Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks.

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node-linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations.

Synthesis and Characterization of Dibenzothieno[a,f]pentalenes Enabling Large Antiaromaticity and Moderate Open-Shell Character through a Small Energy Barrier for Bond-Shift Valence Tautomerization.

Experimental and theoretical rationalization of bond-shift valence tautomerization, characterized by double-well potential surfaces, is one of the most challenging topics of study among the rich electronic properties of antiaromatic molecules. Although the pseudo-Jahn-Teller effect (PJTE) is an essential effect to provide attractive characteristics of 4nπ systems, an understanding of the structure-property relationship derived from the PJTE for planar 4nπ electron systems is still in its infancy. Herein, we describe the synthesis and characterization of two regioisomers of the thiophene-fused diareno[a,f]pentalenes 6 and 7. The magnetic and optoelectronic properties characterize these sulfur-doped diareno[a,f]pentalenes as open-shell antiaromatic molecules, in sharp contrast to the closed-shell antiaromatic systems of 3 and 5, in which these main cores consist of the same number of π electrons as 6 and 7. Notably, thiophene-fused 6b and 7b showed pronounced antiaromaticity, the strongest among the previous systems, as well as moderate open-shell characteristics. Our experimental and theoretical investigations concluded that these properties of 6b and 7b are derived from the small energy barrier Ea‡ for the bond-shift valence tautomerization. The energy profile of the single crystal of 6b showed the temperature-dependent structural variations assigned to the dynamic mutual exchange between the two Cs-symmetric structures, which was also supported by changes in the chemical shifts of variable-temperature 1H NMR spectra in the solution phase. Both experimental and computational results revealed the importance of introducing heteroaromatic rings into 4nπ systems for controlling the PJTE and manifesting the antiaromatic and open-shell natures originating from the high-symmetric structure. The findings of this study advance the understanding of antiaromaticity characterized by the PJTE by controlling the energy barrier for bond-shift valence tautomerizations, potentially leading to the rational design of optoelectronic devices based on novel antiaromatic molecules possessing the strong contributions of their high-symmetric geometries.

Orbital-Energy Modulation of Tetrabenzoporphyrin-Derived Non-Fullerene Acceptors for Improved Open-Circuit Voltage in Organic Solar Cells.

Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage (VOC). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels-Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high VOC of up to 0.94 V when used as a donor or can even work as a new class of nonfullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.

Synthesis of Anthracene Derivatives with Azaacene-Containing Iptycene Wings and the Utilization as a Dopant for Solution-Processed Organic Light-Emitting Diodes.

Substituted acene derivatives are regarded as promising materials for organic electronic devices such as organic light-emitting diodes (OLEDs). In particular, anthracene derivatives are known to exhibit good fluorescence property, with the air stability and solubility in common organic solvents expected to give advantages for solution-processed device fabrication. In this study, a series of bistriisopropylsilyl(TIPS)ethynyl anthracene derivatives with azaacene-containing iptycene wings have been synthesized by using condensation reactions. Effects of size of azaacenes on optical properties and packing structures were investigated. UV/Vis absorption and fluorescence spectra indicate that the π-elongation of iptycene units has small effects on the overall π-system, which is also supported by electrochemical measurements. Secondly, single-crystal X-ray analysis implies that the molecules likely have interactions with the iptycene units of adjacent molecules, while the iptycene wings and TIPSethynyl groups can prevent the central anthracene unit from undesirable non-radiative energy loss. Finally, the most emissive derivative was used as a dopant for solution-processed OLEDs, showing obvious electroluminescence with a luminance of over 920 cd m-2 .

Photoconversion of 6,13-α-diketopentacene single crystals exhibiting light intensity-dependent morphological change.

