"Concentration-in-Control" self-assembly concept at the liquid-solid interface challenged.

Self-assembled molecular networks (SAMNs) on surfaces evoke a lot of interest, both from a fundamental as well as application point of view. When formed at the liquid-solid interface, precise control over different polymorphs can be achieved by simply adjusting the concentration of molecular building blocks in solution. Significant influence of solute concentration on self-assembly behavior has been observed, whether the self-assembly behavior is controlled by either van der Waals forces or hydrogen bonding interactions. In both cases, high- and low-density supramolecular networks have been observed at high and low solute concentrations, respectively. In contrast to this "concentration-in-control" self-assembly concept here we report an atypical concentration dependent self-assembly behavior at a solution-solid interface. At the interface between heptanoic acid (HA) and highly oriented pyrolytic graphite (HOPG), we show, using scanning tunneling microscopy (STM), the formation of a low-density porous network at high solute concentrations, and a high-density compact network at low solute concentrations. This intriguing inverse concentration dependent self-assembly behavior has been attributed to the preaggregation of solute molecules in the heptanoic acid solution as revealed by UV-vis spectroscopy. The observed results have been correlated to the molecular density of self-assembled monolayers attained at the HA/HOPG interface.

High-Mobility Self-Assembling Truxenone-Based n-Type Organic Semiconductors.

The synthesis, self-assembly, and semiconducting properties of a series of disk-like truxenone derivatives, functionalized with three peripheral long alkyl chains, either directly attached or distanced by linking phenyl or ethynyl groups, are reported. The strategy of distancing the alkyl chains from the central aromatic cores induces in these discotics well-ordered columnar assemblies and has a favorable effect on their charge-carrier mobility. Electron mobility values above 1 cm2 V-1 S-1 were determined for a truxenone functionalized with three peripheral decynyl chains by means of the space charge-limited current technique. DFT calculations help to rationalize the high mobility values found for these new truxenone-based systems, indicating efficient intermolecular electronic couplings (fostered by a favorable stacking configuration) and moderate intramolecular reorganization energies for electrons in the origin of such high mobilities.

Two-dimensional crystal engineering using halogen and hydrogen bonds: towards structural landscapes.

Two-dimensional (2D) crystallization on solid surfaces is governed by a subtle balance of supramolecular and interfacial interactions. However, these subtle interactions often make the prediction of supramolecular structure from the molecular structure impossible. As a consequence, surface-based 2D crystallization has often been studied on a case-by-case basis, which hinders the identification of structure-determining relationships between different self-assembling systems. Here we begin the discussion on such structure-determining relationships by comparing the 2D crystallization of two identical building blocks based on a 1,3,5-tris(pyridine-4-ylethynyl)benzene unit at the solution-solid interface. The concepts of supramolecular synthons and structural landscapes are introduced in the context of 2D crystallization on surfaces to identify common structural elements. The systems are characterized using scanning tunneling microscopy (STM). This strategy involves carrying out minor structural modifications on the parent compound to access supramolecular patterns that are otherwise not obtained. We demonstrate that this chemical perturbation strategy translates equally well for 2D co-crystallization experiments with halogen bond donors yielding porous bi-component networks. The holistic approach described here represents a stepping stone towards gaining predictive power over the 2D crystallization of molecules on solid surfaces.

Solvent-dependent truxene-based nanostructures.

Three truxene derivatives functionalized with alkyl chains, either attached directly or distanced by linking phenyl or ethynyl groups, self-assemble in solution and induce the gelation of different solvents in spite of not being endowed with groups able to establish strong directional interactions. A (1)H NMR study points to face-to-face alternating π-stacked motifs at the origin of nucleation. Solvents play an important role in modulating the aggregation of these derivatives giving rise to fibrous or spherical superstructures. Analysis of the influence of different solvents on the morphology of the aggregates provides a better understanding of the various stages of the hierarchical self-assembly. The way in which alkyl chains are attached to the central core also strongly affects the self-assembling properties and gelation ability of this series. Phenyl spacers present the highest association constants in solution and give rise to gelation in a broader range of solvents. This behavior has been rationalized by means of (1)H NMR spectroscopy, X-ray powder diffraction, SEM, and photophysical measurements. Interestingly, it was found that these compounds in the gel state exhibit unusual emission properties most likely arising from the formation of excimers, which evidences that π-π interactions also occur in the excited state.

