Reviewed by Alex SmithApr 13 2022
Organic long persistent luminescence (OLPL) have great potential for overcoming the defects associated with conventional inorganic long persistent luminescence systems of silicate, aluminate and sulfate-based matrices doped along with rare-earth elements.
a, A possible LPL mechanism: during photo-excitation, electrons (black dots) are continuously generated and filled into the LUMO orbits (path i); in the blends of CDs and matrix, electrons transferred from the HOMO of the donor to the HOMO of the acceptor to form charge-transfer (CT) states (path ii); the acceptor radical anions diffuse to isolate the donor radical cations from the acceptor radical anions, forming charge-separated states (CSS) (path iii); gradual recombination of the radical anions and radical cations (path iv) to generate long persistent exciplex emission, i.e., electron transition from the LUMO of the acceptor to the HOMO of the donor (path v). The small energy gaps between the lowest singlet excited state of exciplex (S1*) and S1/T1 of CDs enable RISC and ISC, resulting in multiple LPL emissive processes. b, Schematic illustration of the preparation process of m-CDs@CA and the conversion from urea to CA. c, The high-resolution TEM image of m-CDs@CA. d, The XRD patterns of m-CDs, pCA, eCA and m-CDs@CA. e, FT-IR spectra of m-CDs, CA and m-CDs@CA. f, High resolution XPS spectra and the corresponding fitting curves of N1s of the CA, m-CDs and m-CDs@CA. Image Credit: Kai Jiang, Yuci Wang, Cunjian Lin, Licheng Zheng, Jiaren Du, Yixi Zhuang, Rongjun Xie, Zhongjun Li, and Hengwei Lin.
But the afterglow emission from the majority of the organic materials are thermal activated delayed florescence (TADF) and phosphorescence (Phos), their afterglow durations are generally restricted to seconds level, far from equivalent to the hour-level of inorganic LPL materials.
To obtain OLPL, the production of long-lived intermediates, like charged-separated (CS) states, is one of the possible routes, which had been verified in a few electron-donating and electron-accepting organics blends.
Unluckily, the presently reported OLPL blends have to be engineered under a nitrogen atmosphere, so as to avoid quenching the charge-transfer (CT) states and CS states from water and oxygen. It is still a great challenge to identify OLPL under ambient conditions, especially in aqueous media.
In a new paper published in Light: Science & Application, a research team, headed by Professor Hengwei Lin and Professor Kai Jiang from the International Joint Research Center for Photo-responsive Molecules and Materials, School of Chemical and Material Engineering, Jiangnan University, China, and co-workers reported the making of the first carbon dots (CDs)-based organic blend (named m-CDs@CA) displaying excellent LPL feature.
The afterglow has been noted to be more than one hour of duration through irradiation by a hand-held UV lamp (365 nm). The afterglow intensity of m-CDs@CA following 10 seconds conforms to an inverse power function of time t-1.
This denotes the LPL emission must be originated from newly-formed ultralong-lived emissive states, instead of the common triplet and/or singlet states (Phos and/or TADF).
In a more efficient manner, this LPL material is relevant under ambient conditions and even in an aqueous medium. Thus, it could be utilized as inks and comfortably patterned onto various substrates for different applications (for example, lighting the emergency sign), as well as dispersed in epoxy resin for easily fabricating luminous pearl.
This work not only provides a very unique example of an OLPL system that displays an hour-level of strong afterglow, but it also paves the way for the development and application of rare-earth-free OLPL materials.
Additional studies verified the origin of this strong OLPL.
As the researchers suggested, “Notably, photo-induced electron transfer properties of CDs had been studied previously, which demonstrated that CDs can serve as either electron donors or electron acceptors. Given these findings, photo-induced CT and CS states might be obtained by properly designing CDs-organic molecule blends, consequently producing LPL.”
From the presence of CT and CS states verified by the systematic photophysical investigations, the LPL of m-CDs@CA originated from the exciplexes of m-CDs and CA. More significantly, the CA was in situ generated from urea during the microwave heating process and then bonded with m-CDs through C-N bond.
Such in situ fixing of CDs in CA crystals and the formation of a covalent bond between CA and m-CD play crucial roles in stiffening the micro-environment of CT and CS states of the exciplexes. Hence, it triggers LPL of m-CDs@CA and avoids its quenching from water and oxygen.
Thus, the scientists summarized that, “To realize such a purpose, the following conditions are needed to be considered: i) the energy levels of CDs and organic molecule should be fitted for electron transfer (i.e., the highest occupied molecular orbital (HOMO) and LUMO energy levels of donor being respectively higher than that of the acceptor);
“ii) CDs are better to be employed as the electron donors and immobilized into an appropriate electron-acceptor matrix, providing a rigid micro-environment to stabilize the CT and CS states; iii) formation of covalent bonds is preferred between the CDs and matrix molecule, so as to further stabilize the excited states and avoid quenching by oxygen and water.”
Such considerations could be preliminarily considered as the fundamental principles for developing and making CDs-based OLPL blends.
“This study developed a facile strategy to prepare OLPL materials that being applicable under ambient conditions, which could not only effectively expand the scope of CDs-related research and applications, but also offer a new idea for designing OLPL systems with robust features,” forecasted the researchers.
Journal Reference:
Jiang, K., et al. (2022) Enabling robust and hour-level organic long persistent luminescence from carbon dots by covalent fixation. Light: Science & Applications. doi.org/10.1038/s41377-022-00767-y.