Recently, we revealed that 6,13-dihydro-6,13-ethanopentacene-15,16-dione (PDK) could be quantitatively photoconverted into pentacene even in the crystal phase, accompanied by the destruction of the crystals. In this work, we investigated the relationship between the photoinduced morphological changes and the light intensity for the photoconversion at a single micrometre-sized crystal level. Photoirradiation with a strong intensity (over 100 kW cm-2) resulted in hole formation in a single crystal. When medium intensity (0.5-100 kW cm-2) was irradiated, destruction including separation and jumping of the crystal was observed. Absorption spectrum measurement of the single crystal revealed that when almost same number of pentacene was generated, the destruction was induced by the generated strain within crystal due to the stacking mismatch between the different molecules. Upon photoirradiation with a low intensity (below 0.5 kW cm-2), protruding pillar objects were observed on the crystal surface. This formation is a result of the surface movement of molecules through the relaxation of strain. Our results provide important insight into stimuli-responsive crystal materials and could contribute to the generation and application of remotely controllable smart materials.

Semiconducting π-Extended Tetrathiafulvalene Derivatives.

Tetrathiafulvalene (TTF) has been one of the most studied compounds, since the discovery of electrical conductivity as a charge-transfer complex in combination with tetracyano-p-quinodimethane (TCNQ) in 1973. In 2004, TTF was realized in a new light as an innate semiconductor material, as well as conductor and superconductor material. Because of the ready modification of its core skeleton and better solubility than that of acene compounds, many TTF derivatives have been reported to attain better charge-carrier mobility for field-effect transistor devices. Considering the high charge-carrier mobility of acene and TTF derivatives, annulation of the acene and TTF moieties is expected to improve the charge-carrier mobilities. This Minireview focuses on the syntheses, crystal structures, and electronic properties of state-of-the-art π-extended TTF derivatives, based on the history of the development of TTF derivatives. The relationship between the packing structure and charge-carrier mobilities is also discussed.

An Ethynylene-Bridged Pentacene Dimer: Two-Step Synthesis and Charge-Transport Properties.

A rigid and planar ethynylene-bridged pentacene dimer (PenD) was synthesized from pentacenequinone in two steps, skipping the conventional stepwise approach. A brickwork motif in the single crystal shows two-dimensionally extended electronic interaction in the solid state. Highly crystalline dip-coated films exhibited average hole mobility of 0.24 cm2 V-1 s-1 , comparable to that of the single-crystal organic field-effect transistors. This discovery and understanding of the reaction for the facile synthesis of ethynylene-bridged π-conjugated systems enables to the synthesis of a wide range of organic semiconducting materials.

Dinaphthotetrathiafulvalene Bisimides: A New Member of the Family of π-Extended TTF Stable p-Type Semiconductors.

Air-stable organic semiconductors based on tetrathiafuluvalene (TTF) were developed by synthesising a series of dinaphthotetrathiafulvalene bisimides (DNTTF-Im) using electron-donating TTF, π-extended naphthalene, and electron-withdrawing imide. Electron-spin-resonance spectroscopy and X-ray single-crystal structure analysis of aryl-substituted DNTTF-Im radical cations confirmed that localisation of the spin resides on the electron-donating TTF moiety. The organic field-effect transistor properties derived from the use of highly crystalline n-butyl (C4) and n-hexyl(C6)-substituted DNTTF-Im were assessed. The hole carrier mobility of C6-DNTTF-Im was improved from 3.7×10-3  cm2 V-1 s-1 to 0.30 cm2 V-1 s-1 in ambient conditions. This is attributed to the raise of the substrate temperature from 25 °C to 200 °C during sublimation. The XRD and microscopy analysis suggested that increasing the substrate temperature accelerates the end-on packing resulting in larger grains suitable for hole charge transport parallel to the substrate.

Directing the Crystallization of Dehydro[24]annulenes into Supramolecular Nanotubular Scaffolds.

The self-assembly of a series of dehydro[24]annulene derivatives into columnar stacks has been examined for its latent ability to form π-conjugated carbon-rich nanotubular structures through topochemical polymerizations. We have studied the parameters affecting self-assembly, including the nature of the substituent and crystallization conditions, using 10 different dehydro[24]annulene derivatives. In particular, hydrogen-bonding interactions through carbamate groups were found to be especially useful at directing the formation of nanotubular supramolecular assemblies. We have also evaluated the electronic coupling between neighboring dehydroannulene molecules within these supramolecular assemblies. Density functional calculations on the stacked supramolecular nanotube assemblies show that transfer integrals vary considerably between the three columnar assemblies, ranging from moderate to high (59-98 meV for the highest occupied molecular orbitals, 63-97 meV for the lowest unoccupied molecular orbitals), depending on the local molecular topology. In addition, the dehydro[24]annulene derivatives afforded distinct architectures in the crystal, including nanochannel arrays, sheets with solvent-filled pores, and lamellae. This work is an essential step toward a controlled formation of covalently linked carbon-rich nanostructures generated from molecular precursors with a latent diacetylene reactivity.

