Two-Dimensional Self-Assembly Driven by Intermolecular Hydrogen Bonding in Benzodi-7-azaindole Molecules on Au(111).

The control of molecular structures at the nanoscale plays a critical role in the development of materials and applications. The adsorption of a polyheteroaromatic molecule with hydrogen bond donor and acceptor sites integrated in the conjugated structure itself, namely, benzodi-7-azaindole (BDAI), has been studied on Au(111). Intermolecular hydrogen bonding determines the formation of highly organized linear structures where surface chirality, resulting from the 2D confinement of the centrosymmetric molecules, is observed. Moreover, the structural features of the BDAI molecule lead to the formation of two differentiated arrangements with extended brick-wall and herringbone packing. A comprehensive experimental study that combines scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory theoretical calculations has been performed to fully characterize the 2D hydrogen-bonded domains and the on-surface thermal stability of the physisorbed material.

2D-Self-Assembled Organic Materials in Undoped Hole Transport Bilayers for Efficient Inverted Perovskite Solar Cells.

Interfaces between photoactive perovskite layer and selective contacts play a key role in the performance of perovskite solar cells (PSCs). The properties of the interface can be modified by the introduction of molecular interlayers between the halide perovskite and the transporting layers. Herein, two novel structurally related molecules, 1,3,5-tris(α-carbolin-6-yl)benzene (TACB) and the hexamethylated derivative of truxenotris(7-azaindole) (TTAI), are reported. Both molecules have the ability to self-assemble through reciprocal hydrogen bond interactions, but they have different degrees of conformational freedom. The benefits of combining these tripodal 2D-self-assembled small molecular materials with well-known hole transporting layers (HTLs), such as PEDOT:PSS and PTAA, in PSCs with inverted configuration are described. The use of these molecules, particularly the more rigid TTAI, enhanced the charge extraction efficiency and reduced the charge recombination. Consequently, an improved photovoltaic performance was achieved in comparison to the devices fabricated with the standard HTLs.

Improving the Robustness of Organic Semiconductors through Hydrogen Bonding.

Molecular organization plays an essential role in organic semiconductors since it determines the extent of intermolecular interactions that govern the charge transport present in all electronic applications. The benefits of hydrogen bond-directed self-assembly on charge transport properties are demonstrated by comparing two analogous pyrrole-based, fused heptacyclic molecules. The rationally designed synthesis of these materials allows for inducing or preventing hydrogen bonding. Strategically located hydrogen bond donor and acceptor sites control the solid-state arrangement, favoring the supramolecular expansion of the π-conjugated surface and the subsequent π-stacking as proved by X-ray diffraction and computational calculations. The consistency observed for the performance of organic field-effect transistors and the morphology of the organic thin films corroborate that higher stability and thermal robustness are achieved in the hydrogen-bonded material.

Synthesis and Two-Dimensional Chiral Surface Self-Assembly of a π-Conjugated System with Three-Fold Symmetry: Benzotri(7-Azaindole).

The synthesis of a novel expanded π-conjugated system, namely benzotri(7-azaindole), BTAI, is reported. Its C3h symmetry along with the integration of six complementary donor and acceptor N-H⋅⋅⋅N hydrogen bonds in the conjugated structure promote the 2D self-assembly on Au(111) over extended areas. Besides, a perfect commensurability with the gold lattice endows the physisorbed molecular film with a remarkable stability. The structural features of BTAI result in two levels of surface chirality: Firstly, the molecules become chiral upon adsorption on the surface. Then, due to the favorable N-H⋅⋅⋅N hydrogen bond-directed self-assembly, along with the relative molecular rotation with respect to the substrate, supramolecular chirality manifests in two mirror enantiomorphous domains. Thus, the system undergoes spontaneous chiral resolution. LEED and STM assisted by theoretical simulations have been employed to characterize in detail these novel 2D conglomerates with relevant chiral properties for systems with C3h symmetry.

Rigid π-Extended Boron Difluoride Complex with Mega-Stokes Shift for Bioimaging.

