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Over the past 50 years, a principal approach to controlling conventional photochemical reactions has relied on imposing geometric constraints on reactant or transition state via conducting photochemistry in the organized or constraining media. Herein, we describe a fundamentally different approach to affect the course of photochemical reactions (photochemical rearrangements) by utilizing spatially selective excitation of specific electronic transitions with plane‐polarized light in the reactant molecules uniformly aligned in the nematic liquid crystal phase. In particular, we focused on the Type B enone rearrangement of 4,4‐diarylcyclohexenones – one of the most common photochemical rearrangements. We demonstrated that the aryl migratory aptitude in this reaction was attenuated in response to changing an angle between the polarization plane of the incident light and the alignment direction of the nematic liquid crystal, with the enhanced aryl migration achieved when the polarization plane coincided with the transition dipole moment leading to the excited state responsible for this migration. The spatially‐selective initial excitation therefore was overruling the electronic factors responsible for the relative ratio of the two alternative photoproducts. The experimental findings were further supported by the results of a computational study. This work showcases a new fundamental paradigm in controlling photochemical reactivity and selectivity of photoreactions. 

Thin films of poly(arylene ethynylene) conjugated polymers, including low-energy-gap donor–acceptor polymers, can be prepared via stepwise polymerization utilizing surface-confined Sonogashira cross-coupling. This robust and efficient polymerization protocol yields conjugated polymers with a precise molecular structure and with nanometer-level control of the organization and the uniform alignment of the macromolecular chains in the densely packed film. In addition to high stability and predictable and well-defined molecular organization and morphology, the surface-confined conjugated polymer chains experience significant interchain electronic interactions, resulting in dominating intermolecular π-electron delocalization which is primarily responsible for the electronic and spectroscopic properties of polymer films. The fluorescent films demonstrate remarkable performance in chemosensing applications, showing a turn-off fluorescent response on the sub-ppt (part per trillion) level of nitroaromatic explosives in water. This unique sensitivity is likely related to the enhanced exciton mobility in the uniformly aligned and structurally monodisperse polymer films. 

Controlled polymerization for the synthesis of structurally precise conjugated polymers remains a challenging problem in polymer chemistry. Catalyst-transfer polymerization (CTP) based on Pd-catalyzed Suzuki-Miyaura cross-coupling is one of the promising approaches toward solving this challenge. Recent introduction of N-methyliminodiacetic acid (MIDA) boronates as monomers for Suzuki-Miyaura CTP has extended this approach towards a broader variety of monomer structures and led to improved control over the polymerization, particularly for heteroaromatic systems (such as thiophenes). Previously, we found that MIDA-boronate monomers polymerization could be facilitated by Ag+-mediated reaction conditions due to shifting the Pd catalytic cycle toward a more efficient oxo-Pd transmetalation pathway where MIDA-boronates could participate in transmetalation directly, without prior hydrolysis to boronic acid. In this work, we continued studying this novel process, and investigated the dual role of the MIDA-boronate functional group in the case of less reactive fluorenyl (and potentially other all-carbon aromatic systems) monomers. With such monomers, MIDA-boronate group enables the controlled polymerization but also produces a hydrolysis byproduct hindering the polymerization. We also investigated the role of Ag+ acting to counteract this hindering effect. Steric bulkiness of the MIDA-boronate functional group may also slow down the Suzuki-Miyaura CTP process. These complications could reduce the synthetic value of MIDA-boronate monomers in Suzuki-Miyaura CTP, although better understanding of these implications and a proper choice of polymerization conditions and catalytic initiators could to some extent mitigate such problems. As part of this work, we also uncovered a "critical length" phenomenon which results in a dual molecular weight distribution of the resulting conjugated polymer, both with MIDA-boronate and boronic acid monomers. This phenomenon could account for the experimentally observed loss of polymerization control beyond formation of the polymer chains of a certain "critical length", even despite the formally "living" nature of the polymer chains. The generality of this phenomenon and whether it is restricted to using Pd catalytic systems based on Buchwald-type phosphine ligands remains to be studied. Overall, these new findings paint a sophisticated picture of the Suzuki-Miyaura CTP process with MIDA-boronate monomers where the mere presence of a Pd center on the polymer chain is not sufficient to sustain the polymerization (even if a chain could be considered "living" in a sense of possessing a Pd center), and the choice of phosphine ligand on the Pd center is an effective tool to overcome the "critical length" restriction. 

Controlled preparation of structurally precise complex conjugated polymer systems remains to be a major synthetic challenge still to be addressed, and this push is stimulated by the improved device performance as well as unique fundamental characteristics that the well-defined conjugated polymer materials possess. Catalyst-transfer polymerization (CTP) based on Pd-catalyzed Suzuki-Miyaura cross-coupling reaction is currently one of the most promising methods towards achieving such a goal, especially with the recent implementation of N-methyliminodiacetic acid (MIDA) boronates as monomers in CTP. Further expansion and development of practical applications of CTP methods will hinge on a clear mechanistic understanding of both the entire process and the particular steps involved in the catalytic cycle. In this work, we introduced Ag⁺-mediated Suzuki-Miyaura CTP and demonstrated that presence of Ag⁺ shifted a key transmetalation step toward the oxo-Pd pathway, leading to direct participation of MIDA-boronates in the transmetalation step and hence in the polymerization process, and resulting in the overall more efficient polymerization. In addition, we found that, under Ag⁺-mediated conditions, MIDA-boronates can also directly participate in small-molecule cross-coupling reactions. The direct participation of MIDA-boronates in Suzuki-Miyaura cross-coupling has not been envisaged previously and could enable new interesting possibilities to control this reaction both for small-molecule and macromolecular syntheses. In contrast to MIDA-boronates, boronic acid monomers likely undergo transmetalation through an alternative boronate pathway, although they may also be directed to react via the oxo-Pd transmetalation pathway in Ag⁺-mediated conditions. The interplay between the two transmetalation pathways which are both involved in the catalyst-transfer polymerization, and the opportunity to selectively enhance one of them not only improves mechanistic understanding of Suzuki-Miyaura CTP process but also provides a previously unexplored possibility to gain more effective control over the polymerization to obtain structurally better-defined conjugated polymers. 

The compound ethyl-adenosyl monophosphate ester (ethyl-AMP) has been shown to effectively inhibit acetyl-CoA synthetase (ACS) enzymes and to facilitate the crystallization of fungal ACS enzymes in various contexts. In this study, the addition of ethyl-AMP to a bacterial ACS from Legionella pneumophila resulted in the determination of a co-crystal structure of this previously elusive structural genomics target. The dual functionality of ethyl-AMP in both inhibiting ACS enzymes and promoting crystallization establishes its significance as a valuable resource for advancing structural investigations of this class of proteins.