John D. Tovar

One-Dimensional Optoelectronic Nanostructures Derived from the Aqueous Self-Assembly of π-Conjugated Oligopeptides

The aqueous self-assembly of oligopeptide-flanked pi-conjugated molecules into discrete one-dimensional nanostructures is described. Unique to these molecules is the fact that the pi-conjugated unit has been directly embedded within the peptide backbone by way of a synthetic amino acid with pi-functionality that is compatible with standard Fmoc-based peptide synthesis. The peptide-based molecular design enforces intimate pi-pi communication within the aggregate after charge-screening and self-assembly, making these nanostructures attractive for optical or electronic applications in biological environments. The synthesis and assembly are reported along with spectroscopic and morphological characterization of the new nanomaterials.

Synthesis and Alignment of Discrete Polydiacetylene-Peptide Nanostructures

Oligopeptides bearing internal diacetylene units are shown to self-assemble in water into one-dimensional nanostructures and aligned macroscopic hydrogels. The diacetylene units can be photopolymerized into polydiacetylenes that run coincident to the nanostructure and noodle long axes, and the resulting nanostructures show evidence for ambipolar charge transport. This self-assembly, alignment and polymerization technique provides a rapid way to produce globally aligned collections of conjugated polymer chains.

Synthesis of Functionalizable Boron-Containing π-Electron Materials that Incorporate Formally Aromatic Fused Borepin Rings†

Boron-containing p-electron systems have emerged as exciting subjects in contemporary organic materials chemistry. The strong Lewis acidity of tricoordinate boron has been utilized for the detection of biologically or environmentally relevant anions. 9] Anionic and neutral boron heteroaromatic molecules are important p-donor ligands for organometallics, and the aromaticity of boroncontaining molecules has inspired substantial experimental and theoretical investigations, thus suggesting that the heteroaromaticity of boron will play will play a key role in other areas where polarization may need to distort p-electron frameworks, such as during operation in electronic devices. The vast majority of boron-based p-electron materials was built with mainchain or lateral 18, 19] tricoordinate boron substitution or from locally antiaromatic four-p-electron fragments such as the borole nucleus within 9-borafluorene. Herein, we present the syntheses and characterization of polycyclic aromatic molecules built around the neutral and formally aromatic six-p-electron borepin ring system that is structurally poised for synthetic elaboration into complex molecular structures. The dibenzo[b,f]borepin (DBB) framework continues to attract substantial attention. 22] Van Tamelen et al. reported the first isolation of DBB as its ethanolamine adduct, and Piers and co-workers very recently reported a B-mesityl DBB that slowly oxidized under ambient conditions. Theoretical studies have revealed planarity throughout the B-H DBB; thus planarity was attractive to us as a means to enhance p-electron delocalization, and nucleus-independent chemical shift (NICS) values reveal a weak degree of aromatic character within the borepin ring of benzo-annelated molecules. However, no synthetic routes for robust and stable tricoordinate borepins that could be tailored into complex p-electron systems have been reported to date. The synthesis of the parent compound DBB 1a is shown in Scheme 1. The stilbene precursor 2a was constructed by a Wittig-type process using reactants derived from a-bromo-o-

Thiophene-Fused Borepins As Directly Functionalizable Boron-Containing π-Electron Systems

Synthetic protocols were developed for the gram-scale preparation of two isomeric dithienoborepins (DTBs), boron-containing polycyclic aromatics featuring the fusion of borepin and thiophene rings. DTBs exhibit reversible cathodic electrochemistry and boron-centered Lewis acidity in addition to enhanced electronic delocalization relative to benzo-fused analogues. Borons precise position within the conjugation pathway of DTBs significantly affected electronic structure, most clearly demonstrated by the variation in spectroscopic responses of each isomer to fluoride ion binding. In addition to excellent stability in the presence of air and moisture, DTBs could also be subjected to electrophilic aromatic substitution and metalation chemistry, the latter enabling the direct, regiospecific functionalization of the unsubstituted thiophene rings. Subsequent tuning of molecular properties was achieved through installation of donor and acceptor π-substituents, leading to compounds featuring multistep electrochemical reductions and polarizable electronic structures. As rare examples of directly functionalizable, π-conjugated, boron-containing polycyclic aromatics, DTBs are promising building blocks for the next generation of organoboron π-electron materials whose development will demand broad scope for molecular diversification in addition to chemical robustness.

Peptide π-Electron Conjugates: Organic Electronics for Biology?

