Conversion of Carbon Dots from Fluorescence to Ultralong Room‐Temperature Phosphorescence by Heating for Security Applications

Abstract

Stimuli-responsive optical materials have received tremendous interest in the last several decades due to their numerous promising applications. Here, fluorescence emissive polymer carbon dots (F-CDs), prepared with a simple heating treatment from ethylenediamine and phosphoric acid, are found to produce unexpected ultralong room-temperature phosphorescence (URTP), which lasts for about 10 s with a lifetime of 1.39 s. This is the first example to achieve the conversion of a fluorescence material to URTP by means of an external heating stimulus. Further investigations reveal that the doping of N and P elements and self-immobilization of the excited triplet species are likely mainly responsible for the observed URTP after the heating treatment, due to the facilitation of the intersystem crossing and formation of more compact cores for effective intraparticle hydrogen bonds, respectively. Importantly, this study also demonstrates the potential for aqueous dispersion of the F-CDs as an advanced security ink for information encryption and anticounterfeiting; this is a feature that has not been reported before. This study is believed to open possibilities to extend stimuli-responsive optical materials to rarely exploited phosphorescence-relevant systems and applications, and also to provide a novel strategy to easily prepare URTP materials.

Proton‐Activated Amorphous Room‐Temperature Phosphorescence for Humidity Sensing and High‐Level Data Encryption

“…To theoretically comprehend the efficient ISC process induced by the configuration transformation, the energy gap (ΔE ST ) of two configurations are investigated through TD‐DFT calculations. The calculated singlet‐triplet energy gap (ΔE ST =0.71 ev) for a′′ configuration (Figure ) has an obvious reduction compared with that (1.15 ev) for a configuration, which is well consistent with the experimental value for 1@LP (0.68 ev, Figure S9) evaluated through low temperature (77 K) fluorescence and phosphorescence spectra . To further gain insight into the exact reasons for the decreased ΔE ST resulting from the configuration transformation, we research the electronic distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for two configuration through TD‐DFT calculations.…”

“…The calculated singlet-triplet energy gap (ΔE ST = 0.71 ev) for a'' configuration ( Figure 3) has an obvious reduction compared with that (1.15 ev) for a configuration, which is well consistent with the experimental value for 1@LP (0.68 ev, Figure S9) evaluated through low temperature (77 K) fluorescence and phosphorescence spectra. [14] To further gain insight into the exact reasons for the decreased ΔE ST resulting from the configuration transformation, we research the electronic distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for two configuration through TD-DFT calculations. As shown in Figure 4, a'' configuration possess the larger spatial separation of HOMO and LUMO than that for a configuration induced by their decreased delocalization of LUMO, which may be owing to the higher distorted geometries (À 39.6°between benzene plane and pyridine plane) for a'' configuration with monocation than that (À 34.8°) for a configuration (Scheme 2).…”

Blue‐Light‐Absorbing Thin Films Showing Ultralong Room‐Temperature Phosphorescence

“…[1][2][3][4][5][6] Until very recently, the URTP phenomenon was mostly described for inorganic or organometallic materials containing noble metals or rare-earth elements. Among those, molecules displaying ultralong room-temperature phosphorescence (URTP) are of paramount importance notably for their applications as active materials in bioimaging, anticounterfeiting, information storage, oxygen sensing, etc.…”

“…In 2017, Shoji et al revealed the exceptional phosphorescence properties of a series of simple crystalline arylboronic esters, with lifetimes up to 1.73 s, which is-to the best of our knowledge-one of the longest values ever reported for metal-free organic phosphors. [1][2][3][4][5][6] Until very recently, the URTP phenomenon was mostly described for inorganic or organometallic materials containing noble metals or rare-earth elements. After dispersion into polyvinyl alcohol (PVA) to create drop-coating thin films and long irradiation under strong UV light (65 min, 254 nm), an intense phosphorescence (Φ p up to 11.23%) associated with a long lifetime (up to 0.71 s) could be observed at room temperature in aerated atmosphere.…”

One‐Step Synthesis of Silica‐Coated Carbon Dots with Controllable Solid‐State Fluorescence for White Light‐Emitting Diodes

“…Luminescent carbon dots (CDots) have received much attention in lighting devices due to their distinctive merits, such as excellent photostability, low toxicity, eco‐friendly preparation, and widely available precursors compared with conventional luminescent materials . To realize the real optical device applications of CDots, fabrication of solid‐state fluorescence CDots is requisite.…”