Azoxybenzenes have been widely studied due to their diverse applications, but reports on their detailed structural characterization are rare. It is often the case that even when the structure is obtained, the azoxy group is disordered, which makes it difficult to determine its geometrical parameters correctly and, in some cases, even its chemical identity (due to oxygen disorder at both nitrogen sites). Here, the molecular structure of a nitrile-substituted azoxybenzene derivative, 1,2-bis-(4-cyanophenyl)-diazene oxide (1), was accurately determined by single-crystal X-ray diffraction (SCXRD) performed by using crystals of a dichloromethane solvate (1a), which displays channels that propagate along the a-axis. SCXRD measurements showed no significant changes in the unit cell parameters of 1a, even at 127 °C. TG measurements reveal that 1a remains stable up to ∼190 °C, at which point dichloromethane molecules start to leave the structure, and a disordered structure (1b) forms. Compound 1 was used for the synthesis of azoxy-linked triazine-based polymer (2) by a trifluoromethanesulfonic acid-catalyzed trimerization reaction under microwave conditions. The obtained polymer 2 is an amorphous solid with good thermal stability, which exhibits a rather low BET surface area of 10.2 m2 g-1 and shows no affinity for CO2 adsorption. The preliminary investigation revealed that polymer 2 has a band gap of 2.69 eV, rendering it a wide-bandgap semiconductor.

Azo-linked porphyrin porous organic polymers (APPs) with metal-free frameworks were investigated as sustainable photocatalysts for the visible-light-driven fixation of CO 2 into styrene carbonate in the presence of TBAB and ZnCl 2 ...

Using the dinitrosobenzene polymer (1) as an example, we explore how the electronic, transport, and optical properties of a conjugated organic semiconductor can be modulated. Combining computational and experimental tools, we explore the effects of solid-state packing, backbone torsion, surface adsorption, the conjugation in the aromatic core, and substituents. The band gap (Eg) and optical spectrum of 1 are calculated using both GW-BSE with zero-gap renormalization (ZGR) and hybrid TD-DFT, with the former method predicting a value (2.41 eV) in excellent agreement with our diffuse reflectance spectroscopy measurements (2.39 eV). Using GW-BSE-ZGR, changes occurring upon solid-state packing are separated into a contribution arising from (i) the change in the torsional angle and (ii) the change in the screened Coulombic interaction, which strongly affects the exciton binding energies. Comprehensive hybrid TD-DFT calculations find that the effects of substituents on Eg and on transport properties can mostly be explained through changes in the torsional angle t, and predict a linear dependence between t and Eg. Extending the conjugation in the aromatic core is found to enhance transport properties and narrow Eg, identifying future synthetic targets. Atomic force microscopy and spectroscopic ellipsometry are used to study 1 adsorbed to a (111) gold surface (1@Au), with the latter method showing a significant narrowing of the band gap to 0.68 eV, in good agreement with TD-DFT predictions.

An optimized protocol for the rapid synthesis of azo-linked porous organic polymers (POPs) containing trigonal triphenylpyridine (AZO-P-M), triphenyltriazine (AZO-T-M), and tetragonal tetraphenylethylene (AZO-E-M) central units by microwave-assisted NaBH4-mediated reductive homocoupling of the corresponding aromatic nitro monomers is presented. The structural and functional features of the azo-linked polymers prepared under microwave heating were directly compared to their counterparts obtained by conventional synthesis. Similar to azo-linked polymers synthesized by conventional reductive homocoupling of nitro monomers with NaBH4, the polymers prepared under microwave irradiation are amorphous solids of good thermal stability showing moderate (e.g., AZO-E-M with BET surface area of 302.1 m2 g-1) to modest (e.g., AZO-P-M with BET surface area of 22.9 m2 g-1) porosities. Although the microwave-assisted procedure for the synthesis of azo-linked polymers described in this work did not result in systems with improved porosities, their efficiency for CO2 adsorption (up to 29 mg g-1 at 306 K) is comparable to that of POPs synthesized by conventional heating. Therefore, the herein reported protocol could be used for the fast and efficient synthesis of new azo-linked POPs with potential for CO2 capture.