Seven Post-synthetic Covalent Reactions in Tandem Leading to Enzyme-like Complexity within Metal-Organic Framework Crystals.

The design of enzyme-like complexity within metal-organic frameworks (MOFs) requires multiple reactions to be performed on a MOF crystal without losing access to its interior. Here, we show that seven post-synthetic reactions can be successfully achieved within the pores of a multivariate MOF, MTV-IRMOF-74-III, to covalently incorporate tripeptides that resemble the active sites of enzymes in their spatial arrangement and compositional heterogeneity. These reactions build up H2N-Pro-Gly-Ala-CONHL and H2N-Cys-His-Asp-CONHL (where L = organic struts) amino acid sequences by covalently attaching them to the organic struts in the MOFs, without losing porosity or crystallinity. An enabling feature of this chemistry is that the primary amine functionality (-CH2NHBoc) of the original MOF is more reactive than the commonly examined aromatic amines (-NH2), and this allowed for the multi-step reactions to be carried out in tandem within the MOF. Preliminary findings indicate that the complexity thus achieved can affect reactions that were previously accomplished only in the presence of enzymes.

Aromaticity Relocation in Perylene Derivatives upon Two-Electron Oxidation To Form Anthracene and Phenanthrene.

We prepared perylene dications 1(2+) and 2(2+) by using "capped" perylene derivatives, and for the first time, successfully obtained single crystals of a perylene dication 1(2+) that enabled us to perform its structural analysis. We realized that the substituted aryl groups on perylene control the positions of positive charges, thus the remaining electronic system satisfies Clar's sextet rule toward the highest number of localized sextets. Experimental and theoretical evidence proved that Clar's aromatic π-sextet rule could be applied even for the dicationic perylenes in a very simple way.

Self-Assembled Dehydro[24]annulene Monolayers at the Liquid/Solid Interface: Toward On-Surface Synthesis of Tubular π-Conjugated Nanowires.

We have studied the self-assembly behavior of dehydro[24]annulene (D24A) derivatives 1, 2a-2d, and 3a-3c at the liquid/solid interface using scanning tunneling microscopy (STM). Both the relative placement and the nature of the four D24A substituents strongly influence the self-assembly pattern. Overall, the eight D24A derivatives examined in this study display seven types of 2D packing patterns. The D24A derivatives 1, 2a, and 3a have either two or four stearate groups and adopt face-on configurations of their macrocyclic cores with respect to the highly oriented pyrolytic graphite (HOPG) surface. Their 2D packing pattern is determined by the interchain spacings and number of stearate substituents. The D24A derivatives 2b-2d and 3b-3c bear hydrogen-bonding carbamate groups to further strengthen intermolecular interactions. Face-on patterns were also observed for most of these compounds, while an unstable edge-on self-assembly was observed in the case of 2b at room temperature. Stable edge-on self-assemblies of D24A derivatives were sought for this work as an important stepping stone to achieving the on-surface topochemical polymerization of these carbon-rich macrocycles into tubular π-conjugated nanowires. The overall factors determining the 2D packing patterns of D24As at the liquid/solid interface are discussed on the basis of theoretical simulations, providing useful guidelines for controlling the self-assembly pattern of future D24A macrocycles.

An optically and thermally switchable electronic structure based on an anthracene-BODIPY conjugate.

