Organic Room Temperature Phosphorescence by Confining Isolated Chromophores
Chang Wang, Yafan Ding, Xiao Wang, Wei Huang, Zhongfu An
Abstract
High Resolution Image Download MS PowerPoint Slide Conspectus Phosphorescence, a delayed luminescence phenomenon upon excitation, is defined as a radiative transition between states with differing electronic spin multiplicities. In contrast to fluorescent materials, phosphorescent counterparts offer several advantages, including long lifetimes, large Stokes shifts, and efficient exciton utilization. These attributes make them promising candidates for applications in information encryption, bioimaging, X-ray radiography, and beyond. Within the realm of phosphorescent materials, organic variants have recently piqued widespread interest, owing to their inherent qualities such as abundant resource availability and high mechanical flexibility. To achieve room temperature phosphorescence (RTP) in purely organic systems, two pivotal factors must be considered: one is to accelerate the intersystem crossing (ISC) rates from excited singlet states to excited triplet states, and the other is to inhibit the nonradiative transition pathways of triplet excitons. Currently, a key approach in organic RTP research involves controlling the aggregation state of organic molecules, as strong molecular interactions can help stabilize triplet excitons and reduce nonradiative transitions. Nevertheless, the aggregation may cause emission quenching, and the model of molecular aggregates remains complex, unpredictable, and uncontrollable. To tackle these challenges, low temperatures (such as 77 K) are often employed to restrict molecular motion, facilitating the realization of single-molecule phosphorescence with definite structures and controllable photophysical properties. However, the development of single-molecule phosphorescent materials has been significantly constrained by these low temperature conditions. Consequently, there is an urgent need for innovative design strategies that can improve the luminescent performance of RTP from isolated chromophores under ambient conditions while further elucidating the underlying photophysical mechanisms. In this Account, we provide a comprehensive overview of recent advances in solid-state, single-molecule RTP systems, with a particular emphasis on confinement methodologies and photophysical mechanisms that enable RTP emission. First, we briefly describe the evolution of organic phosphorescence, highlighting the importance and merits of single-molecule RTP materials. Subsequently, the luminescence processes of isolated organic molecules are illustrated, and the strategies for RTP from the isolated chromophores are thoroughly discussed. Essentially, two efficient approaches are proposed: one involves confining isolated chromophores within a rigid environment by chemical bonds, while the other entails the incorporation of chromophores into a polymer matrix or macrocyclic molecules. Effective confinement of chromophores suppresses nonradiative transitions from molecular motions and external quenching by oxygen, enabling extraordinary RTP from isolated chromophores in the solid state. Following this, we delve into typical reports of solid-state single-molecule RTP achieved through covalent bonding, ionic bonding, and noncovalent interactions, encompassing doped and supramolecular self-assembly systems. Finally, we propose challenges and prospects for the future development of RTP from isolated chromophores.