Nonlinear optical thin film device from a chiral octopolar phenylacetylene liquid crystal.

A set of chiral discotic phenylacetylenes have been synthesized by 3-fold Sonogashira coupling between different ethynylbenzenes and triiodobenzenes. The resultant bulk materials are fully characterized by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction. The octopolar nature of the target compounds is studied by UV-vis absorption spectroscopy and hyper-Raleigh scattering in solution. Optimization of the donor-acceptor substitution yields both high hyperpolarizability values and appreciable mesomorphic properties. A simple thin film device for second harmonic generation has been prepared from the nitro-substituted liquid crystalline derivative.

Organic Semiconductors toward Electronic Devices: High Mobility and Easy Processability.

Chemists worldwide are making strong efforts toward finding an organic semiconductor with optimum charge-transport stability and processability properties, approaching the problem from several angles. The number and variety of organic semiconductors have expanded exponentially during the past few years, allowing their classification and identification of the advantages and drawbacks of the different candidates, and leading to an increasingly better understanding of factors affecting charge carrier mobility. In this Perspective, we present different types of materials available, classified according to their supramolecular order. As it will be shown, high intermolecular order, although beneficial for charge transport, may compromise processability. The required balance between processing and high mobility can be achieved only through an adequate design at both the molecular and supramolecular levels.

Microwave-assisted synthesis of naphthalenemonoimides and N-desymmetrized naphthalenediimides.

Naphthalenemonoimides and N-desymmetrized naphthalenediimides were synthesized using a stepwise microwave-assisted protocol. The steric and electronic properties of aliphatic amines determined the outcome of the reactions, while in the amino acid series their ability to solubilize the naphthalene dianhydride starting material was crucial. Molecular modeling was used to rationalize the observed selectivity.

Aggregation behavior of a conjugated C3-symmetric molecule: a description based on chiro-optical experimental and theoretical spectroscopies.

A new chiral octopolar C(3)-symmetric molecule is synthesized and its optical and chiro-optical properties are analyzed with the help of density functional theory. In all the cases, the work relates these spectroscopic properties with the phenomena of aggregation. Ultraviolet-visible absorption, infrared and Raman spectroscopies, and ECD and VCD chiro-optical spectroscopies are presented in combination with theoretical chemistry. In this regard, optical spectroscopies are insensitive to the supramolecular effects, while electronic and vibrational circular dichroism spectroscopies delivered adequate proof for the detection of aggregation. We have simulated the effect of molecular self-assembly by means of a docking process that provides a stable dimer on which all the spectroscopic properties have been theoretically calculated. The shape of this dimer provides us with a guideline to propose a helicoidal aggregate that is the responsibility of the chiro-optical spectra amplified by self-assembly.

Steric and electronic situation in the 4-X-4'-[(4''-Y-phenyl)ethynyl]biphenyl homologous series: a joint theoretical and spectroscopic study.

In this work we have studied the rotational barriers, the polarization of the acetylenic triple bond, and the molecular dipole moments in the 4-X-4'-[(4''-Y-phenyl)ethynyl]biphenyl homologous series using the density functional theory (DFT) and 1D/2D NMR spectroscopy. This series of compounds constitutes an effective base for the acquisition of liquid crystals. The equilibrium angle (theta(eq)) and the torsional barriers DeltaE(0 degrees) and DeltaE(90 degrees) are not very sensitive to the substituent effects. We have found evidence for the similarity in the pi-conjugation of the Y-substituted and X,Y-disubstituted compounds, the latter with mesomorphic properties, by means of the graphic analysis of the relationship between the molecular dipolar moment mu(D) and the difference between the (13)C NMR chemical shifts of the acetylenic carbon atoms (Deltadelta(CI[triple bond]CII) [ppm]). The obtained results contribute to a better understanding of the structure-activity relationship for potential liquid crystalline systems.

Self-assembly of C3-symmetrical hexaaryltriindoles driven by solvophobic and CH-pi interactions.

