π‑Electron Conjugation in Two Dimensions

Organic oligomers and polymers with extended π-conjugation are the fundamental building blocks of organic electronic devices. Novel routes are being explored to create tailor-made organic materials, and recent progress in organic chemistry and surface chemistry has led to the synthesis of planar 2D polymers. Here we show how extending π-conjugation in the second dimension leads to novel materials with HOMO–LUMO gaps smaller than in 1D polymers built from the same parent molecular repeat unit. Density functional theory calculations on <i>experimentally realized</i> 2D polymers grant insight into HOMO–LUMO gap contraction with increasing oligomer size and show fundamental differences between 1D and 2D “band gap engineering”. We discuss how the effects of cross-conjugation and dihedral twists affect the electronic gaps

2D Self-Assembly of Fused Oligothiophenes: Molecular Control of Morphology

We report the synthesis and properties of two π-functional heteroaromatic tetracarboxylic acids (isomeric tetrathienoanthracene derivatives 2-TTATA and 3-TTATA) and their self-assembly on highly oriented pyrolytic graphite. Using scanning tunneling microscopy at the liquid–solid interface we show how slight geometric differences between the two isomers (position of sulfur in the molecule) lead to dramatic changes in monolayer structure. While 3-TTATA self-assembles exclusively in a highly ordered porous network <i>via</i> dimeric R²₂(8) hydrogen-bonding connection (synthon), 2-TTATA is polymorphic, forming a less ordered porous network <i>via</i> R²₂(8) synthons as well as a close-packed network <i>via</i> rare tetrameric R⁴₄(16) synthons. Density functional theory calculations show that the self-assembly direction is governed by the angle between the carboxylic groups and secondary interactions with sulfur atoms

H-Bonding Control of Supramolecular Ordering of Diketopyrrolopyrroles

Diketopyrrolopyrrole (DPP) is a widely used building block for high-mobility ambipolar semiconductors. Hydrogen bonding of N-unsubstituted DPPs has recently been identified as a tool for controlling their solid state structure and properties of semiconducting films, yet little is known about supramolecular packing of H-bonded DPP derivatives. Here we report a comparative study of three archetypical DPP derivatives, difurylDPP (DFDPP), diphenylDPP (DPDPP), and dithienylDPP (DTDPP), at the interface and in bulk crystals. Using scanning tunneling microscopy (STM) combined with X-ray crystallographic analysis, we demonstrate how the interactions of the (hetero)­aromatic substituents interplay with H-bonding, causing dramatic differences in the supramolecular ordering of these structurally similar building blocks. Under all explored conditions, DPDPP exclusively forms H-bonded homoassemblies; DFDPP strongly prefers to co-assemble with alkanoic acids, through a rare lactam···carboxylic acid H-bonded complex, and DTDPP, depending on conditions, either co-assembles with alkanoic acids or self-assembles in one of two H-bonded polymorphs. One of these polymorphs suggests an out-of-plane twist of thiophene rings that form π-stacks running along the surface plane; this is unexpected considering the large energetic penalty of DTDPP deplanarization. The results are explained in terms of inter- versus intramolecular interactions, which are quantified with density functional theory calculations. This work shows that aryl substituents can strongly influence H-bonding assembly of DPP derivatives that is likely to affect their charge-transport properties

Convenient Synthesis of a Highly Soluble and Stable Phosphorescent Platinum Porphyrin Dye

A new highly soluble platinum porphyrin derivative <b>10</b> with suppressed photobleaching is prepared on a multigram scale from inexpensive starting materials. <b>10</b> possesses intense absorption bands at λ = 463 nm (log ε = 5.39) and 633 nm (log ε = 5.20) with near-IR emission at 755 nm. Efficient NIR phosphorescence (PLQY = 0.45) and a large Stokes shift (eliminating self-absorption) make it an attractive and readily available material for a number of applications

Aromatization of Benzannulated Perylene-3,9-diones: Unexpected Photophysical Properties and Reactivity

Highly unusual properties of acene-based quinones <b>1A</b> and <b>1T</b> are reported. They undergo an unexpected combination of Michael and carbonyl additions of aryllithium leading to new triarylated benzoperylenes <b>3A</b> and <b>3T</b>. Uncharacteristically for quinones, <b>1A</b> and <b>1T</b> display vibronically split absorption bands and small Stokes shifts. The absorption/emission spectra of the highly emissive <b>1A</b> are almost indistinguishable from those of the aromatized <b>3A.</b> Additional benzene rings cause a counterintuitive blue shift of the aromatic derivatives (<b>2T</b>/<b>3T</b>), but an expected red shift was observed for the quinone (<b>1T</b>). This behavior is fully supported by DFT calculations and rationalized by considering the longest conjugation path

