Luminescent Neutral Cyclometalated Iridium(III) Complexes Featuring a Cubic Polyhedral Oligomeric Silsesquioxane for Lipid Droplet Imaging and Photocytotoxic Applications

New neutral iridium(III) complexes featuring a cubic polyhedral oligomeric silsesquioxane (POSS) unit, [Ir(N∧C)2(L1-POSS)] [HN∧C = 2-phenylpyridine (Hppy; 1), 2-phenylbenzothioazole (Hbt; 2), and 2-(1-naphthyl)benzothiazole (Hbsn; 3); L1-POSS = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-heptakis(isobutyl)POSS-propyl carbamate], were designed and synthesized. Their POSS-free counterparts, [Ir(N∧C)2(L1)] [L1 = (E)-N-(4-hydroxymethylphenyl)-1-(2-hydroxyphenyl)methanimine; HN∧C = Hppy (1a), Hbt (2a), and Hbsn (3a)], and the poly(ethylene glycol) (PEG) derivatives [Ir(N∧C)2(L1-PEG)] [L1-PEG = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-[2-[ω-methoxypoly(1-oxapropyl)]ethyl]carbamate; HN∧C = Hppy (1b), Hbt (2b), and Hbsn (3b)] were also prepared. The photophysical, photochemical, and biological properties of the POSS complexes were compared with those of their POSS-free and PEG-modified counterparts. Upon irradiation, all of these complexes displayed orange-to-red emission and long emission lifetimes under ambient conditions. The bsn complexes 3, 3a, and 3b exhibited the highest singlet oxygen (1O2) generation quantum yields (ΦΔ = 0.85-0.86) in aerated CH3CN. Laser-scanning confocal microscopy images revealed that complexes 1-3 and 1a-3a showed exclusive lipid-droplet staining upon cellular uptake, while the PEG derivatives 1b-3b displayed lysosomal localization. Complex 3 was utilized to study various lipid-droplet-related biological events including lipid-droplet accumulation under oleic acid stimulation, the movement of lipid droplets, and preadipocyte differentiation. Notably, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays indicated that the ppy complexes 1 and 1b and the bt complexes 2 and 2b were noncytotoxic both in the dark and upon irradiation at 450 nm for 5 min (IC50 > 200 μM), while the bsn complexes 3, 3a, and 3b showed low dark cytotoxicity (IC50 = 52.9 to >200 μM) and high photocytotoxicity (IC50 = 1.1-5.3 μM). The cellular uptake, internalization mechanisms, and cell death pathways of these complexes were also investigated. This work not only offers promising luminescent probes for lipid droplets through the structural modification of iridium(III) complexes but also paves the way to the construction of new reagents for theranostics.