Tetrathiafulvalenes, Oligoacenenes, and Their Buckminsterfullerene Derivatives: The Brick and Mortar of Organic Electronics

A facile approach to hydrophilic oxidized fullerenes and their derivatives as cytotoxic agents and supports for nanobiocatalytic systems

A facile, environment-friendly, versatile and reproducible approach to the successful oxidation of fullerenes (oxC₆₀) and the formation of highly hydrophilic fullerene derivatives is introduced. This synthesis relies on the widely known Staudenmaier’s method for the oxidation of graphite, to produce both epoxy and hydroxy groups on the surface of fullerenes (C₆₀) and thereby improve the solubility of the fullerene in polar solvents (e.g. water). The presence of epoxy groups allows for further functionalization via nucleophilic substitution reactions to generate new fullerene derivatives, which can potentially lead to a wealth of applications in the areas of medicine, biology, and composite materials. In order to justify the potential of oxidized C₆₀ derivatives for bio-applications, we investigated their cytotoxicity in vitro as well as their utilization as support in biocatalysis applications, taking the immobilization of laccase for the decolorization of synthetic industrial dyes as a trial case.

Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridinylidene) form persistent radical cations, whereas others such as the dimer of N,N'-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) with N,N'-dimethyl imidazolin-2-ylidene (NHC), N,N'-dimethyl imidazolidin-2-ylidene (saNHC) and N-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2' positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.

Dibenzo[f,h]furazano[3,4-b]quinoxalines: Synthesis by Intramolecular Cyclization through Direct Transition Metal-Free C-H Functionalization and Electrochemical, Photophysical, and Charge Mobility Characterization

Herein, we describe the synthesis of unsymmetrically substituted dibenzo[f,h]furazano[3,4-b]quinoxalines by intramolecular cyclization through direct transition metal-free C-H functionalization. The electrochemical and photophysical properties for several polycycles have been measured. In thin films of the dibenzo[f,h]furazano[3,4-b]quinoxalines, hole mobility is in the order of 10^-4 cm² V^-1 s^-1. The results show that the HOMO and LUMO energy levels are appropriate for using the compounds as hole-transport materials in thin-film devices, in particular, organic and perovskite solar cells.

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

Understanding the reactivity of polycyclic aromatic hydrocarbons and related compounds

This perspective article summarizes recent applications of the combination of the activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds such as cycloparaphenylenes, fullerenes and doped systems. To this end, we have selected representative examples to highlight the usefulness of this relatively novel computational approach to gain quantitative insight into the factors controlling the so far not fully understood reactivity of these species. Issues such as the influence of the size and curvature of the system on the reactivity are covered herein, which is crucial for the rational design of novel compounds with tuneable applications in different fields such as materials science or medicinal chemistry.

BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis, Aromaticity, and Reactivity

BN-embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X-ray crystallography, optical spectroscopy, and cyclic voltammetry. The B-N bonds show delocalized double-bond characteristics and the conjugation can be extended through the trans-orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN-embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN-embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge-transfer interactions.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

Molecular and Supramolecular Multiredox Systems

The design and synthesis of molecular and supramolecular multiredox systems have been summarized. These systems are of great importance as they can be employed in the next generation of materials for energy storage, energy transport, and solar fuel production. Nature provides guiding pathways and insights to judiciously incorporate and tune the various molecular and supramolecular design aspects that result in the formation of complex and efficient systems. In this review, we have classified molecular multiredox systems into organic and organic-inorganic hybrid systems. The organic multiredox systems are further classified into multielectron acceptors, multielectron donors and ambipolar molecules. Synthetic chemists have integrated different electron donating and electron withdrawing groups to realize these complex molecular systems. Further, we have reviewed supramolecular multiredox systems, redox-active host-guest recognition, including mechanically interlocked systems. Finally, the review provides a discussion on the diverse applications, e. g. in artificial photosynthesis, water splitting, dynamic random access memory, etc. that can be realized from these artificial molecular or supramolecular multiredox systems.

Cyanomethylation of Substituted Fluorenes and Oxindoles with Alkyl Nitriles

The first example of metal-free cyanomethylenation from alkyl nitriles of sp³ C-H bonds to afford quaternary carbon centers is described. This oxidative protocol is operationally simple and features good functional group compatibility. This method provides a novel approach to highly functionalized fluorene and oxindole derivatives, which are commonly used in material and pharmaceutical areas. Control experiments provide evidence of a radical reaction process.

