Photophysical and Cellular Imaging Studies of Brightly Luminescent Osmium(II) Pyridyltriazole Complexes

Abstract

The series of complexes [Os(bpy) 3-n (pytz) n ][PF 6 ] 2 (bpy = 2,2'-bipyridyl, pytz = 1-benzyl-4-(pyrid-2yl)-1,2,3-triazole, 1 n = 0, 2 n = 1, 3 n = 2, 4 n = 3) have been prepared and characterised and are rare examples of luminescent 1,2,3-triazole-based osmium(II) complexes. For 3 we present an attractive and particularly mild preparative route via an osmium(II)  6-arene precursor circumventing the harsh conditions that are usually required. Due to the high spin-orbit coupling constant associated with the Os(II) centre the absorption spectra of the complexes all display absorption bands of appreciable intensity in the range 500-700 nm corresponding to spin-forbidden ground-state-to-3 MLCT transitions, which occur at significantly lower energies than the corresponding spin-allowed 1 MLCT transitions. The homoleptic complex 4 is a bright emitter ( max em = 614 nm) with a relatively high quantum yield of emission of ~ 40 % in deoxygenated acetonitrile solutions at RT. Water soluble chloride salts of 1-4 were also prepared, all of which remain emissive in aerated aqueous solutions at room temperature. The complexes were investigated for their potential as phosphorescent cellular imaging agents whereby efficient excitation into the 3 MLCT absorption bands at the red side of the visible range circumvents 2 autofluorescence from biological specimens which do not absorb in this region of the spectrum. Confocal microscopy reveals 4 to be readily taken up by cancer cell lines (HeLa and EJ) with apparent lysosomal and endosomal localisation, whilst toxicity assays reveal that the compounds have low dark and light toxicity. These complexes therefore provide an excellent platform for the development of efficient luminescent cellular imaging agents with advantageous photophysical properties which enable excitation and emission in the biologically transparent region of the optical spectrum. Optimised ground state geometry atomic coordinates and TDDFT data for complexes 1-4. X-ray crystallographic data for 2 and mer-4. Confocal microscopy images and cellular viability data for cells incubated with 1-4. Crystallographic Data Crystallographic information files are available for complexes 2 (CCDC 1826209) and mer-4 (CCDC 1850035).

Intracellular Photophysics of an Osmium Complex bearing an Oligothiophene Extended Ligand

“…One strategy for shifting the optical window of metal complex phototherapy agents to longer wavelengths while building on successful molecular design concepts for Ru II complexes is to utilize the Os II analogues of the Ru II complexes [26–30] . In fact, Os II complexes are becoming widely appreciated as both therapeutic compounds [31–41] and cellular imaging agents [42–47] . This approach has been explored by McFarland and co‐workers for PDT, [48] but it is has not been applied to PCT given that the Os II polypyridyl counterparts are photoinert [49, 50] .…”

“…[26][27][28][29][30] In fact, Os II complexes are becoming widely appreciateda sb oth therapeutic compounds [31][32][33][34][35][36][37][38][39][40][41] and cellular imaging agents. [42][43][44][45][46][47] This approach has been explored by McFarland and co-workers for PDT, [48] but it is has not been appliedt oP CT given that the Os II polypyridyl counterparts are photoinert. [49,50] Both in vitro and in vivo studies of the Os II photosensitizers OsH2B, OsH2IP, and OsH2dppn)( Scheme 1) publishedb yM cFarland and coworkers showedp anchromatic activation, low dark toxicity, and PDT activity.…”

Triazole-based osmium(ii) complexes displaying red/near-IR luminescence: antimicrobial activity and super-resolution imaging

“…The previously reported complex [Os(pytz) 3 ](PF 6 ) 2 [1 2+ ](PF 6 ) 2 was prepared through an established procedure, 23 Due to the asymmetry of the triazole-containing ligands, analysis by 1 H NMR spectroscopy (Fig. S1-S8 †) reveals the isolated products to be composed of a mixture of meridional and facial isomers, with formation of the former being favoured in all cases with mer : fac ratios of 1.4 : 1 (1 2+ ), 2 : 1 (2 2+ ) and 3 : 1 (3 2+ ).…”

“…By comparison with previous studies on complexes of Os II and Ru II containing this ligand this process is attributed to a ligand-based reduction. 23 Substitution of the pytz ligand with pymtz for both mer-and fac-2 2+ results in the appearance of an irreversible reduction process at more positive potential. This shi is of similar magnitude to the anodic shi in the Os II/III couple relative to 1 2+ , resulting in minimal change in the HOMO-LUMO energy gap.…”

“…20 Very recently Zhu et al have also exploited the deep-red luminescence of [Os(phen) 2 (dppz)] 2+ (phen ¼ 1,10-phenanthroline, dppz ¼ dipyridophenazine) for correlative luminescence and transmission electron microscopy (TEM) imaging of nuclear DNA. 21 In this context, the Scattergood and Elliott groups have previously investigated triazolecontaining Os II complexes that show lysosomal/endosomal and mitochondrial localisation as optical probes, 22,23 whilst the Thomas group has looked at DNA targeting polypyridyl Os II complexes as high-resolution contrast probes for TEM. 24 Yet, whilst there has been interest in Os II complexes as potential anti-cancer therapeutics, 19,[25][26][27][28][29][30][31][32] examples of their use in bacteria are far rarer.…”