The synthesis and aggregation properties of a series of differently substituted star-shaped hexaaryltriindoles both in solution and in the solid state are being reported. While these molecules do not show any significant intermolecular aggregation in CDCl(3), it has been possible to induce aggregation by increasing the polarity of the solvent and therefore facilitating the occurrence of solvophobic forces. A study of the influence of the electronic character of peripheral substituents on the self-association behavior in solution has shown that increasing the electron-donor character of the substituents facilitates self-association while derivatives substituted with electron-acceptor substituents do not self-assemble. The electronic nature of the substituents also has an influence in the geometry of the stacking of these derivatives observed in the solid state. While unsubstituted hexaphenyl triindole self-assemble in a staggered face-to-face arrangement, attaching six cyano functional groups results in an offset stacking. The influence of the substituents in the strength and geometry of the stacking tendency contrasts with the trend expected for an aggregation induced solely by pi-pi interactions, but can be explained considering an important contribution of multiple cooperative CH-pi interactions.

Synthesis, spectroscopy, nonlinear optics, and theoretical investigations of thienylethynyl octopoles with a tunable core.

We have synthesized three new molecules that have three thienylethynyl arms substituting a central benzene core and different electron donor/acceptor groups in the three remaining phenyl positions. The absorption, fluorescence, phosphorescence, and transient triplet-triplet spectra are analyzed in the light of the electronic structure of the ground and excited states obtained from quantum-chemical calculations. From the above, the relevant photophysical data (including quantum yields, lifetimes, and rate constants) could be derived. It was found that the major deactivation pathway is internal conversion, which competes with the fluorescence and intersystem crossing processes. For the three investigated compounds, we provide convincing theoretical support corroborating these findings and further conclusions based on the theoretical information obtained. These molecules are one of the very few cases in which the depolarization ratios, obtained from the NLO optical measurements, clearly reflect the octopolar configuration. Molecular hyperpolarizabilities have been measured and display a typical dependence on the donor-acceptor substitution pattern.

Alkynyl expanded donor-acceptor calixarenes: geometry and second-order nonlinear optical properties.

A number of wide- and narrow-rimmed functionalized alkynylcalix[4]arenes have been synthesized by Sonogashira coupling. With respect to their optical properties, these donor-acceptor systems are treated as ensembles of covalently linked, electronically independent tolane subchromophores. Linear UV/visible and fluorescence spectroscopic investigations revealed that the charge-transfer character of the electronic transitions in calixarenes, and also the second-order nonlinear optical (NLO) properties depend on the electron-withdrawing nature of the terminal ethynylphenyl substituent (NO(2), CF(3), H). The nitro derivatives display high values of the quadratic hyperpolarizability beta. Not only do the (nonlinear) optical properties of the target compounds depend on the number and relative disposition of the subchromophores, but also on the geometry of the calixarenes. In particular, the opening angle of the calixarene cavity can be determined by the substitution pattern of the calixarene scaffold (wide- versus narrow-rim substitution) and the number of the acetylene functions introduced. Both the NLO properties and the conformational issues are conveniently assessed by using hyper-Rayleigh scattering (HRS) in solution, and supported by X-ray crystallography in the solid state.

Octopolar chromophores based on donor- and acceptor-substituted 1,3,5-tris(phenylethynyl)benzenes: impact of meta-conjugation on the molecular and electronic structure by means of spectroscopy and theory.

The molecular and electronic structures of a series of all-meta-substituted phenylacetylene mesitylenes peripherally substituted with donor or acceptor (D-A) groups are studied. The impact of meta- and para-substitution patterns is also analyzed by employing Raman and optical spectroscopies in conjunction with theoretical calculations. Outer phenyl rings display a partial quinoid character induced by two different motifs: (i) outer phenyls --> triple bond charge transfer for the cases where these phenyls are substituted with electron-donors; (ii) double electron withdrawing effect in the molecules with the peripheral phenyls substituted with electron acceptors. A moderate tuning of the optical gap is observed in agreement with the partial blockade of pi-electron conjugation exerted by the meta disposition. The orbital structure of the compounds partially preserves that of the mesitylene group showing extra-conjugation due to the addition of the arms, so that conjugation is not entirely obstructed but partially impeded in the ground electronic state (i.e., electron occupied orbitals). As for the excited states, the low-lying energy empty orbitals offer better conditions for full conjugation over the whole molecular scaffold. Interesting optical properties such as overlapping centers along the lowest energy optical excitations and enhanced optical transparency with importance for the application of these materials in optoelectronics have been justified on the basis of the electronic structure. A greater degree of quinoidization, and more allowed pi-electron delocalization, over the entire molecule is recognized in the case of linear phenylacetylenes substituting in para positions the central core.