Stereoselective synthesis of amino-substituted cyclopentafullerenes promoted by magnesium perchlorate/ferric perchlorate

A facile one-step reaction of [60]fullerene with cinnamaldehydes and amines promoted by magnesium perchlorate/ferric perchlorate under air conditions afforded a series of rare amino-substituted cyclopentafullerenes in moderate to good yields. Stereoselectivity was readily achieved. Secondary amines exclusively produced N,N-disubstituted cyclopentafullerenes as cis isomers, while arylamines gave N-monosubstituted cyclopentafullerenes with a preference of cis isomers as major products. N-Monosubstituted cyclopentafullerenes could be further converted into other scarce cyclopentafullerenes in the presence of acid chloride or paraformaldehyde. A possible reaction pathway was proposed to elucidate the formation of amino-substituted cyclopentafullerenes.

Fused Pyrazine- and Carbazole-Containing Azaacenes: Synthesis and Properties

A new family of azaacenes has been designed and synthesized by incorporating the electron-withdrawing sp² -hybridized nitrogen of pyrazine and electron-donating nitrogen of carbazole in a molecular skeleton. Two different conjugated lengths of 8-ring aza-nonacene and 10-ring aza-undecene have been achieved by an efficient condensation reaction. The unique optoelectronic properties of these molecules were investigated using both experimental and theoretical techniques. The azaacenes show visible-region absorption and near-infrared (NIR) fluorescence. These compounds can serve as hole-transport semiconductors for solution-processed organic field-effect transistors (OFETs). Single-crystal transistor devices of one of the aza-nonacenes exhibit hole charge transport behavior with a hole mobility of 0.07 cm² /Vs and an on/off current ratio of 1.3x10⁶ .

Serendipitous Rediscovery of the Facile Cyclization of Z,Z-3,5-Octadiene-1,7-diyne Derivatives to Afford Stable, Substituted Naphthocyclobutadienes

The serendipitous isolation of very small amounts of two naphthocyclobutadiene (NCB) derivatives has led to the computational re-examination of the electrocyclization of Z,Z-3,5-octadiene-1,7-diyne as well as the experimental and computational study of diethynylindeno[2,1-a]fluorene derivatives that contain the 3,5-octadiene-1,7-diyne motif as part of a larger π-framework. In both cases the calculated potential energy surface strongly implicates two successive electrocyclic reactions to afford the antiaromatic products. With the octadienediyne fragment locked in the reactive conformation, the postulated diethynylindeno[2,1-a]fluorene intermediates afford the NCBs in modest to good yields. X-ray crystallography of four NCBs as well as NICS-XY scan calculations show that the paratropic motif is located primarily in the benzocyclobutadiene fragment within the larger π-scaffold.

Recent advances in dithiafulvenyl-functionalized organic conjugated materials

This review highlights the recent studies of advanced organic π-conjugated materials that contain 1,4-dithiafulvene (DTF) as a redox-active component.

A tetrathiafulvalene–l-glutamine conjugated derivative as a supramolecular gelator for embedded C60 and absorbed rhodamine B

An l-glutamine-containing tetrathiafulvalene gelator could form charge-transfer complex gels in the presence of C₆₀, and also the native gel exhibited excellent absorption properties for the removal of rhodamine B from aqueous solution.

Structural diversity in conducting bilayer salts (CNB-EDT-TTF)4A

The family of recently described salts based on the electron donor CNB-EDT-TTF and different anions A, with general formula (CNB-EDT-TTF)₄A, constitutes an unprecedented type of molecular conductor based on a bilayer structure of the donors.

Embedding a boroxazine ring into a nanographene scaffold by a concise bottom-up synthetic strategy

The stepwise synthesis of a polycyclic aromatic hydrocarbon with an internalized boroxazine ring provides an unusual heterocycle with emitter properties.

Computationally aided design of a high-performance organic semiconductor: the development of a universal crystal engineering core

Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene "universal crystal engineering core". After computationally screening derivatives, a trialkylsilylethyne-based crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm2 V-1 s-1.

Dimesitylboryl-Decorated Azaarene: Synthesis, Enhanced Stability and Optoelectronic Property

The instability of large acenes and analogues usually limits their wide potentials in organic devices. Thus, effectively constructing large acenes or heteroacenes and examining their optoelectronic properties are of great interest. We herein describe the synthesis, optoelectronic behaviors and electroluminescent property of dimesitylboryl-decorated azaarene 5 and its homologue 7. The former emits strong green fluorescence in non-polar solvents but yellow light in polar solvents. The latter emits a blue light in various organic solvents. Moreover, their electrochemical behavior, photostability and electroluminescent property were further studied in a comparative manner, and the experimental findings suggest that the desired heteroarenes are appealing materials for fabricating electroluminescent devices.

