Fundamental Properties of Organic Nanocrystals and Photochemical Intermediates in their Crystal Lattices

This dissertation describes the use of organic nanocrystalline suspensions for the investigation of solid-state photochemical reaction mechanisms, the elucidation of fundamental properties of photochemical intermediates embedded within the crystal lattice, and a novel study of size-dependent photosalience in crystals of a compound subject to a solid state [2+2] photocycloaddition reaction. To perform these studies, the nanocrystalline suspensions formed from rapid precipitation of crystalline material via the addition of water-miscible solutions of the compound of interest to water or dilute surfactant solution uniquely enabled the use of transmission spectroscopy methods such as laser flash photolysis in order to gain detailed information about photochemical intermediates and the kinetics of solid-state reactions. In addition, the relatively new method of microcrystal electron diffraction was utilized for the first report of crystal structures solved before and after a single-crystal-to-single-crystal reaction in a single microcrystal specimen.Chapter One is a brief overview of the history of organic solid-state photochemistry, highlighting key advancements over many decades. A particular focus will be applied to the more recent advancements that have enabled various forms of solid-state photochemical mechanistic analysis, especially the use of nanocrystalline suspensions for transmission spectroscopy. Chapter Two describes a series of adamantylacetophenone compounds which serve as scaffolds for competitive -Hydrogen abstraction from multiple sites leading to the formation of divergent Norrish-Yang cyclization products. This work offered new insight into the nature of Norrish Type II reaction selectivity through a combination of laser flash photolysis, photoproduct determination, and other mechanistic analysis techniques in both solution and solid-state samples. It showed that the reaction selectivity is governed not only by the geometric parameters of the parent ketones, but also on varying C-H bond dissociation energies, and the steric parameters and lifetimes of biradical intermediates. Chapters Three and Four focus on the exceptional lifetimes of acyl-alkyl radical pair intermediates generated in crystals of diphenylmethyl- and triphenylmethyl-containing ketones, respectively. Diphenylmethyl ketones with various -adamantyl substituents were shown to be photostable with respect to decarbonylation, but still generated acyl-alkyl radical pairs which had triplet lifetimes an order of magnitude longer than those of any analogous radical pair or biradical previously reported. Upon this discovery, a series of triphenylmethyl ketones were synthesized in hopes of further extending this lifetime such that crystals of these compounds might serve as a promising platform for the generation of qubit pairs with correlation lifetimes on the millisecond scale. A subset of the triphenylmethyl ketones were photostable and displayed triplet radical pair lifetimes up to >4 ms by leveraging the stability of the iconic Gomberg radical. Chapter Five introduces preliminary results related to the effect of an external magnetic field on the lifetimes of radical pairs and biradicals in solution and the crystalline solid state. A macrocyclic ketone was used to generate an acyl-alkyl biradical tethered by a flexible aliphatic chain. In solution, this biradical has much greater conformational freedom than when it is embedded within a crystal lattice. This has a substantial effect on the singlet-triplet gap, as was shown by laser flash photolysis experiments under varying external magnetic field strengths. Chapter Six describes the discovery of a size-dependent threshold for the observation of photosalience in crystals that facilitate a [2+2] photocycloaddition reaction upon ultraviolet irradiation. Microcrystal electron diffraction was used to establish that the expected single-crystal-to-single-crystal reaction still occurs in small microcrystals, while transmission electron microscopy showed that they do not exhibit photosalience below a particular size. This novel discovery has potentially significant implications for the deployment of these types of crystals for use as nano- and micromechanical actuators controlled by external stimulus

Trajectories of alcohol and cigarette use among sexual minority and heterosexual girls

To examine disparities between sexual minority girls (SMGs) and heterosexual girls in trajectories of substance use over time. Girls were included in the analyses if they were 1218 years of age at wave 1 and did not miss sexual orientation data at wave 4 (n = 7,765). Latent curve models were estimated across all four waves (extending from middle adolescence into young adulthood) to examine trajectories of cigarette and alcohol use. Initial levels of substance use were higher for SMGs than they were for heterosexual girls. SMGs also exhibited sharper escalations in use across all substances over time as they were transitioning into young adulthood. Persistent rates of cigarette and heavy alcohol use among SMGs may increase their risk for a host of mental and physical health problems in adulthood. Clinicians should be prepared to discuss SMG health topics effectively and in private, and discuss prevention and intervention programs with girls at risk

Association of Severe Bronchiolitis during Infancy with Childhood Asthma Development: An Analysis of the ECHO Consortium

Objective: Many studies have shown that severe (hospitalized) bronchiolitis during infancy is a risk factor for developing childhood asthma. However, the population subgroups at the highest risk remain unclear. Using large nationwide pediatric cohort data, namely the NIH Environmental influences on Child Health Outcomes (ECHO) Program, we aimed to quantify the longitudinal relationship of bronchiolitis hospitalization during infancy with asthma in a generalizable dataset and to examine potential heterogeneity in terms of major demographics and clinical factors. Methods: We analyzed data from infants (age Results: The analytic cohort consisted of 11,762 infants, 10% of whom had bronchiolitis hospitalization. Overall, 15% subsequently developed asthma. In the Cox proportional hazards model adjusting for 10 patient-level factors, compared with the no-bronchiolitis hospitalization group, the bronchiolitis hospitalization group had a significantly higher rate of asthma (14% vs. 24%, HR = 2.77, 95%CI = 2.24–3.43, p interaction = 0.02). The magnitude of the association was greater in non-Hispanic White (HR = 3.77, 95%CI = 2.74–5.18, p p p = 0.23). Conclusions: According to the nationwide cohort data, infants hospitalized with bronchiolitis are at a higher risk for asthma, with quantitative heterogeneity in different racial and ethnic groups