Supramolecular-Enhanced Charge Transfer within Entangled Polyamide Chains as the Origin of the Universal Blue Fluorescence of Polymer Carbon Dots

Abstract

The emission of a bright blue fluorescence is a unique feature common to the vast variety of polymer carbon dots (CDs) prepared from carboxylic acid and amine precursors. However, the difficulty to assign a precise chemical structure to this class of CDs yet hampers the comprehension of their underlying luminescence principle. In this work, we show that highly blue fluorescent model types of CDs can be prepared from citric acid and ethylenediamine through low temperature synthesis routes. Facilitating controlled polycondensation processes, the CDs reveal sizes of 1-1.5 nm formed by a compact network of short polyamide chains of about 10 monomer units. Density functional theory calculations of these model CDs uncover the existence of a spatially separated highest occupied molecular orbital and a lowest unoccupied molecular orbital located at the amide and carboxylic groups, respectively. Photoinduced charge transfer between these groups thus constitutes the origin of the strong blue fluorescence emission. Hydrogen-bond-mediated supramolecular interactions between the polyamide chains enabling a rigid network structure further contribute to the enhancement of the radiative process. Moreover, the photoinduced charge transfer processes in the polyamide network structure easily explain the performance of CDs in applications as revealed in studies on metal ion sensing. These findings thus are of general importance to the further development of polymer CDs with tailored properties as well as for the design of technological applications.

Carbon Dots from Citric Acid and its Intermediates Formed by Thermal Decomposition

“…Citric acid (CA) is a weak organic acid, which is largely employed as a non‐toxic and low‐cost precursor, alone or in combination with other compounds, to synthesise fluorescent carbon dots (C‐dots). C‐dots synthesised from CA exhibit, in general, a blue luminescence of lower intensity in comparison to C‐dots prepared from other precursors . CA C‐dots and citrate‐based fluorescent materials have, however, intrinsic advantages, namely low toxicity and good biocompatibility.…”

“…C-dotss ynthesised from CA exhibit, in general, ab lue luminescence of loweri ntensity in compari-son to C-dotsp repared from other precursors. [5][6][7] CA C-dots and citrate-based fluorescent materialsh ave, however,i ntrinsic advantages, namely low toxicity andg ood biocompatibility. They have been, therefore, widely appliedf or developing fluorophores for biomedical applications.…”

Carbon Dots‐in‐Matrix Boosting Intriguing Luminescence Properties and Applications

“…The structures and compositions of CDs are closely related to their synthetic methods and the use of carbon sources or molecular precursors. To date, CDs have been synthesized using two classes of methods: the top‐down route, in which the C‐dots are broken off from a larger carbon species, and the bottom‐up route, in which the CDs are formed from molecular precursors . In addition, the surface passivation or surface functionalization of CDs may further enhance their physical and fluorescence properties .…”

Scalable synthesis of fluorescent organic nanodots by block copolymer templating

“…S6) The stretching at 2934 and 2853 cm −1 corresponded to C─H. Other functionalities, such as CO, C─N, and C─O, were observed at 1653, 1348, and 1050 cm −1 , respectively . The atomic ratios of N/C and O/C were calculated to be 1.9% and 39.3%, respectively, according to the deconvoluted high‐resolution XPS .…”