Hierarchical Modeling of the Nonlinear Optical Response of Composite Materials Based on Tetrathiafulvalene Derivatives

The presented work concerns computational investigations of the physical properties of composite materials based on polymer matrix and nonlinear optical (NLO) active chromophores. The structural, electronic, and optical properties of selected tetrathiafulvalene (TTF)-based chromophores have been calculated using quantum chemical methods. The polymer matrix changes the physical properties of the inserted chromophores influencing their optical parameters. To explain the mechanism of the NLO signal occurrence from the composites based on poly(methyl methacrylate) (PMMA) matrix and TTF chromophores, their structures are modeled using the classical molecular dynamics. In consequence, the structural properties of the composites are discussed according to the NLO requirements. By developing the theoretical model based on a discrete multipole local field approach, the impact of polymer matrix on the optical properties of chromophores is explained.

Small-molecule ambipolar transistors

Ambipolar transistor properties have been observed in various small-molecule materials. Since a small energy gap is necessary, many types of molecular designs including extended π-skeletons as well as the incorporation of donor and acceptor units have been attempted. In addition to the energy levels, an inert passivation layer is important to observe ambipolar transistor properties. Ambipolar transport has been observed in extraordinary π-electron systems such as antiaromatic compounds, biradicals, radicals, metal complexes, and hydrogen-bonded materials. Several donor/acceptor cocrystals show ambipolar transport as well.

High-efficiency photocatalytic degradation of rhodamine 6G by organic semiconductor tetrathiafulvalene in weak acid-base environment

Tetrathiafulvalene (TTF) was successfully applied to degrade rhodamine 6G (Rh6G) in photocatalytic system. The results exhibited that TTF had excellent suitability in the range of weak acid to weak base (pH = 5-9). Multiple reactive oxygen species (˙OH, ¹O₂, H₂O₂ and ˙O₂^-) as well as h⁺ and e^- were generated in photocatalytic process, causing the rapid degradation of Rh6G. This study provides new ideas for the development of catalysts in photocatalytic system and the broader applications of organic semiconductor materials.

High-performance five-ring-fused organic semiconductors for field-effect transistors

Organic molecular semiconductors have been paid great attention due to their advantages of low-temperature processability, low fabrication cost, good flexibility, and excellent electronic properties. As a typical example of five-ring-fused organic semiconductors, a single crystal of pentacene shows a high mobility of up to 40 cm² V^-1 s^-1, indicating its potential application in organic electronics. However, the photo- and optical instabilities of pentacene make it unsuitable for commercial applications. But, molecular engineering, for both the five-ring-fused building block and side chains, has been performed to improve the stability of materials as well as maintain high mobility. Here, several groups (thiophenes, pyrroles, furans, etc.) are introduced to design and replace one or more benzene rings of pentacene and construct novel five-ring-fused organic semiconductors. In this review article, ∼500 five-ring-fused organic prototype molecules and their derivatives are summarized to provide a general understanding of this catalogue material for application in organic field-effect transistors. The results indicate that many five-ring-fused organic semiconductors can achieve high mobilities of more than 1 cm² V^-1 s^-1, and a hole mobility of up to 18.9 cm² V^-1 s^-1 can be obtained, while an electron mobility of 27.8 cm² V^-1 s^-1 can be achieved in five-ring-fused organic semiconductors. The HOMO-LUMO levels, the synthesis process, the molecular packing, and the side-chain engineering of five-ring-fused organic semiconductors are analyzed. The current problems, conclusions, and perspectives are also provided.

Formation of Single Micro- and Nanowires with Extreme Aspect Ratios in Microfluidic Channels

Shown here is the site-specific formation of single extraordinarily long metal-organic micro- and nanowires using a microfluidic device made of poly(dimethylsiloxane) (PDMS). This approach exploits two concepts, i) the diffusion of organic precursor molecules through PDMS and ii) the use of microfluidic channels as a growth template. To initiate wire formation, metal and organic precursor solutions are filled into different supply channels that are separated by PDMS. As the precursor diffuses through PDMS, and thereby infiltrates the adjacent channel, the growth of micro- and nanowires starts at the side walls of this adjacent channel. The formation yields single wires with sizes ranging from several hundreds of micrometers to millimeters at diameters of 0.5-2 µm. The principles of this formation pathway are demonstrated with the reaction of tetrathiafulvalene (TTF) and gold(III) ions that yields Au-TTF wires. The influence of various reaction parameters including the choice of solvents and the chip fabrication protocol on the reaction are evaluated. Based on these findings, a further microfluidic device design with orthogonally arranged channels is developed, and the formation of single wires in a channel-defined pattern is demonstrated. Moreover, the possibility of pulsed precursor supply allows for advanced control over the growth of the wires.

