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]GNR). 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]GNR 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 C 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]GNR, which can now be synthesized on a large scale via two operationally simple and discrete solid-state processes.

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(3pyridyl)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 13 C 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 topologies and metallicities.

Bay-linked diperylenediimide (di-PDI) molecules are finding increasing use in organic electronics because of their steric hindrance that “twists” the two monomer units relative to one another, decreasing molecular aggregation. In this paper, we explore the electronic spectroscopy and ultrafast dynamics of the singly linked β-β-S-di-PDI (2,9′-di­(undecan-5-yl)-2′,9-di­(undecan-6-yl)-[5,5′-bianthra­[2,1,9-def:6,5,10-d′e′f′]­diisoquinolin]-1,1′,3,3′,8,8′,10,10′(2H,2′H,9H,9′H)-octaone). Excitation–emission spectroscopy reveals two distinct emitting species, which are further characterized by time-dependent density functional theory (TD-DFT), demonstrating that the bay-linked PDI dimers exist in two geometrical conformations. These conformations are an “open” geometry, where the two monomer subunits are oriented nearly at right angles, giving them more J-like coupling, and a “closed” geometry, in which the two monomer subunits are nearly π-stacked, resulting in a more H-like coupling. Given the extent of through-space and through-bond coupling, however, neither di-PDI conformer can be well described simply in terms of independently coupled monomers; instead, a full quantum chemistry description is required to understand the electronic structure of this molecule. Temperature-dependent experiments and the TD-DFT calculations indicate that the “closed” conformer is ∼70 meV more stable than the “open” conformer, so that both conformers are important to the behavior of the molecule at room temperature and above. We use a combination of steady-state and femtosecond transient absorption and emission spectroscopies to globally fit the multiple electronic transitions underlying the spectra of both the “closed” and “open” conformers, which agree well with the TD-DFT calculations. The fact that di-PDI molecules are molecular species that adopt two distinct quasi-independent chemical identities has important ramifications for charge trapping and mobility in the organic electronic devices employing these materials.

This paper examines the effect of compositional heterogeneity on the thermal conductivity of transparent, mesoporous silica–titania composites that contain either 10 or 20 mol % titania. The relative hydrolysis rates of the silica and titania precursors were modified to control their compositional heterogeneity, while the ratio of polymer to inorganic precursors (silica + titania) was varied to control the porosity of the films. All films were optically transparent at thicknesses up to 1 μm with transmittance above 90% and haze below 5% at visible wavelengths. It was found that the heterogeneity of the titania species in the 10 mol % titania samples could be easily tailored from highly dispersed titania to a composition with heterogeneous silica-rich and titania-rich domains. By contrast, samples with 20 mol % titania always showed a heterogeneous titania distribution. The results show that mesoporous films with more homogeneously distributed titania had a lower thermal conductivity at all porosities, likely due to increases in propagon and diffuson scattering as a result of the increased number density of titania heteroatom scattering centers. These results increase our understanding of heat carrier propagation in amorphous materials and add to the design rules for creating amorphous, optically clear, low thermal conductivity materials.

Semiconducting polymers are a versatile class of materials that are used in many (opto)­electronic applications, including organic photovoltaics. However, they are inherently disordered and suffer from poor conductivities due to bends and kinks in the polymer chains along the conjugated backbone, as well as disorder at grain boundaries. In an effort to reduce polymer disorder, we developed a method to straighten polymer chains by creating amphiphilic conjugated polyelectrolytes (CPEs) that self-assemble in water into worm-like micelles. The present work refines our design rules for self-assembly of CPEs. We present the synthesis and characterization of a straight, micelle-forming polymer, a derivative of poly­(cyclopentadithiophene-alt-thiophene) (PCT) bearing two ammonium-charged groups per cyclopentadithiophene unit. Solution-phase self-assembly of PCT into micelles is observed by both small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM), while detailed SAXS fitting allows for characterization of intra-micellar interactions and inter-micelle aggregation. We find that PCT displays significant chain straightening thanks to the lack of steric hindrance between its alternating cyclopentadithiophene and thiophene subunits, which increases the propensity for the polymer to self-assemble into straight rod-like micelles. This work extends the availability of micelle-forming semiconducting polymers and points to further enhancements that can be made to obtain homogeneous nanostructured polymer assemblies based on cylindrical micelles.