Theoretical illumination of highly original photoreactive³MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes

Abstract

We have identified highly novel photoreactive MC states of ruthenium(ii) 4,4'-bi-1,2,3-triazolyl (btz) complexes of the form [Ru(N^N)(btz)] and have elucidated the mechanism of the highly unusual experimental observations of photochemical ligand dechelation and concomitant ligand rearrangement reactivity to form unusual photoproducts trans-[Ru(N^N)(κ-btz)(κ-btz)(solvent)]. The triplet metal-to-ligand charge-transfer (MLCT) states and classical Jahn-Teller type triplet metal-centred (MC) states of the series of complexes [Ru(N^N)(btz)] (btz = 4,4'-bi-1,2,3-triazolyl; N^N = 2,2'-bipyridyl (bpy), n = 0 (1), 1 (2), 2 (3), 3 (5); N^N = 4-(pyrid-2-yl)-1,2,3-triazole (pytz), n = 1 (4)) have been optimised by density functional theory (DFT) and characterised. There is a gradual and significant destabilisation of the MLCT states as the triazole content of the complexes increases, which occurs with a slight stabilisation of theMC states. Whilst consistent with the promotion of photochemical reactivity in the heteroleptic complexes of the series relative to 1, these classical MC states fail to account for the extraordinary ligand rearrangement processes that accompany ligand ejection. Thorough theoretical exploration of the lowest excited triplet potential energy surface (PES) here reveals the existence of a new type of MC state and the role it plays in the photochemical reactivity of the complexes. This newly discovered state, called MC(F), displays a flattened geometry (indicated by the 'F' in the parentheses) which makes it clearly on the path to achieving the coplanarity of the bidentate ligands in the experimentally observed trans-photoproduct. Further novel 'pentacoordinate'MC states with coplanar bidentate ligands, called MC(P) (where the 'P' in the parentheses denotes the pentacoordinate character), were then identified and optimised. The energy barriers between the different triplet states were confirmed to be small which makes all triplet states accessible. Solvent trapping, which occurs on the singlet PES according to Wigner's rules, is finally achieved by a singlet pentacoordinate species to yield the monosolvento photoproduct. Thus, our calculations not only reveal highly novel MC states but more significantly demonstrate their crucial role in the formation of the experimentally observed photoproducts.

Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

“…The above emission band shape analysis serves to rationalize nonradiative relaxation occurring directly from the emissive 3 MLCT manifold to the electronic ground state. However, in Ru II polypyridine complexes there is usually additional nonradiative relaxation from the 3 MLCT via the 3 T 1g excited state (Figure 3b) [37,38]. Depending on ligand design [39], this metal-centered state can be energetically very close and nonradiative relaxation becomes very rapid, and this is the reason why [Ru(tpy) 2 ] 2+ (tpy = 2,2 :6 ,2"-terpyridine) is essentially non-emissive in solution at room temperature [40].…”

DFT rationalization of the room-temperature luminescence properties of Ru(bpy) 3 2+ and Ru(tpy) 2 2+ : 3MLCT–3MC minimum energy path from NEB calculations and emission spectra from VRES calculations

“…On the basis of previously described 3 [30], which were reoptimized, we here report for the first time the computation of the 3 MLCT-3 MC minimum energy path for Ru(bpy) 3 2+ and Ru(tpy) 2 2+ , using the nudged elastic band method, a method that is popular in solid state physics and surface science for ground state potential energy surface exploration [31] , [32] but has been reported scarcely in molecular inorganic photochemistry [33] , [34], to the best of our knowledge. Very recently we have reported the successful use of this method in the context of deciphering photoreactivity mechanisms [35].…”

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

“…The emission lifetimes for compounds 1-3 were recorded in aerated acetonitrile solutions and interestingly show reversed ordering to expectations based on energy-gap law considerations. In ruthenium(II) [40][41][42][43][44] and even osmium(II) [45,46] complexes, the presence of a 1,2,3-triazole-based chelate ligand can result in the quenching of The complexes are luminescent in aerated acetonitrile solutions and exhibit broad featureless bands at significantly longer wavelengths than their 1 MLCT absorption bands, indicative of emission from 3 MLCT states ( Figure 4a and Table 2). The emission spectra follow the same trend for the electronic absorption spectra with red-shifting of the 3 MLCT bands in the order 1 (540 nm) < 2 (572 nm) < 3 (638 nm).…”

“…The emission lifetimes for compounds 1-3 were recorded in aerated acetonitrile solutions and interestingly show reversed ordering to expectations based on energy-gap law considerations. In ruthenium(II) [40][41][42][43][44] and even osmium(II) [45,46] complexes, the presence of a 1,2,3-triazole-based chelate ligand can result in the quenching of luminescence and promotion of photochemical reactivity through increased accessibility of 3 MC states from the photoexcited 3 MLCT state. Indeed, the rhenium(I) complex [Re(btz)(CO) 3 (Cl)] is non-emissive in fluid solution [47].…”