Elsevier

Optical Materials

Volume 123, January 2022, 111845
Optical Materials

Excited-state intramolecular proton-transfer-induced dual fluorescence emission in 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and resorcinol-based carbon dots

https://doi.org/10.1016/j.optmat.2021.111845Get rights and content

Highlights

  • Excitation-state intramolecular proton-transfer mechanism is found in carbon dots.

  • A dual-emissive fluorophore is extracted and identified during the synthesis.

  • ESIPT carbon dots are highly sensitive to solution medium.

  • Efficient anti-counterfeiting and picric acid sensing system are explored.

Abstract

Herein, we revealed the excited-state intramolecular proton-transfer (ESIPT) process in 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and resorcinol based carbon dots. The dual-emissive fluorophore, 5,6-diethoxy-4,7-dihydroxyisoindoline-1,3-dione (DDD), is extracted from the products and identified as the main fluorescence origin of the carbon dots. A blue-to-green switching in the emission color of the DDD can be observed as the solution pH decreases and the polarity of the solvent increases. Versatile morphologies including disks, dots and fibers can be formed during the DDD self-assembly in different mediums. By virtue of ESIPT-induced sensitive fluorescence, we not only construct the fluorescent anti-counterfeiting platform based on photoswitching of carbon dots in response to ammonia vapor and dimethyl sulfoxide stimuli, but also develop the picric acid sensing system with high selectivity and sensitivity.

Introduction

In the past decade, the complex structure and fluorescence mechanism of carbon dots (CDs) has received an increasing understanding, especially for those synthesized via bottom-up routes [[1], [2], [3]]. It has been proved that various species would coexist in the products of carbon dots, including molecular fluorophores, supramolecular aggregates, polymers, quasi carbon dots (carbon core bound with molecular fluorophores) and carbogenic nanoparticles [[4], [5], [6], [7], [8]]. Most importantly, many types of molecular fluorophores have been separated and identified as the single fluorescence origin of the carbon dots [[9], [10], [11], [12], [13], [14], [15]], such as the citrazinic acid [16], 2-pyridine compounds [[17], [18], [19], [20]], pyrrolo[3,4-c]pyridine [21], methylenesuccinic acid [11], thiazolo[3,2-a]pyridine [22], etc. Compared to its carbogenic counterpart, the fluorophore-dominated fluorescence is frequently characterized as excitation-independence, high fluorescence quantum yield, single fluorescence lifetime and environment-sensitiveness [23,24].

Recently, dual photoluminescence emission in single carbon dots has attracted increasing attentions of the community. By using phenylenediamine [[25], [26], [27]], ethylene diamine tetraacetic acid [28], poly(vinyl alcohol) [29], glutathione [30], naphthalenediol [31], aniline, ascorbic acid [32], cationic surfactant [33], alizarin carmine [34], aminosalicylic acid [35] as the carbon sources, and doping with heteroatoms such as nitrogen, sulfur, oxygen and phosphorus, the dual-emission carbon dots have been prepared. The dual emission is frequently attributed to the multiple fluorescent centers that are generated by the abundant functional groups and/or the carbon core related electronic states. In some cases, the dual emission of carbon dots is susceptible to solution polarity and pH, which is suitable to construct ratiometric fluorescent nanoprobes. Moreover, the intramolecular interaction between the carbon dots, e.g. hydrogen bonding effect and concentration-dependent aggregation, is also thought to be responsible for the extraordinary spectral shift and fluorescence sensing, which makes the dual emissive carbon dots useful candidates for bioimaging and biosensing [[36], [37], [38], [39], [40], [41], [42], [43]].

Herein, we have explored the dual fluorescence emission characteristics of carbon dots derived from 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and resorcinols. A fluorophore, 5,6-diethoxy-4,7-dihydroxyisoindoline-1,3-dione (DDD), is identified and considered as the main origin of the carbon dots. Excited-state intramolecular proton-transfer (ESIPT) process in DDD is supposed to be responsible for the dual fluorescence emission of the DDD fluorophore as well as the carbon dots. ESIPT is a unique feature of hydrogen-bonded systems, which is has been well investigated since 1950s [[44], [45], [46], [47], [48], [49], [50], [51], [52]]. Most ESIPT fluorophores show dual emission, large Stokes shift, and high sensitivity to the micro-environment. On the basis of a four-level photocycle, the short wavelength emission of dual fluorescence is usually attributed to the excited state enol form (E*) and the longer band arises from the keto form (K*). The structural basis of ESIPT phenomenon in DDD lies in the intramolecular hydrogen bonding between the proton acceptor (–Cdouble bondO) and the donor (–OH) groups adjacent to each other inside the molecule. Not only the fluorescence properties, the morphologies of DDD are also affected by the pH and solvent polarity of the environments. This provides great advantages for the application of the DDQ/resorcinol-based CDs in anti-counterfeiting and chemosensing.

Section snippets

Materials

Resorcinol and DDQ were purchased from Sigma-Aldrich. Picric acid, aniline, bisphenol A, 1,3-naphthalenediol (1,3-DHN), 1,6-DHN, 1,7-DHN, 2,3-DHN, 2,6-DHN, 2,7-DHN, 1,3-DHN, 1,5-DHN, 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP) and 2-NP were purchased from the Sinopharm Chemical Reagent Co., Ltd. Other reagents were analytical-grade.

Synthesis

The DDQ and resorcinol-based CDs are synthesized according to our previous report [10]. 0.11 g (1.0 mmol) of resorcinol, 0.57 g (2.5 mmol) of DDQ were

Structure characterization

Following our previous method, the carbon dots are synthesized by ethanothermal reaction of resorcinol and DDQ at 180 °C. The fluorescent species in raw carbon dots can be roughly divided into three types: carbogenic nanoparticles, carbon cores bound with fluorophores, and molecular fluorophores [8,15]. To extract the molecular fluorophores, the crude solution was separated by the combination of silica gel column chromatography and preparative liquid chromatography (LC) system (Fig. 1), in both

Conclusion

In summary, we explore the excited-state intramolecular proton-transfer (ESIPT) mechanism in 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and resorcinol-based carbon dots. By the normal and reversed phase chromatography, an ESIPT fluorophore, 5,6-diethoxy-4,7-dihydroxyisoindoline-1,3-dione (DDD), is separated and identified. Decreasing the pH of CD solution, or increasing the solvent polarity can induce a transition from blue to green in its emission color, as well as from regularity (disk,

CRediT authorship contribution statement

Jing Bai: performed research, analyzed data, Writing – original draft. Weiwei Zhu: performed research, analyzed data. Futao He: contributed new reagents, analytic tools. Yanyan Cheng: contributed new reagents, analytic tools. Xianrui Meng: analyzed data. Hao Xu: analyzed data. Yuanqing Xu: contributed new reagents/analytic tools. Wenkai Zhang: designed research, analyzed data. Xiaomin Fang: designed research. Hai-Bei Li: conducted theoretical simulation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (U2004179; U1904184; U1904191), China Postdoctoral Science Foundation (2019M662485) and Natural Science Foundation of Henan Province (202300410086).

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