Enabling Control over Mechanical Conformity and Luminescence in Molecular Crystals: Interaction Engineering in Action.

Molecular crystals of π-conjugated molecules are of great interest as the highly ordered dense packing offers superior charge and exciton transport compared with its amorphous counterparts. However, integration into optoelectronic devices remains a major challenge owing to its inherently brittle nature. Herein, control over the mechanical conformity in single crystals of pyridine-appended thiazolothiazole derivatives is reported by modulating the molecular packing through interaction engineering. Two polymorphs were prepared by achieving control over the thermodynamic/kinetic factors of crystallization; one of the polymorphs exhibits elastic bending whereas the other is brittle. The control over the bending ability was achieved by forming co-crystals with hydrogen/halogen bond donors. A seamless extended crisscross pattern with respect to the bend plane through a ditopic hydrogen-bonding motif showed the highest compliance towards mechanical bending, whereas the co-crystals with a layered crisscross arrangement with segregated layers of co-formers exhibit slightly lower bending conformity. These results update the rationale behind the plastic/elastic bending in molecular crystals. The co-crystals of ditopic halogen bond co-assemblies are particularly appealing for waveguiding applications as the co-crystals blend high mechanical flexibility and luminescence properties. The hydrogen bonded co-crystals are non-emissive in nature owing to excited state proton transfer dynamics. The rationale behind the fluorescence properties of these materials was also established from DFT calculations in a quantum mechanics/molecular mechanics (QM/MM) framework.

Tuning Solid-State Luminescence in Conjugated Organic Materials: Control of Excitonic and Excimeric Contributions through π Stacking and Halogen Bond Driven Self-Assembly.

Two polymorphs with distinctly different fluorescence emission (green and yellow; G, Y) emanating from excitonic and excimeric contributions were prepared from solution as well as by using physical vapour transport. Based on crystal structure investigations, the vibrationally-resolved excitonic emission is found to originate from a ß-Sheet arrangement (G), whereas a sandwich herringbone structure is responsible for the excimer emission (Y). The intermolecular interactions and energies were quantified to have a complete picture of the decisive factors that controls the self-assembly. Halogen-bond directed self-assembly was explored to fine-tune the intermolecular interactions through co-crystallization as well as a commercially viable liquid assisted grinding method. A smooth fluorescence shift from G to Y was achieved by co-assembly due to substantial differences in the π orbital overlap in the molecular packing. Our investigation provides a comprehensive understanding of the origin of excitonic and excimeric contributions of emission behaviour in conjunction with the molecular packing and π-π orbital overlap, and might provide a directive towards the engineering of fluorescent functional molecular materials.

A study of respiratory function among the workers engaged in ginning processes.

BACKGROUND: Known respiratory health effects of exposure to cotton dust are mainly altered pulmonary function findings and symptom of chest tightness. A number of studies have been carried out all over the world to enumerate and evaluate the health effects of cotton dust exposed workers in different processes. However, such studies carried out in ginning industry especially in Indian context are scanty. OBJECTIVES: This study was initiated to explore occupational and morbidity details and respiratory functional status of the exposed workers as well as to investigate across the working shift pulmonary function changes. MATERIALS AND METHODS: A cross-sectional study was conducted including workers from ginning units where principal exposure is from cotton dust. All the workers of the selected ginning units were subjected to an interview with a predesigned questionnaire to collect occupational and medical history, clinical examination and pulmonary function test. RESULTS: In this present study, no cross-shift significant change in relation to PEFR and FEV1 values is observed. However, chronic effect on lung function is observed in a few subjects and declining trend of values was observed with increasing job duration as well as age of workers and among smokers. Other health problems among these subjects were backache and joint pain. CONCLUSIONS: Studies on cotton textile workers have shown both cross-shift and chronic decline of values. In this study on ginning workers, chronic effect only is observed. This difference of observation may be explained by different nature of exposure in case of ginning. This study recommends regular periodic clinical examination, lung function test and monitoring of dust, gram-negative bacteria and endotoxins in such workplaces.