Photorearrangement of [8]-2,6-Pyridinophane N-Oxide.

Reaction pathways operative when pyridinophane N-oxides are photoirradiated have been studied using time course analyses and careful isolation of photolabile intermediates with support from DFT calculations. Based on the data and the isolation of two previously unknown heterocyclophanes, we outline a unified mechanistic scheme that explains competing processes under varying photochemical conditions.

Expanded Helicenes as Synthons for Chiral Macrocyclic Nanocarbons.

Expanded helicenes are large, structurally flexible π-frameworks that can be viewed as building blocks for more complex chiral nanocarbons. Here we report a gram-scale synthesis of an alkyne-functionalized expanded [11]helicene and its single-step transformation into two structurally and functionally distinct types of macrocyclic derivatives: (1) a figure-eight dimer via alkyne metathesis (also gram scale) and (2) two arylene-bridged expanded helicenes via Zr-mediated, formal [2+2+n] cycloadditions. The phenylene-bridged helicene displays a substantially higher enantiomerization barrier (22.1 kcal/mol) than its helicene precursor (<11.9 kcal/mol), which makes this a promising strategy to access configurationally stable expanded helicenes. In contrast, the topologically distinct figure-eight retains the configurational lability of the helicene precursor. Despite its lability in solution, this compound forms homochiral single crystals. Here, the configuration is stabilized by an intricate network of two distinct yet interconnected helical superstructures. The enantiomerization mechanisms for all new compounds were probed using density functional theory, providing insight into the flexibility of the figure-eight and guidance for future synthetic modifications in pursuit of non-racemic macrocycles.

Fjord-Edge Graphene Nanoribbons with Site-Specific Nitrogen Substitution.

The synthesis of graphene nanoribbons (GNRs) that contain site-specifically substituted backbone heteroatoms is one of the essential goals that must be achieved in order to control the electronic properties of these next generation organic materials. We have exploited our recently reported solid-state topochemical polymerization/cyclization-aromatization strategy to convert the simple 1,4-bis(3-pyridyl)butadiynes 3a,b into the fjord-edge nitrogen-doped graphene nanoribbon structures 1a,b (fjord-edge N2[8]GNRs). Structural assignments are confirmed by CP/MAS 13C NMR, Raman, and XPS spectroscopy. The fjord-edge N2[8]GNRs 1a,b are promising precursors for the novel backbone nitrogen-substituted N2[8]AGNRs 2a,b. Geometry and band calculations on N2[8]AGNR 2c indicate that this class of nanoribbons should have unusual bonding topology and metallicity.

Solid-State Order and Charge Mobility in [5]- to [12]Cycloparaphenylenes.

We report a computational study of mesoscale morphology and charge-transport properties of radially π-conjugated cycloparaphenylenes ([ n]CPPs) of various ring sizes ( n = 5-12, where n is the number of repeating phenyl units). These molecules are considered structural constituents of fullerenes and carbon nanotubes. [ n]CPP molecules are nested in a unique fashion in the solid state. Molecular dynamics simulations show that while intramolecular structural stability (order) increases with system size, intermolecular structural stability decreases. Density functional calculations reveal that reorganization energy, an important parameter in charge transfer, decreases as n is increased. Intermolecular charge-transfer electronic couplings in the solid state are relatively weak (due to curved π-conjugation and loose intermolecular contacts) and are on the same order of magnitude (∼10 meV) for each system. Intrinsic charge-carrier mobilities were simulated from kinetic Monte Carlo simulations; hole mobilities increased with system size and scaled as ∼ n4. We predict that disordered [ n]CPPs exhibit hole mobilities as high as 2 cm2/(V·s). Our computations show a strong correlation between reorganization energy and hole mobility (µ ∼ λ-4). Quantum mechanical calculations were performed on cofacially stacked molecular pairs for varying phenyl units and reveal that orbital delocalization is responsible for both decreasing reorganization energies and electronic couplings as n is increased.

Impact of morphology, side-chains, and crystallinity on charge-transport properties of π-extended double helicenes.

