Observation of an Inversion in Photophysical Tuning in a Systematic Study of Luminescent Triazole-Based Osmium(II) Complexes

Abstract

In a systematic survey of luminescent bis(terdentate) osmium(II) complexes a tipping-point involving reversal in photophysical tuning is observed whereby increasing stabilisation of the ligand-based LUMO results in a blue-shift in optical absorption and emission bands. The complexes [Os(N^N'^N")2] 2+ (N^N'^N" = 2,6-bis(1-phenyl-1,2,3-triazol-4-yl)pyridine (Os1); 2,6-bis(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os2); 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2'-bipyridyl (Os3); 2-(pyrid-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os4); 2-(pyrazin-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyridine (Os5); 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2'bipyrazinyl (Os6)) have been prepared and characterised and all complexes display phosphorescence ranging from the orange to near-IR regions of the spectrum. Replacement of the central pyridine in the ligands of Os1 by the more -accepting pyrazine in Os2 results in a 55 nm red-shift in the 3 MLCT-based emission band whilst a larger red-shift of 107 nm is observed for replacement of one of the triazole donors in the ligands of Os1 by a second pyridine ring in Os3 ( em max = 702 nm). Interestingly, replacement of the central pyridine ring in the ligands of Os3 by pyrazine Os4 ( em max = 702 nm) fails to result in a further red-shift in the emission band. Reversal of the relative positions of the pyridine and pyrazine donors in Os5 ( em max = 733 nm) compared to Os4 does indeed result in the expected red-shift in emission with respect to that for Os3 based on the increased -acceptor character of the ligands present. However, an inversion in emission tuning is observed for Os6 in which the incorporation of a second pyrazine donor in the ligand architecture results in a blue-shift in optical absorption and emission maxima ( em max = 710 nm). Electrochemical studies reveal that whilst incorporating pyrazine into the ligands indeed results in an expected anodic shift in the first reduction potential through stabilisation of the ligand-based LUMO, there is also concomitant anodic shift in the Os(II)/Os(III)-based oxidation potential. This stabilisation of the metal-based HOMO thus nullifies the effect of the stabilisation of the LUMO in Os4 compared to Os3 resulting in these complexes having coincident emission maxima. For Os6 the stabilisation of the HOMO through incorporation of two pyrazine donors in the ligand structure now exceeds the stabilisation of the LUMO resulting in a larger HOMO-LUMO gap and the counter-intuitive blue-shift in optical properties in comparison with Os5. Whilst it is known that replacement of ligands (e.g. replacing bipyridyl with bipyrazinyl) can result in a larger HOMO-LUMO energy gap through greater stabilisation of the HOMO, these results importantly allow us to capture the tipping-point at which this inversion in photophysical tuning occurs. This therefore enables us to explore the limits available in emission tuning with a relatively simple and minimalist ligand structure. 13 , including functioning as the low-energy aspect within multi-component white light sy...

Photophysical and Electrocatalytic Properties of Rhenium(I) Triazole-Based Complexes

“…Fc/Fc + , all of which are assigned to reduction of the N^N donor fragment. The redox potentials reported herein are in line with those for similar rhenium-triazole complexes investigated by Ching et al and with data from our laboratory for osmium(II) and iridium(III) complexes bearing pyridylversus pyrazinyltriazole ligands [36,37]. In agreement with the stabilisation of the pyrimidine-and pyrazine-localised LUMOs for 2 and 3, respectively, relative to that of 1, the electrochemical reduction features of 2 and 3 are anodically shifted with respect to the corresponding pyridyltriazole complex (1).…”

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

“…Trends in both lifetime and quantum yields are in excellent agreement with the energy gap law 52,53 and typical for similar triazole-containing complexes of Os II . 23,48 Quantum yields increase upon deoxygenating the aqueous solutions, supporting assignment to an emissive MLCT state of triplet character. Signicantly, the low-energy spin-forbidden absorption envelope of the complexes enables direct population of their 3 MLCT states, with emission readily observed using excitation wavelengths as low as 560 (mer/fac-1 2+ and 2 2+ ) and 600 nm (mer/fac-3 2+ ).…”

“…A similar trend was reported recently for a series of bis-terdentate 1,2,3-triazole-based Os II complexes featuring pyridyl and pyrazinyl donors. 48 Scheme 1 Synthesis of 1 2+ -3 2+ . All the complexes display at least one reduction process within the available electrochemical solvent window.…”

Photophysical Properties and DNA Binding of Two Intercalating Osmium Polypyridyl Complexes Showing Light-Switch Effects

“…Such moderately intense absorbances in the visible region are commonly observed for osmium­(II) polypyridyl systems, with 1 MLCT transitions for Os­(bpy) 3 2+ , for example, falling between 430 and 520 nm. − Further absorbances, still of appreciable intensity, are noted beyond 570 nm, which extend into the red region before tailing off at 720 nm. This feature is typical for osmium­(II) coordination complexes ,,− and arises due to the formally spin-forbidden direct population of triplet states with MLCT character ( 3 MLCT) as a consequence of the high spin–orbit coupling constant of the Os center . The circular dichroism (CD) spectra for the Δ and Λ stereoisomers of [ 1 2+ ]­[Cl] 2 and [ 2 2+ ]­[Cl] 2 show opposite (but equal) Cotton effects, with characteristic couplets observed for the 1 LC and 3 MLCT transitions, with the latter extending to the NIR region (Figures S11 and S12).…”