Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short-chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials that can be easily modified with a variety of functional groups. Recent work by a number of research groups has revealed functionalized oligothiophenes to be the up-and-coming generation of advanced materials for organic electronic devices. In this review, we discuss the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We will highlight key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance.

Polycyclic aromatic hydrocarbons (PAHs), consisting of laterally fused benzene rings, are among the most widely studied small-molecule organic semiconductors, with potential applications in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Linear acenes, including tetracene, pentacene, and their derivatives, have received particular attention due to the synthetic flexibility in tuning their chemical structure and properties and to their high device performance. Unfortunately, longer acenes, which could exhibit even better performance, are susceptible to oxidation, photodegradation, and, in solar cells which contain fullerenes, Diels-Alder reactions. This Account highlights recent advances in the molecular design of two-dimensional (2-D) PAHs that combine device performance with environmental stability. New synthetic techniques have been developed to create stable PAHs that extend conjugation in two dimensions. The stability of these novel compounds is consistent with Clar's sextet rule as the 2-D PAHs have greater numbers of sextets in their ground-state configuration than their linear analogues. The ionization potentials (IPs) of nonlinear acenes decrease more slowly with annellation in comparison to their linear counterparts. As a result, 2-D bistetracene derivatives that are composed of eight fused benzene rings are measured to be about 200 times more stable in chlorinated organic solvents than pentacene derivatives with only five fused rings. Single crystals of the bistetracene derivatives have hole mobilities, measured in OFET configuration, up to 6.1 cm(2) V(-1) s(-1), with remarkable Ion/Ioff ratios of 10(7). The density functional theory (DFT) calculations can provide insight into the electronic structures at both molecular and material levels and to evaluate the main charge-transport parameters. The 2-D acenes with large aspect ratios and appropriate substituents have the potential to provide favorable interstack electronic interactions, and correspondingly high carrier mobilities. In stark contrast to the 1-D acenes that form mono- and bis-adducts with fullerenes, 2-D PAHs show less reactivity with fullerenes. The geometry of 2-D PAHs plays a crucial role in determining both the barrier and the adduct stability. The reactivity and stability of the 2-D PAHs with regard to Diels-Alder reactions at different reactive sites were explained via DFT calculations of the reaction kinetics and of thermodynamics of reactions and simple Hückel molecular orbital considerations. Also, because of their increased stability in the presence of fullerenes, these compounds have been successfully used in OPVs. The small-molecule semiconductors highlighted in this Account exhibit good charge-transport properties, comparable to those of traditional linear acenes, while being much more environmentally stable. These features have made these 2-D PAHs excellent molecules for fundamental research and device applications.

Monodispersed conjugated oligothiophenes are receiving attention in fundamental and applied science due to their interesting optical, optoelectronic, and charge transport properties. These "low molecular weight" polymers serve as model structures for the corresponding polymer analogues, which are inherently polydispersed. Here we report the synthesis, electronic structure, molecular packing/morphology, and charge transport properties of monodispersed oligothiophenes with up to six didodecylquaterthiophene (DDQT) building block repeat units (i.e., 24 thiophene units). At the point where the effective conjugation length is reached, the electronic structure showed convergence behavior to the corresponding polymer, poly(3,3"-didodecyl-quaterthiophene) (PQT-12). X-ray crystal structure analysis of the dimer (DDQT-2) showed that terminal thiophenes exhibit syn-conformations, similar to the terminal syn-conformations observed in the trimer (DDQT-3). The dimer also exhibits a rare bending of the terminal alkyl side chains in order to prevent steric hindrance with neighboring hydrogens attached to core thiophenes. Grazing incidence X-ray scattering measurements revealed a morphology evolution from small molecule-like packing to polymer-like packing in thin films, with a morphology transition occurring near the effective conjugation length. Charge transport measurements showed a mobility increase with decreasing chain length. We correlated the molecular packing and morphology to charge transport and determined that carrier mobilities are most sensitive to crystallinity and crystal grain misorientation. This indicates that molecular weight is not a decisive factor for improved carrier mobility in the low molecular weight region, but rather the degree in crystallinity and in-plane crystal orientation. These results represent a fundamental advancement in understanding the relationship between conjugation length and carrier mobilities in oligothiophene semiconductors.

