Molecular Origin and Self-Assembly of Fluorescent Carbon Nanodots in Polar Solvents

Despite numerous efforts, there are several fundamental ambiguities regarding the photoluminescence of carbon dots (CDs). Spectral shift measurements display characteristic of both π–π* and <i>n</i>–π* transitions for the main absorption or excitation band at ∼350 nm, contrary to common assignment of exclusive <i>n</i>–π* transition. Additionally, the generally perceived core-state transition at ∼250 nm, involving sp²-networked carbogenic domains shielded from external environments, needs to be reassessed because it fails to explain the observed fluorescence quenching and spectral shift. These results have been explained based on the molecular origin of PL in CDs invoking the similarity between CD and citrazinic acid. Fluorescent derivatives of the latter are recognized to be produced during citric-acid-based CD synthesis. Concentration-dependent spectral splitting of the main excitation band in combination with the temperature-dependent PL results has been envisioned assuming self-assembly of CDs into various H-aggregates

Origin of Excitation Dependent Fluorescence in Carbon Nanodots

The fascinating aspect of excitation dependent fluorescence in carbon nanodots has led to several hypotheses, starting from particle size distribution to the presence of different emissive states and even to sluggish solvent relaxation around nanodot. In this contribution we provide definitive evidence for the involvement of discrete multiple electronic states for the excitation dependent emission in carbon nanodots. The presence of different types of aggregates even at very dilute solutions used in ensemble fluorescence spectroscopy, where fluorescence intensity shows linear dependence with absorbance, is the origin of these multiple electronic states. Inhomogeneous broadening due to slow solvent relaxation leading to excitation dependent spectral shift has negligible influence in conventional solvents

First Report on the Separation of Trivalent Lanthanides from Trivalent Actinides Using an Aqueous Soluble Multiple N‑Donor Ligand, 2,6-bis(1<i>H</i>‑tetrazol-5-yl)pyridine: Extraction, Spectroscopic, Structural, and Computational Studies

A terdentate multiple N donor ligand, 2,6-bis­(1<i>H</i>-tetrazol-5-yl)­pyridine (H₂BTzP), was synthesized, and its complexation with trivalent americium, neodymium, and europium was studied using single-crystal X-ray diffraction, attenuated total reflectance-fourrier transform infrared spectroscopy, time-resolved fluorescence spectroscopy, UV–vis absorption spectrophotometry. Higher complexation strength of BTzP toward trivalent actinide over lanthanides as observed from UV–vis spectrophotometric study resulted in an effective separation of Am³⁺ and Eu³⁺ in liquid–liquid extraction studies employing <i>N,N,<i>N</i>′,N′</i>-tetra-<i>n</i>-octyl diglycolamide in the presence of BTzP as the aqueous complexant. The selectivity of BTzP toward Am³⁺ over Eu³⁺ was further investigated by DFT computations, which indicated higher metal–ligand overlap in the Am³⁺ complex as indicated from the metal–nitrogen bond order and frontier molecular orbital analysis of the BTzP complexes of Am³⁺ and Eu³⁺