Tuning the Electronics of Bis(tridentate)ruthenium(II) Complexes with Long-Lived Excited States: Modifications to the Ligand Skeleton beyond Classical Electron Donor or Electron Withdrawing Group Decorations

Abstract

A series of homoleptic bis(tridentate) [Ru(L)2](2+) (1, 3) and heteroleptic [Ru(L)(dqp)](2+) complexes (2, 4) [L = dqxp (1, 2) or dNinp (3, 4); dqxp = 2,6-di(quinoxalin-5-yl)pyridine, dNinp = 2,6-di(N-7-azaindol-1-yl)pyridine, dqp = 2,6-di(quinolin-8-yl)pyridine] was prepared and in the case of 2 and 4 structurally characterized. The presence of dqxp and dNinp in 1-4 result in anodically shifted oxidation potentials of the Ru(3+/2+) couple compared to that of the archetypical [Ru(dqp)2](2+) (5), most pronounced for [Ru(dqxp)2](2+) (1) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes' HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex 3 exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in 3, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in 1, where the LUMO experiences large stabilization by the lateral quinoxalines. Owing to the differences in LUMO energies, the complexes' reduction potentials differ by about 900 mV [E(1/2)(1(2+/1+)) = -1.17 V, E(c,p)(3(2+/1+)) = -2.06 V vs Fc(+/0)]. As complexes 1-4 exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes' excited state oxidation strengths over a range of 900 mV. Complex 1 is the strongest excited state oxidant of the series, exceeding even [Ru(bpy)3](2+) by more than 200 mV. At room temperature, complex 3 is nonemissive, whereas complexes 1, 2, and 4 exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of 5 (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power.

Understanding the Excited State Behavior of Cyclometalated Bis(tridentate)ruthenium(II) Complexes: A Combined Experimental and Theoretical Study

“…30−32 Similarly, Hammarstrom and co-workers used di(quinolin-8-yl)pyridine (dqp) as tridentate ligand forming six-membered chelate rings with ruthenium as metal center. 24,33 The homoleptic complex [Ru(dqp) 2 ] 2+ F 2+ (Scheme 1) is phosphorescent at room temperature (ϕ = 0.02) with a remarkably long excited-state lifetime of 3.0 μs. Ruben and coworkers employed the carbonyl analogue of the ddpd ligand, 2,6-di(2-carboxypyridyl)pyridine (dcpp) as chelating ligand with N−Ru−N bite angles of 90°.…”

Predicting Structures of Ru-Centered Dyes: A Computational Screening Tool

“…In addition, homoleptic meridional bis-tridentate complexes incorporated are Ru(TPy) 2 , 41 Ru(PzPyPz) 2 (PzPyPz = 2,6-di(pyrazol-1-yl)pyridine, sometimes called bpp), 42 Ru(DBPzP) 2 (DBPzP = 2,6-dibenzopyrazolyl-pyridine), 43 Ru(DQP) 2 , 23 and its 4-substituted analogs Ru(DQPCOOH) 2 , 44 Ru(DQPFur) 2 , and Ru(DQPNH 2 ) 2 . 44 These are combined with other expanded cage compounds that employ both saturated, Ru(PzbPybPz) 2 (PzbPybPz = 2,6-di(pyrazol-1-ylmethyl)pyridine) 45 and Ru(DCpP) 2 (DCpP = 6- bis (2-carboxypyridyl)pyridine), 46 and aromatic, Ru(DNinP) 2 (DQinP = 2,6-di(N-7-azaindol-1-yl)pyridine), 26 extensions between the central and terminal rings. In addition, the strain in ligands can be reduced by reducing some of the aromatic rings from six-membered rings to five like in Ru(DQxP) 2 (DQxP = 2,6-di(quinoxalin-5-yl)pyridine), 26 Ru(DQPz) 2 (DQPz = 1,3- bis (8-quinolinyl)-pyrazole), 47 and in Ru(DQPl) 2 (DQPl = 1,3- bis (8-quinolinyl)-pyrrole).…”

“…44 These are combined with other expanded cage compounds that employ both saturated, Ru(PzbPybPz) 2 (PzbPybPz = 2,6-di(pyrazol-1-ylmethyl)pyridine) 45 and Ru(DCpP) 2 (DCpP = 6- bis (2-carboxypyridyl)pyridine), 46 and aromatic, Ru(DNinP) 2 (DQinP = 2,6-di(N-7-azaindol-1-yl)pyridine), 26 extensions between the central and terminal rings. In addition, the strain in ligands can be reduced by reducing some of the aromatic rings from six-membered rings to five like in Ru(DQxP) 2 (DQxP = 2,6-di(quinoxalin-5-yl)pyridine), 26 Ru(DQPz) 2 (DQPz = 1,3- bis (8-quinolinyl)-pyrazole), 47 and in Ru(DQPl) 2 (DQPl = 1,3- bis (8-quinolinyl)-pyrrole). This extensive set of complexes gives a broad representation of ligands incorporating Ru–N bonding.…”

“…26,44 Calculations are made based on PM6 and DFT optimized geometries. The calculated spectra are broadened using Lorentzian line shapes with widths of 0.2 eV.…”

Push‐Pull Design of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores as Deep Red Light Emitters in Light‐Emitting Electrochemical Cells

“…[38] Hammarström et al designed a complex {[Ru(dqp) 2 ] 2+ ; dpq: 2,6-di(quinolin-8-yl)pyridine} with sixmembered chelate rings and 90°bite angles featuring high room-temperature lifetime and quantum yield in solution (τ = 3.0 μs, Φ = 2 %). [39,40] The substituted complex [Ru(dpqCOOEt) 2 ] 2+ with ester substituents on the 4Ј-positions reaches even higher values (τ = 5.5 μs, Φ = 7 %). [41] Ruben et al incorporated carbonyl bridges between the pyridine rings of [Ru(tpy) 2 ] 2+ and thus obtained six-membered chelate rings and 90°bite angles leading to high 3 MLCT lifetimes and, to the best of our knowledge, to the highest re- …”