Prospects for electroactive and conducting framework materials

Electroactive and conducting framework materials, encompassing coordination polymers and metal-organic frameworks, have captured the imagination of the scientific community owing to their highly designable nanoporous structures and their potential applications in electrochromic devices, electrocatalysts, porous conductors, batteries and solar energy harvesting systems, among many others. While they are now considered integral members of the broader field of inorganic materials, it is timely to reflect upon their strengths and challenges compared with 'traditional' solid-state materials such as minerals, pigments and zeolites. Indeed, the latter have been known since ancient times and have been prized for centuries in fields as diverse as art, archaeology and industrial catalysis. This opinion piece considers a brief historical perspective of traditional electroactive and conducting inorganic materials, with a view towards very recent experimental progress and new directions for future progress in the burgeoning area of coordination polymers and metal-organic frameworks. Overall, this article bears testament to the rich history of electroactive solids and looks at the challenges inspiring a new generation of scientists. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.

Theoretical study on the charge transport and metallic conducting properties in organic complexes

The charge transfer process between substrate molecular and dopant always appears in doped organic semiconductors, so that molecular doping is a common method to improve the electrical properties by combining appropriate complexes of electron acceptor and donor molecules. At the interface of the doped complexes, the amount of charge-transfer based on the charge analysis method could be affected by various factors, including the stacking structure, the HOMO_D-LUMO_A gaps, the offset defined by the donor ionization potential and the acceptor electron affinity IP_D-|EA_A|, and the strength of the intermolecular orbital interaction. To better understand the charge transport properties in complex crystals, reasonable mobility values were calculated by combining semi-classical Marcus-Hush theory with molecular dynamics simulation, in which the mobility values were on the same order of magnitude as experimental values. The largest and average room-temperature mobility were 4.59 and 0.21 cm² V^-1 s^-1 for TTF-TCNQ based on the anisotropic transport properties and random-walk schemes of the charge diffusion coefficient. The interface of the TTF-TCNQ crystal possesses metallic conducting properties with a predicted resistance of 4.43 kΩ. Charge-transfer complexes exhibit larger mobility and higher conductivity compared to the constituent donor and acceptor molecules.

Study of stacking interactions between two neutral tetrathiafulvalene molecules in Cambridge Structural Database crystal structures and by quantum chemical calculations

Tetrathiafulvalene (TTF) and its derivatives are very well known as electron donors with widespread use in the field of organic conductors and superconductors. Stacking interactions between two neutral TTF fragments were studied by analysing data from Cambridge Structural Database crystal structures and by quantum chemical calculations. Analysis of the contacts found in crystal structures shows high occurrence of parallel displaced orientations of TTF molecules. In the majority of the contacts, two TTF molecules are displaced along their longer C 2 axis. The most frequent geometry has the strongest TTF–TTF stacking interaction, with CCSD(T)/CBS energy of −9.96 kcal mol−1. All the other frequent geometries in crystal structures are similar to geometries of the minima on the calculated potential energy surface.

Hole Mobility Modulation in Single-Crystal Metal Phthalocyanines by Changing the Metal-π/π-π Interactions

Weak intermolecular interactions in organic semiconducting molecular crystals play an important role in determining molecular packing and electronic properties. Single crystals of metal-free and metal phthalocyanines were synthesized to investigate how the coordination of the central metal atom affects their molecular packing and resultant electronic properties. Single-crystal field-effect transistors were made and showed a hole mobility order of ZnPc>MnPc>FePc>CoPc>CuPc>H₂ Pc>NiPc. Density functional theory (DFT) and 1D polaron transport theory reach a good agreement in reproducing the experimentally measured trend for hole mobility. Additional detail analysis at the DFT level suggests the metal atom coordination into H₂ Pc planes can tune the hole mobility via adjusting the intermolecular distances along the shortest axis with closest parallel π stackings.

