Excited-state intramolecular proton-transfer-induced dual fluorescence emission in 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and resorcinol-based carbon dots
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 (–CO) 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).
References (59)
- et al.
The polymeric characteristics and photoluminescence mechanism in polymer carbon dots: a review
Mater. Today Chem.
(2017) - et al.
Ethanothermal synthesis of phenol-derived carbon dots with multiple color emission via a versatile oxidation strategy
Opt. Mater.
(2019) - et al.
Supramolecular nanodots derived from citric acid and beta-amines with high quantum yield and sensitive photoluminescence
Opt. Mater.
(2018) - et al.
Beyond bottom-up carbon nanodots: citric-acid derived organic molecules
Nano Today
(2016) - et al.
Novel ratiometric fluorescent probe for detection of iron ions and zinc ions based on dual-emission carbon dots
Sensor. Actuator. B Chem.
(2019) - et al.
Dual band emission in carbon dots
Chem. Phys. Lett.
(2018) - et al.
Concentration-dependent color tunability of nitrogen-doped carbon dots and their application for iron(III) detection and multicolor bioimaging
J. Colloid Interface Sci.
(2018) - et al.
Luminescent carbon dots with concentration-dependent emission in solution and yellow emission in solid state
J. Colloid Interface Sci.
(2020) - et al.
Tautomeric schiff bases: iono-, solvato-, Thermo- and photochromism
J. Mol. Struct.
(2011) - et al.
Proton transfer and self-association of sterically modified schiff bases
Chem. Phys.
(2003)
Hydrothermal synthesis of carbon quantum dots as fluorescent probes for the sensitive and rapid detection of picric acid
Anal. Methods
Design of metal-free polymer carbon dots: a new class of room-temperature phosphorescent materials
Angew. Chem. Int. Ed.
Supramolecular cross-link-regulated emission and related applications in polymer carbon dots
ACS Appl. Mater. Interfaces
Carbonized polymer dots: a brand new perspective to recognize luminescent carbon-based nanomaterials
J. Phys. Chem. Lett.
the formation mechanism and fluorophores of carbon dots synthesized via a bottom-up route
Mater. Chem. Front.
Paving the path to the future of carbogenic nanodots
Nat. Commun.
Insight into the multiple quasi-molecular states in ethylenediamine reduced graphene nanodots
Phys. Chem. Chem. Phys.
Purification and structural elucidation of carbon dots by column chromatography
Nanoscale
Molecular origin of photoluminescence of carbon dots: aggregation-induced orange-red emission
Phys. Chem. Chem. Phys.
Small molecular organic nanocrystals resemble carbon nanodots in terms of their properties
Chem. Sci.
Supramolecular interactions via hydrogen bonding contributing to citric-acid derived carbon dots with high quantum yield and sensitive photoluminescence
RSC Adv.
Structure and photoluminescence evolution of nanodots during pyrolysis of citric acid: from molecular nanoclusters to carbogenic nanoparticles
J. Mater. Chem. C
Tracking the source of carbon dot photoluminescence: aromatic domains versus molecular fluorophores
Nano Lett.
Aggregated molecular fluorophores in the ammonothermal synthesis of carbon dots
Chem. Mater.
Carbon dots with high fluorescence quantum yield: the fluorescence originates from organic fluorophores
Nanoscale
Novel efficient fluorophores synthesized from citric acid
RSC Adv.
Investigation from chemical structure to photoluminescent mechanism: a type of carbon dots from the pyrolysis of citric acid and an amine
J. Mater. Chem. C
Conformational behavior and optical properties of a fluorophore dimer as a model of luminescent centers in carbon dots
J. Phys. Chem. C
Luminescence phenomena of carbon dots derived from citric acid and urea – a molecular insight
Nanoscale
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These authors contributed equally to this work.