Emergent Properties and Applications of Self- Assembled Benzophenone-Containing Materials

Supramolecular assembly of benzophenone through urea hydrogen bonding interactions facilitates the formation of remarkably persistent triplet radical pairs upon UV-irradiation at room temperature, whereas no radicals are observed in solution. The generation of organic radicals is correlated to the microenvironment around the benzophenone carbonyl, the types of proximal hydrogens, and the rigid supramolecular network. High-Field EPR and variable temperature X-band EPR accompanied by simulations suggest a resonance stabilized radical pair through hydrogen abstraction. Previous work has shown that UV-irradiation of self-assembled benzophenone bis-urea host results in low quantities of radical pairs that can be used to enhance NMR signals by a factor of 4 for both the host and the encapsulated guest using a dynamic nuclear polarization (DNP) technique. This result suggests that even low levels of endogenous radicals can facilitate the study of host-guest relationships in the solid-state.2 Additionally, the photochemical formation of reactive oxygen species (ROS) by the host was examined, which was found to generate both superoxide and singlet oxygen in similar quantities. The host was then applied as a nanoreactor to mediate photooxidations of 1-methyl-1-cyclohexene while suspended in solution and as a solvent free host:guest complex

Probing the Formation of Reactive Oxygen Species by a Porous Self- Assembled Benzophenone Bis-Urea Host

Herein, we examine the photochemical formation of reactive oxygen species (ROS) by a porous benzophenone-containing bis-urea host (1) to investigate the mechanism of photooxidations that occur within the confines of its nanochannels. UV irradiation of the self-assembled host in the presence of molecular oxygen generates both singlet oxygen and superoxide when suspended in solution. The efficiency of ROS generation by the host is lower than that of benzophenone (BP), which could be beneficial for reactions carried out catalytically, as ROS species react quickly and often unselectively. Superoxide formation was detected through reaction with 5,5- dimethyl-1-pyrroline N-oxide in the presence of methanol. However, it is not detected in CHCl3, as it reacts rapidly with the solvent to generate methaneperoxy and chloride anions, similar to BP. The lifetime of airborne singlet oxygen (τΔairborne) was examined at the air−solid outer surface of the host and host·quencher complexes and suggests that quenching is a surface phenomenon. The efficiency of the host and BP as catalysts was compared for the photooxidation of 1-methyl-1-cyclohexene in solution. Both the host and BP mediate the photooxidation in CHCl3, benzene, and benzene-d6, producing primarily epoxide-derived products with low selectivity likely by both type I and type II photooxidation processes. Interestingly, in CHCl3, two chlorohydrins were also formed, reflecting the formation of chloride in this solvent. In contrast, UV irradiation of the host·guest crystals in an oxygen atmosphere produced no epoxide and appeared to favor mainly the type II processes. Photolysis afforded high conversion to only three products: an enone, a tertiary allylic alcohol, and a diol, which demonstrates the accessibility of the encapsulated reactants to oxygen and the influence of confinement on the reaction pathway

Probing the Formation of Reactive Oxygen Species by a Porous Self-Assembled Benzophenone Bis-Urea Host

Herein, we examine the photochemical formation of reactive oxygen species (ROS) by a porous benzophenone-containing bis-urea host (1) to investigate the mechanism of photooxidations that occur within the confines of its nanochannels. UV irradiation of the self-assembled host in the presence of molecular oxygen generates both singlet oxygen and superoxide when suspended in solution. The efficiency of ROS generation by the host is lower than that of benzophenone (BP), which could be beneficial for reactions carried out catalytically, as ROS species react quickly and often unselectively. Superoxide formation was detected through reaction with 5,5-dimethyl-1-pyrroline N-oxide in the presence of methanol. However, it is not detected in CHCl3, as it reacts rapidly with the solvent to generate methaneperoxy and chloride anions, similar to BP. The lifetime of airborne singlet oxygen (τΔairborne) was examined at the air–solid outer surface of the host and host·quencher complexes and suggests that quenching is a surface phenomenon. The efficiency of the host and BP as catalysts was compared for the photooxidation of 1-methyl-1-cyclohexene in solution. Both the host and BP mediate the photooxidation in CHCl3, benzene, and benzene-d6, producing primarily epoxide-derived products with low selectivity likely by both type I and type II photooxidation processes. Interestingly, in CHCl3, two chlorohydrins were also formed, reflecting the formation of chloride in this solvent. In contrast, UV irradiation of the host·guest crystals in an oxygen atmosphere produced no epoxide and appeared to favor mainly the type II processes. Photolysis afforded high conversion to only three products: an enone, a tertiary allylic alcohol, and a diol, which demonstrates the accessibility of the encapsulated reactants to oxygen and the influence of confinement on the reaction pathway

Synergistic effects of hydrogen and halogen bonding in co-crystals of dipyridylureas and diiodotetrafluorobenzenes

<p>Herein, we investigate co-crystallization of three linear co-formers that contain urea and pyridyl groups with three regioisomers of diiodotetrafluorobenzene (DITFB) to afford eleven co-crystals. The linear <i>o</i>-<i>, m</i>-<i>,</i> and <i>p</i>- dipyridylureas vary distance and geometry between the urea carbonyl oxygen and two pyridyl nitrogen acceptors, while the donors consist of urea NH groups and the activated halides in DITFB. Electrostatic potential calculations suggest that the <i>o</i>-dipyridylurea co-former presents two significantly different acceptors. In comparison, the acceptors in the <i>m</i>- and <i>p</i>-dipyridylurea co-formers display electrostatic potentials within 5–6 kJ/mol and should be competitive, potentially leading to altered assembly motifs. Overall, ten of the co-crystals consistently display the urea assembly motif as the best acceptor/donor pair. Seven structures were obtained as the predicted 1:1 ratio with halogen bonding interactions linking ditopic halogen bond donors and the pyridyl units through N···I interactions ranging from 78.4 to 83.1% of the van der Waals radii. Modified structures were more likely when there was a structural mismatch with the geometrically challenging <i>o</i>-DITFB donor and <i>m</i>- or <i>p</i>-dipyridylurea co-former. The majority of the co-crystal structures (10/11) demonstrated fully satisfied hydrogen and halogen bonding interactions suggesting that these synthons can be used synergistically to generate complex solid-state structures.</p