Comparison of the photophysical properties of three phenothiazine derivatives: transient detection and singlet oxygen production.

New methylene blue (NMB+) and methylene violet (MV) are known for their photosensitizing properties for singlet oxygen ((1)O2) generation upon visible-light irradiation, and various examples of their use in the photodynamic inactivation of microorganisms and for photomedicinal purposes have been reported. However, their photophysical properties have never been extensively and systematically analyzed and compared. In the current work, we studied their absorption and fluorescence behavior relative to their parent compound, methylene blue (MB+), detected the transient species generated upon excitation of the photosensitizers and determined their quantum yields of singlet oxygen production. We could measure very high quantum yields of singlet oxygen production for all the studied compounds. NMB+ appeared similar to MB+, even though it produces (1)O2 much more efficiently, and was slightly influenced by the solvent. MV, in contrast, was much more sensitive to the chemical environment, and the transient species formed upon irradiation were different in methanol and acetonitrile. It appeared to be a very good singlet oxygen sensitizer, but the influence of the chemical environment should be carefully considered for any application. The comparative characterization of these sensitizers will represent a support for the determination and the understanding of the photochemical mechanisms occurring by using these phenothiazine dyes for various photobiological applications.

Efficient cyanoaromatic photosensitizers for singlet oxygen production: synthesis and characterization of the transient reactive species.

In order to graft cyanoaromatic molecules onto various inert supports, we designed two new cyanoanthracene derivatives of benzo[b]triphenylene-9,14-dicarbonitrile (DBTP, 1), which already demonstrated good photosensitizing properties. We synthesized 3-(N-hydroxypropyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 3 and 3-(N-N0-Boc-aminohexyl)carboxamido-9,14-dicyanobenzo[b]triphenylene, 4 and compared their photophysical properties in acetonitrile relative to those of the parent compound 1 and its carboxylic derivative 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid, 2. The transient species were analysed and the quantum yields of singlet oxygen production (ΦΔ) determined in acetonitrile. The effect of chemical functionalization can be considered negligible, since absorption spectra, fluorescence emission spectra and fluorescence lifetimes do not significantly change with the substituent. The triplet-triplet absorption spectra and the triplet excited state lifetimes are similar for the whole series. For compounds 1-4 high values of ΦΔ, close to that of the standard sensitizer 1H-phenalen-1-one (PN, ΦΔ ≈ 1), and higher than that of the well-known photosensitizer 9,10-dicyanoanthracene (DCA), are due to very efficient intersystem crossing from the singlet to the triplet excited state and subsequent energy transfer to ground state oxygen ((3)O2). They belong to a class of very efficient photosensitizers, absorbing visible light and stable under irradiation, they may be functionalized without significant changes to their photophysical behaviour, and grafted onto various supports.

Photoreactivity of the fungicide chlorothalonil in aqueous medium.

The photoreactivity of chlorothalonil was studied by means of steady-state irradiation and laser-flash photolysis. Experiments were conducted in water containing acetonitrile as a co-solvent. This fungicide undergoes very slow phototransformation in the first stages of the reaction, but the consumption profile is auto-accelerated. To understand the reaction mechanism, we undertook a detailed study of the rates, products and transient species. The rates and photoproduct distribution vary greatly with the oxygen concentration. Concerning the transient species, we measured the absorption of the triplet, its yield of formation, and its reactivity with oxygen in various water-acetonitrile mixtures and with isopropanol. The reduced radical, CTH˙, could be produced and its transient spectrum was recorded. Combining all the experimental data, it is hypothesized that in the first step of the reaction CT is excited to the triplet state. The triplet has several possible fates including reduction by organic constituents to form the radical which gives photoproducts. Another characteristic of the CT triplet is its capacity to generate singlet oxygen. The production of this species was measured by phosphorescence and compared to the percentage of the triplet trapped by oxygen in air-saturated solutions. The yield varies from 0.88 in pure acetonitrile to 0.48 in water-acetonitrile (95 : 5, v/v). Therefore, in surface waters, chlorothalonil is expected to sensitize the photooxidation of micropollutants, and to be competitively phototransformed through reaction with any H donor or electron donor water constituents.

Perinaphthenone phototransformation in a model of leaf epicuticular waxes.

