o-Phenylene Octamers as Surface Modifiers for Homeotropic Columnar Ordering of Discotic Liquid Crystals

Evaluation of 1 H-NMR Spectroscopy-Based Quantification Methods of the Supramolecular Aggregation of a Molecular Photosensitizer

The supramolecular dimerization of a ruthenium polypyridyl precursor of a well-developed family of hydrogen evolving photocatalysts via π-π-interactions of the polyheteroaromatic bridging ligand was quantified with concentration dependent ¹ H-NMR-spectroscopy. The data sets were analyzed with different calculation and fit methods. A comparison between the results of direct calculation, linear and nonlinear approaches showed that the application of a global nonlinear fit procedure yields the best results. The presented methods are also applicable for dimerization processes in solution of other molecular moieties.

Mechanical single-molecule potentiometers with large switching factors from ortho-pentaphenylene foldamers

Molecular potentiometers that can indicate displacement-conductance relationship, and predict and control molecular conductance are of significant importance but rarely developed. Herein, single-molecule potentiometers are designed based on ortho-pentaphenylene. The ortho-pentaphenylene derivatives with anchoring groups adopt multiple folded conformers and undergo conformational interconversion in solutions. Solvent-sensitive multiple conductance originating from different conformers is recorded by scanning tunneling microscopy break junction technique. These pseudo-elastic folded molecules can be stretched and compressed by mechanical force along with a variable conductance by up to two orders of magnitude, providing an impressively higher switching factor (114) than the reported values (ca. 1~25). The multichannel conductance governed by through-space and through-bond conducting pathways is rationalized as the charge transport mechanism for the folded ortho-pentaphenylene derivatives. These findings shed light on exploring robust single-molecule potentiometers based on helical structures, and are conducive to fundamental understanding of charge transport in higher-order helical molecules.

Competing forces in the self-assembly of amide-functionalized discotic mesogens

The effect of incorporating a single amide group on the self-assembly of discotic mesogens was examined. Two series of tetraalkoxydibenzophenazines amides were prepared: tertiary diethyl amides, dEt(n) incapable of hydrogen bonding, and secondary amides HBu(n) that can act as both H-bond donors and acceptors. These compounds exhibit markedly different behavior in solution; NMR studies of dEt(n) show no evidence of self-association, whereas HBu(n) strongly associate via H-bonding and π-stacking. Compounds HBu(n) also act as small molecule gelators in a range of solvents, a property not observed for the corresponding tertiary amides. All compounds were found to form Colh liquid crystal phases; variable temperature XRD experiments indicate that each column has a diameter approximately equal to that of a single molecule. A comparison of the phase behavior of HBu(n) and dEt(n) suggests that the columnar phases of the former are stabilized by hydrogen bonding, likely at the expense of local parallel alignment of these molecules. Single crystal X-ray diffraction studies revealed that dEt(6) adopts an antiparallel arrangement in the solid state, in keeping with previous theories of packing within columnar LC phases. These studies highlight the interplay between competing factors, such as hydrogen bonding, π-stacking and dipole-dipole interactions that affect the stability of the LC phases.

Manipulation of Supramolecular Columnar Structures of H-Bonded Donor-Acceptor Units through Geometrical Nanoconfinement

Ambipolar organic semiconductors are considered promising for organic electronics because of their interesting electric properties. Many hurdles remain yet to be overcome before they can be used for practical applications, especially because their orientation is hard to control. We demonstrate a method to control the orientation of columnar structures based on a hydrogen (H)-bonded donor-acceptor complex between a star-shaped tris(triazolyl)triazine and triphenylene-containing benzoic acid, using physicochemical nanoconfinement. The molecular configuration and supramolecular columnar assemblies in a one-dimensional porous anodic aluminium oxide (AAO) film were dramatically modulated by controlling the pore-size and by chemical modification of the inner surface of the porous AAO film. In situ experiments using grazing-incidence X-ray diffraction (GIXRD) were carried out to investigate the structural evolution produced at the nanometer scale by varying physicochemical conditions. The resulting highly ordered nanostructures may open a new pathway to effectively control the alignment of liquid crystal ambipolar semiconductors.

Electron Transfer across o-Phenylene Wires

Photoinduced electron transfer across rigid rod-like oligo- p-phenylenes has been thoroughly investigated in the past, but their o-connected counterparts are yet entirely unexplored in this regard. We report on three molecular dyads comprised of a triarylamine donor and a Ru(bpy)₃²⁺ (bpy =2,2'-bipyridine) acceptor connected covalently by 2 to 6 o-phenylene units. Pulsed excitation of the Ru(II) sensitizer at 532 nm leads to the rapid formation of oxidized triarylamine and reduced ruthenium complex via intramolecular electron transfer. The subsequent thermal reverse charge-shift reaction to reinstate the electronic ground-state occurs on a time scale of 120-220 ns in deaerated CH₃CN at 25 °C. The conformational flexibility of the o-phenylene bridges causes multiexponential transient absorption kinetics for the photoinduced forward process, but the thermal reverse reaction produces single-exponential transient absorption decays. The key finding is that the flexible o-phenylene bridges permit rapid formation of photoproducts storing ca. 1.7 eV of energy with lifetimes on the order of hundreds of nanoseconds, similar to what is possible with rigid rod-like donor-acceptor compounds. Thus, the conformational flexibility of the o-phenylenes represents no disadvantage with regard to the photoproduct lifetimes, and this is relevant in the greater context of light-to-chemical energy conversion.

Twist sense control in terminally functionalized ortho-phenylenes

Chiral groups induce opposite twist senses of o-phenylene helices depending on their positions in dynamic mixtures.