Molecular Crystal Engineering: Tuning Organic Semiconductor from p-type to n-type by Adjusting Their Substitutional Symmetry

Focusing on the bottleneck of molecularly engineered organic semiconductors, a breakthrough is made to tune the electronic properties of organic semiconductors from p-type to n-type by using fluorinated metal phthalocyanines as examples. The experimentally observed p-type to n-type transition characteristics of single-crystal field-effect devices result from a combination of extrinsic and intrinsic properties of materials with different fluoridation substitution.

Structural and electronic characterisation of π-extended tetrathiafulvalene derivatives as active components in field-effect transistors

The electronic and structural properties of two tetrathiafulvalene derivatives bearing aromatic benzene rings are reported. Thin film transistors of these materials show p-type characteristics with comparable mobility values. It is found that the rigidification of the molecule is beneficial for reducing the reorganisation energy but also has an unfavorable impact on the electronic structure dimensionality.

Induced smectic phases of stoichiometric liquid crystal mixtures

We revealed the detailed structures of induced smectic liquid crystal (LC) phases composed of a binary mixture of charge-transfer (CT) LC substances. Although neither of the constituents had highly ordered smectic phases, the mixture exhibited smectic-E (SmE) or smectic-B (SmB) phases when mixed at ratios of 1 : 1 and 2 : 3, respectively. The results of polarized optical microscopy, differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy indicated that the induced smectic phases were stabilized by an exquisite balance between the CT interactions, dipolar interactions, and excluded volume effects. We proposed a possible model for the molecular arrangements in the SmE and SmB phases, which consistently explained the experimental results including the stoichiometric ratios.

Thiazole-induced rigidification in substituted dithieno-tetrathiafulvalene: the effect of planarisation on charge transport properties

Two novel tetrathiafulvalene (TTF) containing compounds 1 and 2 have been synthesised via a four-fold Stille coupling between a tetrabromo-dithienoTTF 5 and stannylated thiophene 6 or thiazole 4. The optical and electrochemical properties of compounds 1 and 2 have been measured by UV–vis spectroscopy and cyclic voltammetry and the results compared with density functional theory (DFT) calculations to confirm the observed properties. Organic field effect transistor (OFET) devices fabricated from 1 and 2 demonstrated that the substitution of thiophene units for thiazoles was found to increase the observed charge transport, which is attributed to induced planarity through S–N interactions of adjacent thiazole nitrogen atoms and TTF sulfur atoms and better packing in the bulk.

Advances in the synthesis of functionalised pyrrolotetrathiafulvalenes

The electron-donor and unique redox properties of the tetrathiafulvalene (TTF, 1) moiety have led to diverse applications in many areas of chemistry. Monopyrrolotetrathiafulvalenes (MPTTFs, 4) and bispyrrolotetrathiafulvalenes (BPTTFs, 5) are useful structural motifs and have found widespread use in fields such as supramolecular chemistry and molecular electronics. Protocols enabling the synthesis of functionalised MPTTFs and BPTTFs are therefore of broad interest. Herein, we present the synthesis of a range of functionalised MPTTF and BPTTF species. Firstly, the large-scale preparation of the precursor species N-tosyl-(1,3)-dithiolo[4,5-c]pyrrole-2-one (6) is described, as well as the synthesis of the analogue N-tosyl-4,6-dimethyl-(1,3)-dithiolo[4,5-c]pyrrole-2-one (7). Thereafter, we show how 6 and 7 can be used to prepare BPTTFs using homocoupling reactions and functionalised MPTTFs using cross-coupling reactions with a variety of 1,3-dithiole-2-thiones (19). Subsequently, the incorporation of more complex functionality is discussed. We show how the 2-cyanoethyl protecting group can be used to afford MPTTFs functionalised with thioethers, exemplified by a series of ethylene glycol derivatives. Additionally, the merits of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an alternative to the most common deprotecting agent, CsOH·H2O are discussed. Finally, we show how a copper-mediated Ullman-type reaction can be applied to the N-arylation of MPTTFs and BPTTFs using a variety of aryl halides.

exo-Methylene-BEDT-TTF and alkene-functionalised BEDT-TTF derivatives: synthesis and radical cation salts

The syntheses of novel BEDT-TTF donors with either an exo-alkene group or a terminal alkene side chain are described along with the crystal structures of their first radical cation salts which include a new semiconductor.