Perinaphthenone (1H-phenalen-1-one, PN) is a reference photosensitizer producing singlet oxygen with a quantum yield close to one in a large variety of solvents. It is also the basic structure of a class of phototoxic phytoalexins. In this work, the PN photoreactivity was studied for the first time in a paraffinic wax, used as model of leaf epicuticular waxes. The PN photodegradation was monitored by UV-Vis spectroscopy. The triplet excited state, singlet oxygen and the hydroxyperinaphthenyl radical were detected by diffuse reflectance laser flash photolysis, near infrared phosphorescence and by EPR spectroscopy, respectively. The PN phototransformation was found to be fivefold faster in the wax than in n-heptane under steady-state irradiation. The hydroxyperinaphthenyl radical formation was observed in aerated irradiated paraffin wax while in n-heptane solution the radical was observed only in the absence of oxygen. These results show that under continuous irradiation, PN is much more easily phototransformed in a solid environment than in solution. Several photoproducts were identified, in particular phenalanone, PN dimers, and oxidized PN-alkanes adducts. Finally, when pyrethrum extract is added into the wax along with PN, the hydroxyperinaphthenyl radical concentration was increased by a factor of 2.4. Such photochemical reactions may occur when systemic pesticides enter the plant cuticle.

Immobilized organic photosensitizers with versatile reactivity for various visible-light applications.

Various photosensitizers were grafted by conventional peptide coupling methods to functionalized silica with several macroscopic shapes (powders, films) or embedded in highly transparent and microporous silica xerogel monoliths. Owing to the transparency and free-standing shape of the monoliths, the transient species arising from irradiation of the PSs could be analyzed and were not strikingly different from those observed in solutions. The observed reactivity for either liquid-solid (α-terpinene oxygenation vs dehydrogenation) or gas-solid (dimethylsulfide, DMS, solvent-free oxidation) reactions was consistent with the properties of the excited states of the PSs under consideration. Immobilized anthraquinone-derived materials preferentially react in both cases by electron transfer from the substrate to the triplet state of the sensitizer, in spite of an efficient singlet oxygen production. The recently developed 9,14-dicyanobenzo[b]triphenylene-3-carboxylic acid, DBTP-COOH, efficiently reacts via energy transfer to yield singlet oxygen from its triplet state. It was shown to perform better than 9,10-dicyanoanthracene and rose bengal for DMS oxidation and α-terpinene photooxygenation to ascaridole, respectively. Thus, by a proper choice of the organic immobilized photocatalyst, it is possible to develop efficient and reusable materials, activated under visible light, for various applications and to tune the reaction pathway, opening the way to green oxidation processes.

Transparent organosilica photocatalysts activated by visible light: photophysical and oxidative properties at the gas-solid interface.

The photophysical properties of several photosensitizers (PSs) included or grafted in silica monoliths were compared to their properties in solution. The effects of the solid support on their steady-state and transient absorption spectra, on their quantum yields of singlet oxygen ((1)O2) production, and on their ability to photoinduce the oxidation of dimethylsulfide (DMS) were investigated. Two cyanoanthracene derivatives (9,14-dicyanobenzo[b]triphenylene, DBTP, and 9,10-dicyanoanthracene, DCA), as well as three phenothiazine dyes (methylene blue, MB(+), new methylene blue, NMB(+), methylene violet, MV), were encapsulated in silica, analyzed and compared to two reference PSs (perinaphthenone, PN and rose bengal, RB). A DBTP derivative (3-[N-(N″-triethoxysilylpropyl-N'-hexylurea)]carboxamido-9,14-dicyanobenzo[b]triphenylene, 3) was also prepared and grafted onto silica. Thanks to the transparency and the free-standing shape of the monoliths, the complete spectroscopic characterization of the supported PSs was carried out directly at the gas-solid interface. The influence of the silica network, the PS, and the adsorption/grafting link between the PS and silica was investigated. The effects of PS concentration, gaseous atmosphere, humidity, and hydrophobicity on the production of (1)O2 were analyzed. With all PSs, (1)O2 production was very efficient (quantum yields of (1)O2 production, relative to PN, between 0.6 and 1), and this species was the only one involved in the pollutant photooxidation. The influence of the matrix on the PSs' photophysics could be considered as negligible. In contrast, the matrix effect on DMS photooxidation was extremely important: the gas diffusion inside the porous structure, and thus, the photoactivity of the materials, strictly depended on silica's surface area and porosity. Our results highlight the suitability of these silica structures as inert supports for the study of the photosensitizing properties at the gas-solid interface. Moreover, thanks to the adsorption properties of the matrix, the synthesized materials can be used as microphotoreactor for the (1)O2-mediated oxidation of volatile pollutants.