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Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received great interest owing to their unusual electronic structure and potential applications in non-linear optics and photovoltaics. Here, we report the generation of unsubstituted indeno[1,2-a]fluorene, the final and yet unreported parent indenofluorene regioisomer, on various surfaces by cleavage of two C-H bonds in 7,12-dihydro indeno[1,2-a]fluorene through voltage pulses applied by the tip of a combined scanning tunneling microscope and atomic force microscope. On bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state, while it exhibits charge bistability between neutral and anionic states on the lower work function surfaces of bilayer NaCl on Ag(111) and Cu(111). In the neutral state, indeno[1,2-a]fluorene exhibits either of two ground states: an open-shell {\pi}-diradical state, predicted to be a triplet by density functional and multireference many-body perturbation theory calculations, or a closed-shell state with a para-quinodimethane moiety in the as-indacene core. Switching between open- and closed-shell states of a single molecule is observed by changing its adsorption site on NaCl., Comment: 12 pages, 3 figures and supplementary information containing additional data

Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received great interest owing to their unusual electronic structure and potential applications in non-linear optics and photovoltaics. Here, we report the generation of unsubstituted indeno[1,2-a]fluorene, the final and yet unreported parent indenofluorene regioisomer, on various surfaces by cleavage of two C-H bonds in 7,12-dihydro indeno[1,2-a]fluorene through voltage pulses applied by the tip of a combined scanning tunneling microscope and atomic force microscope. On bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state, while it exhibits charge bistability between neutral and anionic states on the lower work function surfaces of bilayer NaCl on Ag(111) and Cu(111). In the neutral state, indeno[1,2-a]fluorene exhibits either of two ground states: an open-shell π-diradical state, predicted to be a triplet by density functional and multireference many-body perturbation theory calculations, or a closed-shell state with a para-quinodimethane moiety in the as-indacene core. Switching between open- and closed-shell states of a single molecule is observed by changing its adsorption site on NaCl., 12 pages, 3 figures and supplementary information containing additional data

Nobelovu nagradu iz kemije za 2018. godinu podijelilo je troje znanstvenika, što se ustalilo kao uobičajena praksa u posljednjih desetak godina. Polovicu nagrade dobila je Frances H. Arnold s Caltecha za usmjerenu evoluciju enzima, a drugu polovicu 9 milijuna švedskih kruna podijelili su George P. Smith (University of Missoury Columbia) i Sir Gregory P. Winter (Cambridge), za tehniku prikaza peptida na površini faga (phage display).

Using the dimerization-dissociation of aromatic C-nitroso compounds as a molecular model (Scheme) we have opened the discussion about the possible conceptualization of the reaction mechanisms in the molecular crystals. By detailed studies of the thermodynamics and kinetics of these thermally induced reactions occurring in crystalline solids we have found that the most important parameter which modify the chemical reactivity is activation entropy. Variation in activation entropy that could be caused by softening of the reactive zone within the crystal can in most cases compensate the changes in the enthalpy of activation, keeping the rate coefficient (and the free energy of activation) within the same range of values under different conditions. Since similar idea appeared also in studying thermodynamics and kinetics of the enzyme catalysis, investigations of the solid-state reaction mechanisms by using convenient models could be of more general importance in physical organic chemistry.

Aromatic C-nitroso compounds can appear as nitroso monomers, and Z- and E-azodioxy dimers. In solution, there is an equilibrium between monomers and dimers which is usually shifted towards the monomers at ambient conditions. However, lowering of solution temperature or crystallization favor dimerization. We studied possible dimerization and/or oligomerization of several new aromatic dinitroso derivatives differing in the spacer group between the aromatic rings. Compounds with multiple aromatic nitroso groups are interesting because thay can form oligomeric and polymeric azodioxides. Solution-state structures and nitroso monomer-azodioxide equilibria of aromatic dinitroso compounds were investigated by one- and two-dimensional variable temperature 1H NMR spectroscopy and quantum chemical calculations.1 Inspection of NMR spectra revealed that aromatic dinitroso compounds dimerize at low temperatures yielding the mixture of monomers and Z- and/or E- azodioxides. The proton chemical shifts of nitroso monomer-azodioxide mixtures were assigned and validated by DFT calculations. In most cases, a preference towards the formation of Z-dimer was observed. The ability of aromatic C-nitroso compounds to dimerize could be used for design of self-assembled bilayers on metal surfaces (Figure 1). Our studies suggested that adsorption of nitrosobenzene derivatives, functionalized with sulfur headgroups, on an Au(111) surface results with the formation of ordered self-assembled monolayers and bilayers with molecules arranged into hexagonal structures.2 More recently, we investigated the possibility of self-assembly and dimerization of sulfur-containing aromatic dinitroso compounds, with nitroso groups in mutual meta-position, on gold surface.3 The results demonstrated that aromatic dinitroso compounds dimerize on an Au(111) surface by forming bilayers through E-azodioxide bonds. However, adlayers of dinitroso compounds are less ordered when compared to those of mononitroso compounds probably due to the steric hindrance of meta-situated nitroso groups.

Dimerizacija aromatskih nitrozo spojeva koristan je modelni sustav za praćenje kemijskih reakcija u čvrstom stanju.

Using the dimerization-dissociation of aromatic C-nitroso compounds as a molecular model (Scheme) we have developed the method for studying the reaction mechanisms in the molecular crystals. By detailed studies of the thermodynamics and kinetics of these thermally induced reactions we have found that the most important parameter, which modify the chemical reactivity is activation entropy. Variation in activation entropy could be explained by softening of the reactive zone around the transition state within the crystal. Since the analogous entropic effect of the variable rigidity has been found also for the transition state encapsulated in the enzyme cavity, our investigations of the solid-state reaction mechanisms by using convenient models could be of more general importance in physical organic chemistry.

Aromatic C-nitroso compounds and their Z- and/or E-azodioxy dimers exist in equilibrium in solution, which is usually shifted towards the monomers at ambient conditions.[1] Dimers can be formed by lowering the solution temperature or by crystallization. Compounds with two or more nitroso groups are able to form oligomers and polymers by interactions through multiple azodioxide bonds. Recently, we prepared several new aromatic dinitroso derivatives differing in spacer between two phenyl rings. Inspection of IR spectra revealed that in solid state these compounds are mostly present as E- or Z-oligomers. In order to investigate their possible dimerization or oligomerization in solution, we recorded one- and two-dimensional 1H NMR spectra at ambient and low temperatures. Analysis of NMR spectra indicated that aromatic dinitroso derivatives dimerize at low temperatures affording a mixture of nitroso monomers and Z- and/or E- azodioxides. The proton chemical shifts of nitroso monomer-azodioxide mixtures were assigned and validated by DFT calculations. In almost all cases, a preference towards the formation of Z- azodioxy isomer was found. In addition, we recently investigated self-assembly and possible dimerization of sulfur-containing aromatic dinitroso derivatives on gold surface by using polarization modulation infrared reflection- absorption spectroscopy (PM-IRRAS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), ellipsometry and water contact angle goniometry.[2] The results obtained suggested that aromatic dinitroso compounds are able to dimerize on gold surface through E-azodioxide bonds. In comparison to their mononitroso counterparts, adsorption of aromatic dinitroso derivatives on gold surface results in the formation of less ordered layers.