The NIR-IIb (1500-1700 nm) window is ideal for deep-tissue optical imaging in mammals, but lacks bright and biocompatible probes. Here, we developed biocompatible cubic-phase (α-phase) erbium-based rare-earth nanoparticles (ErNPs) exhibiting bright downconversion luminescence at ~ 1600 nm for dynamic imaging of cancer immune-therapy in mice. We used ErNPs functionalized with cross-linked hydrophilic polymer layers attached to anti-PD-L1 antibody for molecular imaging of PD-L1 in a mouse model of colon cancer and achieved tumor to normal tissue signal ratios of ~ 40. The long luminescence lifetime of ErNPs (~ 4.6 ms) enabled simultaneous imaging of ErNPs and lead sulfide quantum dots (PbS QDs) emitting in the same ~ 1600 nm window. In vivo NIR-IIb molecular imaging of PD-L1 and CD8 revealed cytotoxic T *

Ultrafast transient absorption spectroscopy serves to identify the (3)dd state as intermediate quencher state of the (3)MLCT luminescence in the non-luminescent ruthenium complexes [Ru(m-bpy)3](2+) (m-bpy = 6-methyl-2,2'-bipyridine) and [Ru(tm-bpy)3](2+) (tm-bpy = 4,4',6,6'-tetramethyl-2',2'-bipyridine). For [Ru(m-bpy)3](2+), the population of the (3)dd state from the (3)MLCT state occurs within 1.6 ps, while the return to the ground state takes 450 ps. For [Ru(tm-bpy)3](2+), the corresponding values are 0.16 and 7.5 ps, respectively. According to DFT calculations, methyl groups added in the 6 and 6' positions of bipyridine stabilize the (3)dd state by ~4000 cm(-1) each, compared to [Ru(bpy)3](2+).

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Deep-tissue three-dimensional (3D) optical imaging of live mammals with high spatiotemporal resolution in non-invasive manners has been challenging due to light scattering. Here, we developed near-infrared II (NIR-II, 1000–1700 nm) light sheet microscopy (LSM) with excitation and emission up to ~ 1320 nm and ~ 1700 nm respectively for optical sectioning through live tissues at ~ 750-μm penetration depth without any invasive surgery. Suppressed light scattering allowed imaging at ~ 2 mm depth in glycerol-cleared brain tissues. NIR-II LSM in normal and oblique configurations enabled in vivo imaging of live mice through intact tissue, revealing abnormal blood flow and T cell motion in tumor microcirculation and mapping out programmed-death ligand 1 (PD-L1) and programmed cell death protein 1 (PD-1) in tumors with cellular resolution. 3D imaging through intact mouse head resolved vascular channels between skull and brain cortex, and monitored recruitment of macrophages/microglia to traumatic brain injury site post injury.

TRIR spectroscopy identifies the low-energy ligand-field state in the relaxation cascade for ruthenium(ii) polypyridyl complexes.