Highly ordered arrays of π-conjugated molecules are often viewed as a prerequisite for effective charge-transporting materials. Studies involving these materials have traditionally focused on organic electronic devices, with more recent emphasis on biological systems. In order to facilitate the transition to biological environments, biomolecules that can promote hierarchical ordering and water solubility are often covalently appended to the π-electron unit. This review highlights recent work on π-conjugated systems bound to peptide moieties that exhibit self-assembly and aims to provide an overview on the development and emerging applications of peptide-based supramolecular π-electron systems.

Conjugated “B-Entacenes”: Polycyclic Aromatics Containing Two Borepin Rings

The synthesis and characterization of functionalized bora-acenes (B-entacenes) where Stille and Sonogashira cross-couplings were used to attach a series of electron-donating and -withdrawing substituents is reported. Photophysical, electrochemical, and computational analyses revealed that the LUMO level can be tuned by changing the para-conjugated substituent. Furthermore, the dimethylamino-functionalized molecule exhibited intense solvatochromism due to the intramolecular charge-transfer interaction.

Functionalized Dibenzoborepins as Components of Small Molecule and Polymeric π-Conjugated Electronic Materials

We present the synthesis and characterization of dibenzo[b,f]borepins (DBBs) functionalized at the para and meta position with respect to the boron center in order to understand how regiochemical issues influence photophysical and electrochemical properties. An expanded synthetic repertoire is presented, using palladium catalysis (to perform Stille, Suzuki, Buchwald-Hartwig, and Sonogashira cross-coupling reactions) and lithium-halogen exchange to synthesize a series of extended π-conjugated DBBs. These chemistries are enabled by the use of a sterically bulky Mes* (2,4,6-tri-tert-butylphenyl) group on boron and the inclusion of reactive bromide handles on the DBB core. Photophysical, electrochemical, and computational analyses of these compounds indicate that relative to the protio-DBB the installation of groups at the meta positions decreases the optical band gap while para substitution raises the electron affinity of the system. Thus, both the HOMO-LUMO gap and specific frontier molecular orbital levels can be tuned by the installation of different conjugated substituents.

Comparative survey of conducting polymers containing benzene, naphthalene, and anthracene cores: interplay of localized aromaticity and polymer electronic structures.

We present a systematic study to understand to what extent the localization of aromaticity in an orthogonal sense to the main polymer conjugation pathway will influence the observed optical and electrical properties as the polymers undergo oxidation and doping into conductive materials. Three classes of electropolymerizable monomers were prepared where the critical electronic unit was chosen to foster different degrees of aromatic localization pendant to the conjugation pathway: specifically, those based upon benzene, naphthalene, and anthracene cores. The expectation was that the benzene unit would foster extensive intramolecular delocalization upon adoption of the quinoidal electronic structure on account of the strong polyene character. On the other hand, resonance contributors can be rationalized for naphthalene and anthracene whereby one or two aromatic sextets evolve within the quinoidal structure thereby leading to a more localized electronic structure. Monomer and polymer electronics were probed with UV-vis spectroscopy and cyclic voltammetry as well as through in situ profiling of the conductive states of the respective polymers. A semiempirical analysis of the frontier orbital wave functions was employed to further understand the influences of competing aromaticity pendant to the polymer backbones. Our findings indicate the potential for complex and tunable pi-conjugated polymers whose properties can be externally controlled through local alterations of aromatic character within units fused or cross-conjugated to polymer main chains.

Supramolecular Polymorphism: Tunable Electronic Interactions within π-Conjugated Peptide Nanostructures Dictated by Primary Amino Acid Sequence

We present a systematic study of the photophysical properties of one-dimensional electronically delocalized nanostructures assembled from π-conjugated subunits embedded within oligopeptide backbones. The nature of the excited states within these nanostructures is studied as a function of primary amino acid sequence utilizing steady-state and time-resolved spectroscopies, and their atomistic structure is probed by molecular simulation. Variations introduced into the amino acid side chains at specific residue locations along the molecular peptide backbone lead to pronounced changes in the observed photophysical behavior of the fibrillar structures (spanning H-like excitonic coupling and disordered excimeric coupling) that arise from subtle changes in the π-stacking within them. These results indicate that residue modification-in terms of relative size, solvation properties, and with respect to the distance from the central π-electron core-enables the ability to tune chromophore packing and the resulting photophysics of supramolecular assemblies of π-conjugated bioelectronic materials in a rational and systematic manner.

Meta-B-entacenes: new polycyclic aromatics incorporating two fused borepin rings

The synthesis of new boron-containing acenes (meta-B-entacenes) is reported. These compounds exhibit slightly non-planar core geometries with blue-shifted spectral properties and more negative electrochemical reduction potentials relative to known para isomers. Polarizable π-extended architectures were realized via cross-coupling procedures with chloro-functionalized precursors.