The rational design of a rigid π-extended ligand, suitable for the formation of four-coordinate boron complexes, has led to the synthesis of the fused hexacyclic structure of carbazolo[2,1-c]phenanthridine. The photophysical characterization of the novel fluorophore revealed a significant Stokes shift whose intramolecular charge transfer origin has been corroborated by computational calculations. The usefulness of the reported N,N-difluoroboryl complex as fluorescent probe with large Stokes shift has been demonstrated for cancer cells imaging.

A Self-Assembled Small-Molecule-Based Hole-Transporting Material for Inverted Perovskite Solar Cells.

Hybrid organic-inorganic perovskite solar cells have recently emerged as one of the most promising low-cost photovoltaic technologies. The remarkable progress of perovskite photovoltaics is closely related to advances in interfacial engineering and development of charge selective interlayers. Herein, we present the synthesis and characterization of a fused azapolyheteroaromatic small molecule, namely anthradi-7-azaindole (ADAI), with outstanding performance as a hole-transporting layer in perovskite solar cells with inverted architecture. Its molecular arrangement, induced by hydrogen-bond-directed self-assembly, favors a suitable morphology of the perovskite layer, reducing the effects of recombination as revealed by light intensity dependence, photoluminescence, and electroluminescence studies.

Adjusting Aggregation Modes and Photophysical and Photovoltaic Properties of Diketopyrrolopyrrole-Based Small Molecules by Introducing B←N Bonds.

The packing mode of small-molecular semiconductors in thin films is an important factor that controls the performance of their optoelectronic devices. Designing and changing the packing mode by molecular engineering is challenging. Three structurally related diketopyrrolopyrrole (DPP)-based compounds were synthesized to study the effect of replacing C-C bonds by isoelectronic dipolar B←N bonds. By replacing one of the bridging C-C bonds on the peripheral fluorene units of the DPP molecules by a coordinative B←N bond and changing the B←N bond orientation, the optical absorption, fluorescence, and excited-state lifetime of the compounds can be tuned. The substitution alters the preferential aggregation of the molecules in the solid state from H-type (for C-C) to J-type (for B←N). Introducing B←N bonds thus provides a subtle way of controlling the packing mode. The photovoltaic properties of the compounds were evaluated in bulk heterojunctions with a fullerene acceptor and showed moderate performance as a consequence of suboptimal morphologies, bimolecular recombination, and triplet-state formation.

Effects of fluorination and thermal annealing on charge recombination processes in polymer bulk-heterojunction solar cells.

We investigate the effect of fluorination on the photovoltaic properties of an alternating conjugated polymer composed of 4,8-di-2-thienylbenzo[1,2-b:4,5-b']dithiophene (BDT) and 4,7-bis([2,2'-bithiophen]-5-yl)-benzo-2-1-3-thiadiazole (4TBT) units in bulk-heterojunction solar cells. The unsubstituted and fluorinated polymers afford very similar open-circuit voltages and fill factor values, but the fluorinated polymer performed better due to enhanced aggregation which provides a higher photocurrent. The photovoltaic performance of both materials improved upon thermal annealing at 150-200 °C as a result of a significantly increased fill factor and open-circuit voltage, counteracted by a slight loss in photocurrent. Detailed studies of the morphology, light intensity dependence, external quantum efficiency and electroluminescence allowed the exploration of the effects of fluorination and thermal annealing on the charge recombination and the nature of the donor-acceptor interfacial charge transfer states in these films.

Single Heteroatom Fine-Tuning of the Emissive Properties in Organoboron Complexes with 7-(Azaheteroaryl)indole Systems.

The application of organoboron compounds as light-absorbing or light-emitting species in areas as relevant as organic electronics or biomedicine has motivated the search for new materials which contribute to the progress of those applications. This article reports the synthesis of four-coordinate boron complexes based on the unexplored 7-(azaheteroaryl)indole ligands. An easy synthetic approach has enabled the fine-tuning of the electronic structure of the organoboron species by modifying a heteroaromatic component in the conjugated system. Furthermore, a comprehensive characterization by X-ray diffraction, absorption and emission spectroscopy, both in solution and in the solid state, cyclic voltammetry, and computational methods has evidenced the utility of this simple strategy. Large Stokes shifts have been achieved in solid thin-films which show a range of emitted light from blue to orange. The synthesized compounds have been used as biocompatible fluorophores in cell bioimaging.

