Twins of Minimalistic Carbon Dots: Uniform Emitting‐Units and Molecular Level Repeatable Photoluminescence

Abstract

Despite of these marvelous achievements, the exact photoluminescence processes of C-dots, one of fundamental and pivotal concerns, has remained an open challenge. To date, considerable efforts have been put into such intricate but fascinating scientific issue, and obtained a few valuable results and insightful perceptions. [19][20][21][22][23][24][25][26] These studies undoubtedly help us to understand their photoluminescence properties indeed. However, there still has an obvious gap between the present cognition and a clear photophysical picture on C-dots' photoluminescence mechanism. For example, where does the photoluminescence of C-dots come from, particles' surface or cores? Do C-dots have quantum confinement effects and sizedependent emission? Whether and how do heteroatoms (O, N, S, P, etc.) affect the emission behaviors of C-dots? To date, these critical problems on C-dots' photoluminescence are still far from resolved. What's more, in terms of a few published reports, different and even completely contradictory conclusions are often obtained, causing longstanding confusion about the origin of C-dots' photoluminescence.The barriers to understanding C-dots' photoluminescence processes are the indeterminate constitution and complicated composition/structure of the samples. First, C-dots are often highly heterogenetic. For a long time, exploring the separation and purification of C-dots has been substantially neglected. To date, the most popular and frequently used purification method is dialysis. [4,27] Due to limited separation capacity, the obtained products usually are mixture containing particles, oligomeric and/or polymeric like molecules, etc. With the employment of various column separation techniques, this situation has been improved to some extent. [20,28,29] However, new separation approaches for highly purified C-dots are still sought after. Second, C-dots often possess complex chemical compositions and/or surface structures, whether they are fabricated by bottom-up or top-down methods. Generally, almost all C-dots contain lots of oxygen-containing functional (−OH and −COOH) groups. [6,8,10,13,15,16,21] Furthermore, if a few heteroatoms (N, B, S, P, etc.) are intentionally incorporated, [7,8,12,29,30] the corresponding products would be more complicated. Even more unfortunately, the above two situations often closely interweave with each other, which causes not only the uncertainty of their composition/structure, but the lack of suitable samples for contrastive analysis. As a result, it is nearly impossible to Understanding the photoluminescence mechanism of carbon dots (C-dots) is both of fundamental and technical significance. However, it has remained an extreme challenge due to C-dots' indeterminate constitution and complicated composition/structure. Herein, a combined processing system, namely extraction and subsequent preparative thin layer chromatography for C-dots separation is presented. Benefiting from its powerful separation performances, two minimalistic C-dots with almost identical st...

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“…In our recent work, we have presented a photophysical picture for understanding C-dots' fluorescence processes: Namely, each C-dot contains multiple emitting units on its surface, which are named as multicenter-emitting or unitemitting C-dots depending on their full width at half-maximum (fwhm) of the emission spectra. 21,25 In terms of the asprepared C-dots, their optical behaviors (Figure 1g−j) are also in accordance with the proposed photophysical picture. So, we combine XPS measurements and DFT calculations to deduce the possible emitting structure.…”

“…28,29 The distinct absorption band at ∼500−600 nm (peak at 564 nm) probably comes from the polyaromatic emitting units (see below). 21,25 The emission peak is located at 590 nm, and it is independent of excitation wavelengths (Figure 1h, Figure S3). The fluorescent lifetimes of the UE-C-dots are 2.67 ns (Figure 1i), which are comparable with that of organic molecules.…”

“…Figure g shows the optical spectra of the UE-C-dots. The absorption spectrum starts from ∼200 to more than 610 nm, and the peaks at 236 and 283 nm are assigned to π–π* and n−π* transitions, respectively. , The distinct absorption band at ∼500–600 nm (peak at 564 nm) probably comes from the polyaromatic emitting units (see below). , The emission peak is located at 590 nm, and it is independent of excitation wavelengths (Figure h, Figure S3). The fluorescent lifetimes of the UE-C-dots are 2.67 ns (Figure i), which are comparable with that of organic molecules.…”

“…So, the C-dots’ emissions probably comes from uniform polyaromatic structures instead of a series of polyaromatic ones . In this regard, they can be classified as UE-C-dots …”

“…The UE-Cdots functionalized by dual recognition sites, namely phenolic hydroxyl and amino groups, are bottom-up fabricated, using paminophenol as the sole precursors. Notably, we herein employ a combination system composed by extraction and the PTLC technique, reported by our group recently, 25 for separating and purifying the UE-C-dots. As shown in Figure 1a, as the pre-extracted samples are conducted by PTLC, using the mixtures of methanol, EAC, and n-hexane as developing agents, several distinct bands are separated on the plate.…”