Exploiting\nthe Potential of Iridium(III) <i>bis</i>-Nitrone Complexes\nas Phosphorogenic Bifunctional Reagents for Phototheranostics

Cross-linking\nstrategies have found wide applications in chemical\nbiology, enabling the labeling of biomolecules and monitoring of protein–protein\ninteractions. Nitrone exhibits remarkable versatility and applicability\nin bioorthogonal labeling due to its high reactivity with strained\nalkynes via the strain-promoted alkyne–nitrone cycloaddition\n(SPANC) reaction. In this work, four cyclometalated iridium(III) polypyridine\ncomplexes functionalized with two nitrone units were designed as novel\nphosphorogenic bioorthogonal reagents for bioimaging and phototherapeutics.\nThe complexes showed efficient emission quenching, which is attributed\nto an efficient nonradiative decay pathway via the low-lying T<sub>1</sub>/S<sub>0</sub> minimum energy crossing point (MECP), as revealed\nby computational studies. However, the complexes displayed significant\nemission enhancement and lifetime extension upon reaction with (1<i>R</i>,8<i>S</i>,9<i>s</i>)-bicyclo[6.1.0]non-4-yne\n(BCN) derivatives. In particular, they showed a remarkably higher\nreaction rate toward a <i>bis</i>-cyclooctyne derivative\n(<i>bis</i>-BCN) compared with its monomeric counterpart\n(<i>mono</i>-BCN). Live-cell imaging and (photo)cytotoxicity\nstudies revealed higher photocytotoxicity in <i>bis</i>-BCN-pretreated\ncells, which is ascribed to the enhanced singlet oxygen (<sup>1</sup>O<sub>2</sub>) photosensitization resulting from the elimination\nof the nitrone-associated quenching pathway. Importantly, the cross-linking\nproperties and enhanced reactivity of the complexes make them highly\npromising candidates for the development of hydrogels and stapled/cyclized\npeptides, offering intriguing photophysical, photochemical, and biological\nproperties. Notably, a nanosized hydrogel (<b>2-gel</b>) demonstrated\npotential as a drug delivery system, while a stapled peptide (<b>2-<i>bis</i>-pDIKK</b>) exhibited p53-Mdm2 inhibitory\nactivity related to apoptosis and a cyclized peptide (<b>2-<i>bis</i>-RGD</b>) showed cancer selectivity.