Photoluminescence Mechanism Studies On Crosslink-Enhanced Emission Effect In Carbonized Polymer Dots

As new carbon-based luminescent nanomaterials,carbon dots(CDs)possess significant advantages like facile synthesis,bright emission,low toxicity,good biocompatibility,thus attracting much attention.In recent years,researchers have carried out a lot of work on the selection of raw materials and the optimization of synthesis methods.Such efforts greatly enriched the species of CDs(graphene quantum dots,carbon quantum dots,and carbonized polymer dots,CPDs),but led to a complex relationship of structures and properties.More and more studies point out that CDs(especially for carbonized polymer dots obtained by “bottom-up” method)are not completely graphitized.Their photoluminescence(PL)is obviously distinct from that of graphene oxide or quantum dots with similar structures.Therefore,the PL mechanism of CDs has always been controversial,which is related to how people understand the essence of materials.In addition,it is still a tough task to make the synthesis of CDs more designable and controllable,instead of relying too much on the nature of the raw materials.Aiming at the above two key issues,this paper took CPDs as the research object,and deeply explored the mechanism of crosslink-enchanced emission effect(CEE).In terms of research significance,the innovations of this paper are as follows: proposed a new research method to explore the PL mechanism by constructing simplified model systems of CPDs;achieved room temperature phosphorescence(RTP)in CPDs by CEE for the first time and exploited a new class of metal-free RTP materials;proposed “addition-condensation polymerization”synthesis strategy and confined-domain CEE theory,realizing the concise adjustment of both fluorescence and RTP performance in CPDs;proposed to understand and design CPDs from the perspective of polymerization and carbonization,and developed the fundamental PL theory.Specifically,the detailed work of this paper can be described as the following three parts:Firstly,we investigated the CEE on the fluorescence of CPDs.We comprehensively considered the formation factors of CPDs,and speculated the possible luminescence center and fluorescence mechanism by constructing a simplified model system(hydrothermal system of polyacrylic acid and ethylenediamine).According to the results of acid-base experiment,oxidation experiment,and the self-hydrothermal product analysis of raw materials,we proposed that the structures produced by amine oxidation(including amide,imide and-N=O group)were the main fluorescence sources of CPDs.Besides,the crosslinked polymeric structures in CPDs were proved to provide with a new and stable chemical environment for luminescence centers.It could not only play a protective role to enhance fluorescence,but also change the emission wavelength to a certain extent.Secondly,after having some understanding of the fluorescence mechanism,we further speculated that the CEE could also promote the generation of RTP in CPDs.Combining the self-characteristics of CPDs and structure design,we achieved RTP performance by one-step method in CDs-class materials for the first time.Sub-luminophores and efficient CEE(here mainly covalent-bond CEE and supramolecular-interaction CEE)were the essential factors for the generation of RTP in CPDs.Following this principle,a variety of RTP CPDs could be synthesized.It expanded the application field of CPDs in anti-counterfeiting and information encryption,and also realized the facile synthesis of metal-free RTP materials.Finally,we discussed the effect of spatial interaction in the confined domain of CPDs on PL,after fully studying the contribution of covalent-bond CEE and supramolecular-interaction CEE to fluorescence and phosphorescence.We proposed the “addition-condensation polymerization” synthesis strategy and designed a model system of CPDs with well-defined structure to explore such abstract interactions.Methyl groups with tunable content served as substituents to exert different degrees of steric hindrance,thereby changing the spatial interaction between the luminophores in CPDs.The results showed the successful introduction of methyl groups achieved the continuous adjustment of PL quantum yields and RTP lifetimes of CPDs.Therefore,the confined-domain CEE can be adopted as a new strategy to modulate the PL,thereby improving the designability and controllability of the synthesis of CPDs.This paper was dedicated to in-depth exploration of the structure and PL mechanism of CPDs.By constructing an appropriate model system,we proved the important contribution of CEE to the fluorescence and RTP in CPDs(covalent-bond CEE and supramolecular-interaction CEE),and proposed a general strategy to further regulate the PL of CPDs(confined-domain CEE).This paper provided a new research method and perspective for the formation process,structural characteristics and PL origin of CDs materials,which had a certain reference value.