Colin G. Coates

Spectroscopic studies of structurally similar DNA-binding Ruthenium (II) complexes containing the dipyridophenazine ligand

Abstract Nanosecond transient resonance Raman and picosecond transient absorption spectroscopic investigations of the two structurally analogous Ru-polypyridyl complexes, [Ru(phen) 2 dppz] 2+ ( 1 ) and [Ru(tap) 2 dppz] 2+ ( 2 ), are presented (phen=1,10-phenanthroline, dppz=dipyrido [3,2- a :2′,3′- c ] phenazine; tap=1,4,5,8 tetraazaphenanthrene). The findings offer insight into the differing nature of the lowest excited states of the two complexes, and describe the role of these states within the very distinct photophysical behaviour of each, both in relation to solvent response and their interaction with DNA (facilitated in each case through the intercalating dppz ligand). The active, solvent-sensitive, dppz-based 3 MLCT states involved in the ‘light-switch’ behaviour of ( 1 ) are probed, alongside evidence of a progression through a precursor transient state when the complex is in non-aqueous environment. Evidence has been provided of a photophysical pathway for ( 2 ), involving formation of a tap-based lowest 3 MLCT state. When ( 2 ) is bound to DNA through the dppz ligand, a photo-driven electron transfer process ensues between the guanine base of DNA and the lowest 3 MLCT state.

Photophysics of mixed-ligand polypyridyl ruthenium(II)complexesimmobilised in silica sol–gel monoliths

A series of compounds [Ru(bpy) 3-x (dpp) x ] 2+ (bpy=2,2′-bipyridyl, dpp=4,7-diphenyl-1,10-phenanthroline) have been synthesised and physically immobilised in silica sol–gel monoliths. Transient resonance-Raman studies show that for the immobilised mixed-ligand complexes, as in solution, the emitting ( 3 MLCT) state is dpp based. The resonance-Raman evidence also suggests that the structure of this state is the same in both environments. The emission lifetimes of the immobilised complexes cannot be analysed as single exponential decays and the values obtained are dependent on the initial pH of the sol–gel. The temperature-dependent emission behaviour of the compounds is substantially changed upon immobilisation in the sol–gel. Both the variation of the emission lifetime and energy are significantly different from that observed for the solution-based species. For the bpy containing complexes no evidence is obtained for the population of the photoactive ligand field state, whereas for the complex [Ru(dpp) 3 ] 2+ population of this state is observed. The relevance of these observations for the design of optical sensors for oxygen is discussed.

Resonance-Raman probing of the interaction between dipyridophenazine complexes of ruthenium(II) and DNA

The resonance-Raman spectroscopic technique is an effective probe of the interaction between dipyridophenazine (dppz) complexes of ruthenium(II) and calf-thymus DNA, providing evidence that DNA addition results in changes to electronic transitions of the intercalating dppz ligand in both ground and excited states.

Time-resolved spectroscopic studies of the influence of the electronic environment on the charge-transfer excited states of mono- and di-nuclear Ru(II) complexes

Abstract Resonance Raman (RR) spectroscopic studies, supplemented by excited state absorption techniques, were used to probe the effect of modifications to the electronic environment on the excited states of mono- and di-nuclear Ru(II) polypyridyl complexes containing triazole-based bridging ligands. Ground state RR spectroscopy showed that coordination of the σ-donating bridging ligands to a second Ru(bpy) 2 moiety resulted in stabilization of the metal d( π )-orbitals, whereas excited state RR spectroscopy revealed a concomitant stabilization of the π ∗ - level of the bridging ligand. With 3,5-bis(pyrazin-2-yl)-1,2,4-triazole) as bridging ligand (BL), the latter effect was sufficient to cause a switching from a bpy-based to a BL-based 3 MLCT state, with evidence suggesting charge polarisation towards a pyrazine fragment. Further, for pyrazine-containing bridging ligands, protonation of the triazole fragment resulted in a switching of the lowest excited state to a BL-based excited state.

Ultrafast transient absorption studies of ruthenium and rhenium dipyridophenazine complexes bound to DNA and polynucleotides

We report on ultrafast pump and probe studies of biological systems, in the form of polynucleotide and calf thymus DNA complexes. Molecules for study are bound to the polynucleotides and probed in the visible region to observe changes in the absorption over time. Various dipyridophenazine metal complexes are studied alone and complexed with DNA or synthetic polynucleotides to investigate changes occurring in their excited states upon interacting with nucleobases. Transient absorption measurements are performed pumping at 400nm and probing from 450-700nm with pulse duration of 400fs.

Transient Resonance RamanStudies of Ru(II) Complexes in DNAand in Homogeneous Media

Transient resonance Raman (TR2) spectroscopy has been used to investigate the metalligand charge-transfer (MLCT) excited states of Ru(II) polypyridyl complexes inDNAand in homogeneous solution. In DNA, complexes of the type [Ru(L)2(L′)]2