Proton-triggered octopolar NLO chromophores.

C(3)-symmetric, octopolar 1,3,5-alkynylbenzenes with terminal pyridyl substituents are presented which show an increase up to 17-fold in the second-order NLO activity upon protonation while maintaining their octopolar character, as confirmed by the depolarization ratios measured.

Tetraalkynyl calix[4]arenes with advanced NLO properties.

Rigid, highly conjugated tetraalkynyl-calix[4]arenes synthesised via Sonogashira coupling give rise to improved second-order hyperpolarizability values as determined by hyper-Rayleigh scattering--a technique that in addition to X-ray crystallography also allows for the conformational analysis of the calixarene structures in solution.

Tuning octopolar NLO chromophores: synthesis and spectroscopic characterization of persubstituted 1,3,5-tris(ethynylphenyl)benzenes.

The synthesis of a series of octopolar 1,3,5-tris(ethynylphenyl)benzenes via Sonogashira coupling is described, varying the substituents on both the central benzene core as well as the acetylenic periphery. In particular, systems bearing an electron-rich core and an electron-poor periphery are obtained that display advanced optical properties. The linear (by UV-vis and fluorescence spectroscopy) and second-order nonlinear optical properties are studied, the latter by hyper-Rayleigh scattering (HRS) (Hendrickx, E.; Clays, K.; Persoons, A. Acc. Chem. Res. 1998, 31, 675-683). The influence of different core and periphery substitution is revealed by the optical properties and confirms the possibility of fine-tuning those. Because of the presence of (one-photon) fluorescence for all compounds, femtosecond hyper-Rayleigh scattering has been applied (Olbrechts, G.; Strobbe, R.; Clays, K.; Persoons, A. Rev. Sci. Instrum. 1998, 69, 2233-2241). The implementation of the deconvolution in the frequency domain allows for a demodulation and a phase shift between immediate (nonlinear) scattering and time-delayed (multiphoton) fluorescence for high modulation frequencies (Wostyn, K.; Binnemans, K.; Clays, K.; Persoons, A. Rev. Sci. Instrum. 2001, 72, 3215-3220). In accordance with the linear optical properties, the second-order NLO properties can also be tuned by varying the core and peripheric substitutents.

1,3,5-2,4,6-functionalized, facially segregated benzenes-exploitation of sterically predisposed systems in supramolecular chemistry.

In the present article, we introduce the unique steric features of 1,3,5-substituted triethylbenzenes as a design principle for host molecules and supramolecular systems. Due to steric gearing, all three functional substituents are disposed on the same side of the phenyl plane. The use of this benzene scaffold has lead to the synthesis of various receptor molecules for a wide range of different guest species. Furthermore, the facially segregated systems can be used as building blocks with a controlled conformation in the synthesis of macrocycles or self-assembled supramolecular systems.

Novel C3-symmetric molecular scaffolds with potential facial differentiation.

The conversion of 1,3,5-substituted benzene and mesitylene by electrophilic aromatic substitution and Sonogashira cross-coupling, respectively, furnished the C3-symmetric, hexasubstituted benzene derivatives 1 and 2 with an alternating substitution pattern. Based on the molecular scaffolds obtained, the two systems serve as model compounds for novel receptor molecules with distinct geometric features. X-ray structures have been obtained for 1 and 2, which are discussed in regard to their aptitude as receptor platforms or supramolecular building blocks. By looking at the rotational barriers for the functional groups placed around the molecular scaffolds by variable temperature 1H NMR spectroscopy, 1 and 2 turn out to exist in rapidly interconverting conformations. The alignment of these potential binding groups around the molecular scaffolds should be strongly biased by specific interactions with suitable guest molecules.