Abstract
Synthesis, characterization, electrochemistry, and photophysics of homo‐ and heteroleptic ruthenium(II) complexes [Ru(cpmp)2]2+ (22+) and [Ru(cpmp)(ddpd)]2+ (32+) bearing the tridentate ligands 6,2’’‐carboxypyridyl‐2,2’‐methylamine‐pyridyl‐pyridine (cpmp) and N,N’‐dimethyl‐N,N’‐dipyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) are reported. The complexes possess one (32+) or two (22+) electron‐deficient dipyridyl ketone fragments as electron‐accepting sites enabling intraligand charge transfer (ILCT), ligand‐to‐ligand charge transfer (LL'CT) and low‐energy metal‐to‐ligand charge transfer (MLCT) absorptions. The latter peak around 544 nm (green light). Complex 22+ shows 3MLCT phosphorescence in the red to near‐infrared spectral region at room temperature in deaerated acetonitrile solution with an emission quantum yield of 1.3 % and a 3MLCT lifetime of 477 ns, whereas 32+ is much less luminescent. This different behavior is ascribed to the energy gap law and the shape of the parasitic excited 3MC state potential energy surface. This study highlights the importance of the excited‐state energies and geometries for the actual excited‐state dynamics. Aromatic and aliphatic amines reductively quench the excited state of 22+ paving the way to photocatalytic applications using low‐energy green light as exemplified with the green‐light‐sensitized thiol–ene click reaction.
Highlights
Polypyridyl ruthenium(II) complexes[1,2,3,4,5,6] often display phosphorescence from triplet metal-to-ligand charge transfer (3MLCT) states
Structure, electrochemical as well as steady-state and time-resolved photophysical properties of the novel homoleptic [Ru(cpmp)2][PF6]2 (2[PF6]2) and heteroleptic ruthenium(II) complex [Ru(cpmp)(ddpd)][PF6]2 (3[PF6]2)
The coordinated CH3CN ligand in 1[PF6] displays a broad band for the CN stretching vibration at 2275 cmÀ1 in the IR spectrum (KBr disk). 1H and 13C NMR resonances for the CH3 groups of the CH3CN ligands in the NMR spectra agree with a single CH3CN ligand coordinated to ruthenium
Summary
Polypyridyl ruthenium(II) complexes (low spin d6 electron configuration)[1,2,3,4,5,6] often display phosphorescence from triplet metal-to-ligand charge transfer (3MLCT) states. Eliminating thermal deactivation via the classical Jahn–Teller type and other distorted 3MC states is a key to higher phosphorescence quantum yields and longer excited-state lifetimes.[14] Complementary concepts to increase the energy gap between the emissive 3MLCT and the deactivating 3MC state have been devised, namely i) shifting the 3MC states to higher energy by an increase of the ligand field splitting with sixmembered ring chelate ligands allowing 1808 NÀRuÀN bond angles and optimal RuÀN orbital overlap and with strong s-donating/p-accepting ligands and ii) lowering the energy of the 3MLCT state with electron-deficient pyridine ligands Examples successfully employing these concepts are [Ru(dcpp)2]2+ (dcpp = 2,6-bis(2-carboxypyridyl)pyridine)[46] and the heteroleptic complex [Ru(ddpd)(tpyR)]2+ (ddpd = N,N’-dimethyl-N,N’-dipyridin-2-ylpyridine-2,6-diamine) (Scheme 1).[30,37] [Ru(dcpp)2]2+ with accepting dipyridyl ketone moieties and sixmembered chelate rings shows a very high phosphorescence quantum yield of 30 % in CH3CN at 298 K.[46] [Ru(ddpd)(tpyR)]2+ with an electron-donating ddpd ligand with six-membered chelate rings and an electron-deficient ester substituted tpyR ligand has a phosphorescence quantum yield of 1.1 % in CH3CN.[30,37] On the other hand, cyclometalating tridentate ligands N^C^N or C^N^N in ruthenium(II) complexes with fivemembered chelate rings were unsuccessful in improving quantum yield and lifetime. The new ruthenium(II) complexes 2[PF6]2 and 3[PF6]2 are applied in a light-sensitized thiol–ene click reaction using low-energy green light for excitation
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