Showing papers by "Madeleine Helliwell published in 2015"
TL;DR: It is found that enolate formation is possible within the protein environment and associated with a large bathochromic shift, thus offering a cogent explanation for the blue-black color and the response to thermal denaturation and revealing the chemistry of astaxanthin upon complex formation.
Abstract: The chemical basis of the blue-black to pink-orange color change on cooking of lobster, due to thermal denaturation of an astaxanthin–protein complex, α-crustacyanin, in the lobster carapace, has so far been elusive. Here, we investigate the relaxation of the astaxanthin pigment from its bound enolate form to its neutral hydroxyketone form, as origin of the spectral shift, by analyzing the response of UV-vis spectra of a water-soluble 3-hydroxy-4-oxo-β-ionone model of astaxanthin to increases in pH, and by performing extensive quantum chemical calculations over a wide range of chemical conditions. The enolization of astaxanthin is consistent with the X-ray diffraction data of β-crustacyanin (PDB code: 1GKA) whose crystals possess the distinct blue color. We find that enolate formation is possible within the protein environment and associated with a large bathochromic shift, thus offering a cogent explanation for the blue-black color and the response to thermal denaturation and revealing the chemistry of astaxanthin upon complex formation.
33 citations
TL;DR: In order to gain insights into the factors that govern the emission properties, density functional theory (DFT) and time-dependent DFT calculations have been carried out and confirm that the emission arises largely from triplet excited states of the C^N ligand ((3)LC), with some triplet metal-to-ligand charge-transfer contributions.
Abstract: Six new tricationic IrIII complexes of cyclometalating ligands derived from 1-methyl-2-(2′-pyridyl)pyridinium or 1-methyl-4-(2′-pyridyl)pyridinium are described. These complexes of the form [IrIII(C^N)2(N^N)]3+ (C^N = cyclometalating ligand; N^N = α-diimine) have been isolated and characterised as their PF6− and Cl− salts. Four of the PF6− salts have been studied by X-ray crystallography, and structures have been obtained also for two complex salts containing MeCN and Cl− or two Cl− ligands instead of N^N. The influence of the position of the quaternised N atom in C^N and the substituents on N^N on the electronic/optical properties are compared with those of the analogous complexes where C^N derives from 1-methyl-3-(2′-pyridyl)pyridinium (B. J. Coe, et al., Dalton Trans., 2015, 44, 15420). Voltammetric studies reveal one irreversible oxidation and multiple reduction processes which are mostly reversible. The new complexes show intramolecular charge-transfer absorptions between 350 and 450 nm, and exhibit bright green luminescence, with λmax values in the range 508–530 nm in both aqueous and acetonitrile solutions. In order to gain insights into the factors that govern the emission properties, density functional theory (DFT) and time-dependent DFT calculations have been carried out. The results confirm that the emission arises largely from triplet excited states of the C^N ligand (3LC), with some triplet metal-to-ligand charge-transfer (3MLCT) contributions.
25 citations
TL;DR: New blue or blue-green emitting iridium complexes have been synthesised with cyclometalating ligands derived from the 1-methyl-3-(2'-pyridyl)pyridine cation, with a large range of lifetimes in the μs region and relatively high quantum yields.
Abstract: New blue or blue-green emitting iridium complexes have been synthesised with cyclometalating ligands derived from the 1-methyl-3-(2′-pyridyl)pyridinium cation. Efficient luminescence is observed in MeCN or aqueous solutions, with a large range of lifetimes in the μs region and relatively high quantum yields.
20 citations