

Antiparallel-aligned neutral-ground-state and zwitterionic chromophores as a nonlinear optical material

Efficient noncentrosymmetric arrangement of nonlinear optical (NLO) chromophores with high first-order hyperpolarizability (beta) for increased electro-optical (EO) efficiency has proven challenging as strong dipolar interactions between the chromophores encourage antiparallel alignment, attenuating the macroscopic EO effect. This work explores a novel approach to simultaneously achieve large beta values while providing an adjustable dipole moment by linking a strong neutral-ground-state (NGS) NLO chromophore with positive beta to a zwitterionic (ZWI) chromophore with negative beta in an antiparallel fashion. It is proposed that the overall beta of such a structure will be the sum of the absolute values of the two types of chromophores while the dipole moment will be the difference. Molecules 1-3 were synthesized to test the feasibility of this approach. Molecular dynamics calculations and NMR data supported that the NGS chromophore component and the ZWI chromophore component self-assemble to an antiparallel conformation in chloroform. Calculations showed that the dipole moment of 1 is close to the difference of the two component chromophores. Hyper-Rayleigh scattering (HRS) studies confirmed that the first hyperpolarizability of 1 is close to the sum of the two component chromophores. These results support the idea that an antiparallel-aligned neutral-ground-state chromophore and a zwitterionic chromophore can simultaneously achieve an increase in beta and a decrease of the dipole moment.

Polyelectrolyte templating strategy for the fabrication of multilayer hemicyanine Langmuir-Blodgett films showing enhanced and stable second harmonic generation

Polyelectrolyte templating effectively suppresses the aggregation of cationic hemicyanine-based amphiphiles in monolayer Langmuir-Blodgett (LB) films leading to enhanced and stable optical second harmonic generation (SHG). The current study explores the impact of different polyelectrolytes (salts of poly(4-styrenesulfonic acid), deoxyribonucleic acid, and carboxymethylcellulose) on the mode of formation of multilayer LB films of the hemicyanine amphiphile and their SHG response. Pressure-area isotherms and Brewster angle microscopy reveal the impact of the polyelectrolyte complexation on the Langmuir films. Transfer ratios observed during film deposition, supported by electronic absorption spectra and atomic force microscope images of the multilayer LB films, suggest that the polyanions influence the deposition sequence, leading to significant variations in the SHG. Carboxymethylcellulose is identified as an optimal template that induces favorable z-type deposition, leading to the formation of stable multilayer films. These films exhibit the expected quadratic increase of SHG with the extent of deposition; significantly the film response is very stable under extended laser irradiation. It is proposed that structural adjustments of the sandwiched polymer layer lead to the observed deposition sequence and film stability. Polyelectrolyte templating is demonstrated to be a simple and effective strategy for the fabrication of multilayer LB films to elicit efficient quadratic nonlinear optical response.

Chiral recognition of PVBA on Pd(111) and Ag(111) surfaces

An asymmetric planar molecule, 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA), has been used to establish the organic chiral recognition on fcc(111) metal surfaces. The strong correlation between the orientation and chiral recognition of PVBA on both Ag(111) and Pd(111) guides the choice of a model potential, which determines the relative binding energy of PVBA on fcc(111). An angle-dependent calculation of relative binding energy reproduces the experimental observation of the chiral recognition of PVBA on Ag(111) but not on Pd(111).

Ambidentate Ligands Capable of Variable Bond Angles in the Coordination-Driven Self-Assembly of Discrete Pt Macrocycles

Flexible, ambidentate pyridyl-carboxylate based donor ligands such as sodium 3-(3-pyridyl)benzoate, sodium 4-(3-pyridyl)benzoate, and potassium 4-(3-pyridyl)ethynylbenzoate self-assemble into discrete [2 + 2] macrocyclic species instead of infinite networks when combined with a 90 degrees platinum-containing acceptor. In each case, only one isomeric ensemble is selectively formed in high yield. All products are characterized by electrospray ionization mass spectrometry (ESI-MS) and 31P{1H} and 1H NMR spectroscopy. They are the first examples of discrete supramolecules incorporating flexible, ambidentate donor ligands. Despite their potential versatility, these pyridyl-carboxylate donors adjust their bonding directionality to accommodate a rigid platinum acceptor in the formation of one discrete ensemble.

