Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to π-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (π*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.

Two-electron multicenter (2e/mc) bonding of phenalenyl (PHYL) pi-dimers was found to be significantly affected by the electron density on the bonding active sites. The computational analysis shows that, upon appropriate beta-substitutions, the newly introduced dimers have the shortest and strongest covalent bonding interactions seen in any neutral pi-dimer. The unusual strengthening of the bonding was attributed to the reduced lone pair bond weakening effect, LPBWE, upon substitutions with electron-withdrawing groups.

Room-temperature ionic liquids (RTILs) are regarded as green solvents due to their low volatility, low flammability, and thermal stability. RTILs exhibit wide electrochemical windows, making them prime candidates as media for electrochemically driven reactions such as electro-catalysis and electro-plating for separations applications. Therefore, understanding the factors determining edges of the electrochemical window, the electrochemical stability of the RTILs, and the degradation products is crucial to improve the efficiency and applicability of these systems. We present here computational investigations of the electrochemical properties of a variety of RTILs covering a wide range of electrochemical windows. We proposed four different approaches with different degrees of approximation and computational cost from gas-phase calculations to full explicit solvation models. It was found that, whereas the simplest model has significant flaws in accuracy, implicit and explicit solvent models can be used to reliably predict experimental data. The general trend of electrochemical windows of the RTILs studied is well reproduced, showing that it increases in the order of imidazolium < ammonium < pyrrolidinium < phosphonium giving confidence to the methodology presented to use it in screening studies of ionic liquids.

wileyonlinelibrary.com COMMUNICATIONreported mechanochromic luminescent materials contain a single luminophore with two fl uorescent states, and thus their fl uorescence can be switched between two colors or two different intensities. [ 5a , 6 ] However, examples exhibiting mechanochromic phosphorescence are rather rarely seen in literature, except for a few metal complexes showing mechanochromic phosphorescence with short lifetimes. [ 7 ] It is known that room temperature phosphorescent (RTP) emissions often occur in metal complexes, but rare for pure organic luminogens. [ 8 ] The development of organic mechanochromic phosphorescence is of great importance from both a scientifi c and a practical point of view.Our group has observed RTP in organic crystals of nonplanar molecules containing carbonyl groups, while phosphorescence is absent in the corresponding amorphous aggregates. This phenomenon has been attributed to crystallization-induced phosphorescence (CIP). [ 9 ] Inspired if mechanochromic or thermochromic phosphorescence can be achieved by means of reversible morphology transformation between the crystalline and amorphous states of those luminescent organic materials. Unfortunately, quick crystallization of those RTP luminogens makes it diffi cult to obtain their amorphous aggregates by mechanical stimulus or thermal method. Therefore, to achieve mechanochromic phosphorescence, it is desirable to attenuate the crystallization ability of those luminegens, which can be realized through a suitable molecular design.In this paper, we present the unique photoluminescent (PL) properties of solid materials based on luminogen 1 ( Figure 1 ), where carbazolyl groups are introduced at the p -positions of phenyl rings in benzophenone molecule. In the molecular crystals of luminogen 1 , the fl uorescent processes can be significantly inhibited with the assistance of chloroform (TCM). However, luminogen 1 exhibiting RTP with ultralong lifetime and the RTP decay process is observable even with the naked eyes, which is unusual for organic luminogens. Remarkably, the persistent RTP of 1 can be repeatedly switched on and off facilely by repeating the fuming-heating or fuming-grinding process. Meanwhile, when phosphorescence was shut down, the solid materials can exhibit multiple fl orescent states depending on the morphologies. The different fl uorescent states can be reversibly switched between each other when vapor, thermal, and mechanical stimuli were applied. Owing to the versatile

The structures of linearly and ortho-fused helical polyaromatic hydrocarbon oligomers and polymers and their isoelectronic thiophene variants are studied using density functional theory (DFT). Structural and optical absorption data are compared with experiments where possible and excellent agreement is obtained. The results are interpreted with reference to orbital interaction diagrams. Infinite helicene tends to adopt a symmetry close to 61. C2S helicene is predicted to have an approximately 263 symmetry leading to an interdigitated S···S network parallel to the helical axis. Thiaheterohelicene has an approximately 72 helical structure. Periodic boundary condition (PBC) calculation and highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap extrapolation at the B3LYP/6-31G* level indicate a smaller band gap for helicene compared to phenacene. This difference is mainly due to the gap reducing effects of the transannular π−π interactions across the helical pitch in helicene. The calculated band gap is much smaller for linear thienoacene than that for isomeric C2S helicene due to the lack of effective conjugation pathway for the latter system. While thiaheterohelicene is structurally between the two large gap systems, helicene and C2S helicene, its band gap is significantly lower than either of the two.

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Inspired by experimental evidence of the thermally accessible pi-dimer of the title compound, DAzPh (7), we propose that the sigma-dimer (8) can undergo a variety of sigma-bond shifts representing very unusual multi-faceted fluxional bonding between two neutral pi-radicals. In this paper, we present a theoretical study of the sigmatropic rearrangement of the DAzPh sigma-dimers. Out of the six sigma-bonded tautomers three are competitive: a degenerate pair resulting from a [5,5] sigmatropic rearrangement and a non-degenerate product of a [3,3] sigmatropic rearrangement with barriers of 10.21 kcal mol(-1) and 10.00 kcal mol(-1), respectively. Both of these rearrangements occur stepwise through a pi-dimer intermediate (9), which is 1.33 kcal mol(-1) higher in energy than the sigma-dimer (8). These data are consistent with optical and paramagnetic susceptibility experiments and offer a natural interpretation for the unusual C-C contact distance of 2.153 A obtained by X-ray diffraction by Morita et al. Another new sigma-dimer (15) with a different dipole-dipole stacking pattern is predicted, the energy of which is very close to that of 8, and is likely to be isolable under suitable conditions. The new sigma-dimer (15) is expected to undergo stepwise [7,7] sigmatropic rearrangement. Thus we observed a complete spectrum of sigmatropic rearrangement reactions in these DAzPh dimers. The pi-dimers 4, 9 and 17 show decreasing order of SOMO-SOMO splittings consistent with the UV-vis absorbance. The calculated paramagnetism is in good agreement with experiments providing further evidence for the presented interpretation of fluxional bonding in the DAzPh sigma-dimers.

Recently, Nishibayashi et al. reported a dimolybdenum-dinitrogen complex that is catalytic for complete reduction of dinitrogen to ammonia. This catalyst is different from the Schrock molybdenum catalyst in two fundamental aspects: it contains two metal centers, and the oxidation state is Mo(0) instead of Mo(III). We show that a remarkable feature of the bimetallic complex is the bond-mediated delocalized electronic states, resulting from the two metal centers bridged by a dinitrogen ligand. Using first-principles calculations, we found that this property makes the bimetallic complex the effective catalyst, as opposed to the originally postulated monometallic fragment. A favorable reaction pathway is identified, and the nature of the intermediates is examined. Furthermore, studies of the intermediate states led us to propose possible deactivation processes of the catalyst. The finding that the central bimetallic unit (Mo-N2-Mo) is relevant for catalytic activity may provide a guideline for the development of more efficient dinitrogen-reducing catalysts.