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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 Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π‐conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (CH…O, OH…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance‐assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented. This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Reaction Mechanisms and Catalysis 

Abstract We examine the effects of fusing two benzofurans tos‐indacene (indacenodibenzofurans, IDBFs) and dicyclopenta[b,g]naphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity and diradical character of these core units. Synthesis via 3‐functionalized benzofuran yieldssyn‐IDBF andsyn‐IIDBF.syn‐IDBF possesses a high degree of paratropicity, exceeding that of the parent hydrocarbon, which in turn results in strong diradical character forsyn‐IIDBF. In the case of theanti‐isomers, synthesized via 2‐substituted benzofurans, these effects are decreased; however, both derivatives undergo an unexpected ring‐opening reaction during the final dearomatization step. All the results are compared to the benzothiophene‐fused analogues and show that the increased electronegativity of oxygen in thesyn‐fused derivatives leads to enhancement of the antiaromatic core causing greater paratropicity. Forsyn‐IIDBF increased diradical character results from rearomati‐zation of the core naphthalene unit in order to relieve this paratropicity. 

The self-trapping nano-loop structures of [1]rotaxanes exhibited multiple Förster resonance energy transfer (FRET) OFF/ON patternsviadual and sequential locking/unlocking upon UV exposure. 

Aromaticity is one of the most deeply rooted concepts in chemistry. But why, if two-thirds of existing compounds can be classified as aromatic, is there no consensus on what aromaticity is? σ−, π−, δ−, spherical, Möbius, or all-metal aromaticity… why are so many attributes needed to specify a property? Is aromaticity a dubious concept? This perspective aims to reflect where the aromaticity community is and where it is going. 

Computed nucleus-independent chemical shifts (NICS), contour plots of isotropic magnetic shielding (IMS), and gauge-including magnetically induced current (GIMIC) plots suggest that polarization of the π-system of acridones may perturb the numbers and positions of Clar sextet rings. Decreasing numbers of Clar sextets are connected to experimental observations of a narrowing HOMO–LUMO gap and increased charge mobility in solid-state assemblies of quinacridone and epindolidione.