We synthesized a series of azo-linked porphyrin-based porous organic polymers (APPs) with linear, bent, and trigonal linkers (APP-1 to APP-6) and with directly connected tetraphenylporphyrin units (APP-7a, APP-7b and APP-8). The synthesized APPs are amorphous solids demonstrating good thermal stability and diverse BET surface areas. APPs with linkers showed significantly higher surface areas (469 to 608 m2 g-1) compared to those with directly connected tetraphenylporphyrin units (0.3 to 23 m2 g-1). Higher surface areas correlated with enhanced CO2 adsorption, particularly for APP-1, APP-2, and APP-5 with experimental CO2 uptake values of 41 mg g-1, 38 mg g-1, and 38 mg g-1, respectively, at 306 K. The computational study supported the experimental findings and provided insights on how surface area and the local landscape affect the CO2 adsorption. Although the computational models were based on ideal structures, while the experiments revealed the materials were amorphous, the calculated CO2 adsorption capacities were roughly comparable to the experimental results, particularly for the 3D systems (APP-5 and APP-6) and the 2D systems with directly connected building units (APP-7 and APP-8). Porphyrin units in the framework serve as additional binding sites for CO2, especially when unhindered and available on either side of the porphyrin plane. This work highlights the potential of 2D layered APPs and 3D topologies for efficient CO2 capture.

The possibility of on-surface intermolecular interactions of aromatic C-nitroso derivatives and the formation of azodioxy polymer thin films were studied by ellipsometry, atomic force microscopy (AFM) and nanoscale Fourier transform infrared (nano-FTIR) spectroscopy. The nitroso terminal groups of monolayers of 3-thiocyanatopropyl-4-nitrosobenzoate on Au(111) surface were used as initiation sites for formation of azodioxy linkages by interactions with the six structurally different aromatic dinitroso derivatives prone to polymerization. Ellipsometry showed increased thicknesses of films formed by interactions of nitroso groups at the monolayer interface and aromatic dinitroso derivatives in solution suggesting the formation of azodioxy oligomer films. The obtained thickness values indicated that the films are composed of only a few monomeric subunits or that poorly organized surface structures are formed with possibly tilted and intertwined chains. Ellipsometry data were corroborated with AFM topography images which revealed the appearance of a high number of islands, attributed to domains of azodioxy oligomers, and increased local RMS roughness values. Both ellipsometry and AFM indicated a greater tendency towards the azodioxy oligomers on the Au(111) surface at longer adsorption times. Nano-FTIR spectroscopy enabled chemical identification of the films at the nanoscale. Bands attributed to the E-azodioxy groups were detected in nano-FTIR spectra, which strongly supported the conclusion that the islands in the AFM images represent azodioxy oligomers, the formation of which was initiated by interactions of nitroso groups at the monolayer interface with dinitroso derivatives in solution.

Porous organic polymers incorporating nitrogen-rich functionalities have recently emerged as promising materials for efficient and highly selective CO2 capture and separation. Herein, we report synthesis and characterization of new two-dimensional (2D) benzene- and triazine-based azo-bridged porous organic polymers. Different synthetic approaches towards the porous azo-bridged polymers were tested, including reductive homocoupling of aromatic nitro monomers, oxidative homocoupling of aromatic amino monomers and heterocoupling of aromatic nitro monomers and a series of aromatic diamines of different lengths and rigidity. IR spectroscopy, 13C CP/MAS NMR spectroscopy, powder X-ray diffraction, elemental analysis, thermogravimetric analysis, nitrogen adsorption–desorption experiments and computational study were used to characterize structures and properties of the resulting polymers. The synthesized azo-bridged polymers are all amorphous solids of good thermal stability, exhibiting various surface areas (up to 351 m2 g−1). The obtained results indicated that the synthetic methods and building units have a pronounced effect on the porosity of the final materials. Reductive and oxidative homocoupling of aromatic nitro and amino building units, respectively, lead to 2D azo-bridged polymers of substantially higher porosity when compared to those produced by heterocoupling reactions. Periodic DFT calculations and Grand-canonical Monte Carlo (GCMC) simulations suggested that, within the used approximations, linear linkers of different lengths do not significantly affect CO2 adsorption properties of model azo-bridged polymers.

CO 2 adsorption properties of azo, azoxy and azodioxy-linked porous organic polymers can be predicted from the calculated electrostatic potential values.