Low-Energy Electronic Excitations of N-Substituted Heteroacene Molecules: Matrix Isolation Spectroscopy in Concert with Quantum-Chemical Calculations

N-Heteropolycycles are attractive as materials in organic electronic devices. However, a detailed understanding of the low-energy electronic excitation characteristics of these species is still lacking. In this work, the matrix isolation technique is applied to obtain high-resolution absorbance spectra for a series of tetracene and core-substituted N-analogues. The experimental electronic excitation spectra obtained for matrix-isolated molecules are then analysed with the help of quantum-chemical calculations. Additional lower energy excitation bands in the spectrum of the core-substituted N-derivatives of tetracene could be explained in terms of intensity borrowing from dipole-forbidden transitions due to Herzberg-Teller vibronic coupling. In the case of tetracene, evidence for the additional formation of London dimers (J aggregates) is found at higher tetracene concentrations in the matrix.

Charge Transfer Salt and Graphene Heterostructure-Based Micro-Supercapacitors with Alternating Current Line-Filtering Performance

The rapid development of lightweight and wearable devices requires electronic circuits possessing compact, high-efficiency, and long lifetime in very limited space. Alternating current (AC) line filters are usually tools for manipulating the surplus AC ripples for the operation of most common electronic devices. So far, only aluminum electrolytic capacitors (AECs) can be utilized for this target. However, the bulky volume in the electronic circuits and limited capacitances have long hindered the development of miniaturized and flexible electronics. In this work, a facile laser-assisted fabrication approach toward an in-plane micro-supercapacitor for AC line filtering based on graphene and conventional charge transfer salt heterostructure is reported. Specifically, the devices reach a phase angle of 73.2° at 120 Hz, a specific capacitance of 151 µF cm^-2 , and relaxation time constant of 0.32 ms at the characteristic frequency of 3056 Hz. Furthermore, the scan rate can reach up to 1000 V s^-1 . Moreover, the flexibility and stability of the micro-supercapacitors are tested in gel electrolyte H₂ SO₄ /PVA, and the capacitance of micro-supercapacitors retain a stability over 98% after 10 000 cycles. Thus, such micro-supercapacitors with excellent electrochemical performance can be almost compared with the AECs and will be the next-generation capacitors for AC line filters.

The importance of intramolecular conductivity in three dimensional molecular solids

Recent years have seen tremendous progress towards understanding the relation between the molecular structure and function of organic field effect transistors. The metrics for organic field effect transistors, which are characterized by mobility and the on/off ratio, are known to be enhanced when the intermolecular interaction is strong and the intramolecular reorganization energy is low. While these requirements are adequate when describing organic field effect transistors with simple and planar aromatic molecular components, they are insufficient for complex building blocks, which have the potential to localize a carrier on the molecule. Here, we show that intramolecular conductivity can play a role in controlling device characteristics of organic field effect transistors made with macrocycle building blocks. We use two isomeric macrocyclic semiconductors that consist of perylene diimides linked with bithiophenes and find that the trans-linked macrocycle has a higher mobility than the cis-based device. Through a combination of single molecule junction conductance measurements of the components of the macrocycles, control experiments with acyclic counterparts to the macrocycles, and analyses of each of the materials using spectroscopy, electrochemistry, and density functional theory, we attribute the difference in electron mobility of the OFETs created with the two isomers to the difference in intramolecular conductivity of the two macrocycles.

Electronic, Structural and Functional Versatility in Tetrathiafulvalene-Lanthanide Metal-Organic Frameworks

Tetrathiafulvalene-lanthanide (TTF-Ln) metal-organic frameworks (MOFs) are an interesting class of multifunctional materials in which porosity can be combined with electronic properties such as electrical conductivity, redox activity, luminescence and magnetism. Herein a new family of isostructural TTF-Ln MOFs is reported, denoted as MUV-5(Ln) (Ln=Gd, Tb, Dy, Ho, Er), exhibiting semiconducting properties as a consequence of the short intermolecular S⋅⋅⋅S contacts established along the chain direction between partially oxidised TTF moieties. In addition, this family shows photoluminescence properties and single-molecule magnetic behaviour, finding near-infrared (NIR) photoluminescence in the Yb/Er derivative and slow relaxation of the magnetisation in the Dy and Er derivatives. As such properties are dependent on the electronic structure of the lanthanide ion, the immense structural, electronic and functional versatility of this class of materials is emphasised.

Facile and versatile access to substituted hexabenzoovalene derivatives: characterization and optoelectronic properties

We describe the design and modular synthesis of a library of substituted hexabenzoovalene derivatives (SHBO), along with the key precursor dinaphthopyrenes (3), highlighting the influence of a wide array of substituent variation on the photophysical properties via UV-vis absorption, fluorescence spectra and electrochemical methods. The results show that the cyclized hexabenzoovalenes present a stronger spectroscopic red-shift than the corresponding dinaphthopyrenes. X-ray diffraction analysis suggests that intermediate 3hx containing two nitro groups forms a trans-configuration with twisted structures. Our systematic investigation might provide a realistic design strategy to construct large one-dimensional and two-dimensional materials via bottom-up approaches.

