Switching between Fluorescence and Room Temperature Phosphorescence in Carbon Dots: Key Role of Heteroatom Functionalities

Abstract

Herein, we have synthesized three different types of carbon dots, i.e., (a) CDs from citric acid, (b) nitrogen atom functionalized CDs from citric acid and ammonia, and (c) phosphorus atom functionalized CDs from citric acid and sodium dihydrogen orthophosphate through a simple bottom-up carbonization technique. Detailed morphological and elemental features are investigated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy study, and it is further correlated to the ongoing photophysical properties. To investigate the specific role of the heteroatom functionalities on the room temperature phosphorescence, we have incorporated all these three types of CDs in a boric acid matrix to diminish the flexibility of surface functional groups and decrease the nonradiative relaxation processes. Various heteroatom functionalities play a very specific role to tune the afterglow properties by altering the energy gap (ΔE ST ) between the lowest excited singlet (S 1 ) and triplet state (T 1 ) and the spin−orbit coupling constant which eventually control the radiative recombination from the triplet state. Finally, a switchable fluorescence and room temperature phosphorescence have been observed depending on the specific heteroatom functionalities. A detailed temperature-dependent study has been performed to investigate the tunability between prompt fluorescence and phosphorescence properties. This is further correlated to the conversion of phosphorescence with thermally activated delayed fluorescence (TADF). Computational studies based on time-dependent density functional theory (TD-DFT) have been performed by using optimized model systems in connection to the elemental study, which nicely support our experimental findings.

Stable Room-Temperature Phosphorescence from MXene-Derived Carbon Dots: Ultralong Afterglow Emission without an External Matrix

“…It is due to the quenching of the thermal nonradiative relaxations at lower temperature. 12,19,43 In addition, phosphorescence lifetime decay profiles also support the same (Figure S9). The schematic illustration of the band diagram of the as-synthesized CDs is shown in Figure 5d.…”

“…As the temperature is lowered to 90 K, the peak intensity at ∼545 nm increases, which suggests the enhancement of phosphorescence emission, while the prompt fluorescence at ∼495 nm almost disappears. It is due to the quenching of the thermal nonradiative relaxations at lower temperature. ,, In addition, phosphorescence lifetime decay profiles also support the same (Figure S9). The schematic illustration of the band diagram of the as-synthesized CDs is shown in Figure d.…”

“…The origin of the afterglow and RTP emission of the as-synthesized CD powders is somewhat complicated, especially without any external matrix. Generally, the n−π* transition of CDs has a major contribution to the emission properties, and notably, the aromatic CO and CN groups possess a strong spin–orbit coupling, resulting in RTP emission through harvesting triplet state. ,,,, Furthermore, commonly embedding CDs in an external rigid matrix is needed to obtain RTP. However, in the present case, we have achieved very intense ultralong afterglow emission from solid-state CDs without using any external matrix.…”

“…Materials with prolonged lifetime and stable room-temperature phosphorescence (RTP)/prominent afterglow have attracted a great deal of attention because of their multimodal applicabilities in photonics, optoelectronics, sensing, and catalytic devices. − However, accomplishing long-term afterglow at room temperature is a very challenging task due to the instability of excited triplet species, oxygen-induced phosphorescence quenching, and inefficient intersystem crossing (ISC). ,− In general, afterglow emissions have been mainly observed in some specific organic and inorganic materials, along with heavy metal-based organometallic complexes, and these materials are seriously suffering from some drawbacks, including heavy metal cytotoxicity, scarcity of their precursors, tedious or complicated synthesis procedures, and limited phosphorescence lifetimes to a range of several microseconds, which limits their practical applicability as long-term afterglow materials. ,,− Henceforth, metal-free sustainable materials with ultralong afterglow emissions offer potential alternatives. In this context, carbon dots (CDs) have gained significant interest because of their prominent features, including tunable photoluminescence (PL), high quantum yield, biocompatibility, ease of synthesis, low cost, excellent water solubility, good chemical stability, and low toxicity as compared with the other conventional fluorescent materials, and their substantial applications in various fields including optoelectronics, catalysis, anticounterfeiting and security devices, energy storage, biomedicines, etc. − ,− However, harvesting triplet state in CDs and generating room-temperature phosphorescence/long afterglow emission have not been explored much.…”

“…In general, afterglow emission can only be generated by embedding CDs in a rigid matrix, where we can suppress the nonradiative deactivation processes. − , However, embedding CDs in a solid matrix limits their further applicability. On the other hand, without a matrix, solidification of CDs decreases the radiative recombination through aggregation-induced quenching. ,− Therefore, it is always challenging to generate afterglow emissions from CDs by solely improving the core as well as surface functionality without using any external matrix.…”