This journal is © The Royal Society of Chemistry 2022 J. Mater. Chem. C, 2022, 10, 2551–2555 | 2551 Cite this: J. Mater. Chem. C, 2022, 10, 2551 Exciton funneling amplified photoluminescence anisotropy in organic radical-doped microcrystals† Zhonghao Zhou, Chan Qiao, Jiannian Yao, Yongli Yan * and Yong Sheng Zhao * We demonstrate a controllable photoluminescence anisotropy amplification in organic luminescent radical-doped microcrystals via exciton funneling. The widely tunable doping ratio resulting from very similar molecular structures between hosts and guests leads to a freely tailorable exciton funneling process, which paves an avenue for the construction of high-performance polarizing optical elements. Introduction Polarization, which is a fundamental characteristic of light, plays an increasingly important role in various fields such as biosensing, quantum communication and 3D displays. 1–3 Traditionally, polarized light is obtained using bulky linear polarizers, which not only limits the conversion efficiency due to the greatly reduced vertical component of light, but also hinders the development of miniaturized polarized optical devices. 4,5 Utilizing polarization-sensitive materials that can directly generate polarized light is a promising route to manip- ulate photons at the micro/nanoscale. 6–12 Benefiting from tailorable excited-state processes, abundant intermolecular interactions and high optical orientation, 13–15 organic micro- crystals provide an ideal platform to precisely control the polarization of photoluminescence (PL). 16–19 Nevertheless, the emitted light from these materials generally suffers from a limited degree of polarization (DOP) owing to the relatively small projection of the transition dipole moment (m) at the optical excitation plane, which impedes their practical applica- tions in modern optoelectronics. A promising approach to improve the DOP is to concentrate polarized excitons by constructing light-harvesting systems composed of donor–acceptor pairs, 20 which has been widely utilized to enhance acceptor emission through exciton funnelling. 21–24 In general, aggregation of organic chromo- phores enables the formation of lower-energy excimer states surrounded by monomers with high-lying energy levels, 25–28 which can serve as artificial light-harvesting antennas. 29–31 Excitation light energy can be harvested by the antennas as linearly polarized excitons and subsequently concentrated in acceptors via exciton funneling (Fig. 1a), 32,33 possibly triggering an amplification of PL anisotropy. However, the general differ- ences between the molecular structures of guest and host compounds result in a limited doping concentration, 34 which impedes the formation and modulation of energy trans- fer processes in crystalline states. Recently, it was reported that excimers with lower-energy were formed in organic radical precursor crystals doped with the corresponding lumi- nescent radicals. 35 The very similar molecular structures of the radical and corresponding precursor lead to a widely tunable doping ratio for free tailoring of excimer states, which is promising for the amplification and in situ modulation of PL anisotropy. Herein, we propose a strategy to amplify PL anisotropy in organic radical-doped microcrystals, in which exciton funneling was exploited to modulate the DOP of excimer emission. Radical precursor microcrystals doped with the corresponding luminescent radicals were synthesized through a controllable liquid-phase self- assembly, where interactions between monomers and excimers were well-modulated by the doping concentration of the radical guests. In these composites, the radical monomers acted as antennas to efficiently capture the excitation energy, which was subsequently delivered to the excimers. Benefiting from the effi- cient coupling between exciton funneling and PL anisotropy trans- fer, the concentration of polarized excitons in the excimer states resulted in a distinct amplification of PL polarization and the DOP was successfully modulated by varying the temperature. These results not only offer a comprehensive understanding of exciton funneling in organic radical excimers but also provide guidance for the rational design of high-performance micro/nanoscale polariz- ing optical elements with specific functionalities. CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: ylyan@iccas.ac.cn, yszhao@iccas.ac.cn † Electronic supplementary information (ESI) available: Details of synthetic procedures and additional data. See DOI: 10.1039/d1tc02638k Received 8th June 2021, Accepted 3rd August 2021 DOI: 10.1039/d1tc02638k rsc.li/materials-c Journal of Materials Chemistry C COMMUNICATION Published on 04 August 2021. Downloaded on 11/19/2022 2:12:58 PM. View Article Online View Journal | View Issue