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Abstract A recent publication by Wu and co‐workers demonstrated that the workhorse functional B3LYP tends to overemphasize the delocalization in antiaromatic molecules, leading to poor agreement between the calculated proton NMR values and the experimentally obtained numbers. Rather, they showed that the M11 functional affords the best agreement between theory and experiment. We have computationally re‐examined our previously published NICS‐XY scan data using M11‐determined geometries and find that, aside from the placement ofs‐indacene, the antiaromaticity trends for both sets of data are essentially identical. 

Abstract Herein we report the synthesis and characterization of four donor/acceptor‐fuseds‐indacenes via the late‐stage oxidation of a family of unsymmetrical benzofuran/benzothiophene‐s‐indacene regioisomers. A thorough study of their properties through experimental and computational analysis has revealed the effect of asymmetry on the molecular properties associated with antiaromaticity, as well as a strong correlation between antiaromaticity and intramolecular charge transfer (ICT). The strength of the charge transfer depends on the fusion orientation of the donor and acceptor motifs relative to thes‐indacene core. The two most antiaromatic oxidized isomers exhibit strong evidence of ICT with 30 and 40 nm solvatochromic shifts. 

Abstract Fusion of aromatic subunits to stabilize an antiaromatic core allows the isolation and study of otherwise unstable paratropic systems. A complete study of a series of six naphthothiophene‐fuseds‐indacene isomers is herein described. Additionally, the structural modifications resulted in increased π–π overlap in the solid state, which was further explored through changing the sterically blocking mesityl group to (triisopropylsilyl)ethynyl in three derivatives. The computed antiaromaticity of the six isomers is compared to the observed physical properties, such as NMR chemical shift, UV‐vis, and CV data. We find that the calculations predict the most antiaromatic isomer and give a general estimation of the relative degree of paratropicity for the remaining isomers, when compared to the experimental results. 

Abstract The phosphaquinolinone scaffold has been previously studied as a modular core for a variety of fluorescent species where use of substituent effects has focused on increasing or decreasing electron density in the core rings. We now report the synthesis and analysis of several pyridine‐containing phosphaquinolinone species exhibiting notable linear conjugation from the aryl‐substituent to electron‐withdrawing pyridyl nitrogen. Varying the nature of the aryl substituent from electron‐withdrawing to electron‐donating leads to the generation of an internal charge‐transfer (ICT) band in the absorbance spectrum, which becomes the dominant absorbance in terms of intensity in the most electron‐rich ‐NMe₂example. This heterocycle exhibits improved photophysical properties compared to others in the set including high quantum yield and considerably red‐shifted emission. The enhanced ICT can be observed in the X‐ray data where a rare example of molecule co‐planarity is observed. Computational data show increased localization of negative charge on the pyridyl nitrogen as the electron‐donating character of the aryl‐substituent increases. 

Abstract The electronic, optical, and solid state properties of a series of monoradicals, anions and cations obtained from starting neutral diradicals have been studied. Diradicals based ons‐indacene and indenoacenes, with benzothiophenes fused and in different orientations, feature a varying degree of diradical character in the neutral state, which is here related with the properties of the radical redox forms. The analysis of their optical features in the polymethine monoradicals has been carried out in the framework of the molecular orbital and valence bond theories. Electronic UV‐Vis‐NIR absorption, X‐ray solid‐state diffraction and quantum chemical calculations have been carried out. Studies of the different positive‐/negative‐charged species, both residing in the same skeletalπ‐conjugated backbone, are rare for organic molecules. The key factor for the dual stabilization is the presence of the starting diradical character that enables to indistinctively accommodate a pseudo‐hole and a pseudo‐electron defect with certainly small reorganization energies for ambipolar charge transport. 

Abstract Inclusion of a second nitrogen atom in the aromatic core of phosphorus‐nitrogen (PN) heterocycles results in unexpected tautomerization to a nonaromatic form. This tautomerization, initially observed in the solid state through X‐ray crystallography, is also explained by computational analysis. We prepared an electron deficient analogue (2 e) with a fluorine on the pyridine ring and showed that the weakly basic pyridine resisted tautomerization, providing key insights to why the transformation occurs. To study the difference in solution vs. solid‐state heterocycles, alkylated analogues that lock in the quinoidal tautomer were synthesized and their different¹H NMR and UV/Vis spectra studied. Ultimately, we determined that all heterocycles are the aromatic tautomer in solution and all but2 eswitch to the quinoidal tautomer in the solid state. Better understanding of this transformation and under what circumstances it occurs suggest future use in a switchable on/off hydrogen‐bond‐directed receptor that can be tuned for complementary hydrogen bonding. 

Abstract We describe two novel hybrid receptors combining a phosphorus‐/nitrogen‐containing (PN) phosphonamidate heterocycle with urea recognition units in an arylethynyl backbone. Structural, spectroscopic and computational studies reveal that the origin of superior binding for hydrogen sulfate (HSO₄⁻) anion is correlated with the formation of strong hetero‐complementary hydrogen bonds with the phosphonamidate motif. We further demonstrate that the hybrid host system is capable of capturing/transporting the HSO₄⁻anion from an aqueous, biphasic system. 

Abstract A set of fully‐conjugated indenofluorenes has been synthesized and confirmed by solid‐state structure analysis. The indeno[2,1‐c]fluorenes and their benzo‐fused analogues all contain the antiaromaticas‐indacene core. The molecules possess high electron affinities and show a broad absorption that reaches into the near‐IR region of the electromagnetic spectrum. All of the featured compounds reversibly accept up to two electrons as revealed by cyclic voltammetry. Analysis of molecule tropicity using NICS‐XY scan calculations shows that, while theas‐indacene core is less paratropic thans‐indacene, benz[a]‐annulation further reduces the antiaromaticity of the core. Antiaromatic strength of theas‐indacene core can also be tuned by the position of fusion of additional arenes on the outer rings.