Organic polymers achieving smart room-temperature phosphorescence

Abstract

Room-temperature phosphorescence (RTP) has drawn widespread attention recently owing to the unique long-lived triplet excited states and potential applications in lightemitting diodes (LEDs) [1], optical imaging [2], photodynamic therapy [3], and information security [4]. Particularly, RTP materials can effectively utilize the triplet excitons to maximize the internal quantum yield in electroluminescent fields. However, to date, the most efficient phosphors are still limited to noble metal-containing compounds (such as iridium and platinum complexes), which have some intrinsic obstacles, such as metal scarcity and high bio-toxicity. Therefore, it is still very important to establish high-efficiency RTP materials based on pure organic compounds. For the design of organic RTP systems, two pivotal scientific problems should be taken into consideration: (1) how to facilitate intersystem crossing (ISC) from excited singlet to triplet through strengthening spin-orbital coupling; (2) how to reduce non-radiative transitions by providing rigid environments. During last few years, crystallization-induced phosphorescence (CIP) has been proved as an ideal strategy to enhance RTP, since the rigid crystals contribute to suppression of non-radiation and isolation of quenchers like oxygen. For instance, a series of benzophenone (BP) derivatives present efficient RTP upon crystal formation [5]. Besides, the introduction of halogen atoms into rigid crystals can also greatly improve RTP efficiency due to heavy-atom effect directed by halogen bonding [6]. However, it is relatively difficult for crystalline materials to prepare solid thin films as photoelectronic devices due to the lack of facile processability and good repeatability, which have hindered the commercial application of these crystalline RTP materials. Moreover, the realization of heavy-atom free system is quite important, which could offer several desirable advantages such as low cost, abundant sources and ultra-long RTP detection [7]. In order to solve problems in the preparation process of pure organic RTP materials, quite recently, Tian and Ma’s group [8] from East China University of Science & Technology presents a concise and simple chemical method to construct heavy-atom-free pure organic amorphous polymers for realizing RTP with a decent quantum yield and ultra-long RTP lifetime. These RTP materials can be further used into smart sensor and information safety fields. They have designed ten polymers copolymerized by acrylamide and different phosphors containing oxygen units (P1–P10) (Figure 1(a)). The polyacrylamide could serve as a polymer matrix to stabilize phosphors through the construction of hydrogen-bonding network structure to decease the nonradiative relaxation, and the oxygen-containing functional groups could favor the n→π* transition to facilitate ISC by strengthening the spin-orbital coupling. Among the all polymers, P3 exhibits the longest lifetime (τp=537 ms) and highest quantum yield (Φ=15.39%) in the solid state, with a blue-colored persistent luminescence observed in the time range of 0–4 s by naked eye (Figure 1(b)). Interestingly, the RTP intensity of polymers is enormously