Ayman A. Abdel-Shafi

Effect of β-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate

The photophysical properties of 1-naphthol-2-sulfonate (1-NOH-2-S) in various solvents and in aqueous beta-cyclodextrin (CD) solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.5, while in water the fluorescence quantum yield is 0.1. The fluorescence quantum yield doubled on the addition of beta-CD. In aqueous solution, proton transfer to water takes place efficiently leading to the formation of the anion form with its longer wavelength emission broad band at about 460 nm. Any environmental changes have been found to affect the rate of deprotonation and subsequently the band intensity at 460 nm. In non-aqueous solution the anion emission band disappears completely. Upon the addition of beta-CD to the aqueous solution of 1-NOH-2-S, the anion emission decreases with an increase in the intensity of the neutral form at 362 nm. Fluorescence measurements show 1:1 inclusion of 1-NOH-2-S in the beta-CD cavity with an association constant of 1915 M(-1) using Benesi-Heldbrand treatment. 1H NMR studies are used to confirm the inclusion and to provide information on the orientation of 1-NOH-2-S inside the cavity of beta-CD.

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven RuII complexes containing two 2,2′-bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′-carboxamide-disubstituted 2,2′-bipyridines, for one complex containing no 2,2′-bipyridine, but 2 of these different ligands, for three multinuclear RuII complexes containing 2 or 4 [Ru(bpy)2] moieties and also coordinated via 4,4′-carboxamide-disubstituted 2,2′-bipyridine ligands, and for the complex [(Ru(bpy)2(L)]2+ where L is N,N′-([2,2′-bipyridine]-4,4′-diyl)bis[3-methoxypropanamide]. Absorption maxima are red-shifted with respect to [Ru(bpy)3]2+, as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal-to-ligand charge transfer states 3MLCT in de-aerated MeCN are equal to or longer than for [Ru(bpy)3]2+ and vary considerably, i.e., from 0.86 to 1.71 μs. Rate constants kq for quenching by O2 of the 3MLCT states were measured and found to be well below diffusion-controlled, ranging from 1.2 to 2.0⋅109 dm3 mol−1 s−1. The efficiencies f of singlet-oxygen formation during oxygen quenching of these 3MLCT states are relatively high, namely 0.53 – 0.89. The product of kq and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and kq−k equals k, the net rate constant for quenching due to energy dissipation of the excited 3MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔGCT, the free-energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.

Electronic to vibrational energy conversion and charge transfer contributions during quenching by molecular oxygen of electronically excited triplet states

The bimolecular rate constants kqT for quenching of some substituted naphthalene triplet states by molecular oxygen (O2(3Σg−)) in acetonitrile and cyclohexane and the efficiencies, fΔT, with which singlet oxygen (O2*(1Δg)) is thereby produced are reported for naphthalene derivatives with a wider range of oxidation potentials than those previously measured. The magnitude of kqT and fΔT are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (EMOX) of the naphthalene derivative and to the solvent polarity. The modified charge transfer mediated mechanism of quenching based on singlet and triplet channels for oxygen quenching is invoked to discuss these results. In cyclohexane the maximum value for fΔT of one is observed for compounds with high oxidation potentials indicating no contribution from the triplet channel whilst in acetonitrile the limit for fΔT of 0.25 expected when singlet and triplet channels give equal contributions, when spin statistics is taken into account, is observed for 2,6-dimethoxynaphthalene, which is the derivative with the lowest oxidation potential. These results are combined with those previously reported by ourselves in cyclohexane in order to examine the dependence of the quenching rate constants due to energy transfer on the energy gap (ET–E1Σ). This allows the quenching rate constants due to energy transfer in cyclohexane to be separated into contributions with and without charge transfer assistance. The latter contribution shows a smooth dependence on (ET–E1Σ) and the same dependence on the electronic energy, which has to be converted into vibrational energy, probably applies when the solvent is acetonitrile since making this assumption results in similar dependences of the charge transfer contributions in both singlet and triplet channels in acetonitrile. The free energy of activation for charge transfer assisted quenching is shown to have a linear dependence on the free energy change for full charge transfer but the indications are that quenching is via singlet and triplet charge transfer complexes with only partial charge transfer character being 12.5% and 17% in acetonitrile and cyclohexane respectively.

