Synthesis of highly functionalized 9,10-phenanthrenequinones by oxidative coupling using MoCl5

The strong oxidative power of molybdenum pentachloride gives rise to an efficient oxidative C-C bond formation of benzil derivatives to the corresponding 9,10-phenanthrenequinones. A highly complementary method to previous approaches was developed. The required derivatives are accessible in a modular fashion and in excellent yields. By this approach the orchid-derived natural product cypripediquinone A was synthesized for the first time.     

Photophysical properties of 9,10-disubstituted anthracene derivatives in solution and films

We have carried out absorption, time-resolved fluorescence, and fluorescence quantum yield measurements of four new soluble anthracene derivatives. They show natural radiative lifetimes in the range of 2.5-4.4 ns, which is 5-10 times shorter than those reported for unsubstituted anthracene. The 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives show the largest fluorescence transition dipoles, which is attributed to extended π-conjugation between anthracene and phenyls through acetylene linkages. Spin-cast films of the BPEA derivatives show strong fluorescence quenching by weakly emitting low energy excitations, which is attributed to excimer-like traps. Quenching is significantly reduced when bulky dendrons are attached so that they give maximum coverage of the emitting chromophore and prevent their aggregation. The results show that anthracene derivatives can be developed into efficient solution-processable fluorescent emitters for the blue and green spectral regions.     

Photophysical studies on antimalarial drugs

Most drugs used in the treatment of malaria produce phototoxic side effects in both the skin and the eye. Cutaneous and ocular effects that may be caused by light include changes in skin pigmentation, corneal opacity, cataract formation and other visual disturbances including irreversible retinal damage (retinopathy) leading to blindness. The mechanism for these reactions in humans is unknown. We irradiated a number of antimalarial drugs (amodiaquine, chloroquine, hydroxychloroquine, mefloquine, primaquine and quinacrine) with light (lambda > 300 nm) and conducted electron paramagnetic resonance (EPR) and laser flash photolysis studies to determine the possible active intermediates produced. Each antimalarial drug produced at least one EPR adduct with the spin-trap 5,5-dimethyl-1-pyrroline N-oxide in benzene: superoxide/hydroperoxyl adducts (chloroquine, mefloquine, quinacrine, amodiaquine and quinine), carbon-centered radical adducts (all but primaquine), or a nitrogen-centered radical adduct only (primaquine). In ethanol all drugs except primaquine produced some superoxide/hydroperoxyl adduct, with quinine, quinacrine, and hydroxychloroquine also producing the ethoxyl adduct. As detected with flash photolysis and steady-state techniques, mefloquine, quinine, amodiquine and a photoproduct of quinacrine produced singlet oxygen ([symbol: see text]delta = 0.38; [symbol: see text]delta = 0.36; [symbol: see text]delta = 0.011; [symbol: see text]delta = 0.013 in D2O, pD7), but only primaquine quenched singlet oxygen efficiently (2.6 x 10(8) M-1 s-1 in D2O, pD7). Because malaria is a disease most prevalent in regions of high light intensity, protective measures (clothing, sunblock, sunglasses or eye wraps) should be recommended when administering antimalarial drugs.     

Photophysical studies of the pheophorbide a dimer

This work reports on a monomer-dimer equilibrium state of pheophorbide a in solution. A methodology for controlling the equilibrium constant by use of temperature and solvent variation is described. The absorption spectrum of the dimer is calculated, using different prepared equilibria of monomer and dimer in solution. We propose that these aggregates provide a good model for understanding the dimerization process in tetrapyrroles.     

Photophysical and photochemical studies of thifensulfuron-methyl herbicide in aqueous solution

The photophysical and photochemical studies of a sulfonylurea herbicide, thifensulfuron-methyl (THM), have been investigated in a buffered aqueous solution. In the first part, the influence of pH on the spectroscopic properties was studied. This allowed the determination of the ground and excited state acidity constants, pK_a = 4 and 4.4, respectively, thus exhibiting the potential existence of a photoinduced protonation in the singlet state. In the second part, the photolysis kinetics was studied at different pH and varying oxygen concentrations, using an HPK 125 W lamp and followed up by the identification of photoproducts formed under continuous photo-irradiation. The kinetics results suggest that the photolysis process is faster in acidic (k = 3 × 10^?4 s^?1) than in basic medium (k = 9.8 × 10^?5 s^?1). The photolysis products were identified by high performance liquid chromatography HPLC-DAD, HPLC–MS and HPLC–MS–MS. In order to obtain a better understanding of the photodegradation mechanism, a laser flash photolysis study was performed. By comparing the quenching rate constant (k_q = 9.64 × 10⁸ mol^?1 l s^?1) obtained from triplet state quenching by molecular oxygen and from the Stern–Volmer relation (k_q = 0.41 × 10⁸ mol^?1 l s^?1), the role of the singlet state in the photodegradation process was demonstrated. The photoproducts originating from both singlet and triplet excited states have been identified and hypothetical photodegradation pathways of the thifensulfuron-methyl in aqueous solution are proposed.     

