Excited-State Behavior of <i>ortho</i>-Phenylenes

The excited-state properties of unsubstituted ortho-phenylene oligomers have been studied using TD-DFT. Calculations of vertical transitions at the helical ground-state geometries are in good qualitative agreement with the experimental UV–vis spectra. In the excited state, the spring-like compounds compress; for the longer oligomers, this compression is localized at one end of the oligomer. This behavior explains the unusual experimentally observed hypsochromic shifts in fluorescence spectra with increasing oligomer length

Folding of <i>ortho</i>-Phenylenes

ConspectusIn 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 <i>foldamers</i>: 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 <i>ortho</i>-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 <i>o</i>-phenylenes showed that they reliably fold into helices in solution (and in the solid state). This conformational behavior is of fundamental interest: <i>o</i>-Arylene and <i>o</i>-heteroarylene structures are found embedded within many other systems, part of an emerging interest in sterically congested polyphenylenes. Further, <i>o</i>-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 <i>o</i>-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 ¹H 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 <i>o</i>-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 <i>o</i>-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 <i>o</i>-phenylenes attractive structural motifs for incorporation into more-complex architectures, an important part of the next phase of foldamer research

Folding of <i>ortho</i>-Phenylenes

ConspectusIn 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 <i>foldamers</i>: 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 <i>ortho</i>-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 <i>o</i>-phenylenes showed that they reliably fold into helices in solution (and in the solid state). This conformational behavior is of fundamental interest: <i>o</i>-Arylene and <i>o</i>-heteroarylene structures are found embedded within many other systems, part of an emerging interest in sterically congested polyphenylenes. Further, <i>o</i>-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 <i>o</i>-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 ¹H 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 <i>o</i>-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 <i>o</i>-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 <i>o</i>-phenylenes attractive structural motifs for incorporation into more-complex architectures, an important part of the next phase of foldamer research

Conformational Analysis of <i>o</i>-Phenylenes: Helical Oligomers with Frayed Ends

The o-phenylenes represent a fundamental class of conjugated polymers that, unlike the isomeric p-phenylenes, should exhibit rich conformational behavior. Recently, we reported the synthesis and characterization of a series of o-phenylene oligomers featuring unusual electronic properties, including surprisingly long-range delocalization as measured by UV−vis spectroscopy and hypsochromic shifts in fluorescence maxima with increasing length. To rationalize these properties, we hypothesized that the oligomers predominantly assume a stacked helical conformation in solution. This assertion, however, was supported by only indirect evidence. Here we present a thorough investigation of the conformational behavior of this series of o-phenylenes by dynamic NMR spectroscopy and computational chemistry. EXSY experiments, in combination with other two-dimensional NMR techniques, provided full 1H chemical shift assignments for at least the two most prevalent conformers for each member of the series (hexamer to dodecamer). GIAO density functional theory calculations were then used to relate the NMR data to specific molecular geometries. We have found that the o-phenylenes do indeed assume stacked helical conformations with disorder occurring at the ends. Thus, the o-phenylene motif appears to have great potential as a means to organize arenes into predictable three-dimensional arrangements. Our results also illustrate the power of 1H NMR GIAO predictions in the solution-phase conformational analysis of oligomers, particularly those with a high density of aromatic subunits

A Push–Pull Macrocycle With Both Linearly Conjugated and Cross-Conjugated Bridges

A series of shape-persistent macrocycles featuring both m-phenylene and 2,5-thiophene linkers has been synthesized, including an example where they bridge electron-rich (veratrole) and electron-poor (phthalimide) units. Charge transfer in this “push–pull macrocycle” has been investigated by UV–vis and fluorescence spectroscopies and DFT calculations. The effect of pairing structurally distinct conjugated bridges is discussed in the context of acyclic and symmetrical macrocyclic analogs

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Programmed Dynamic Covalent Assembly of Unsymmetrical Macrocycles

Unsymmetrical shape-persistent macrocycles have been prepared from diphenylacetylene monomers using imine formation and metathesis. A sequence-directed approach, in which each monomer is uniquely labeled by its N-donor/C-donor sequence, has been used to control the self-assembly and makes possible an added level of complexity in the final structures. To illustrate the potential of this strategy, a series of macrocycles with different side-chain substitution patterns have been prepared, including monofunctionalized and Janus-type structures. We believe this to be the first example of sequence control of dynamic covalent self-assembly and that it will enable the fully covalent synthesis of more complex nanostructures

Parent <i>o</i>-Phenylene Oligomers: Synthesis, Conformational Behavior, and Characterization

The o-phenylenes are an unusual class of conjugated polymer, defined largely by substantial steric twisting along their backbones. Consequently, they exhibit limited conjugation but also interesting conformational behavior: they have been shown to adopt well-defined helical secondary structures, both in the solid state and in solution. While several examples of functionalized o-phenylene oligomers have been reported, most of the basic properties of the parent compounds are unknown. Here we report the synthesis and characterization of the series of unsubstituted o-phenylene oligomers up to the octamer. Through a combination of NMR spectroscopy, including dynamic NMR (EXSY), and computational chemistry, we have found that these compounds adopt compact helical conformations in solution with three repeat units per turn. Although formally conjugated, the oligomers have a very short effective conjugation length of necl ≈ 4 (based on UV–vis spectra), significantly shorter than most other conjugated systems. Also, unlike other (substituted) o-phenylenes, no hypochromicity is observed in their UV–vis spectra. The fluorescence spectra of the series exhibit a systematic blue shift with increasing length. We believe this unusual property results from increased steric congestion in the longer oligomers, which are therefore less able to accommodate structural relaxation in the excited state

A Push–Pull Macrocycle With Both Linearly Conjugated and Cross-Conjugated Bridges

A series of shape-persistent macrocycles featuring both <i>m</i>-phenylene and 2,5-thiophene linkers has been synthesized, including an example where they bridge electron-rich (veratrole) and electron-poor (phthalimide) units. Charge transfer in this “push–pull macrocycle” has been investigated by UV–vis and fluorescence spectroscopies and DFT calculations. The effect of pairing structurally distinct conjugated bridges is discussed in the context of acyclic and symmetrical macrocyclic analogs

Visualizing Molecular Chirality in the Organic Chemistry Laboratory Using Cholesteric Liquid Crystals

Although stereochemistry is an important topic in second-year undergraduate organic chemistry, there are limited options for laboratory activities that allow direct visualization of macroscopic chiral phenomena. A novel, guided-inquiry experiment was developed that allows students to explore chirality in the context of cholesteric liquid crystals. As part of the experiment, which requires no specialized equipment, students visually distinguish two enantiomers. A chiral imine is synthesized in one step from an assigned (but unknown to students) enantiomer of 1-phenylethylamine and then dissolved in a nematic liquid crystal host, inducing a helical structure. The resulting cholesteric liquid crystalline material selectively reflects circularly polarized light with a handedness that depends on the absolute configuration of the starting amine, easily detected using circularly polarizing filters from disposable 3D glasses. Working in teams, students examine the behavior of both dopant enantiomers and the racemic mixture. Analysis of our students’ responses to post-lab questions indicates comprehension of most of the ideas introduced in lab