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–1 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.