Porous Azatruxene Covalent Organic Frameworks for Anion Insertion in Battery Cells.

Covalent organic frameworks (COFs) containing well-defined redox-active groups have become competitive materials for next-generation batteries. Although high potentials and rate performance can be expected, only a few examples of p-type COFs have been reported for charge storage to date with even fewer examples on the use of COFs in multivalent ion batteries. Herein, we report the synthesis of a p-type highly porous and crystalline azatruxene-based COF and its application as a positive electrode material in Li- and Mg-based batteries. When this material is used in Li-based half cells as a COF/carbon nanotube (CNT) electrode, a discharge potential of 3.9 V is obtained with discharge capacities of up to 70 mAh g-1 at a 2 C rate. In Mg batteries using a tetrakis(hexafluoroisopropyloxy)borate electrolyte, cycling proceeds with an average discharge voltage of 2.9 V. Even at a fast current rate of 5 C, the capacity retention amounts to 84% over 1000 cycles.

Low-lying LUMO Boosts Conductance in Antiaromatic Dibenzopentalene Versus Aromatic Analogues.

Antiaromaticity is a fundamental concept in chemistry, but the study of molecular wires incorporating antiaromatic units is limited. Despite initial predictions, very few studies show that antiaromaticity has a beneficial effect on electron transport. Dibenzo[a,e]pentalene (DBP) is a stable structure that displays appreciable antiaromaticity within the five-membered rings of the pentalene core. We have investigated derivatives of DBP furnished with pyridyl (Py) and F4-pyridyl (PyF4) anchor groups, and compared the conductance with purely aromatic phenyl and anthracene analogues. We find that the low-bias conductance of DBP-Py is approximately 60 % larger than that of the anthracene analogue Anth-Py and 250 % larger compared to the phenyl derivative Ph-Py. This is due to a better alignment of the LUMO with the gold Fermi level, which we confirm by conductance-voltage spectroscopy where the conductance of DBP-Py shows the greatest voltage-dependence. The F4-pyridyl compounds, which have lower LUMO energies compared to the pyridyl analogues, did not, however, form detectable molecular junctions. The strongly electron-withdrawing fluorine atoms reduce the donor capability of the nitrogen lone-pair to the point where stable N-Au bonds no longer form.

Reducing undesired solubility of squarephaneic tetraimide for use as an organic battery electrode material.

Locally aromatic alkyl-N-substituted squarephaneic tetraimide (SqTI) conjugated macrocycles are four-electron reducible, owing to global aromaticity and presumed global Baird aromaticity of the dianion and tetraanion states, respectively. However, their good solubility inhibits their application as a battery electrode material. By applying sidechain removal as a strategy to reduce SqTI solubility, we report the development of its unsubstituted derivative SqTI-H, which was obtained directly from squarephaneic tetraanhydride by facile treatment with hexamethyldisilazane and MeOH. Compared to alkyl-N-substituted SqTI-Rs, SqTI-H exhibited further improved thermal stability and low neutral state solubility in most common organic solvents, owing to computationally demonstrated hydrogen-bonding capabilities emanating from each imide position on SqTI-H. Reversible solid state electrochemical reduction of SqTI-H to the globally aromatic dianion state was also observed at -1.25 V vs. Fc/Fc+, which could be further reduced in two stages. Preliminary testing of SqTI-H in composite electrodes for lithium-organic half cells uncovered imperfect cycling performance, which may be explained by persistent solubility of reduced states, necessitating further optimisation of electrode fabrication procedures to attain maximum performance.

Antiaromatic Covalent Organic Frameworks Based on Dibenzopentalenes.

Despite their inherent instability, 4n π systems have recently received significant attention due to their unique optical and electronic properties. In dibenzopentalene (DBP), benzanellation stabilizes the highly antiaromatic pentalene core, without compromising its amphoteric redox behavior or small HOMO-LUMO energy gap. However, incorporating such molecules in organic devices as discrete small molecules or amorphous polymers can limit the performance (e.g., due to solubility in the battery electrolyte solution or low internal surface area). Covalent organic frameworks (COFs), on the contrary, are highly ordered, porous, and crystalline materials that can provide a platform to align molecules with specific properties in a well-defined, ordered environment. We synthesized the first antiaromatic framework materials and obtained a series of three highly crystalline and porous COFs based on DBP. Potential applications of such antiaromatic bulk materials were explored: COF films show a conductivity of 4 × 10-8 S cm-1 upon doping and exhibit photoconductivity upon irradiation with visible light. Application as positive electrode materials in Li-organic batteries demonstrates a significant enhancement of performance when the antiaromaticity of the DBP unit in the COF is exploited in its redox activity with a discharge capacity of 26 mA h g-1 at a potential of 3.9 V vs. Li/Li+. This work showcases antiaromaticity as a new design principle for functional framework materials.