Photophysical studies on binding of curcumin to bovine serum albumins

The excited-state photophysical properties of curcumin in the presence of bovine serum albumin (BSA) have been studied. The absorption and fluorescence changes in curcumin on binding to BSA have been followed at varying concentrations of either curcumin or BSA to determine the binding constant, which has been found to be approximately 10(4) to 10(5) M(-1). Stopped-flow kinetics studies suggested at least two distinct kinetic steps for the binding of curcumin to BSA. The photophysical properties of the singlet-excited state of the curcumin-BSA complex have also been studied. Whereas the absorption spectrum of curcumin is redshifted, the fluorescence spectrum of curcumin was blueshifted in the presence of BSA. The fluorescence quantum yield of curcumin on complexing with BSA was approximately 0.05. Steady-state fluorescence anisotropy studies showed a significant increase in the anisotropy value of 0.37 in BSA-bound curcumin. The fluorescence decay of the curcumin-BSA complex followed a biexponential decay with fluorescence lifetimes of 413 ps (33%) and 120 ps (67%). On the basis of these complementary results, it has been concluded that curcumin shows very high binding to BSA, probably at the hydrophobic cavities inside the protein.     

Photophysical and photochemical processes of 9,10-dihydro-9-silaphenanthrene derivatives: photochemical formation and electronic structure of 9-silaphenanthrenes

Photophysical and photochemical processes of 9-methyl- and 9-phenyl-9,10-dihydro-9-silaphenanthrene derivatives have been studied at room temperature and 77 K in comparison with the carbon analogue, 9,10-dihydrophenanthrene. These 9,10-dihydro-9-silaphenanthrene derivatives show smaller fluorescence quantum yield and remarkably larger Stokes shifts than those of the carbon analogue. In contrast, their phosphorescence quantum yields are two times larger than those of the carbon analogue, although the absolute value is not so large (approximately 0.1). Reaction products and intermediates produced by the 266 nm light photolysis have been studied, and it has been confirmed that 9-methyl- and 9-phenyl-9-silaphenanthrenes have been photochemically formed in methylcyclohexane at 77 K, in addition to the formation of radical cations of 9,10-dihydro-9-silaphenanthrene derivatives and the carbon-centered radical: 9-hydro-9-silaphenanthrenyl radical.     

Photophysical properties of phenanthro[9,10-d]imidazole-type fluorescent hosts upon inclusion of organic solvent molecules

The crystal of phenanthro[9,10-d]imidazole-type fluorescent clathrate host (1) exhibits fluorescence enhancement behavior with a blue-shift of the emission maximum upon enclathration of organic solvent molecules such as 1-butanol and morpholine, whereas the crystal of 1 exhibits fluorescence enhancement behavior with a red-shift of the emission maximum upon enclathration of carboxylic acid such as acetic acid and propionic acid. The crystal structures of the guest-free and the guest-inclusion compounds of 1 have been determined by X-ray analysis. On the bases of the spectral data and the crystal structures, the effects of the enclathrated guest molecule on the solid-state photophysical properties of the guest-inclusion compounds are discussed.     

Photophysical studies and excited-state structure of a blue phosphorescent gold-thallium complex

The dinuclear complex [[Tl(eta6-toluene)][Au(C6Cl5)2]] (1) displays very intense blue phosphorescence and a rigidochromic behavior in the solid state. The photophysical measurements in a glassy solution display an oligomerization process via metal-metal interactions. Density functional theory calculations show a distortion of the aurate-thallium T shape in the lowest triplet excited state, leading to a triplet metal-to-metal charge-transfer state.     

Photophysical studies of some dyes in aqueous solution of triton X-100

The spectral (both absorption and fluorescence) and photoelectrochemical studies of a few selective dyes, namely, anionic erythrosin B, neutral riboflavin and cationic safranin O have been carried out in aqueous solution of triton X-100, a neutral surfactant. The results show that the ionic dyes, erythrosin B and safranin O form 1:1 electron donor-acceptor (EDA) or charge-transfer (CT) complexes with triton X-100 both in the ground and excited states, whereas neutral dye riboflavin in its excited state forms 1:1 complex with triton X-100. In these complexes, the dyes act as electron acceptors whereas triton X-100 acts as an electron donor. The fluorescence spectra of erythrosin B and safranin O in presence of triton X-100 show enhancement of fluorescence intensity with red and blue shifts respectively while riboflavin shows normal quenching of fluorescence. A good correlation has been found among photovoltage generation of the systems consisting of these dyes and triton X-100, spectral shift due to complex formation and thermodynamic properties of these complexes.     

Electrochemical studies of 9,10-anthraquinone interacting with hemoglobin and determination of hemoglobin

Electrochemical investigation of the interaction of 9,10-anthraquinone (AQ) with hemoglobin (HB) on a mercury electrode is reported for the first time. On addition of hemoglobin to an anthraquinone solution, both the reduction and oxidation currents decrease, with increasing peak separation. In the presence of hemoglobin, no new peaks appear, but the electrochemical parameters (standard rate constantk _s and diffusion coefficientD) change significantly. Reaction of anthraquinone with hemoglobin forms an electrochemically active complex HB-AQ. The equilibrium constant for this complex is calculated to be 3.27 × 10⁵ l/mol. A satisfactory result has been obtained for the determination of hemoglobin in clinical blood samples.     

Photophysical studies of oxicam group of NSAIDs: piroxicam,meloxicam and tenoxicam

Oxicam group of non steroidal anti-inflammatory drugs has been chosen as a prototype molecular group that shows diverse biological functions and dynamic structural features. Photophysical studies of three drugs from this group viz., piroxicam, meloxicam and tenoxicam have been carried out in different solvents with varying polarity, H-bond character and viscosity. The spectral responses of different prototropic forms of these drugs towards varying solvent parameters have been studied, with the aim to characterize their interaction in biomimetic environment non-invasively. The nature of the lowest transition has been identified. The extinction coefficient, quantum yield and viscosity dependence on the nature of the solvents, all indicate the extreme sensitivity of these drugs to their microenvironment.