Effect of Ligands with Extended π-System on the Photophysical Properties of Ru(II) Complexes

Abstract

Density functional theory calculations were performed on a series of six ruthenium complexes possessing tridentate ligands: [Ru(tpy)(2)](2+) (1; tpy = [2,2';6',2'']-terpyridine), [Ru(tpy)(pydppx)](2+) (2; pydppx = 3-(pyrid-2'-yl)-11,12-dimethyldipyrido[3,2-a: 2',3'-c]phenazine), [Ru(pydppx)(2)](2+) (3), [Ru(tpy)(pydppn)](2+) (4; pydppn = 3-(pyrid-2'-yl)-4,5,9,16-tetraazadibenzo[a,c]naphthacene), [Ru(pydppn)(2)](2+) (5), and [Ru(tpy)(pydbn)](+) (6; pyHdbn = 3-pyrid-2'-yl-4,9,16-triazadibenzo[a,c]naphthacene). The calculations were compared to experimental data, including electrochemistry and electronic absorption spectra. The theoretical results reveal that the lowest-lying singlet and triplet states in 4 and 5 are pydppn-based ππ* in character, which are remarkably different from the lowest-lying metal-to-ligand charge transfer (MLCT) states in 1-3. The calculated lowest triplet states in 4 and 5 are consistent with the (3)ππ* states observed experimentally. However, although the extended π-system of pydbn(-) is similar to that of pydppn, the HOMO of 6 lies above those of 4 and 5, resulting in strikingly different spectroscopic properties. Calculations show that the lowest triplet excited state of 6 is a combination of (3)MLCT and (3)ππ*. This work demonstrates that the electronic structure of the tridentate ligand has a pronounced effect on the photophysical properties of ruthenium(II) complexes and that DFT and TD-DFT methods are a useful tool that can be used to predict photophysical and redox properties of transition metal complexes.

Strained, Photoejecting Ru(II) Complexes that are Cytotoxic Under Hypoxic Conditions

“…Anthracene is the only organic fragment with a triplet energy level below the 3 MLCT energy (1.82 vs 2.1 eV; Chart ), supporting the assertion that population of the organic triplet is highly inefficient for all systems with 3 IL energies equal to or greater than 2.1 eV. This is substantively different from prior studies on dyads that contained extended π‐systems but lacked competition from relaxation pathways involving 3 MC states. The relatively low 1 O 2 quantum yields for this series support the contribution of dissociative 3 MC excited states to the excited‐state decay pathways of these complexes, which can be potentially harnessed for PCT effects in low oxygen conditions.…”

“…In this work, we investigate a family of novel bis‐heteroleptic strained Ru(II) complexes for their potential to act as phototoxic agents in normoxia—but more importantly as PCT agents in hypoxia . We and others have an interest in developing systems that will be effective under these conditions, as most solid tumors contain hypoxic regions which are commonly resistant to radio‐ and chemotherapy. A key structural feature of the series is the modification of one organic ligand with different aromatic groups R (Chart ).…”

Quantitative wave function analysis for excited states of transition metal complexes

“…A very interesting class of TMCs is based on the complex [Ru(bipy) 2 (dppz)] 2+ (bipy = 2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]-phenazine), which has been reported as a molecular light switch for DNA in 1990 [150]. Since then, a huge research activity has developed in this eld, both experimentally [151][152][153][154][155][156][157][158][159][160] and theoretically [59,[160][161][162][163][164][165][166][167]. The main goal of these studies was to understand the impact of the ligand sphere on the photophysics of [Ru(L) 2 (dppzlike)] 2+ molecules and to assess the character of the low-lying excited states responsible for the luminescence properties.…”

Chemical consequences of pyrazole orientation in RuII complexes of unsymmetric quinoline–pyrazole ligands

“…Ruthenium polypyridine complexes have attracted much attention as sensors or probes in biology, [1][2][3] as catalysts and sensitizers in photochemical applications, [4][5][6][7][8][9][10][11] and as interesting model complexes in theoretical chemistry. [12][13][14][15] The specific functions or properties of a complex can typically be understood in terms of electronic and steric factors. 16,17 This kind of structure-function relationship is well established for the prototypical polypyridine complexes [Ru(bpy) 3 ] 2+ and [Ru(tpy) 2 ] 2+ , (bpy is 2,2′-bipyridine and tpy is 2,2′:6′,2″-terpyridine) and their derivatives.…”

“…16,17 This kind of structure-function relationship is well established for the prototypical polypyridine complexes [Ru(bpy) 3 ] 2+ and [Ru(tpy) 2 ] 2+ , (bpy is 2,2′-bipyridine and tpy is 2,2′:6′,2″-terpyridine) and their derivatives. 12,[16][17][18][19] In comparison, Ru-complexes with other polyheteroaromatic ligands have received less attention, despite the fact that incorporation of non-pyridine type polyheteroaromatic ligands can substantially affect both structural and electronic properties of the complex. 11,20,21 One way of forming such polyheteroaromatic ligands is by the combinations of quinoline (Q) and pyridine (Py).…”