Conjugated Nanohoop Polymers based on Antiaromatic Dibenzopentalenes for Charge Storage in Organic Batteries.

With their bent π-systems, cyclic conjugation and inherent cavities, conjugated nanohoops are attractive for organic electronics applications. For ease of processing and morphological stability, an incorporation into polymers is desirable, but to date was hampered with few exceptions by synthetic difficulties. We herein present a unique strategy for the synthesis of conjugated nanohoop polymers using a dibenzo[a,e]pentalene (DBP) as central connector. We demonstrate this versatility by synthesizing three electronically diverse copolymers with dithienyldiketo(pyrrolopyrrol), fluorene and carbazole comonomers, and report the first donor-acceptor nanohoop polymer. Optoelectronic investigations reveal the prevalence of cyclic or linear conjugation, depending on the comonomer unit, and ambipolar electrochemical properties through the antiaromatic character of the DBP units. As the first report on using conjugated nanohoops for charge storage as positive electrode materials, we show a significant improvement in battery performance in a nanohoop-containing polymer compared to an equivalent nanohoop-free reference polymer. We believe this study will pave the way for the synthesis of a diverse range of nanohoop polymers and further stimulate their exploration for charge storage in batteries.

Spiroconjugated Tetraaminospirenes as Donors in Color-Tunable Charge-Transfer Emitters with Donor-Acceptor Structure.

Charge-transfer emitters are attractive due to their color tunability and potentially high photoluminescence quantum yields (PLQYs). We herein present tetraaminospirenes as donor moieties, which, in combination with a variety of acceptors, furnished 12 charge-transfer emitters with a range of emission colors and PLQYs of up to 99 %. The spatial separation of their frontier molecular orbitals was obtained through careful structural design, and two DA structures were confirmed by X-ray crystallography. A range of photophysical measurements supported by DFT calculations shed light on the optoelectronic properties of this new family of spiro-NN-donor-acceptor dyes.

Phenothiazine-Based Donor-Acceptor Polymers as Multifunctional Materials for Charge Storage and Solar Energy Conversion.

The increasing energy demand for diverse applications requires new types of devices and materials. Multifunctional materials that can fulfill different roles are of high interest as they can allow fabricating devices that can both convert and store energy. Herein, organic donor-acceptor redox polymers that can function as charge storage materials in batteries and as donor materials in bulk heterojunction (BHJ) photovoltaic devices are investigated. Based on its reversible redox chemistry, phenothiazine is used as the main building block in the conjugated copolymer design and combined with diketopyrrolopyrrol and benzothiadiazole as electron-poor comonomers to shift the optical absorption into the visible region. The resulting polymers show excellent cycling stability as positive electrode materials in lithium-organic batteries at discharge potentials of 3.6-3.7 V versus Li/Li+ as well as good performances in BHJ solar cells with up to 1.9% power conversion efficiency. This study shows that the design of such multifunctional materials is possible, however, that it also faces challenges, as essential properties for good device function can lead to diametrically opposite requirements in materials design.

[n]Cyclodibenzopentalenes as Antiaromatic Curved Nanocarbons with High Strain and Strong Fullerene Binding.

Conjugated nanohoops provide a platform to study structure-property relationships; they are attractive hosts for supramolecular chemistry as well as promising candidates as new organic materials. We herein present [n]cyclodibenzopentalenes ([n]CDBPs) as antiaromatic analogues of [n]cycloparaphenylenes. Platinum-mediated macrocyclization of dibenzopentalene boronic esters provided the trimer and tetramer with strain energies of up to 80 kcal mol-1. In the solid state, the cylindrical [4]CDBP molecules align to form columnar structures. The larger hoop [4]CDBP binds both fullerenes C60 and C70 with temperature-dependent exchange behavior, providing higher activation energies for the exchange compared to [10]CPP. The antiaromatic character of the [n]CDBPs paired with the cyclic conjugation leads to high HOMO energies and lowered LUMO energies with band gaps below 2 eV. This work presents a new class of the antiaromatic and nonalternant curved nanocarbons with intriguing supramolecular and ambipolar optoelectronic properties.

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane.

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.

Dibenzo[a,e]Cyclooctatetraene-Functionalized Polymers as Potential Battery Electrode Materials.

