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Investigating electron transfer behavior under external electric fields in molecular electronics is crucial for understanding the function of each component and for improving molecular design. Notably, the one-electron transfer is inevitable in molecular wires and switches, for which traditional density functional theory (DFT) and long-range corrected self-consistent-charge density functional tight binding (LC-DFTB) meet significant challenges. Inspired by previous studies on constrained configuration interaction schemes, we present restriction-based configuration interaction (RCI) LC-DFTB, a novel extension of LC-DFTB to deliver an accurate description of one-electron transfer under external electric fields. This approach retains the low cost of LC-DFTB while accurately capturing charge-resonance, localization versus delocalization, and field-induced response in large, structurally complex systems. We demonstrate its performance on a benzene assembly and a polyfluorene, showing that RCI-LC-DFTB efficiently describes the effects of molecular conformation and applied bias on electron localization and transfer. Our method thus provides a robust, scalable tool for the design of molecular electronic and organic photovoltaic materials. 

Multicolor carbon dots (CDs) have been developed recently and demonstrate great potential in bio-imaging, sensing, and LEDs. However, the fluorescence mechanism of their tunable colors is still under debate, and efficient separation methods are still challenging. Herein, we synthesized multicolor polymeric CDs through solvothermal treatment of citric acid and urea in formamide. Automated reversed-phase column separation was used to achieve fractions with distinct colors, including blue, cyan, green, yellow, orange and red. This work explores the physicochemical properties and fluorescence origins of the red, green, and blue fractions in depth with combined experimental and computational methods. Three dominant fluorescence mechanism hypotheses were evaluated by comparing time-dependent density functional theory and molecular dynamics calculation results to measured characteristics. We find that blue fluorescence likely comes from embedded small molecules trapped in carbonaceous cages, while pyrene analogs are the most likely origin for emission at other wavelengths, especially in the red. Also important, upon interaction with live cells, different CD color fractions are trafficked to different sub-cellular locations. Super-resolution imaging shows that the blue CDs were found in a variety of organelles, such as mitochondria and lysosomes, while the red CDs were primarily localized in lysosomes. These findings significantly advance our understanding of the photoluminescence mechanism of multicolor CDs and help to guide future design and applications of these promising nanomaterials.