Three-Photon Absorption of Coordination Polymer Transforms UV-to-VIS Thermometry into NIR-to-VIS Thermometry.

Lanthanide-based metal-organic frameworks (MOFs) and coordination polymers (CPs) attract much attention as candidates for optical ratiometric thermometry applications. Thus far, excitation of these materials was mainly performed in the ultraviolet that drastically limits their applicability as sensors, e.g., in tissue biological thermometry. As a remedy for this constraint, for the first time, we leverage a nonlinear optical process, the three-photon absorption property of Eu,Tb-CPs to shift the excitation wavelength from ultraviolet into near-infrared region. Experiments demonstrate that three-photon induced thermometric responses of Eu,Tb-CPs follow excellent optical characteristics similar to those determined for one-photon excitation, yet are not identical. The relative sensitivity reaches a very high value of 2.91%K in the physiological temperature region. We put forward a notion that utilizing multiphoton absorption is a general strategy for realizing NIR-to-VIS remote temperature sensing in practically any CP that is designed for UV-to-VIS thermometry.