To date the self-assembly of ordered metal nanoparticle (NP)/block copolymer hybrid materials has been limited to NPs with core diameters (D(core)) of less than 10 nm, which represents only a very small fraction of NPs with attractive size-dependent physical properties. Here this limitation has been circumvented using amphiphilic brush block copolymers as templates for the self-assembly of ordered, periodic hybrid materials containing large NPs beyond 10 nm. Gold NPs (D(core) = 15.8 ± 1.3 nm) bearing poly(4-vinylphenol) ligands were selectively incorporated within the hydrophilic domains of a phase-separated (polynorbornene-g-polystyrene)-b-(polynorbornene-g-poly(ethylene oxide)) copolymer via hydrogen bonding between the phenol groups on gold and the PEO side chains of the brush block copolymer. Well-ordered NP arrays with an inverse cylindrical morphology were readily generated through an NP-driven order-order transition of the brush block copolymer.

COMMUNICATIONthis system for the fabrication of metamaterials that have useful metallic photonic behaviors such as strong localized surface plasmon resonance (LSPR), and the tuning range of its PBG was also limited (560-640 nm).In this work, we demonstrate a simple strategy for rapid and scalable fabrication of well-ordered metallodielectric 1D PhCs using amphiphilic brush BCPs as the templates and H-bonding as a driving force for the selective incorporation of gold NPs into hydrophilic domains (see Figure 1 ). We express the concentrations of the gold NPs as the weight percentage (wt%) of the nanocomposites based on the mass of the NP core and ligand shell, while the wt% of the NP core (Au%) was determined using the thermogravimetric analysis (TGA) of the composite and refl ects the Au metal content of the composite. The content of the gold NPs of ≈2 nm in a core diameter was up to 67.2 wt% (Au% = 48.4 wt%), corresponding to 80.4 wt% of the gold NPs (Au% = 58.1 wt%) in the hydrophilic domain. By varying the gold NP loading quantity or the molecular weight of the brush BCP, highly ordered metallodielectric structures with the domain spacing controlled from 120 to 260 nm were readily created resulting in widely tunable PBGs from the visible to near-IR region (458-1010 nm). Moreover, the PBG is also tunable via thermally induced ripening of the NPs, which infl uences their size as well as their distribution (see Figure 1 ). A network structure of gold NPs with diameters greater than 10 nm was formed after thermal annealing leading to greatly increased conductivity and higher effective refractive index of the gold NP layer. The optical properties of the metallodielectric PhCs were signifi cantly infl uenced by the network structure of gold NPs, which is tunable simply through the variation of annealing time. Numerical modeling using transfer matrix methods was employed to quantify the optical characteristics infl uenced by the effective conductivity of the gold NP network and showed a good agreement with the experimental results.Recently, chemists have made a signifi cant progress in design and synthesis of brush BCPs which show substantially reduced polymer chain entanglements relative to their linear analogues. This enables rapid self-assembly to yield nanostructures with domain sizes on the order of several hundreds of nanometers. [19][20][21][22][23][24][25] Introducing metal NPs into the large microdomains of phase-separated brush BCPs is of great interest from both fundamental research and application viewpoints. In our work, the brush BCPs were well-defi ned (polynorbornene-g -polystyrene)-b -(polynorbornene-g -poly(ethylene oxide)) block copolymers ((PNB-PS 3.5k ) n -b -(PNB-PEO 2k ) m ) synthesized by sequential ring opening metathesis polymerization (ROMP) (see the Supporting Information for details). The molecular weight of the Photonic crystals (PhCs) prepared via the self-assembly of block copolymers (BCPs) offer new opportunities for simple fabrication of fl exible photonic devices or coatings in an in...