An optically and thermally responsive boron dipyrromethene (BODIPY) dye, namely, meso-2-(9,10-dihydro-9,10-ethanoanthracene-11,12-dione) (DK)-linked, bicyclo[2.2.2]octadiene (BCOD)-fused BODIPY (BCOD-DK), was synthesized. The weakly luminous structure of BCOD-DK can be changed quantitatively to that of the strongly fluorescent BODIPY BCOD-Ant by optical excitation at the DK unit, which induces double decarbonylation of the DK unit to give an anthracene unit. The solvent effect on the fluorescence properties of BCOD-DK suggests that the dramatic change in fluorescence intensity is controlled by intramolecular electron transfer from the BODIPY moiety to the meso-DK substituent. BCOD-DK is converted to meso-DK benzene-fused BODIPY (Benzo-DK) by heating at 220 °C with 64-70 nm redshift of absorption and fluorescence peaks without changing the fluorescence quantum yield of ΦF =0.08 in dichloromethane. Benzo-DK can be converted to strongly fluorescent meso-anthracene benzene-fused BODIPY Benzo-Ant by optical excitation. Thus, BCOD-DK can show four different optical performances simply by irradiation and heating, and hence may be applicable for optical data storage and security data encryption.

Evaluation of the charge transfer efficiency of organic thin-film photovoltaic devices fabricated using a photoprecursor approach.

Recently, a unique 'photoprecursor approach' was reported as a new option to fabricate a p-i-n triple-layer organic photovoltaic device (OPV) through solution processes. By fabricating the p-i-n architecture using two kinds of photoprecursors and a [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the donor and the acceptor, the p-i-n OPVs afforded a higher photovoltaic efficiency than the corresponding p-n devices and i-devices, while the photovoltaic efficiency of p-i-n OPVs depended on the photoprecursors. In this work, the charge transfer efficiency of the i-devices composed of the photoprecursors and PC71BM was investigated using high-sensitivity fluorescence microspectroscopy combined with a time-correlated single photon counting technique to elucidate the photovoltaic efficiency depending on the photoprecursors and the effects of the p-i-n architecture. The spatially resolved fluorescence images and fluorescence lifetime measurements clearly indicated that the compatibility of the photoprecursors with PC71BM influences the charge transfer and the photovoltaic efficiencies. Although the charge transfer efficiency of the i-device was quite high, the photovoltaic efficiency of the i-device was much lower than that of the p-i-n device. These results imply that the carrier generation and carrier transportation efficiencies can be increased by fabricating the p-i-n architecture.

Tetrabenzoperipentacene: Stable Five-Electron Donating Ability and a Discrete Triple-Layered ß-Graphite Form in the Solid State.

An oxidative ring-closure reaction of a tetranaphthylpyrene derivative led to the synthesis of a 56 all-carbon conjugated tetrabenzoperipentacene. In the single-crystal X-ray structure, three molecules make a triple-layered cluster by π-stacking, wherein each layer rotates by 120°, and is thus considered a petit ß-graphite. As for the optical properties, the Stokes shift is extremely small (10 cm(-1) ), thus indicating its remarkably rigid framework. The tetrabenzoperipentacene exhibits reversible five-electron oxidation waves in cyclic voltammetry, and is regarded as a counterpart to the fullerene C60 in terms of stable multicharge-storage nanocarbon materials.

Metal-organic frameworks with precisely designed interior for carbon dioxide capture in the presence of water.

The selective capture of carbon dioxide in the presence of water is an outstanding challenge. Here, we show that the interior of IRMOF-74-III can be covalently functionalized with primary amine (IRMOF-74-III-CH2NH2) and used for the selective capture of CO2 in 65% relative humidity. This study encompasses the synthesis, structural characterization, gas adsorption, and CO2 capture properties of variously functionalized IRMOF-74-III compounds (IRMOF-74-III-CH3, -NH2, -CH2NHBoc, -CH2NMeBoc, -CH2NH2, and -CH2NHMe). Cross-polarization magic angle spinning (13)C NMR spectra showed that CO2 binds chemically to IRMOF-74-III-CH2NH2 and -CH2NHMe to make carbamic species. Carbon dioxide isotherms and breakthrough experiments show that IRMOF-74-III-CH2NH2 is especially efficient at taking up CO2 (3.2 mmol of CO2 per gram at 800 Torr) and, more significantly, removing CO2 from wet nitrogen gas streams with breakthrough time of 610 ± 10 s g(-1) and full preservation of the IRMOF structure.

