Ultralong Organic Phosphorescence: From Material Design to Applications

Abstract

Organic phosphorescence is defined as a radiative transition between the different spin multiplicities of an organic molecule after excitation; here, we refer to the photoexcitation. Unlike fluorescence, it shows a long emission lifetime (∼μs), large Stokes shift, and rich excited state properties, attracting considerable attention in organic electronics during the past years. Ultralong organic phosphorescence (UOP), a type of persistent luminescence in organic phosphors, shows an emission lifetime of over 100 ms normally according to the resolution limit of the naked eye. According to the Jablonski energy diagram, two prerequisites are necessary for UOP generation and enhancement. One is to promote intersystem crossing (ISC) of the excitons from the excited singlet to triplet states by enhancing the spin-orbit coupling (SOC); the other is to suppress the nonradiative transitions of the excitons from the excited triplet states.In this Account, we will give a summary of our research on ultralong organic phosphorescence, including the design of materials, manipulation of properties, fabrication of nano/microstructures, and function applications. First, we give a brief introduction to the UOP development. Then, we discuss the constructed methods of UOP materials from the inter/intramolecular interaction types, including π-π interactions, intermolecular hydrogen bonds, halogen bonds, ionic bonds, covalent bonds, and so on. These effective interactions can build a rigid environment to restrain the nonradiative transitions from the molecular motions or external quenching by oxygen, moisture, or heat, and thus enhance the UOP performance. Next, the manipulation of UOP properties, containing excitation wavelength, emission colors, lifetimes, and quantum efficiency (QE), through molecular or crystal engineering will be summarized. Recently, the excitation wavelengths of the materials for UOP can be regulated in different regions, such as UV, visible light, and X-ray; the emission colors of UOP can cover the whole visible-light region, from deep blue to red; the phosphorescence lifetime of UOP materials can reach 2.5 s, and the quantum efficiency can be achieved up to 96.5%. Moreover, we will present the fabrication of micro/nanoscale UOP materials, including the preparation of micro/nanostructure, optical performance, and device fabrication. Afterward, we will review the potential applications of UOP materials in organic/bio-optoelectronics, such as information encryption, bioimaging, sensing, afterglow display, etc. Finally, an outlook on the development of UOP materials and applications will be proposed.