Controlling the responsive behavior of smart room temperature phosphorescent materials through orthogonal stimuli

Manipulating the responsive behaviors of the room temperature phosphorescence (RTP) materials on demand will profoundly improve and advance current state-of-the-art photonic applications, but it remains a formidable challenge.
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Controlling the responsive behavior of smart room temperature phosphorescent materials through orthogonal stimuli
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Ultralong room temperature phosphorescence (RTP) is an appealing optical phenomenon that can last at least for several seconds after the removal of excitation source. Persistent RTP materials have attracted considerable attention due to its great potential applications in bioimaging, display, sensor, and anti-counterfeiting. In particular, those who can exhibit reversible changes in optical properties upon exposure to external stimuli possess advantages of fast, convenient, and high-efficient tuning RTP properties. These advantages suggest a potentially bright future in frontier photonic applications. Over past few years, numerous stimuli-responsive persistent RTP materials exhibiting emission intensity, color, and lifetime switching have been developed.

We have step by step investigated stimuli-responsive persistent RTP materials in depth for years. For example, we have developed a series of triphenylphosphine oxide derivatives, whose phosphoresecne intensity and decay time can be siginificantly enhanced under countious UV irradiation (Cell Reports Physical Science 2021, 2, 100505). By tailoring substituent groups in those molecules, dynamic persistent RTP behavior with different photo-activation and deactivation times can be controllably tuned under ambient conditions. In addition, we have prepared a pure organic phosphor of tris(4-chlorophenyl)phosphine oxide with a large energy gap between T1 and T2 triplet states is developed (Advanced Optical Materials 2022, 10, 2102706). Tunable afterglow colors from green to yellow can be achieved due to the different distribution of the triplet excitons from low- and high-lying triplet excited states. Next, the triphenylphosphine skeleton can be greatly expanded to numerous dual RTP materials by rational chemical modification because these molecules are easily available with low cost and simple synthetic procedures.

Nevertheless, less attention has been paid on the study of the control of response behaviors of stimuli-responsive RTP materials. On-demand regulation of persistent RTP behaviors will profoundly improve and advance current state-of-the-art photonic applications of organic persistent RTP materials. For example, modulating the photoactivation speeds and emission decay times of photoactivated organic persistent RTP molecules is crucial to achieve high-level information encryption. The regulation of persistent RTP colors of excitation-wavelength-dependent (Ex-De) materials is beneficial to realizing multicolor displays. Although our previous works have achieved control of persistent RTP behavior with different responsive behaviors through rational molecular design. However, complex chemical synthesis is often required to achieve the regulation of response behaviors, making the manipulation process inconvenient and inefficient. Therefore, it is highly desireable to control responsive behavior of smart RTP materials through orthogonal stimuli.

One major reason for the difficulty in controlling response behaviors of smart RTP materials is due to their poor extensibility. Small changes on the chemical structures of those molecules would result in completely different photophysical properties. Small changes on the chemical structures of those molecules would result in completely different photophysical properties. In this work, we have provided a novel molecular skeleton of triphenylphosphine derivative with smart responsive persistent RTP effect. Excitingly, it has great extensibility and various photoactivated RTP molecules can be obtained based on this skeleton by chemical modification. In specific, aromatic groups were directly bonded to the phosphorus center in quaternary phosphonium derivatives to form a typical molecular rotor; therefore, they may adopt various conformations in rigid surroundings since the rotating aromatic groups will be fixed at different angles, which are closely related to different triplet energy levels. This feature makes controlling persistent RTP colors possible by employing different excitation energies. In addition, the thermal energy might cause molecular rotations of quaternary phosphonium derivatives and thus result in conformational changes. On this basis, we envisage that it is possible to regulate the Ex-De RTP behavior of quaternary phosphonium derivative-based polymers by manipulation of the molecular conformations via thermal energy (Fig. 1).

Fig.1 Schematic representation for the dynamic manipulation of Ex-De RTP behavior.
Fig.1 Schematic representation for the dynamic manipulation of Ex-De RTP behavior.

The key advances in this work are reported as the following:

  • A RTP polymer (P1) was synthesized by copolymerization of (but-3-en-1-yl)triphenylphosphonium bromide and acrylamide. The photophysical property investigation of P1 showed that the delayed photoluminescence maxima could be adjusted from 474 nm to 506 nm by changing the excitation wavelength from 300 nm to 360 nm, accompanied with afterglow colors change from sky blue to green. Furthermore, by copolymerization of different triphenylphosphonium based phosphorescent rotors and acrylamide, a series of Ex-De RTP polymers (P2-P5) were obtained, demonstrating the universality of our strategy.
  • Both experimental results and theoretical calculations revealed that various molecular conformations of monomers are responsible for the Ex-De RTP behavior of these polymers (P1-P5). Thus, we have demonstrated that the RTP bands of P1-P5 were red shifted with the increase of temperature, because it is known that the thermal energy can cause the rotatory motions of monomers. Based on this feature, dynamic control of Ex-De RTP behavior was unprecedentedly achieved through thermal stimulus.
  • Based on the excellent processability of these polymers, high-level anti-counterfeiting labels were produced through ink-jet printing technique, and their Ex-De afterglow color changed with the temperature varied. In addition, large-area transparent films and fibers with Ex-De afterglow emissions were prepared for security applications. It is expected that this work can move early-stage demonstrations of security applications into mature commercialization for organic persistent RTP materials.

Look into the future, our strategy on the control of responsive behaviors can be extended to various RTP materials for different advanced photonic applications.

More details can be found in our paper "Conformation-dependent dynamic organic phosphorescence through thermal energy driven molecular rotations" published in Nature Communications.

Link to article: Conformation-dependent dynamic organic phosphorescence through thermal energy driven molecular rotations | Nature Communications

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