Organic redox polymers are attractive electrode materials for more sustainable rechargeable batteries. To obtain full-organic cells with high operating voltages, redox polymers with low potentials (<2 V versus Li|Li+ ) are required for the negative electrode. Dibenzo[a,e]cyclooctatetraene (DBCOT) is a promising redox-active group in this respect, since it can be reversibly reduced in a two-electron process at potentials below 1 V versus Li|Li+ . Upon reduction, its conformation changes from tub-shaped to planar, rendering DBCOT-based polymers also of interest to molecular actuators. Here, the syntheses of three aliphatic DBCOT-polymers and their electrochemical properties are presented. For this, a viable three-step synthetic route to 2-bromo-functionalized DBCOT as polymer precursor is developed. Cyclic voltammetry (CV) measurements in solution and of thin films of the DBCOT-polymers demonstrate their potential as battery electrode materials. Half-cell measurements in batteries show pseudo capacitive behavior with Faradaic contributions, which demonstrate that electrode composition and fabrication will play an important role in the future to release the full redox activity of the DBCOT polymers.

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero- or Polycyclic Aromatics.

In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size-dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.

Chiral Dibenzopentalene-Based Conjugated Nanohoops through Stereoselective Synthesis.

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π-systems. They can possess unexpected optoelectronic properties and their radially oriented π-system makes them attractive for host-guest chemistry. Bending the π-subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel "corner unit" in the synthesis of the hoops. The [6]DBP[4]Ph-hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO-LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.

Spiroconjugated Donor-σ-Acceptor Charge-Transfer Dyes: Effect of the π-Subsystems on the Optoelectronic Properties.

Charge-transfer-based materials with intramolecular donor-acceptor structures are attractive for technological applications. Herein, a series of donor-σ-acceptor dyes has been prepared in a modular approach. The design of these intramolecular charge-transfer dyes is based on the concept of spiroconjugation, which leads to unique materials with special optical properties. The optical transitions are based on intramolecular charge transfer, as shown by solvatochromic measurements and density functional theory (DFT) calculations. Crystallographic, computational, electrochemical, and optical studies were performed to clarify the effect of different perpendicular π-moieties on the optoelectronic properties. Our molecular tuning allowed for the synthesis of molecules exhibiting strong visible-range absorption. The compounds are not fluorescent due to structural changes in the excited state, as revealed by DFT calculations. Finally, our study describes enantiomerically pure spiroconjugated absorber molecules using 1,1'-binaphthyl-2,2'-diol (BINOL) units on the donor part.

Synthesis and Properties of Conjugated Nanohoops Incorporating Dibenzo[a,e]pentalenes: [2]DBP[12]CPPs.

Conjugated nanohoops allow studying the effect of cyclic conjugation and bending on the incorporated π-systems. To date, no such system containing antiaromatic units has been reported. We herein present [12]cycloparaphenylenes incorporating two dibenzo[a,e]pentalene units: [2]DBP[12]CPP nanohoops. Dibenzo[a,e]pentalene is a nonalternant hydrocarbon with antiaromatic character. The syntheses and optoelectronic properties of two different [2]DBP[12]CPP nanohoops with electronically modifying substituents are reported, accompanied by TDDFT calculations.

Thiocoumarin Caged Nucleotides: Synthetic Access and Their Photophysical Properties.

Photocages have been successfully applied in cellular signaling studies for the controlled release of metabolites with high spatio-temporal resolution. Commonly, coumarin photocages are activated by UV light and the quantum yields of uncaging are relatively low, which can limit their applications in vivo. Here, syntheses, the determination of the photophysical properties, and quantum chemical calculations of 7-diethylamino-4-hydroxymethyl-thiocoumarin (thio-DEACM) and caged adenine nucleotides are reported and compared to the widely used 7-diethylamino-4-hydroxymethyl-coumarin (DEACM) caging group. In this comparison, thio-DEACM stands out as a phosphate cage with improved photophysical properties, such as red-shifted absorption and significantly faster photolysis kinetics.

Raising the Bar in Aromatic Donor-Acceptor Interactions with Cyclic Trinuclear Gold(I) Complexes as Strong π-Donors.

Aromatic donor-acceptor interactions are of high importance in supramolecular chemistry, materials science and biology. Compared to other noncovalent interactions, such as hydrogen bonding, the binding is often weak. Here we show that strong donor-acceptor interactions between planar aromatics with binding free energies down to -10.1 kcal mol-1 and association constants of up to 2.34 × 107 L mol-1 for 1:1 complexes can be realized using cyclic trinuclear complexes of gold(I) with pyridinate, imidazolate, or carbeniate ligands. Data were obtained through NMR and UV/vis absorption spectroscopic studies and supported by quantum chemical calculations for a variety of acceptors. By using a specifically designed bridged naphthalene diimide-based acceptor with only one binding site, we furthermore show that a 1:2 (donor:acceptor) binding model is best suited to quantify the donor and acceptor/complex equilibrium. Scanning electron microscopy on selected donor-acceptor pairs shows crystalline supramolecular assemblies. We anticipate this study to be relevant for the future design of supramolecular systems and chemical sensors and the determination of binding energies between planar donors and acceptors.

