Non-traditional lighting technology has a long history and has only been revived in recent years. Unlike ACQ and AIE emitters, unconventional emitters contain only single pairs of electrons (such as N, O, S, and P) or isolated unsaturated bonds (such as C=O, C=C, C=N, Electron-rich heteroatoms (C=O, and C≡N) are not generally considered to be typical luminophores. Unconventional or non-conjugated luminescent materials that do not contain polycyclic aromatic hydrocarbons or extended π conjugation are a rising star in the field of luminescent materials. However, continuously tuning the emission color over a broad visible region through rational molecular design remains quite challenging because the mechanism of unconventional luminescence is not yet fully understood for .
Recently, Institute of Chemistry, Chinese Academy of Sciences Researcher Zhang Jun , Associate Researcher Tian Weiguo and Associate Researcher Song Guangjie et al. proposed a new type of unconventional luminophore, polymaleimide (PMs) , which has full-color emission and can be finely tuned by anionic polymerization of even at ambient temperature. Interestingly, general features of unconventional luminescence—clustered luminescence (CTE)—such as concentration-enhanced emission, are not observed in PMs. In contrast, PMs have characteristics similar to the aggregation-induced quenching of by due to enhanced intra/intermolecular charge transfer in . This biocompatible luminescent material synthesized from low-cost monomers has broad prospects for large-scale production and application in the fields of anti-counterfeiting printing, fingerprint identification, metal ion identification, etc. . This also provides a new rational molecular design platform for achieving full-color unconventional luminescence without any aromatic hydrocarbons. The related work was published in the latest issue of "Nature Communications" under the title "Anionic polymerization of nonaromatic maleimide to achieve full-color nonconventional luminescence".
Figure 1. Synthesis and characterization of fluorescent polymaleimides (PMs).
[Synthesis and characterization of A-PM and Fr-PM]
Use a mild Lewis base (such as triethylamine , TEA) instead of AIBN to initiate the polymerization of maleimide, which can be easily obtained by controlling the polymerization conditions. Non-conjugated PMs with full color emission (red, green, blue). The characterization results confirm that Fr-PM contains –C–C– connections, and A-PM is chemically connected by –C–C– and –C–N–. Essentially, the change in the connection mode of repeating units caused by this polymerization should be the source of the different fluorescence properties of Fr-PM and A-PM.
Figure 2. Polymerization of anion and regulates A-PM full-color emission.
[Anionic polymerization regulates A-PM full-color emission]
Continuous and precise tuning of fluorescence emission in the visible region and even larger ranges has been the main driving force for the structural design and synthesis of fluorescent materials . Lewis base species and reaction times relevant to anionic polymerization are studied here to modulate the emission of PMs. As shown in Figure 2a, the full-color fluorescence emission in the visible region (red, green, and blue) of A-PM powder or solution (DMF is the solvent) is achieved by using different Lewis bases as initiators during the polymerization process. The normalized fluorescence spectra in Figure 2c,d show that the emission λmax of PM powder ranges from 465 nm (A-PM-HA) to 608 nm (A-PM-TEA) and the solution from 465 nm (A-PM-HA) A continuous red shift occurs to 624 nm (A-PM-TEA). Furthermore, the fluorescence intensity of the PM solution and the photoluminescence quantum yield (PLQY) of the PM powder decreased exponentially with the red shift of the emission λmax (Fig. 2b). In addition, the researchers monitored the in-situ fluorescence spectrum of reaction solutions triggered by Lewis bases (such as HA and TEA) at 80°C in real time to study the changes in emission with reaction time.
Figure 3. Fluorescence properties of PM are concentration and solvent dependent.
[Concentration-dependent A-PM emission]
CTE is currently the most popular non-traditional luminescence system. The general characteristics of concentration-enhanced emission are the basic criteria for CTE. Therefore, the concentration-dependent emission of PM induced by TEA has been intensively studied. When the concentration of the A-PM-TEA/DMF solution gradually increases, the emission intensity at ~450 nm first increases, and when the concentration reaches 3.56 mg/mL, the emission λmax appears blue-shifted (lower than 450 nm). In contrast to the blue shift of λmax in the short wavelength direction (~450 nm), λmax undergoes a red shift at ~600 nm.The results show that the extension of the reaction time equivalently increases the concentration of PM in the solution. The concentration-dependent emission of A-PM-TEA completely contradicts the general characteristics of CTE and AIE luminescence.. On the contrary, is more similar to the luminescence properties of ACQ luminophore due to the internal filtering effect. Therefore, more theoretical assumptions and experimental verifications are needed to arrive at a general unconventional luminescence mechanism, especially for CTE systems.
Figure 4. Pressure-dependent fluorescence characteristics of A-PM
Figure 5. Schematic diagram of unconventional luminescence of PMs.
[Pressure-dependent emission and luminescence mechanism of A-PM]
In order to gain a deeper understanding of the unconventional luminescence properties of PM, researchers used the diamond pressure chamber (DAC) to study typical PM (A-PM-TEA) during anionic polymerization. pressure-dependent emission. results indicate that molecular deformation and recovery under high pressure are responsible for the reversible change in emission intensity .
Based on the unique luminescence of polymers with different polymer structures (–C–C– and –C–N–) under different environments, a clear mechanism can be established to explain the unconventional luminescence of polymers. The unconventional panchromatic emission of PMs originates from varying degrees of intramolecular charge transfer through bonds and/or intermolecular charge transfer through space, TBCT and TSCT, mainly by the connection modes of PMs (–C–C– and –C–N–) , molecular weight and aggregation state determine .
Figure 6. A-PM’s biocompatibility and applications.
[Summary]
In summary, a new type of unconventional luminescent material that does not contain any aromatic hydrocarbons - polymaleimide - was synthesized through anionic polymerization of maleimide. By continuously tuning the emission spectrum of PMs under different polymerization conditions (even at room temperature), full-color emission is achieved. However, typical CTE features, such as concentration-enhanced emission and excitation-dependent luminescence, are not observed in PMs. Compared with the limited spatial conjugation in typical non-conjugated luminophore heteroatom clusters that have been reported, PMs extend electron delocalization through intra- and/or intermolecular charge transfer, which can be induced by PMs under different conditions such as Chain mode (–C–C– and –C–N–), molecular weight and aggregation state control during polymerization under different reagents and reaction times. This biocompatible luminophore synthesized from low-cost monomers provides a unique example for perfecting the CTE theory and provides a new platform for rational molecular design, which can achieve far beyond its application in anti-counterfeiting printing, fingerprint recognition, metal Full-color unconventional luminescence provides a new platform for potential applications such as ion recognition.
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Original link:
https://www.nature.com/articles/s41467-022-31547-2
Source: Frontiers of Polymer Science
