Ruthenium(II) Complexes with Improved Photophysical Properties Based on Planar 4‘-(2-Pyrimidinyl)-2,2‘:6‘,2‘ ‘-terpyridine Ligands

Abstract

A series of new tridentate polypyridine ligands, made of terpyridine chelating subunits connected to various substituted 2-pyrimidinyl groups, and their homoleptic and heteroleptic Ru(II) complexes have been prepared and characterized. The new metal complexes have general formulas [(R-pm-tpy)Ru(tpy)]2+ and [Ru(tpy-pm-R)2]2+ (tpy = 2,2':6',2' '-terpyridine; R-pm-tpy = 4'-(2-pyrimidinyl)-2,2':6',2' '-terpyridine with R = H, methyl, phenyl, perfluorophenyl, chloride, and cyanide). Two of the new metal complexes have also been characterized by X-ray analysis. In all the R-pm-tpy ligands, the pyrimidinyl and terpyridyl groups are coplanar, allowing an extended delocalization of acceptor orbital of the metal-to-ligand charge-transfer (MLCT) excited state. The absorption spectra, redox behavior, and luminescence properties of the new Ru(II) complexes have been investigated. In particular, the photophysical properties of these species are significantly better compared to those of [Ru(tpy)2]2+ and well comparable with those of the best emitters of Ru(II) polypyridine family containing tridentate ligands. Reasons for the improved photophysical properties lie at the same time in an enhanced MLCT-MC (MC = metal centered) energy gap and in a reduced difference between the minima of the excited and ground states potential energy surfaces. The enhanced MLCT-MC energy gap leads to diminished efficiency of the thermally activated pathway for the radiationless process, whereas the similarity in ground and excited-state geometries causes reduced Franck Condon factors for the direct radiationless decay from the MLCT state to the ground state of the new complexes in comparison with [Ru(tpy)2]2+ and similar species.

A High Molar Extinction Coefficient Bisterpyridyl Homoleptic Ru(II) Complex with trans-2-Methyl-2-butenoic Acid Functionality: Potential Dye for Dye-Sensitized Solar Cells

“…At the aliphatic region, a singlet peak at 2.09 ppm was unambiguously assigned to the CH 3 signal. In a pseudo-octahedral geometry, a tridentate ligand like terpyridine coordinates to a metal center in a meridional fashion [32] thus, in the proton NMR spectrum of the homoleptic [Ru(L1) 2 (PF 6 ) 2 ] complex in CDCl 3 , a slight complication in the aromatic region was observed. The complication may be due to a different geometrical arrangement occasioned by the two outer pyridine rings of the terpyridine and/or the coordination of the nitrogen atom lone-pair electrons to the ruthenium ion centre leading to an upfield shifts in the chemical shifts of protons (Figure 2).…”

Harnessing Reversible Electronic Energy Transfer: From Molecular Dyads to Molecular Machines

“…[7] The photophysical properties may be improved by design,o nd efavouring thermally-activated loss by increasing the energy gap between states on lowering emissive 3 MLCT levels,a saresult of incorporating highly electron poor tpy-like ligands. [8] Furthermore, integration of an organic auxiliary led to luminescence lifetimes as long as 1.8 msb ut with as imilarly low quantum yield. [9] More recently,r uthenium complexes exhibiting microsecondl uminescence lifetimes were reported on replacing the two externalp yridinem oieties of tpy with quinolines in constituent ligands, which offersa no ptimized octahedral coordination environment.…”

Redox and Photochemistry of Bis(terpyridine)ruthenium(II) Amino Acids and Their Amide Conjugates – from Understanding to Applications

“…Thus, the reduced π-conjugation mitigates the positive effect of donor-acceptor substitution. [65] In these complexes the enhanced π-conjugation efficiently stabilizes the 3 MLCT state with respect to the 3 MC state, giving long excited-state lifetimes up to τ = 231 ns and quantum yields up to Φ = 0.17 % (Table 1). [65] In these complexes the enhanced π-conjugation efficiently stabilizes the 3 MLCT state with respect to the 3 MC state, giving long excited-state lifetimes up to τ = 231 ns and quantum yields up to Φ = 0.17 % (Table 1).…”