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An excited-state intramolecular proton transfer (ESIPT) reaction triggered by the second excited singlet S2 state is found by ab initio calculations to be an ultrafast process for a novel fluorophore.

In this research, a multimodal imaging platform guided photodynamic theranostics under 808 nm was designed using a NaErF 4 :Tm@NaYF 4 :Yb@NaLuF 4 :Nd,Yb-ZnPc structure. Unlike conventional codoped Yb 3+ /Er 3+ system, Er 3+ ions as activator and sensitizer were used to improve the up-conversion energy transfer processes. Furthermore, higher energy transfer processes between Er 3+ ions could be obtained through doped 1% Tm 3+ ions as an energy trapping center in the NaErF 4 . This platform could emit much brighter upconversion luminescence (UCL) (124-fold enhancement for red emission) and short wavelength infrared (SWIR) emission under single 808 nm laser excitation. Importantly, the SWIR imaging with higher resolution and better signal-to-noise ratio can pass the blood−brain barrier to see the brain vessels. Because of the enhanced red emission, the UCL nanoparticles were combined with ZnPc agent to exhibit photodynamic therapy (PDT) effect, and its distribution and excretion could be detected by the photoacoustic (PA) imaging under single near-infrared (NIR) laser. Thus, this platform could be used as multimodal imaging (SWIR, PA, CT, and UCL) guided PDT agent under single 808 nm laser.

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