Ruthenium, osmium and rhodium-2,3-bis(2¢-pyridyl)quinoxaline complexes

Ru3(CO)12 reacts with 2,3-bis (2′-pyridyl)quinoxaline (dpq) in benzene in the presence of either 2,2′(dipyridine (dpy) or pyridine (py) to give the mononuclear complexes Ru(CO)3(dpq) (1) and Ru(CO)2(dpq)(py) (2), respectively. On the other hand, reactions of Os3(CO)12 with dpq alone, or in the presence of dipyridine, yield only Os(CO)3(dpq) (3). Spectroscopic studies of the three complexes were consistent with the proposed structures. Reactions of RuCl3 with dpq, under reduced pressure in PhH/EtOH gave bis-[Ru(dpq)2Cl2]Cl (4). Magnetic measurements showed RuIII, d5, to be in a low-spin electronic configuration. Three complexes, Rh2(dpq)2Cl6(5), [Rh(dpq)2Cl2]Cl (6) and Rh(dpq)(py)Cl3(7), were isolated from reactions of RhCl3 with dpq. The type of product was dependent on the reactants and conditions. The complexes exhibit either irreversible or quasi-reversible ligand-based reductions. In addition, the zerovalent complexes (1)–(3) displayed one metal(based oxidation in their cyclic voltammograms, due to the formation of M+ species.

Photosensitized generation of singlet oxygen from ruthenium(II)-substituted benzoaza-crown-bipyridine complexes

Photophysical properties in dilute acetonitrile solution are reported for a number of vinyl-linked benzoaza-15-crown-5-bipyridine ruthenium(II) complexes and for three multinuclear Ru(II) bipyridine complexes. Absorption and emission spectra are found to depend on the number of conjugated benzoaza-15-crown-5-bipyridine ligands present in the complex. The bi-, tri- and tetranuclear Ru(II) complexes show absorption and emission maxima very close to those of the parent mono-complex, Ru(II) tris-bipyridine. For those complexes with similar phosphorescence maxima, in the range 607–615 nm, the lifetimes of the lowest excited triplet metal to ligand charge-transfer (3MLCT) states in de-aerated acetonitrile are also very similar, i.e., in the range 0.71 to 0.88 μs. However, for two of the studied compounds, where the phosphorescence maxima shift to 692 and 699 nm, the phosphorescence lifetimes increase to 2.2 and 3.0 μs, respectively. Rate constants, kq, for quenching by molecular oxygen of the lowest excited 3MLCT states are in the range (2.4–4.6)×109 dm3 mol−1 s−1. Efficiencies of singlet oxygen production, fΔT, sensitized by these ruthenium complexes are in the range of 0.26–0.69, lower values being associated with those compounds showing low potentials for oxidation of conjugated ligands. The product of kq and fΔT gives the net rate constant for quenching due to energy transfer to produce singlet oxygen kq1, and kq−kq1 equals kq3, the net rate constant for quenching due to energy dissipation of the excited 3MLCT states without energy transfer. Quenching rate constants, kq and kq3, were found to give an inverse correlation with the energy of the excited state being quenched. However, the dependence of kq1 values on the energy of the excited states being quenched by oxygen was more complicated, with complexes falling into two groups showing either high or low efficiencies for quenching with energy transfer.

Inclusion of Paracetamol into β-cyclodextrin nanocavities in solution and in the solid state

We report on steady-state UV-visible absorption and emission characteristics of Paracetamol, drug used as antipyretic agent, in water and within cyclodextrins (CDs): β-CD, 2-hydroxypropyl-β-CD (HP-β-CD) and 2,6-dimethyl-β-CD (Me-β-CD). The results reveal that Paracetamol forms a 1:1 inclusion complex with CD. Upon encapsulation, the emission intensity enhances, indicating a confinement effect of the nanocages on the photophysical behavior of the drug. Due to its methyl groups, the Me-β-CD shows the largest effect for the drug. The observed binding constant showing the following trend: Me-β-CD>HP-β-CD>β-CD. The less complexing effectiveness of HP-β-CD is due to the steric effect of the hydroxypropyl-substituents, which can hamper the inclusion of the guest molecules. The solid state inclusion complex was prepared by co-precipitation method and its characterization was investigated by Fourier transform infrared spectroscopy, 1H NMR and X-ray diffractometry. These approaches indicated that Paracetamol was able to form an inclusion complex with CDs, and the inclusion compounds exhibited different spectroscopic features and properties from Paracetamol.

