Photon avalanche in metal-organic frameworks: converting infrared light into visible emission and motion

Upconversion converts two or more low-energy photons into a single photon of higher energy. Among the upconversion mechanisms, photon avalanche (PA) is the most nonlinear one, producing an intense burst of emission once the laser excitation power exceeds a critical threshold. PA has been observed only in lanthanide-doped inorganic matrices where the emission arises from electron accumulation in the 4f levels of lanthanide ions. Applications are in deep-tissue imaging [1]. Here, we generate a broadband PA in lanthanide-trimesate metal-organic frameworks (MOFs) [2]. The PA mechanism is new: PA occurs within the organic molecules. The IR photons are first absorbed and upconverted by a pair of lanthanide ions; then the electrons are funneled into electronically coupled triplet states of the ligand where they accumulate and subsequently are re-emitted (Fig. 1a). The nonlinearity factor is comparable to that of state-of-the-art inorganic PA matrices. Importantly, by selecting different metal ions, the alignment between lanthanide levels and the ligand triplet can be tuned, thus adjusting PA properties. Moreover, the excitation laser pulse features seem to govern the population of diverse vibrational levels of the ligand triplet. Remarkably, preliminary results also showed that IR irradiation triggers a phase change accompanied by microscopic MOF contraction (Fig. 1b), likely due to non- radiative pathways. To our knowledge, this is the first example of IR triggered dynamic crystal. These results broaden PA material design and highlight MOFs as platforms for converting IR light into concurrent controlled motion and intense visible emission.