Many abiotic foldamers are based on achiral repeat units but adopt chiral geometries, especially helices. In these systems, there is no inherent preference for one handedness of the fold; however, it is well-established that the point chirality of substituents can be communicated to the helix. This capability represents a basic level of control over folding that is necessary for applications in molecular recognition and in the assembly of higher-order structures. The ortho-phenylenes are a structurally simple class of aromatic foldamers that fold into helices driven by arene–arene stacking interactions. Although their folding is now reasonably well-understood, access to o-phenylenes enriched in one twist sense has been limited to resolution, yielding conformationally dynamic samples that racemize over the course of minutes to hours. Here, we report a detailed structure–property study of chiral induction from o-phenylene termini using a combination of NMR spectroscopy, CD spectroscopy, and computational chemistry. We uncover mechanistic details of chiral induction and show that the same substituents can give effective twist sense control in opposite directions in mixtures of interconverting conformers; that is, they are “ambidextrous”. This behavior should be general and can be rationalized using a simple model based on sterics, noting that arene–arene stacking is, to a first approximation, unaffected by flipping either partner. We demonstrate control over this mechanism by showing that chiral groups can be chosen such that they both favor one orientation and provide effective chiral induction.

Controlled synthesis of ortho, para-alternating linked polyarenes via catalyst-transfer Suzuki coupling polymerization

A novel class of ortho, para-alternating linked polyarenes is synthesized via catalyst-transfer Suzuki coupling polymerization with Pd₂(dba)₃/t-Bu₃P/p-BrC₆H₄COPh as initiator.

Mobility versus Alignment of a Semiconducting π-Extended Discotic Liquid-Crystalline Triindole

The p-type semiconducting properties of a triphenylene-fused triindole mesogen, have been studied by applying two complementary methods which have different alignment requirements. The attachment of only three flexible alkyl chains to the nitrogen atoms of this π-extended core is sufficient to induce columnar mesomorphism. High hole mobility values (0.65 cm² V^-1 s^-1) have been estimated by space-charge limited current (SCLC) measurements in a diode-like structure which are easily prepared from the melt, rendering this material a good candidate for OPVs and OLEDs devices. The mobility predicted theoretically via a hole-hopping mechanism is in very good agreement with the experimental values determined at the SCLC regime. On the other hand the hole mobility determined on solution processed thin film transistors (OFETs) is significantly lower, which can be rationalized by the high tendency of these large molecules to align on surfaces with their extended π-conjugated core parallel to the substrate as demonstrated by SERS. Despite the differences obtained with the two methods, the acceptable performance found on OFETs fabricated by simple drop-casting processing of such an enlarged aromatic core is remarkable and suggests facile hopping between neighboring molecular columns owing to the large conducting/isolating ratio found in this discotic compound.

Folding of ortho-Phenylenes

In nature, the folding of oligomers and polymers is used to generate complex three-dimensional structures, yielding macromolecules with diverse functions in catalysis, recognition, transport, and charge- and energy-transfer. Over the past 20-30 years, chemists have sought to replicate this strategy by developing new foldamers: oligomers that fold into well-defined secondary structures in solution. A wide array of abiotic foldamers have been developed, ranging from non-natural peptides to aromatics. The ortho-phenylenes represent a recent addition to the family of aromatic foldamers. Despite their structural simplicity (chains of benzenes connected at the ortho positions), it was not until 2010 that systematic studies of o-phenylenes showed that they reliably fold into helices in solution (and in the solid state). This conformational behavior is of fundamental interest: o-Arylene and o-heteroarylene structures are found embedded within many other systems, part of an emerging interest in sterically congested polyphenylenes. Further, o-phenylenes are increasingly straightforward to synthesize because of continuing developments in arene-arene coupling, the Asao-Yamamoto benzannulation, and benzyne polymerization. In this Account, we discuss the folding of o-phenylenes with emphasis on features that make them unique among aromatic foldamers. Interconversion between their different backbone conformers is slow on the NMR time scale around room temperature. The (1)H NMR spectra of oligomers can therefore be deconvoluted to give sets of chemical shifts for different folding states. The chemical shifts are both highly sensitive to conformation and readily predicted using ab initio methods, affording critical information about the conformational distribution. The picture that emerges is that o-phenylenes fold into helices with offset stacking between every third repeat unit. In general, misfolding occurs primarily at the oligomer termini (i.e., "frayed ends"). Because of their structural simplicity, the folding can be described by straightforward models. The overall population can be divided into two enantiomeric pools, with racemization and misfolding as two distinct processes. Examination of substituent effects on folding reveals that the determinant of the relative stability of different conformers is (offset) aromatic stacking interactions parallel to the helical axis. That is, the folding of o-phenylenes is analogous to that of α-helices, with aromatic stacking in place of hydrogen bonding. The folding propensity can be tuned using well-known substituent effects on aromatic stacking, with moderate electron-withdrawing substituents giving nearly perfect folding. The combination of a simple folding mechanism and readily characterized conformational populations makes o-phenylenes attractive structural motifs for incorporation into more-complex architectures, an important part of the next phase of foldamer research.

Scanning tunnelling microscopy analysis of octameric o-phenylenes on Au(111)

STM microscopy allowed direct observation of perfectly- and partially-folded conformers of OP₈Br and OP₈NO₂on Au(111). The metastable partially-folded conformation was stabilized by their more efficient electronic coupling with the Au substrate.

Synthesis of poly(1,10-phenanthroline-5,6-diyl)s having a π-stacked, helical conformation

5,6-Dibromo-1,10-phenanthroline and 2,9-di-n-butyl-5,6-dibromo-1,10-phenanthroline were polymerized using a Ni catalyst to afford helical polymers in which the phenanthroline moieties are densely stacked on top of each other.