Correction to "Performance Improvement with an Ultrathin p-Type Interfacial Layer in n-Type Vertical Organic Field-Effect Transistors Based on Reduced Graphene Oxide Electrode".

[This corrects the article DOI: 10.1021/acsomega.2c02085.].

Nonpolymer Organic Solar Cells: Microscopic Phonon Control to Suppress Nonradiative Voltage Loss via Charge-Separated State.

Recent remarkable developments on nonfullerene solar cells have reached a photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this regard, understanding the impact of small donor molecules on nonpolymer solar cells is essential. Here, we systematically investigated mechanisms of solar cell performance using diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP) conjugates of C4-DPP-H2BP and C4-DPP-ZnBP, where C4 represents the butyl group substituted at the DPP unit as small p-type molecules, while an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester is employed. We clarified the microscopic origins of the photocarrier caused by phonon-assisted one-dimensional (1D) electron-hole dissociations at the donor-acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking. This ensures carrier transport through stacking molecular conformations to suppress nonradiative voltage loss capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. We show that, while disordered lattice motions by the π-π stackings via zinc ligation are essential to enhance the entropy for charge dissociations at the interface, too much ordered crystallinity causes the backscattering phonon to reduce the open-circuit voltage by geminate charge-recombination.

Performance Improvement with an Ultrathin p-Type Interfacial Layer in n-Type Vertical Organic Field-Effect Transistors Based on Reduced Graphene Oxide Electrode.

Vertical organic field-effect transistors (VOFETs) with a large current on/off ratio and easy fabrication process are highly desirable for future organic electronics. In this paper, we proposed an ultrathin p-type copper (II) phthalocyanine (CuPc) interfacial layer in reduced graphene oxide (rGO)-based VOFETs. The CuPc interfacial layer was sandwiched between the rGO electrode and the N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) organic layer. The introduced CuPc interfacial layer not only decreased the off-current density of the device but also slightly enhanced the on-current density. The threshold voltage of the device was also effectively improved and stabilized at around 0 V. The obtained device exhibited a current on/off ratio exceeding 106, which is the largest value reported for rGO-based VOFETs. The vertical electron mobility of the PTCDI-C8 layer estimated by the space-charge-limited current technique was 1.14 × 10-3 cm2/(V s). However, it was not the main limiting factor for the current density in this device. We totally fabricated 48 devices, and more than 75% could work. Besides, the device was stable with little performance degradation after 1 month. The use of low-cost, solution-processable rGO as work-function-tunable electrode and the application of an ultrathin CuPc interfacial layer in VOFETs may open up opportunities for future organic electronics.

Exploration of Alkyl Group Effects on the Molecular Packing of 5,15-Disubstituted Tetrabenzoporphyrins toward Efficient Charge-Carrier Transport.

The high design flexibility of organic semiconductors should lead to diverse and complex electronic functions. However, currently available high-performance organic semiconductors are limited in variety; most of p-type materials are based on thienoacenes or related one-dimensionally (1D) extended π-conjugated systems. In an effort to expand the diversity of organic semiconductors, we are working on the development of tetrabenzoporphyrin (BP) derivatives as active-layer components of organic electronic devices. BP is characterized by its large, rigid two-dimensionally (2D) extended π-framework with high light absorptivity and therefore is promising as a core building unit of organic semiconductors for optoelectronic applications. Herein, we demonstrate that BP derivatives can afford field-effect hole mobilities of >4 cm2 V-1 s-1 upon careful tuning of substituents. Comparative analysis of a series of 5,15-bis(n-alkyldimethylsilylethynyl)tetrabenzoporphyrins reveals that linear alkyl substituents disrupt the π-π stacking of BP cores, unlike the widely observed "fastener effect" for 1D extended π-systems. The n-octyl and n-dodecyl groups have the best balance between high solution processability and minimal π-π stacking disruption, leading to superior hole mobilities in solution-processed thin films. The resulting thin films show high thermal stability wherein the field-effect hole mobility stays above 1 cm2 V-1 s-1 even after heating at 160 °C in air, reflecting the tight packing of large BP units. These findings will serve as a good basis for extracting the full potential of 2D extended π-frameworks and thus for increasing the structural or functional diversities of high-performance organic semiconductors.