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Laser flash photolysis of ketone 1 in argon-saturated methanol yields triplet biradical 1BR (τ = 63 ns) that intersystem crosses to form photoenols Z-1P (λ max = 350 nm, τ ~ 10 μs) and E-1P (λ max = 350 nm, τ > 6 ms). The activation barrier for Z-1P re-forming ketone 1 through a 1,5-H shift was determined as 7.7 ± 0.3 kcal mol -1 . In contrast, for ketone 2, which has a less sterically hindered carbonyl moiety, laser flash photolysis in argon-saturated methanol revealed the formation of biradical 2BR (λ max = 330 nm, τ ~ 303 ns) that intersystem crosses to form photoenol E-2P (λ max = 350 nm, τ > 42 μs), but photoenol Z-2P was not detected. However, in more viscous basic H-bond acceptor (BHA) solvent, such as hexamethylphosphoramide, triplet 2BR intersystem crosses to form both Z-2P (λ max = 370 nm, τ ~ 1.5 μs) and E-2P. Thus, laser flash photolysis of ketone 2 in methanol reveals that intersystem crossing from 2BR to form Z-2P is slower than the 1,5-H shift of Z-2P, whereas in viscous BHA solvents, the 1,5-H shift becomes slower than the intersystem crossing from 2BR to Z-2P. Density functional theory and coupled cluster calculations were performed to support the reaction mechanisms for photoenolization of ketones 1 and 2., (© 2018 The American Society of Photobiology.)

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm -3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA-acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet-state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet-triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile., (© 2018 The American Society of Photobiology.)

Bile salts form supramolecular aggregates with two binding sites with different properties. The guest binding dynamics to the aggregates and guest protection from species in the aqueous phase were investigated using fluorescence and laser flash photolysis experiments. Sodium cholate, deoxycholate and taurodeoxycholate were used as bile salts and acetonitrile or ethylene glycol were added as co-solvents to water in order to alter the binding properties of 1-ethylnaphthalene and 1-naphthyl-1-ethanol with the aggregates. The binding dynamics are faster and protection efficiencies are lower for guests bound to cholate and in the presence of either co-solvent.

Bile salt aggregates are supramolecular structures with two types of binding sites, called primary and secondary sites. The objective of this work was to explore how the nonplanarity and size of guests (biphenyl [BP], 1-1'-binaphthyl [BNP] and dibenz[b,f]oxepin [DBX]) affected their binding affinity and dynamics to sodium cholate (NaC) aggregates. Fluorescence and laser-flash photolysis experiments were performed to obtain information on the binding environment for the guests, the accessibility of quenchers to guests in the aggregate and the dissociation rate constants of the guests from the aggregates. All guests were bound to the more hydrophobic primary aggregate, showing that this site can accommodate nonplanar molecules. However, the structure of the guest affects the structure of the primary aggregates, leading to changes in the accessibility of anions to aggregate-bound guests and to changes for the guest dissociation rate constants from the aggregates.

Xanthones with amino substituents were synthesized to diminish the photoreactivity of the xanthone chromophore with DNA, with the objective of using these molecules to study their binding dynamics with DNA. The aminoxanthones showed a strong solvatochromic effect on their singlet and triplet excited-state photophysics, where polar solvents led to a decrease of the energies for the excited states. Quenching of the triplet excited states by nitrite anions was used to determine the binding dynamics, and a residence time in the microsecond time domain was estimated for the bound 2-aminoxanthone with DNA. The quenching experiments performed showed that this methodology will not be applicable to study the binding dynamics of a wide variety of guests with DNA.

The photochemistry of ninhydrin in benzene and water was studied by laser flash photolysis and electron paramagnetic resonance. Its photochemistry was shown to be dependent on the solvent. In benzene, a triplet excited state was observed, which underwent hydrogen abstraction reactions or reduction to the radical anion. In water, the radical anion of ninhydrin was formed within the laser pulse (15 ns) at neutral pH, whereas the neutral ketyl radical was formed by protonation of the radical anion at low pH. A pKa of 0.77 was determined for the protonation equilibrium. The formation of hydrindantin is proposed to occur through the dimerization of the ketyl radical or the radical anion (or both). In addition, ninhydrin was shown to be a poor precursor for the photogeneration of hydroxyl radicals.

The complexation dynamics of radical cations with cyclodextrins (CD) was studied using photophysical techniques. Radical cations of 4-vinylanisole and trans-anethole were formed within alpha- and beta-CD cavities by two-photon photolysis of the respective styrene precursors. Exit of the radical cations from alpha-CD complexes with 1: 1 and 1:2 (guest: CD) stoichiometries and beta-CD complexes with 1:1 stoichiometries occurred with lifetimes shorter than 100 ns. Most of the radical cations formed escape from the CD cavities, but a small portion do react with alpha-CD when this host is present in high concentrations.

Short lived reaction intermediates such as triplet states and free radicals can be detected in vivo using laser photolysis techniques with time-resolved diffuse reflectance detection. This novel approach is illustrated for bacterial suspensions of Bacillus subtilis.