Photon Upconversion Cooperates with Downshifting in Chiral Systems: Modulation, Amplification, and Applications of Circularly Polarized Luminescence

Circularly polarized luminescence (CPL)-active materials are increasingly recognized for their potential applications such as 3D imaging, data storage, and optoelectronic devices. Typically, CPL materials have required high-energy (HE) photons for excitation to emit low-energy (LE) circularly polarized light, a process known as downshifting CPL (DSCPL). However, the emergence of upconverted CPL (UCCPL), where the absorption of multi LE photons results in the emission of a single HE photon with circular polarization, has recently attracted considerable attention. This minireview highlights the intricate relationship between upconversion and CPL phenomena. During upconversion, the dissymmetry factor (glum) value can be improved in certain systems. Additionally, the integration of both LE and HE photons in upconversion-downshifting-synergistic systems offers avenues for dual-excitation or dual-emission CPL functionalities. More in detail, the emerging UCCPL based on various photon upconversion mechanisms and their synergy with DSCPL are introduced. Additionally, several examples that demonstrate the applications of UCCPL are presented to highlight the future opportunities. Photon upconversion, cooperating with downshifting in chiral systems, offers a powerful method to modulate circularly polarized luminescence (CPL) in several ways: (1) Dual upconverted CPL (UCCPL) from the acceptor and downshifting CPL (DSCPL) from the donor can be simultaneously emitted upon excitation with low-energy light. (2) In dual-excitation system, the acceptor can produce both UCCPL and DSCPL, each with distinct dissymmetry factor values, depending on whether it is excited by low energy or high energy. image