We report a computational study on the effect of side-chain substitution, heteroaromatic substitution and unique crystal packing on the charge transport and mobility of three double helicene molecules. These double helicene (DH) molecules, having curved π-conjugation, are structural hybrids of non-planar [6]helicene and planar tribenzo[b,n,pqr]perylene (TBP). We find that side-chain substitution has only a effect on intrinsic electronic properties in DHs but dramatically impacts the packing arrangement, morphologies and transport network, exhibited in calculated charge transport parameters. Interestingly, the dimensionality of the transport evolves from one dimensional to three dimensional with side-chain substitution (DH2) and heteroaromatic substitution (DH3). Using two different well-known transport models, we have established a direct link between the morphology, transport connectivity, and hole mobilities. While both unsubstituted and substituted DHs exhibit high hole mobilities in the ordered phase, the results show that with inclusion of positional disorder, the mobilities of disordered DH1 and DH3 are lower while the mobility of DH2 remain nearly unchanged. We relate this effect to the dimensionality of their unique transport networks. These DH molecules are promising organic semiconductors with high mobilities in ordered and disordered phases, with predicted values that lie in the range of ∼1 to 10 cm2 V-1 s-1.

Conjugated Trimeric Scaffolds Accessible from Indolyne Cyclotrimerizations: Synthesis, Structures, and Electronic Properties.

We report the design and synthesis of a new class of indole-based conjugated trimers. The targeted compounds are accessed from in situ generated, highly reactive indolyne intermediates using Pd-catalyzed cyclotrimerization reactions. By harnessing three indolyne isomers, six isomeric indole trimers are accessible, none of which have been previously synthesized. Using computational analysis, we describe the structural and photophysical properties of these unique compounds. This study showcases the use of indolynes in transition metal-catalyzed reactions, while providing access to a new class of conjugated trimers, including highly bent heteroaromatic compounds. Computations indicate that, despite differences in planarity between the molecules, the photophysical properties of each trimer are derived from the N-methylindole building block. Excited state behavior follows predicable patterns.

Synthesis of N = 8 Armchair Graphene Nanoribbons from Four Distinct Polydiacetylenes.

We demonstrate a highly efficient thermal conversion of four differently substituted polydiacetylenes (PDAs 1 and 2a-c) into virtually indistinguishable N = 8 armchair graphene nanoribbons ([8]AGNR). PDAs 1 and 2a-c are themselves easily accessed through photochemically initiated topochemical polymerization of diynes 3 and 4a-c in the crystal. The clean, quantitative transformation of PDAs 1 and 2a-c into [8]AGNR occurs via a series of Hopf pericyclic reactions, followed by aromatization reactions of the annulated polycyclic aromatic intermediates, as well as homolytic bond fragmentation of the edge functional groups upon heating up to 600 °C under an inert atmosphere. We characterize the different steps of both processes using complementary spectroscopic techniques (CP/MAS 13C NMR, Raman, FT-IR, and XPS) and high-resolution transmission electron microscopy (HRTEM). This novel approach to GNRs exploits the power of crystal engineering and solid-state reactions by targeting very large organic structures through programmed chemical transformations. It also affords the first reported [8]AGNR, which can now be synthesized on a large scale via two operationally simple and discrete solid-state processes.

Chlorinated Spiroconjugated Fused Extended Aromatics for Multifunctional Organic Electronics.

The synthesis of a new molecule, SFIC-Cl, is reported, which features enhanced π-electron delocalization by spiroconjugation and narrowed bandgap by chlorination. SFIC-Cl is integrated into a single-crystal transistor (OFET) and organic light-emitting diode (OLED). The material demonstrates remarkable transport abilities across various solution-processed OFETs and retains efficient radiance in a near-infrared OLED emitting light at 700 nm. Furthermore, the intermolecular multi-dimensional connection of SFIC-Cl enables the fabrication of a single-component large-area (2 × 2 cm2 ) near-infrared OLED by spin-coating. The SFIC-Cl-acceptor-based solar cell shows excellent power conversion efficiency of 10.16% resulting from the broadened and strong absorption and well-matched energy levels. The study demonstrates that chlorinated spiroconjugated fused systems offer a novel direction toward the development of high-performance organic semiconductor materials for hybrid organic electronic devices.

Torsional Barriers to Rotation and Planarization in Heterocyclic Oligomers of Value in Organic Electronics.

In order to understand the conformational behavior of organic components in organic electronic devices, we have computed the torsional potentials for a library of thiophene-based heterodimers. The accuracy and efficiencies of computational methods for these organic materials were benchmarked for 11 common density functionals with three Pople basis sets against a Focal Point Analysis (FPA) on a model oligothiophene 2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]-thiophene (BTTT) system. This study establishes a set of general trends in regards to conformational preferences, as well as planarization and rotational barriers for a library comprised of common fragments found in organic materials. These gas phase structures are compared to experimental crystal structures to determine the effect of crystal packing on geometry. Finally, we analyze the structure of hole-transporting material DERDTS-TBDT and design a new oligomer likely to be planar in the solid state.