Aromatization of Benzannulated Perylene-3,9-diones: Unexpected Photophysical Properties and Reactivity

Highly unusual properties of acene-based quinones <b>1A</b> and <b>1T</b> are reported. They undergo an unexpected combination of Michael and carbonyl additions of aryllithium leading to new triarylated benzoperylenes <b>3A</b> and <b>3T</b>. Uncharacteristically for quinones, <b>1A</b> and <b>1T</b> display vibronically split absorption bands and small Stokes shifts. The absorption/emission spectra of the highly emissive <b>1A</b> are almost indistinguishable from those of the aromatized <b>3A.</b> Additional benzene rings cause a counterintuitive blue shift of the aromatic derivatives (<b>2T</b>/<b>3T</b>), but an expected red shift was observed for the quinone (<b>1T</b>). This behavior is fully supported by DFT calculations and rationalized by considering the longest conjugation path

Synthesis and Divergent Electronic Properties of Two Ring-Fused Derivatives of 9,10-Diphenylanthracene

Two new contorted polycyclic aromatic hydrocarbons (PAHs) <b>1</b> and <b>2</b> were synthesized by acid-catalyzed benzannulation of a substituted anthracene. The isomers reveal dissimilar photophysical and redox properties with <b>2</b> having a much smaller HOMO–LUMO gap than <b>1</b>. In the solid state, <b>2</b> packs in a unique two-dimensional herringbone motif that gives rise to efficient ambipolar charge transport in OFET devices, a feature not previously observed in contorted PAHs. On the other hand, <b>1</b> packs in one-dimensional dimerized π-stacks and displays insulating properties

Tuning the Electronic Properties of Poly(thienothiophene vinylene)s via Alkylsulfanyl and Alkylsulfonyl Substituents

The use of alkylsulfanyl and alkylsulfonyl side chains are demonstrated to be a useful synthetic strategy for tuning the electronic properties of organic semiconductors, as shown in thienothiophene vinylene polymers. By changing the oxidation state of sulfanyl to sulfonyl, we lower the HOMO and LUMO energy levels of our substituted polymers, as well as enhance their fluorescence. Fine-tuning of the energy levels was achieved by combining sulfanyl and sulfonyl substituted thienothiophene monomers through random polymerization, yielding polymers with low-band gaps (1.5 eV) yet benefiting from a structurally uniform conjugated backbone. The effects of these functional side chains are presented through DFT calculations, UV–vis, fluorescence, and electrochemical measurements, as well as crystallographic analysis of a sulfanyl-substituted oligomer. The semiconducting properties of the new polymers are studied in OFET and OPV devices

Directing the Assembly of Gold Nanoparticles with Two-Dimensional Molecular Networks

Lamellar patterns resulting from the adsorption of <i>p</i>-dialkoxybenzene derivatives on HOPG have been investigated as molecular templates for directing the assembly of thiol-capped gold nanoparticles (AuNP). STM characterization at the liquid–solid interface reveals the periodic arrangement of AuNP on top of the self-assembled molecular network (SAMN), spanning hundreds of nanometers. The resulting superlattices are notably different from the close-packed structures formed by spherical nanoparticles during evaporative drying. The templating effect is based on van der Waals interactions of the alkyl chains of the SAMN and AuNP, and the assembly efficiency is greatest when these chains are of similar length

Tuning the Electronic Properties of Poly(thienothiophene vinylene)s via Alkylsulfanyl and Alkylsulfonyl Substituents

The use of alkylsulfanyl and alkylsulfonyl side chains are demonstrated to be a useful synthetic strategy for tuning the electronic properties of organic semiconductors, as shown in thienothiophene vinylene polymers. By changing the oxidation state of sulfanyl to sulfonyl, we lower the HOMO and LUMO energy levels of our substituted polymers, as well as enhance their fluorescence. Fine-tuning of the energy levels was achieved by combining sulfanyl and sulfonyl substituted thienothiophene monomers through random polymerization, yielding polymers with low-band gaps (1.5 eV) yet benefiting from a structurally uniform conjugated backbone. The effects of these functional side chains are presented through DFT calculations, UV–vis, fluorescence, and electrochemical measurements, as well as crystallographic analysis of a sulfanyl-substituted oligomer. The semiconducting properties of the new polymers are studied in OFET and OPV devices