Afterglow efficiency of 45%! So far afterglow highest efficiency organic materials
The organic material refers to the afterglow after excitation may continue to stop light emitting organic material. Which can be used in various fields, such as bio-imaging and information storage, and security sensors. However, there are only a few reports of afterglow efficiency (absorbed photons converted into afterglow efficiency) can exceed 10%, the afterglow of organic materials to achieve high efficiency of the afterglow is still a huge challenge, which is due to organic long room temperature phosphorescence ( OURTP) required by the spin-forbidden triplet excited state (T1) obtained barrier radiative transitions. Recently, Huang academician, Professor Chen Runfeng on \”Nature Communications\” published an article entitled \”Thermally activated triplet exciton release for highly efficient tri-mode organic afterglow\” of the article, has been successfully efficiency up to 45% of organic material afterglow. They stabilized by heat activation triplet excitons (T1 *) into the lowest triplet (T1), T1 is then converted to the singlet excited state (Sl), to give a spin allowed to emit light, thereby significantly reinforcing organic afterglow. This afterglow is room temperature S1, T1 and T1 * radiation attenuation caused by unconventional tri-mode emission. Afterglow since it has the highest efficiency reported to date, tri-mode afterglow concept represents a significant progress in the release of thermal activation by promoting stable triplet excitons to design efficient organic material afterglow. 
Photo REVIEW
1. How to significantly improve the efficiency of afterglow?
Let\’s take a look at the causes of organic material afterglow afterglow low efficiency. Typically, to produce observable emission afterglow is required to facilitate the incorporation of hetero atoms excited spin orbit coupling between the singlet and triplet (the SOC), in order to enhance intersystem crossing (ISC) and build a stable triplet excited state (T1 *) to suppress non-radiative transitions (Figure 1a). Since SOC weak effect of purely organic molecules, only a small portion of the light excited singlet excitons may be converted into triplet excitons through the ISC, Africa and the radiation decay of triplet excitons accounted for most transitions, low temperature phosphorescence efficiency , the organic afterglow therefore less efficient. How can significantly improve the efficiency of the organic afterglow afterglow material it? On activation delay from heatDelayed fluorescence (TADF) was inspired on. TADF can crossing (RISC) will be converted into triplet excitons allowed spin singlet excitons by inverse intersystem give delayed fluorescence (FIG. 1b). In order to improve the efficiency of afterglow, they will be introduced into the thermal activation afterglow organic molecules. Different T1 into the exciton in the S1 TADF exciton, the exciton they need from a stable low release triplet * T1 is T1, T1 is then converted to S1 allow to obtain an organic spin-afterglow. Therefore, the material of the following requirements: firstly requires less exciton capture depth (ETD), so that it can be released by thermal energy excitons T1 * fluctuation; secondly, ΔEST need small, can be obtained by the RISC T1 of excitons excitons into S1 (FIG. 1c). According to the above mechanism, they designed a donor twisted – the molecular structure of the donor (D-A-D), such molecules (FIG. 1d) was prepared using a two-dione and boron trifluoride β- carbazole units – receptors. P has an activity which non-bonding electrons, can significantly promote inter-ISC process and build more intramolecular / intermolecular hydrogen bonds to suppress non-radiative transitions. Carbazole having a low tendency to form a strong T1 * can be stabilized in the triplet exciton state of aggregation. Further, boron difluoride β- dione strong receptor, directly bonded to the carbazole donor will produce strong intramolecular charge transfer (the ICT), to give a characteristic typical TADF, including small and efficient ISC ΔEST and RISC process. Twisted structure destroys carbazole π-π stacking unit H- and aggregation, resulting in less ETD.
2. Photophysical properties
of from 77 K DCzB fluorescence (450 nm) and a phosphor (475 nm) of the spectrum can be deduced ΔEST 0.15 eV, indicating that DCzB been successfully designed to TADF molecule. However, unlike the typical points TADFSub weak phosphorescence, DCzB showed significant phosphorescence. DCzB crystal fluorescence, and phosphorescence emission OURTP (525 and 570nm) at room temperature showed a long life of more than 230 ms (Fig. 2F), while resulting in the observed spectrum S1 in the phosphor, the emission T1 and T1 * band (FIG. 2d), with three mold afterglow emission behavior. The most important thing is, DCzB crystalline organic afterglow efficiency can be as high as 45%. This is the highest efficiency of organic afterglow pure single-component organic molecules reported to date.
3. Imaging and applied to the cells temperature detection
in order to achieve cell imaging, using the bottom-up approach, using polyethylene glycol – block – polyethylene glycol – block – polyethylene glycol (F127) amphiphilic phospholipid prepared DCzB nanoparticles (NPs ), to wrap afterglow hydrophobic organic molecule, it has a good water dispersibility and stability (FIG. 3a). The average size of about 100-120 nm (FIG. 3b, c). Since the size of NPs in the aggregates is significantly reduced, as compared with the solid film aggregate (FIG. 2A), the absorption spectrum blue shift (Fig. 3d). Because the transparent aqueous solution DCzB NPs AIE effect exhibits strong fluorescence and afterglow is generated due to the TAA, it is used for cell imaging. The phosphate buffer DCzB NPsAfter Hela cells were cultured with 2 h, confocal images show DCzB NPs easily stained viable Hela cells, and visible light excitation shows strong luminescence (FIG. 3f) by at 405 nm. To take advantage of long-lived luminescence DCzB NP to completely eliminate the background fluorescence in live cells, we were phosphorescence lifetime imaging (PLIM) (FIG. 3g). DCzB NPs cells exhibit a long lifetime emission, the average life of about 500 [mu], collected by photons is greater than 100μs long life can be obtained luminance image time-gated, shows high signal to noise ratio, as confirmed DCzB NPs cell life long probes is very effective for eliminating short-lived autofluorescence interference. Furthermore, considering the afterglow color DCzB temperature-dependent, and which may be applied from a colorful display specific temperature of 77 to 300 K visual detection (FIG. 3h). Pattern \”8\” DCzB filled with powder. Excitation at 365 nm, the pattern of blue at different temperatures, and after the excitation is stopped, the pattern is from 300 K to 77 K becomes blue-green afterglow green yellow. Application afterglow cell imaging and visualization of the temperature detection:
Highlights Summary
In summary, the authors propose a method of significantly improve the efficiency of single-component organic molecules of afterglow. The method is carried out by reducing the ETD and have ΔESTTAER and efficient RISC procedure to long life T1 and T1 * excitons release activated by heat, T1 is then converted to the singlet state S1 spin allowed thereby achieve efficient afterglow luminescence. Organic afterglow highest efficiency up to 45%. By means of an efficient and long-life mode, and three temperature sensitive afterglow, high performance can be achieved PLIM live cell imaging and multi-color temperature sensing visualized. This study marks a fundamental progress has been made in improving the efficiency of organic persistence to achieve multi-functional applications, and provides a common approach for the molecular design of organic phosphorescent material at room temperature. The full text link: https://www.nature.com/articles/s41467-020-14669-3