Phosphorus Post-Functionalization of Diphosphahexaarenes

Diphosphahexaarenes are highly stable π‐extended frameworks containing two six‐membered phosphorus heterocycles that have emerged recently. Herein, we present a detailed investigation on the post‐functionalization reactions of their phosphorus centers with special emphasis on the selectivity of the processes and the impact of the phosphorus functionalizations into the physicochemical properties. These studies reveal that indeed the phosphorus atoms of the diphosphahexaarenes are readily available to be functionalized with quaternizing and oxidizing agents as well as borane groups without compromising the stability of the system. In addition, the optoelectronic properties of the diphosphahexaarenes are impacted by the phosphorus post‐modifications.

Bis(1,3-dithiol-2-ylidene)-Substituted Subtriazachlorin: A Subphthalocyanine Analogue with Redox Properties

Bis(1,3-dithiol-2-ylidene)-substituted subtriazachlorin was formed because of an unusual reaction of a 1,3-dithiole-2-one-fused subphthalocyanine in a triethylphosphite-mediated tetrathiafulvalene synthesis. In this novel molecule, the bis(1,3-dithiol-2-ylidene)ethane moiety and subtriazachlorin structure are fused, resulting in an electron-donating ability and broad absorption in the near-infrared region.

Crystal structure of 4,4'-bis-(4-bromo-phen-yl)-1,1',3,3'-tetra-thia-fulvalene

The mol-ecule of the title compound, C18H10Br2S4, has a C-shape, with C s mol-ecular symmetry. The dihedral angle between the planes of the di-thiol and phenyl rings is 8.35 (9)°. In the crystal, mol-ecules form helical chains along [001], the shortest inter-actions being π⋯S contacts within the helices. The inter-molecular inter-actions were investigated by Hirshfeld surface analysis. Density functional theory (DFT) was used to calculate HOMO-LUMO energy levels of the title compound and its trans isomer.

A DFT Study of the Modulation of the Antiaromatic and Open-Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation, BN/CC Isosterism, and Substitution

Dibenzo[a,f]pentalene ([a,f]DBP) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a,f]DBP, in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a,f]DBP. Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.

BN-Embedded Tetrabenzopentacene: A Pentacene Derivative with Improved Stability

Considerable efforts have been devoted to achieving stable acene derivatives for electronic applications; however, the instability is still a major issue for such derivatives. To achieve higher stability with minimum structural change, CC units in the acenes were replaced with isoelectronic BN units to produce a novel BN-embedded tetrabenzopentacene (BNTBP). BNTBP, with a planar structure, is highly stable to air, moisture, light, and heat. Compared with its carbon analogue tetrabenzopentacene (TBP), BN embedment lowered the highest occupied molecular orbital (HOMO) energy level of BNTBP, changed the orbital distribution, and decreased the HOMO orbital coefficients at the central carbon atoms, which stabilize BNTBP molecules upon exposure to oxygen and sunlight. The single-crystal microribbons of BNTBP exhibited good performance in field-effect transistors (FETs). The high stability and good mobility of BNTBP indicates that BN incorporation is an effective approach to afford stable large-sized acenes with desired properties.

Tetrathiafulvalene: effective organic anodic materials for WO3-based electrochromic devices

Finding a new, effective anodic species is a challenge for achieving simpler low-voltage tungsten trioxide (WO₃)-based electrochromic devices (ECDs). In this work, we utilize tetrathiafulvalene (TTF) and demonstrate its reversible redox behaviors as an electrolyte-soluble anodic species. The concentration of TTF in the electrolyte is varied to optimize device performance. When the TTF concentration is low (0.01 M), a smaller maximum transmittance difference (ΔT_max ∼ 34.2%) and coloration efficiency (η ∼ 59.6 cm² C⁻¹) are measured. Although a better performance of ΔT_max ∼ 93.7% and η ∼ 74.5 cm² C⁻¹ is achieved at 0.05 M TTF, the colored state could no longer return to its original form. We conclude that 0.03 M of TTF is the appropriate concentration for high-performance WO₃ ECDs with high optical contrast and reversible EC behaviors. The irreversible EC transition at high concentrations of TTF is attributed to the agglomeration of TTF molecules.

Tetrathiafulvalene (TTF) is employed as an effective electrolyte-soluble anodic species for achieving low-voltage tungsten trioxide (WO₃)-based electrochromic devices (ECDs).

Radical cations of twisted acenes: chiroptical properties and spin delocalization

We introduce the first series of enantiopure twistacene radical cations, which form reversibly upon chemical or electrochemical oxidation. Their vis-NIR chiroptical properties (Cotton effect and anisotropy factor) increase systematically with the backbone twist. The hyperfine constants observed by EPR demonstrate significant spin delocalization even for large backbone twist angles.