Background Introduction Organic room-temperature phosphorescence (ORP) materials, due to their long luminescence lifetime, high triplet exciton utilization, and large Stokes shift, have broad application prospects in optoelectronic devices and biomedicine, and have received widespread attention and research from researchers in recent years. Methods for obtaining high-efficiency ORP materials typically include crystal engineering, H-aggregation, metal-organic frameworks, and host-guest doping. Among these, host-guest doping focuses primarily on small molecule and polymer doping, which can yield amorphous luminescent materials with good processability and flexibility.
Compared to well-developed crystalline organic small molecule systems, amorphous powders or films exhibit more active molecular motion, making the development of amorphous ORP materials extremely challenging. Currently, physical blending of organic fluorescent molecules into rigid poly(vinyl alcohol) (PVA) matrices has become the most common and effective design strategy for obtaining amorphous organic room-temperature phosphorescent materials. This is because the hydrogen bonding between the guest luminescent molecules and the PVA matrix helps suppress molecular motion, thereby inducing persistent phosphorescence emission in the PVA-doped system at room temperature. However, research on PVA-doped systems mainly focuses on improving luminescence properties, while the actual contribution of hydrogen bonding and its influence on luminescence properties have not been reported. Hydrogen bonding is essentially an electrostatic force, but in complex doped or composite systems, the non-covalent interactions between molecules are quite complex, necessitating systematic and in-depth research to reveal the more realistic luminescence mechanism of doped PVA systems.
Article Summary: Recently, Professor Wang Dong and Assistant Professor Xiong Yu, from the AIE Research Center of Shenzhen University, led by Academician Tang Benzhong, reported a series of hydrogen-linked doped PVA films with ultralong room-temperature phosphorescence properties. Through detailed experimental studies and theoretical calculations, they revealed for the first time the significant contributions of electrostatic and dispersive forces to the ultralong room-temperature phosphorescence of hydrogen-linked doped PVA films.
In this work, three star-shaped molecules containing carboxyl, hydroxyl, and amino active groups were designed and incorporated into a PVA matrix via physical blending. These three star-shaped molecules can form multiple hydrogen bonds with the hydroxyl groups on the PVA chains through their carboxyl, hydroxyl, and amino groups, constructing a cross-linked hydrogen bond network within the PVA matrix. This effectively suppresses nonradiative transitions to achieve sustained room-temperature phosphorescence emission. Experimental results show that by controlling the formation of different types of multiple hydrogen bonds between the active groups and the PVA matrix, amorphous ultralong organic room-temperature phosphorescence with a lifetime of 1.74 seconds was successfully obtained.
To analyze the source of the ultralong room-temperature phosphorescence in the doped PVA film, the low-temperature (77 K) phosphorescence spectra of the doped PVA film, the guest molecule solid, and the dilute solution of the guest molecule were compared. The phosphorescence spectra of the doped PVA film and the low-temperature (77 K) phosphorescence spectra of the dilute solution of the guest molecule overlapped well, indicating that the ultralong room-temperature phosphorescence in the doped PVA film originates from the intrinsic single-molecule phosphorescence of the guest molecule. This result indicates that the guest molecules in the doped PVA film are monodisperse at extremely low concentrations. Simultaneously, single-crystal structure analysis also reveals that these three guest molecules possess highly distorted molecular configurations and do not exhibit significant π…π stacking interactions, further demonstrating that the guest molecules tend to be monodisperse when doped into the PVA matrix.
After assigning the ultralong room-temperature phosphorescence properties of the doped PVA film, the interaction between the guest luminescent molecules and the PVA matrix was further investigated in detail using theoretical calculations. Two different binding modes were observed between the guest luminescent molecules and the PVA matrix. In binding mode A, the guest luminescent molecules and the PVA matrix are primarily bound together through hydrogen bonds, while in binding mode B, they are primarily bound together through non-hydrogen bond interactions. Binding energy calculations and energy decomposition analyses for binding modes A and B revealed that in binding mode A, electrostatic attraction contributes the most to the overall binding energy, with the highest binding energy observed in the TPB-3COOH@PVA doped film. This is because hydrogen bonding is essentially an electrostatic force, and the carboxyl groups can form stronger intermolecular hydrogen bonds with the PVA matrix. In binding mode B, both electrostatic attraction and dispersion contribute significantly to the overall binding energy, again with the highest binding energy found in the TPB-3COOH@PVA doped film.
Following the calculations of the binding energy and energy decomposition data for the PVA doped system, the relationship between binding energy and luminescence lifetime was further analyzed. The binding energy trend of binding mode B shows a positive correlation with the room-temperature phosphorescence lifetime, indicating a greater contribution from binding mode B. This is likely because binding mode B involves more guest molecules interacting with the PVA matrix, resulting in a higher overall binding energy. Therefore, the ultralong room-temperature phosphorescence of these doped PVA films is not solely induced by hydrogen bonding between the guest molecules and the PVA matrix; rather, the electrostatic and dispersive interactions between the guest molecules and the PVA matrix play a significant role.
The ultralong room-temperature phosphorescence of hydrogen-bonded cross-linked doped PVA films exhibits reversible responses to temperature and humidity, as well as responsiveness to excitation wavelength. Therefore, it holds potential applications in multi-layered security protection and high-end anti-counterfeiting.
In summary, this work constructed an extended hydrogen-bonded cross-linked network by doping multiple star-shaped molecules containing active groups such as carboxyl, hydroxyl, and amino groups into the PVA matrix, achieving ultralong-lifetime room-temperature phosphorescence emission, reaching a maximum of 1.74 s. Through well-designed and controlled experiments combined with theoretical calculations, it was demonstrated that the strong electrostatic and dispersive interactions between the guest molecules and the PVA matrix are key to achieving the ultralong RTP characteristic. This work is also the first to reveal the nature of non-covalent intermolecular interactions in PVA doped systems.
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