PULSED RADIATION STUDIES OF PHOTODYNAMIC SENSITISERS: THE NATURE OF DHE

Abstract Laser flash photolysis has previously been used to study the nature of DHE via measurements of photophysical parameters which are dependent on the molecular weight of the system being studied. These results to date allow only a lower limit to be established for DHE which imply that in some environments such as detergents more than two porphyrin units are linked. We have now determined the triplet extinction coefficient of DHE by the pulse radiolysis technique via an energy transfer method which allows the triplet extinction of DHE to be estimated independent of the molecular weight. The combined techniques allow the actual molecular weight of DHE to be established at about 4200. Laser flash techniques have also now been used to determine, for a number of potential photodynamic sensitisers, the quantum yield of triplet state formation (θ_T) and, using the direct luminescence of singlet oxygen at 1270 nm, the quantum yield of singlet oxygen formation (θ_δ). For many of the porphyrins studied θ_δ is less than θ_T. For DHE itself there is a substantial increase in θ_δ in detergent compared to buffer. The θ_δ yields for a number of related systems including 'simple'systems such as haematoporphyrin, for linked porphyrin-chlorin systems, (including DHE in which the end porphyrin is reduced to a chlorin–DHEC), and for phthalocyanines are compared. For the DHEC the θ_δ is close to that of DHE itself which may imply that such chlorins could be of use in photodynamic therapy (PDT).     

Influence of ligand rigidity and ring substitution on the structural and electronic behavior of trivalent iron and gallium complexes with asymmetric tridentate ligands

Species 1-6 are [M(III)(L)2]ClO4 complexes formed with the PhO--CH=N-CH2-Py imines, (L(I))- and (L(tBuI))-, and PhO--CH2-NH-CH2-Py amines, (L(A))- and (L(tBuA))-, in which PhO- is a phenolate ring and Py is a pyridine ring and the prefix tBu indicates the presence of tertiary butyl groups occupying the positions 4 and 6 of the phenol ring. Monometallic species with d5 high-spin iron (1, 2, 3, 4) and d10 gallium (5, 6) were synthesized and characterized to assess the influence of the ligand rigidity and the presence of tertiary butyl-substituted phenol rings on their steric, electronic, and redox behavior. Characterization by elemental analysis, mass spectrometry, IR, UV-visible, and EPR spectroscopies, and electrochemistry has been performed, and complexes [FeIII(L(tBuI))2]ClO4 (2), [FeIII(L(tBuA))2]ClO4 (4), and [Ga(III)(L(tBuI))2]ClO4 (5) have been characterized by X-ray crystallography. The crystal structures show the imine ligands meridionally coordinated to the metal centers, whereas the amine ligands are coordinate in a facial mode. Cyclic voltammetry shows that the complexes with the ligands (L(tBuI))- and (L(tBuA))- were able to generate ligand-based phenoxyl radicals, whereas unsubstituted ligands displayed ill-defined redox processes. EPR spectroscopy supports high-spin configurations for the iron complexes. UV-visible spectra are dominated by charge-transfer phenomena, and imine compounds exhibit dramatic hyperchromism when compared to equivalent amines. The tertiary butyl groups on the phenolate ring enhance this trend. Detailed B3LYP/6-31G(d)-level calculations have been used to account for the results observed.     

Structural and electronic behavior of unprecedented five-coordinate iron(III) and gallium(III) complexes with a new phenol-rich electroactive ligand

A new asymmetric pentadentate ligand was designed to impose low symmetry to trivalent ions. Five-coordinate Fe3+ and Ga3+ complexes were investigated by crystallographic, electrochemical, and electron paramagnetic resonance methods showing enhanced redox reversibility. Calculations were performed to account for the observed trends.     

Ground- and excited-state electronic structure of an emissive pyrazine-bridged ruthenium(II) dinuclear complex

The synthesis, characterization, and electrochemical, photophysical, and photochemical properties of the binuclear compounds [(Ru(H8-bpy)2)2((Metr)2Pz)](PF6)2 (1) and [(Ru(D8-bpy)2)2((Metr)2Pz)](PF6)2 (2), where bpy is 2,2'-bipyridine and H2(Metr)2Pz is the planar ligand 2,5-bis(5'-methyl-4'H-[1,2,4]triaz-3'-yl)pyrazine, are reported. Electrochemical and spectro-electrochemical investigations indicate that the ground-state interaction between each metal center is predominantly electrostatic and in the mixed-valence form only a low level of ground-state delocalization is present. Resonance Raman, transient, and time-resolved spectroscopies enable a detailed assignment to be made of the excited-state photophysical properties of the complexes. Deuteriation is employed to both facilitate spectroscopic characterization and investigate the nature of the lowest excited states.     

The application of diffuse reflectance laser photolysis to study photochemistry at interfaces and in dyed fabrics

Recently we have extended to heterogeneous, opaque and often highly scattering systems, the application of flash photolysis by using diffuse reflected light instead of transmitted light as the analysing source on timescales extending from several seconds to picoseconds. Laser-induced transient spectra and decay kinetics have been observed from a wide variety of samples including fractions of monolayers of organic molecules adsorbed on catalytic metal oxide surfaces, within inorganic and organic microcrystals, from dyed fabrics and from paper pulp. The potential of the technique to study photochemical reactions at interfaces is demonstrated with particular reference to ion-electron recombination on porous silica surfaces and to photochemical and photophysical processes occurring in dyed fabrics.     

Studies on the stereoselective hydroboration of vinyl ethers

The diastereoselective hydroboration/oxidation of several asymmetric vinyl ethers is presented.     

Synthesis, structure, and single-crystal EPR study of [Cp(t-Bu3PN)Ti(mu-Cl)]2

The reduction of CpTi(NPt-Bu3)Cl2 with Mg powder in benzene proceeds slowly to give [CpTi(NPt-Bu3(mu-Cl)]2 (1). An X-ray crystallographic study confirmed 1 is a centrosymmetric chloro-bridged dimer in the solid state. A single-crystal EPR study of 1 permitted the determination of the best-fitted values for the diagonal spin Hamiltonian parameters. This appears to be the first EPR single-crystal study of a Ti(III)-Ti(III) dimer. The implications of these data are considered and discussed in light of extended Hückel molecular orbital calculations.     

Kinetic studies of retinol addition radicals

Retinol neutral radicals (RS-retinol˙), generated from the reaction of retinol with 4-pyridylthiyl and 2-pyridylthiyl radicals in argon-saturated methanol, undergo β-elimination, which can be monitored via the slow secondary absorption rise at 380 nm attributed to the rearrangement of the unstable retinol neutral addition radicals to the more stable addition radicals. Rate constants for the β-elimination reactions (k(β)) of 4-PyrS-retinol˙ were measured at different temperatures and the Arrhenius equation for the reaction is described by log (k(β)/s(-1)) = (12.7 ± 0.2) - (54.3 ± 1.3)/θ, where θ = 2.3RT kJ mol(-1). The reactivities of retinol addition radicals (RS-retinol˙), generated from the reaction of retinol with various thiyl radicals, towards oxygen have also been investigated in methanol. In the presence of oxygen, the decay of RS-retinol˙ fits to biexponential kinetics and both observed rate constants for the RS-retinol˙ decay are oxygen-concentration dependent. This suggests that at least two thiyl addition radicals, formed from the reaction of RS˙ with retinol, undergo oxygen addition reactions. In light of the estimated rate constants for oxygen addition to RS-retinol˙ and RS-CAR˙ (CAR: carotenoid), the antioxidant-prooxidant properties of retinol are discussed.