Rik R. Tykwinski

Synthesis of polyynes to model the sp-carbon allotrope carbyne

Carbyne is an allotrope of carbon composed of sp-hybridized carbon atoms. Although its formation in the laboratory is suggested, no well-defined sample is described. Interest in carbyne and its potential properties remains intense because of, at least in part, technological breakthroughs offered by other carbon allotropes, such as fullerenes, carbon nanotubes and graphene. Here, we describe the synthesis of a series of conjugated polyynes as models for carbyne. The longest of the series consists of 44 contiguous acetylenic carbons, and it maintains a framework clearly composed of alternating single and triple bonds. Spectroscopic analyses for these polyynes reveal a distinct trend towards a finite gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for carbyne, which is estimated to be ∼485 nm (∼2.56 eV). Even the longest members of this series of polyynes are not particularly sensitive to light, moisture or oxygen, and they can be handled and characterized under normal laboratory conditions. The structure of the carbon allotrope ‘carbyne’ is based on a framework of sp-hybridized carbon. To model its properties, a series of conjugated polyynes — the longest of which contains 44 contiguous acetylenic carbons — have been synthesized and their spectroscopic properties investigated.

Singlet fission in pentacene dimers

Significance In the present work, we show compelling evidence for the unprecedented intramolecular singlet fission at room temperature and in dilute solutions within a set of three different regioisomeric pentacene dimers. Pump–probe experiments, which were complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods, corroborate triplet quantum yields as high as 156 ± 5%. To this end, electronic couplings between the two pentacenes in the dimers, by virtue of through-bond or through-space interactions, are decisive in tuning the rates of singlet fission. Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.

Acetylene Chemistry: Chemistry, Biology, and Material Science

SYNTHESIS.Addition of Terminal Acetylides to C=O and C=N Electrophiles.Synthesis of Heterocycles and Carbocycles via Electrophilic Cyclization of Alkynes.Transition Metal Acetylides.ADVANCED MATERIALS-ORIENTED.Semiconducting Poly(arylene ethylene)s.Polyynes via Alkylidene Carbenes and Carbenoids.PROPERTIES AND THEORY.Theoretical Studies on Acetylenic Scaffolds.Macrocycles Based on Phenyl-Acetylene Scaffolding.Carbon-Rich Compounds: Acetylene-Based Carbon Allotropes.Chiral Acetylenic Macromolecules.Shape-Persistent Acetylenic Macrocycles for Ordered Systems.BIOLOGY-ORIENTED.Acetylenosaccharides.

The surprising nonlinear optical properties of conjugated polyyne oligomers

Polyynes represent a unique class of conjugated organic compounds. The third-order nonlinear optical response of polyynes has been extensively modeled theoretically, and it is generally believed that the increase in molecular second hyperpolarizability (gamma) as a function of length for polyynes should be lower than that for polyenes. Experimental evidence to test this prediction, however, has been absent. We have synthesized conjugated polyynes that contain up to 20 consecutive sp-hybridized carbons, and we have determined their nonresonant gamma-values as a function of the number of acetylene repeat units (n). These gamma-values demonstrate a power-law behavior versus n(gamma approximately n(4.28+/-0.13)), with an exponent that is both larger than theoretically predicted for polyynes and substantially higher than that observed for polyenes or polyenynes. Furthermore, no saturation of the linear or nonlinear optical properties is observed.

Carbon-rich compounds : from molecules to materials

Foreword.Preface.List of Contributors.1. Pioneers of Carbon-rich Compounds.2. Electronic Conduction in Photoactive Metallo-wires.3. All-benzenoid Polycyclic Aromatic Hydrocarbons: Synthesis, Self-assembly and Applications in Organic Electronics.4. [N]Phenylenes: a Novel Class of Cyclohexatrienoid Hydrocarbons.5. A Solid State Strategy for the Preparation of Carbon-rich Polymers.6. Chiral Carbon-rich Macrocycles and Cyclophanes.7. Carbon-rich Cycles with Two and More 1,3-Butadiyne Units ? Syntheses, Structures and Reactivities.8. Carbon-rich Structures: Computational Considerations.9. Fullerene Reactivity - Fullerene Cations and Open-Cage Fullerenes.10. Polyynes.11. Defined-length Carbon-rich Conjugated Oligomers.12. Synthesis and Chemistry of Polycyclic Aromatic Hydrocarbons with Curved Surfaces: Buckybowls.13. Reduction of Carbon-rich Compounds.Index.

Modern supramolecular chemistry : strategies for macrocycle synthesis

Cyclophanes Resorcinarene and calixarene synthesis Supramolecular Metallomacrocycles Supramolecular macrocycle synthesis by H-bonding assembly Supramolecular macrocycle synthesis by metal ion assembly Rotaxane and catenane synthesis Macrocyles via the Dynamic Covalent Bond Cucurbiturils Porphyrins Macrocyclic drugs (including macrocyclic Macrocycles by ring-closure metathesis Anion-binding macrocycles Macrocycles by acetylenic scaffolding

Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures (as well as from solution) yielded significant variations in morphology, resulting in different optical-absorption spectra and tran...

Anisotropy of transient photoconductivity in functionalized pentacene single crystals

We report on the anisotropy of transient photoconductivity in functionalized pentacene single crystals using ultrafast optical pump–terahertz probe techniques. Functionalized pentacene crystals with tri-isopropylsilylethynyl (TIPS) and tri-ethylsilylethynyl (TES) side groups were studied, characterized by crystal structures favoring two-dimensional and one-dimensional charge transports, respectively. Charge carrier mobility anisotropies in the a-b plane of 3.5±0.6 and 12±6 were obtained in the TIPS and TES crystals, respectively, consistent with the degree of π overlap along different directions in the crystals. A photogeneration efficiency anisotropy was also observed in both types of crystals.

Synthesis of Polyyne Rotaxanes

Active-metal templating has been used to synthesize rotaxanes consisting of a phenanthroline-based macrocycle threaded around a C8, C12, or C20 polyyne chain. The crystal structure of the C12 rotaxane has been determined. In the rhenium(I) carbonyl complex of this rotaxane, with Re(CO)(3)Cl coordinated to the phenanthroline macrocycle, the proximity of the polyyne chain quenches the luminescence of the rhenium. These rotaxanes offer a new approach to controlling the environment and interactions of a polyyne chain.

Isomerically Pure syn-Anthradithiophenes: Synthesis, Properties, and FET Performance

The synthesis of isomerically pure syn-anthradithiophene derivatives (syn-ADTs) is described. X-ray crystallography is used to compare the solid-state arrangement of syn-ADT derivatives 2a,b to the analogous mixture of syn- and anti-ADTs. Single-crystal OFETs based on isomerically pure syn-ADTs 2a,b display device performance comparable to those based on a mixture of ADT isomers syn/anti-2a,b with mobilities as high as 1 cm(2)/(V s).