Photosensitized generation of singlet oxygen from ruthenium(II) and osmium(II) bipyridyl complexes

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.

Flow injection fluorimetric determination of chromium(VI) in electroplating baths by luminescence quenching of tris(2,2'-bipyridyl) ruthenium(II).

A sensitive and selective luminescence quenching method is developed and used for manual and flow injection analysis (FIA) of chromium(VI) by reaction with [Ru(bpy)(3)](2+). The emission peak of ruthenium(II) at 595 nm is linearly decreased as a function of Cr(VI) concentration. This permits determination of chromium(VI) ion over the concentration range 0.1-20 microg ml(-1) with a detection limit of 33 ng ml(-1). The quenching process is due to an electron transfer from the luminescent [Ru(bpy)(3)](2+) complex ion to Cr(VI) resulting in the formation of the non-luminescent [Ru(bpy)(3)](3+) complex ion. Selectivity for Cr(VI) over many anions and transition, alkali and alkaline earth metal cations is demonstrated. High concentration levels of sulphate, chloride, borate, acetate, phosphate, nitrate, cyanide, Pb(2+), Zn(2+), Hg(2+), Cu(2+), Cd(2+), Ni(2+) and Mn(2+) ions are tolerated. The effects of solution pH and [Ru(bpy)(3)](2+) reagent concentration are examined and the reaction conditions are optimized. Validation of the method according to the quality assurance standards show suitability of the proposed method for use in the quality control assessment of Cr(VI) in complex matrices without prior treatment. The method is successfully applied to determine chromium(VI) in electroplating baths using flow injection analysis. Results with a mean standard deviation of +/-0.6% are obtained which compare fairly well with data obtained using atomic absorption spectrometry.

Solvent effects on the photophysical properties of 9,10-dicyanoanthracene

The quantum yields of fluorescence, ΦF and of triplet state production, ΦT of 9,10-dicyanoanthracene and the fluorescence lifetimes have been measured in fourteen different solvents. It has been shown that ΦF + ΦT is less than one in all the solvents except bromobenzene. Delayed fluorescence of 9,10-dicyanoanthracene arising following triplet–triplet annihilation is observed in all solvents. It consists of excimer as well as monomer emission, the relative intensities of which were shown to be different from the ratio for prompt fluorescence and to vary with solvent polarity. The rate constants for fluorescence quenching by the three heavy-atom-containing quenchers, iodopropane, iodobenzene and dimethyliodobenzene, kIPS, kIBS and kDMIBS, respectively have been measured and in all solvents it was found that kIPS<kIBS<kDMIBS. Reasons for this are discussed in terms of the charge transfer interactions of the first excited singlet state of 9,10-dicyanoanthracene with the quenchers and with the different solvents.

Photosensitised production of singlet oxygen, O2*(a1Δg), in the unique `heavy-atom' solvent, supercritical fluid xenon. Pressure dependence of electronic to vibrational energy conversion during quenching of O2*(a1Δg) by xenon and by ground state oxygen

Abstract Photosensitised generation of singlet oxygen, O 2 * ( a 1 Δ g ) , in supercritical xenon, SCF-Xe at 325 K is reported for the first time. Rate constants for quenching by ground state oxygen, and by xenon, vary from 8.9±0.3 to 13±1×10 2 and from 7±1 to 4.4±0.5 dm 3 mol −1 s −1 , respectively, as the pressure increases from 7.8 to 44 MPa. The measured decay constant shows little temperature dependence in the range 295–325 K at 29.6 MPa. The extrapolated lifetime of O 2 * ( a 1 Δ g ) , expected in the absence of O 2 ( X 3 Σ g − ) , in SCF-Xe, at 325 K and 8.8 MPa is 22±3 ms . Explanations for observed variations are proposed and factors governing the decay of singlet oxygen in SCF-Xe are discussed.