Binding studies and anion-selective electrodes with neutral isophthalamide-based receptors.

Two acyclic isophthalamide-based hosts have been synthesised and their anion binding properties have been evaluated by (1)H-NMR titrations. Different binding modes have been detected for the series of tested anions. The attachment of aminomethylpyrrole groups resulted in an improved binding selectivity. Additionally, the receptors have been incorporated as ionophores in plasticised polymeric membrane-based anion-selective electrodes. The potentiometric studies were in agreement with the NMR experiments and revealed a good sensing ability, considering the structural simplicity of the receptors and their interactions purely based on hydrogen bonding. These preliminary experiments have revealed an interesting selectivity towards highly hydrophilic anions such as fluoride and sulfate. Moreover, a particularly low detection limit (9 × 10(-7) M) has been determined for the fluoride anion.

New carbazolo[1,2-a]carbazole derivative as ionophore for anion-selective electrodes: remarkable recognition towards dicarboxylate anions.

A new carbazolo[1,2-a]carbazole derivative was synthesized by expanding the binding cavity to explore the possibility of hosting larger anions such as dicarboxylate anions. The compound was incorporated as an ionophore into a membrane for an anion-selective electrode. The response of the electrode was evaluated for oxalate, malonate, succinate, glutarate and adipate in terms of calibration characteristics (slope, limit of detection and linear range of the response), response time, repeatability, reproducibility and selectivity. Nernstian reproducible responses, with very good detection limits, fast responses and selectivity not previously observed, were found for all the dicarboxylates anions, and the results were especially good in the case of glutarate. In order to obtain additional structural information about the complex formed between the ionophore and the dicarboxylate anions, (1)H NMR and fluorescence studies were carried out. The observed potentiometric selectivity depends on the good correspondence between the size of the carbazolocarbazole cavity and the length of the dicarboxylate anion, as supported by the NMR and fluorescence studies.

Rational design of a fluorescent receptor for the recognition of anthrax biomarker dipicolinate.

A new carbazole-2,7-dicarboxamide derivative has been synthesised and has been proved to effectively bind the dipicolinate anion, which is commonly used as an anthrax biomarker. The fluorescent response from this synthetic receptor offers a selective colour change in an organic-aqueous environment that is of valuable analytical use.

Indolocarbazole-based ligands for ladder-type four-coordinate boron complexes.

A novel class of π-conjugated systems, which combine the indolo[3,2-b]carbazole unit with the formation of four-coordinate boron complexes, is presented. The resulting conjugated compounds have a double-laddered structure that provides interesting optical and electrochemical properties. The wide absorption range, covering most of the visible spectrum, along with the narrowing of the HOMO-LUMO energy gap, due to the presence of diphenylboryl centers, reinforces the potential of these molecules within the area of organic electronics.

A new building block for anion supramolecular chemistry. Study of carbazolocarbazole as anion receptor.

A novel carbazolocarbazole system has been evaluated as an anion receptor in DMF solution. A good affinity has been detected for oxoanions. The anion binding studies have been contrasted by different experimental techniques.

Synthesis and characterization of new carbazolocarbazoles: toward pi-extended N-fused heteroacenes.

A regiospecific route has been developed for the synthesis of carbazolo[1,2-a]carbazole. The synthetic strategy is based on the use of appropriately tetrasubstituted naphthalene. Optical and electrochemical characterization of the fused heteroaromatic system reveals that this optically transparent heteroacene, with a low-lying HOMO, might be used as an organic material.

A new open benzodipyrrole-based chemosensor for hydrogenpyrophosphate anion in aqueous environment.

Benzodipyrrolic derivatives have been synthesised and evaluated as neutral receptors for anions, in a mixed acetone-water media.