This review delineates the developments in triplet–triplet annihilation based NIR to Vis molecular photon upconversion including recent progress in conceptual design, applications, existing challenges, possible future directions and opportunities.

Structural changes of a globular protein, bovine serum albumin (BSA), as a consequence of interaction with the surface active ionic liquids (ILs)-3-methyl-1-octylimidazolium chloride, [C(8)mim][Cl], and 1-butyl-3-methylimidazolium octylsulfate, [C(4)mim][C(8)OSO(3)]-have been investigated using various physicochemical and spectroscopic techniques such as tensiometry, conductometry, steady-state fluorescence, far-UV circular dichroism spectroscopy (CD), and dynamic light scattering (DLS). The interactional behavior of ILs (monomers and self-assembled structures) toward BSA in different IL concentration regimes at the air/solution interface as well as in the bulk is investigated and discussed depending upon the nature of ions of ILs. CD combined with the steady state fluorescence spectroscopy provided valuable insights into the unfolding of BSA as a consequence of IL binding. The complementary results obtained from the multitechnique approach proved very useful in drawing out the mechanism of interaction between ILs and BSA in different IL concentration regimes.

Photon upconversion (UC) from near-infrared (NIR) light to visible light has enabled optogenetic manipulations in deep tissues.H owever,m aterials for NIR optogenetics have been limited to inorganic UC nanoparticles.Herein, NIR-light-triggered optogenetics using biocompatible,organic TTA-UC hydrogels is reported. To achieve triplet sensitization even in highly viscous hydrogel matrices,aNIR-absorbing complex is covalently linked with energy-pooling acceptor chromophores,w hichs ignificantly elongates the donor triplet lifetime.T he donor and acceptor are solubilized in hydrogels formed from biocompatible Pluronic F127 micelles,a nd heat treatment endows the excited triplets in the hydrogel with remarkable oxygen tolerance.Combined with photoactivatable Cre recombinase technology,N IR-light stimulation successfully performs genome engineering resulting in the formation of dendritic-spine-like structures of hippocampal neurons.