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Crystal and molecular structure of poly(1,4-phenyleneazine- N , N -dioxide) (PDNB, 1), obtained from polymerization of 1,4-dinitrosobenzene, is investigated by X-ray powder diffraction (XRPD) and DFT calculations. Starting with the determined crystal structure, we have calculated the electronic structure of the polymer, which reveals that 1 is potentially a wide-bandgap (WBG) semiconductor with a band gap of 2.3 eV. Its calculated electronic spectrum with the absorbance at 400 nm is in good agreement with the experimental data. A highly organized crystal structure with linear chains of polymer indicates possible anisotropic electronic properties that could be useful in molecular photoelectronics (e.g OLEDs). Photolysis of 1 at cryogenic temperatures yields dinitrosobenzene monomers, which re-polymerize at higher temperatures. After this reversible photo-thermal cycle, the starting polymorph of 1 is recovered. Thermal analysis (TG/DSC/DTA) and in situ XRPD measurements show the high thermal stability of 1 until around 150 °C. Image 1 • The crystal structure is determined via X-ray powder diffraction. • The ON-OFF polymerization mechanism is studied at low temperature. • Reversible polymerization always gives the same polymorphic form. • Calculation of electronic properties reveals wide-bandgap semiconductivity. • Thermal analyses and in situ studies show stability up to circa 150 °C. [ABSTRACT FROM AUTHOR]

A family of seven symmetrical molecular rods, which are sharing 4-ethynylpyridyl terminal groups and differ in the structure of the linker have been investigated. Although structurally related they show unique electrochemical behavior, which clearly correlates with their conformational flexibility. Two flexible compounds are reduced by two electrons. Their reduction yields intensive charge transfer UV-vis absorption bands and is accompanied by characteristic EPR spectra. In contrast, all conformationally rigid structures are reduced by four electrons and do not yield neither charge transfer bands nor distinct EPR signals. Simple irreversible voltammetry of all derivatives is similar to the reduction of 4-ethynylpyridine at -2.1 V. The experimental observations were supported by the extensive DFT calculations. Electrochemical techniques proved to be valuable in distinction between molecular rigidity and flexibility. [Display omitted] [ABSTRACT FROM AUTHOR]