The polymerization property of aromatic polynitroso compounds could be used to create azodioxy porous networks with possible application for the adsorption of CO2, the main greenhouse gas. Herein, we report the synthesis and characterization of new aromatic polynitroso compounds, with para-nitroso groups attached to the triphenylbenzene, triphenylpyridine, triphenyltriazine and triphenylamine moiety. The synthesis of the pyridine-based trinitroso compound was performed by reduction of the corresponding trinitro derivative to N-arylhydroxylamine followed by oxidation to the trinitroso product. For the synthesis of the benzene- and triazine-based trinitroso compounds, a novel synthetic strategy was implemented, which included cyclotrimerization of the 4-nitrosoacetophenone and 4-nitrosobenzonitrile, respectively. Reduction of the trinitro compound with triphenylamine unit produced the dinitroso product. In a solid state, all synthesized compounds form E-azodioxy oligomers or polymers. While azodioxy polymer with triphenylbenzene moiety is an amorphous solid, other azodioxy oligomers and polymers displayed sharp diffraction peaks pointing to their crystalline nature. A computational study indicated that eclipsed AA configurations are preferred over staggered AB and inclined AA’ configurations. The serrated layers may be the most likely outcome when/if 2D layers form an organized polymer network of azodioxy linked triphenyltriazine-based building blocks.

The possibilities for tuning of electronic, transport, and optical properties of the linear dinitrosobenzene polymer (1) are explored. The band gap (Eg) and optical spectrum of 1 are calculated using both GW-BSE corrected for zero-point vibrations and hybrid TD-DFT, with the former method predicting a value (2.41 eV) in excellent agreement with diffuse reflectance spectroscopy measurements (2.39 eV). GW-BSE is also used to evaluate the effects of solid-state packing, while comprehensive TD-DFT calculations are employed to study the effects of intra-polymer torsion, gold surface adsorption, substitution, and changes in the aromatic core of 1. Torsion is found to be an important factor in determining Eg and transport properties, and a strong effect of the environment on the exciton binding energies is identified. Extending the conjugation in the aromatic core is found to enhance transport properties and narrow Eg, identifying future synthetic targets. Atomic force microscopy and spectroscopic ellipsometry are used to study 1 adsorbed to a (111) gold surface (1@Au), with the latter method showing a significant narrowing of the band gap to 0.68 eV, in good agreement with TD-DFT predictions.

Dimerization of nitrosobenzenes also occurs at metal-solution interface which enables the design of azodioxy self-assembled bilayers. Formation of self-assembled adlayers is a dynamic process, dependent on various parameters. To gain new insights into the key experimental parameters governing a complex process of on-surface self-assembly and dimerization of nitrosobenzenes, here we studied the growth of adlayers of nitrosobenzene derivatives containing thiocyanate headgroup for adsorption on an Au(111) as a function of solution concentration and adsorption time. Ellipsometry revealed an increase in film thicknesses with increasing solution concentration and adsorption times, suggesting formation of more ordered adlayers and a greater tendency towards bilayers. This was supported by AFM and STM microscopy which showed variations in local surface morphology of adlayers in dependence on experimental parameters. It was found that higher solution concentration and longer adsorption times lead to formation of more ordered separate domains which were attributed to monolayers and bilayers.

Three types of organic solid-state reactions, dimerizations, dissociations, and Z-E isomerizations were investigated by using the transformations of aromatic C-nitroso compounds in crystalline solids as a convenient molecular model. Here we propose a conceptual frame for solid-state organic reaction mechanisms by examining activation parameters obtained from kinetic measurements under specific experimental conditions. The possibility of the appearance of a sort of short-lived intermediate liquid phase that constitutes a critical condition for initiating chemical reaction in crystalline solids, similarly to the mechanism for the thermal solid-state reactions proposed by Paul and Curtin is discussed. The analogy of the proposed concept with the recent hypothesis about the variable rigidity/ softness of the reaction cavity in the enzyme reactions, and with the newest molecular dynamic simulation studies of solid phase transformations was considered.

In this diploma thesis, kinetics of dimerization reactions of m-halogenonitrosobenzenes in solid state were studied. m-Chloro- and m-bromonitrosobenzene were prepared by oxidation of the corresponding amino derivatives and were isolated in the form of E-dimers by crystallization from the solution. Mixture of Z- and E-dimers was prepared by sublimation of m-halogenonitrosobenzenes under reduced pressure. Monomers of mhalogenonitrosobenzenes were prepared by cryogenic photodissociation of dimers in KBr pellet. Kinetics of dimerization reactions after cryogenic photodissociation were followed by using time-resolved IR spectroscopy at various temperatures. From the temperature dependence of the reaction rate constants, the activation parameters for the reaction of dimerization of m-halogenonitrosobenzenes monomers were estimated in solid state.