Designing tridentate ligands for ruthenium(II) complexes with prolonged room temperature luminescence lifetimes

Coordination complexes have been used extensively as the photoactive component of artificial photosynthetic devices. While polynuclear arrays increase the probability of light absorption, the incorporation of the stereogenic Ru(2,2'-bipyridine)(3)(2+) motif gives rise to diastereomeric mixtures whereas the achiral Ru(2,2':6',2"-terpyridine)(2)(2+) motif creates stereopure polynuclear complexes. Thus, polynuclear arrays composed of ruthenium(II) complexes of tridentate ligands are the targets of choice for light-harvesting devices. As Ru(II) complexes of tridentate ligands have short excited state lifetimes at room temperature (r. t.), considerable effort has been focused on trying to increase their r. t. luminescence lifetime for practical applications. This tutorial review will report on the sophisticated synthetic strategies currently in use to enhance the room temperature photophysical properties of Ru(II) complexes of tridentate ligands.     

Bilayer-by-bilayer antiferroelectric ordering in freely suspended films of an achiral polymer-monomer liquid crystal mixture

Thin freely suspended films of a mixture of an achiral side-chain liquid crystal polymer and its monomer have been studied with depolarized reflected light microscopy. We observe that regions with an odd number of bilayers exhibit a net spontaneous polarization in the tilt plane of the molecules, while regions with an even number of bilayers have no net polarization. These odd-even effects are direct evidence that the tilted smectic bilayers are anticlinic at the polymer backbone and synclinic at bilayer interface and confirm that the phase is bilayer-by-bilayer antiferroelectric.     

The effect of steric hindrance on the Fe(II) complexes of triazine-containing ligands

Fe(II) complexes based on ligands containing a 1,3,5-triazine core have been synthesised and characterised and their electrochemical, spectroscopic and magnetic properties have also been investigated. The incorporation of various substituted metal-binding heterocycles into the 2,4-positions of the triazine allows the properties of the complexes to be modified considerably. Whereas the magnetic moments of the pyridyl (1b: 2.4 B.M.) and pyrazyl (1c: 4.2 B.M.) substituents lead to spin transitions, the 6-picolyl (1d: 5.4 B.M.) substituent is a pure Curie paramagnet at 300 K. Interestingly, the Fe(II) complex of bromo-phenyl-2,2′:6″,2?-terpyridine (1a) displays significant anion effects in the solid state, with the ClO₄ salt exhibiting major diamagnetic contributions (2.1 B.M. at 300 K) and the PF₆ salt has more significant paramagnetic contributions (3.5 B.M. at 300 K). The Fe–N bond distances observed in the 6-picolyl complex [Fe(1d)₂](ClO₄)₂ for both the central rings 2.084 (5) Å and the distal rings 2.254 (4) and 2.369 (4) Å are significantly longer than those of the pyridyl complex [Fe(1b)₂](ClO₄)₂ (centre: 1.869 (4) Å; distal: 1.992 (4) Å; distal: 2.005 (4) Å) due to steric strain, thus generating high-spin [Fe(1d)₂](ClO₄)₂.     

Approaches to wired terpyridine: Bithienyl alkynyl derivatives of 2,2′:6′,2″-terpyridine and their ruthenium(II) complexes

Two approaches to the formation of ruthenium(II) complexes containing ligands with conjugated 2,2′:6′,2″-terpyridine (tpy), alkynyl and bithienyl units have been investigated. Bromination of 4′-(2,2′-bithien-5′-yl)-2,2′:6′,2″-terpyridine leads to 4′-(5-bromo-2,2’-bithien-5′-yl)-2,2′:6′,2″-terpyridine (1), the single crystal structure of which has been determined. The complexes [Ru(1)₂][PF₆]₂ and [Ru(tpy)(1)][PF₆]₂ have been prepared and characterized. Sonogashira coupling of the bromo-substituent with (TIPS)CCH did not prove to be an efficient method of preparing the corresponding complexes with pendant alkynyl units. The reaction of 4′-ethynyl-2,2′:6’,2″-terpyridine with 5-bromo-2,2′-bithiophene under Sonogashira conditions yielded ligand 2, and the heteroleptic ruthenium(II) complex [Ru(2)(tpy)][PF₆]₂ has been prepared and characterized.