Dibenzo[a,e]pentalenophanes: Bending a Non-Alternant Hydrocarbon.

In cyclophanes, an aromatic moiety is incorporated into a (strained) cyclic structure. Of particular interest as model systems for bent carbon nanostructures are those containing polycyclic aromatic hydrocarbons. Dibenzo[a,e]pentalene (DBP) is a non-alternant polycyclic hydrocarbon with small band gap and tunable optoelectronic properties. However, changing these properties by bending of the DBP structure has yet to be investigated. Herein, we report the synthesis, optoelectronic, and structural properties of (2,7)dibenzo[a,e]pentalenophanes with four different bridge sizes and bending angles of the DBP unit, accompanied by (TD)DFT calculations. The last, strain-inducing dehydration reaction was accomplished by using Burgess' reagent. The HOMO and LUMO levels and the magnetic shielding of protons pointing inside the cyclophane cavity grew stepwise with increasing ring strain. Single-crystal X-ray structures of the smallest three derivatives revealed a near semi-circle and a bend angle of the DBP unit of almost 88° for the smallest derivative. We demonstrated the synthetic versatility of our approach by varying the substituents at the DBP unit, allowing for further tuning of optoelectronic properties. The synthetic strategy presented herein may pave the way for the synthesis of conjugated DBP nanorings.

Dibenzo[a,e]pentalenes with Low-Lying LUMO Energy Levels as Potential n-Type Materials.

Ambipolar organic semiconductors are of high interest for organic field-effect transistors. For n-type conduction, low LUMO energies are required. Dibenzo[a,e]pentalenes (DBPs) are promising compounds; however, few derivatives exist with energetically low-lying LUMO levels. Here, we present DBP derivatives with LUMO energies down to -3.73 eV and small bandgaps down to 1.63 eV determined through cyclic voltammetry, UV/vis absorption spectroscopy, and TDDFT calculations. Single-crystal X-ray diffraction analysis revealed a 1D π-stacking mode. The addition of arylalkynyl substituents at the five-membered rings in a facile and versatile synthetic route allowed for tuning of the band gaps and LUMO energies. The synthetic route can easily be modified to access a variety of DBP derivatives. The LUMO energies of the DBP derivatives presented herein make them attractive for an application in n-type or ambipolar field-effect transistors.

Theoretical analysis of [5.5.6]cyclacenes: electronic properties, strain energies and substituent effects.

A novel class of cyclic conjugated molecules, composed of annelated five- and six-membered rings, is proposed and theoretically investigated using density functional theory and multireference methods with regards to their structures, strain energies, aromaticity (NICS values), electronic ground states, band gaps, and the effect of substituents. These [5.5.6]ncyclacenes are predicted to be low band gap materials (below 1 eV) with, depending on their size, closed-shell singlet ground states. The strain energies from n = 4 upwards lie in the range of the synthetically known [n]cycloparaphenylenes. An investigation of the effect of rim-substitution by methyl, alkynyl, thiomethyl or phenyl groups on the electronic ground states showed that thiomethyl-substitution leads to [5.5.6]ncyclacenes with closed-shell singlet ground states for all sizes n investigated.

Anion-Rocking Chair Batteries with Tuneable Voltage using Viologen- and Phenothiazine Polymer-based Electrodes.

Organic battery electrode materials offer the unique opportunity for full cells to operate in an anion-rocking chair mode. For this configuration a pair of p-type redox-active electrode materials is required with a substantial potential gap between their redox processes. We herein investigate viologen-functionalized polystyrenes as negative electrode paired with a phenothiazine polymer as positive electrode in all-organic full cells. The 10 % crosslinked viologen polymer X10 -PVBV gave better performance than the linear PVBV and was employed in a full cell as negative electrode with cross-linked poly(3-vinyl-N-methylphenothiazine) (X-PVMPT) as positive electrode. Three cell configurations regarding the voltage range were investigated, of which one with an operating potential of 0.9 V gave the highest performance. The full cell delivered a specific discharge capacity of 64 mA h g-1 (of X-PVMPT) in the first cycle and a capacity retention of 79 % after 100 cycles. This is one of only few reported anion rocking chair all-organic cells and the first employing a phenothiazine-based positive electrode material.