Strategies for Electrooptic Film Fabrication. Influence of Pyrrole−Pyridine-Based Dibranched Chromophore Architecture on Covalent Self-Assembly, Thin-Film Microstructure, and Nonlinear Optical Response

The new dibranched, heterocyclic "push-pull" chromophores bis{1-(pyridin-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}methane (1), 1-(pyrid-4-yl)-2-(N-methyl-5-formylpyrrol-2-yl)ethylene (2), {1-(N-methylpyridinium-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}{(1-(pyridin-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}methane (3), N-methyl-2-[1-(N-methylpyrid-4-yl)ethen-2-yl]-5-[pyrid-4-yl]ethen-2-yl]pyrrole iodide (4), bis{1-(N-methyl-4-pyridinio)-2-[2-(N-methylpyrrol-5-yl)]ethane}methane iodide (5), and N-methyl-2,5-[1-(N-methylpyrid-4-yl)ethen-2-yl]pyrrole iodide (6) have been synthesized and characterized. The neutral (1 and 2) and monomethyl salts (3 and 4) undergo chemisorptive reaction with iodobenzyl-functionalized surfaces to afford chromophore monolayers SA-1/SA-2 and SA-3/SA-4, respectively. Molecular structures and other physicochemical properties have been defined by (1)H NMR, optical spectroscopy, and XRD. Thin-film characterization by a variety of techniques (optical spectroscopy, specular X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and angle-dependent polarized second harmonic generation) underscore the importance of the chromophore molecular architecture as well as film growth method on film microstructure and optical/electrooptic response.

Very large electro-optic responses in H-bonded heteroaromatic films grown by physical vapour deposition

A crucial goal for modern telecommunications systems is development of very high-speed components for broadband (>100 GHz), all-optical signal/information processing. The core component of such technologies is the electro-optic modulator, which encodes electrical signals onto fibre-optic transmissions. A significant challenge therefore is obtaining materials that have large electro-optic responses and that can be readily fabricated into devices at low cost. We report here on the realization of high-response heteroaromatic organic chromophores that can be straightforwardly self-organized from the vapour phase into intrinsically acentric, high-quality, micrometre-scale films. These pi-conjugated electro-optically active films (with second-order susceptibilities up to approximately 100 pm V(-1)) are thermally stable and conveniently grown by a simple physical vapour deposition process in a few hours. Supramolecular acentricity is achieved without electric field poling, enforced by biomimetic heterocycle-hydroxycarbonyl head-to-tail hydrogen-bonding.

Ambidentate Pyridyl-carboxylate Ligands in the Coordination-Driven Self-Assembly of 2D Pt Macrocycles: Self-Selection for a Single Isomer

The coordination-driven self-assembly of discrete 2D macrocyclic species from ambidentate pyridyl-carboxylate-based donor ligands and platinum-containing acceptors is presented. All these species are characterized by electrospray ionization mass spectrometry (ESIMS), multinuclear NMR, and in one example, X-ray crystallography. In each case only one isomeric assembly is selectively formed in high yield, despite the potential for more than one product as a consequence of differences in connectivity.

Stereochemical effects in supramolecular self-assembly at surfaces: 1-D versus 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111)

We present investigations on noncovalent bonding and supramolecular self-assembly of two related molecular building blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic acid (PEBA). These rigid, rodlike molecules comprising the same complementary moieties for hydrogen bond formation are comparable in shape and size. For PVBA, the ethenylene moiety accounts for two-dimensional (2-D) chirality upon confinement to a surface; PEBA is linear and thus 2-D achiral. Molecular films were deposited on a Ag(111) surface by organic molecular beam epitaxy and characterized by scanning tunneling microscopy. At low temperatures (around 150 K), both species form irregular networks of flat lying molecules linked via their endgroups in a diffusion-limited aggregation process. In the absence of kinetic limitations (adsorption or annealing at room temperature), hydrogen-bonded supramolecular assemblies form which are markedly different. With PVBA, enantiomorphic twin chains in two mirror-symmetric species running along a high-symmetry direction of the substrate lattice form by diastereoselective self-assembly of one enantiomer. The chirality signature is strictly correlated between neighboring twin chains. Enantiopure one-dimensional (1-D) supramolecular nanogratings with tunable periodicity evolve at intermediate coverages, reflecting chiral resolution in micrometer domains. In contrast, PEBA assembles in 2-D hydrogen-bonded islands, which are enantiomorphic because of the orientation of the supramolecular arrangements along low-symmetry directions of the substrate. Thus, for PVBA, chiral molecules form 1-D enantiomorphic supramolecular structures because of mesoscopic resolution of a 2-D chiral species, whereas with PEBA, the packing of an achiral species causes 2-D enantiomorphic arrangements. Model simulations of supramolecular ordering provide a deeper understanding of the stability of these systems.