6-Hydroxy-3-oxo-3H-xanthene-9-carboxylic acid is introduced as the first transition-metal-free carbon monoxide releasing molecule activated by visible light (photoCORM). This water-soluble fluorescein analogue releases carbon monoxide in both water and methanol upon irradiation at 500 nm. When selectively irradiated in the presence of hemoglobin (Hb) under physiological conditions, released CO is quantitatively trapped to form carboxyhemoglobin (COHb). The reaction progress can be accurately monitored by characteristic absorption and emission properties of the reactants and products.

The photophysical and photochemical properties of rose bengal (RB) in degassed aqueous and acetonitrile solutions were studied using steady-state and transient absorption spectroscopies. This comprehensive investigation provides detailed information about the kinetics and the optical properties of all intermediates involved: the triplet excited state and the oxidized and reduced forms of RB. A full kinetic description is used to control the concentrations of these intermediates by changing the initial experimental conditions.

The design, synthesis, and both experimental and theoretical studies of several novel 9-(acylimino)- and 9-(sulfonylimino)pyronin derivatives containing either an oxygen or a silicon atom at position 10 are reported. These compounds, especially the Si analogues, exhibit remarkably large Stokes shifts (around 200 nm) while still possessing a high fluorophore brightness, absorption bands in the near-UV and visible part of the spectrum, and high thermal and photochemical stabilities in protic solvents. The reason for the observed large Stokes shifts is an intramolecular charge-transfer excitation of an electron from the HOMO to the LUMO of the chromophore, accompanied by elongation of the C9-N bond and considerable solvent reorganization due to hydrogen bonding to the solvent. Due to the photophysical properties of the studied compounds and their facile and high-yielding synthesis, as well as a simple protocol for their bioorthogonal ligation to a model saccharide using a Huisgen alkyne-azide cycloaddition, they represent excellent candidates for biochemical and biological applications as fluorescent tags and indicators for multichannel imaging. 9-(Acylimino)pyronins alter their optical properties upon protonation and may also be used as pH sensors.

A new photoremovable protecting group, (6-hydroxy-3-oxo-3H-xanthen-9-yl)methyl (1), with a molar absorption coefficient ε of ∼4 × 10(4) m(-1) cm(-1) at ∼520 nm for the release of carboxylates or phosphates is reported. Three derivatives of 1 (diethyl phosphate, acetate, and bromide) were isolated as complexes with DDQ and shown to release the ligands with quantum yields ≤2.4% in aqueous solution.

The syntheses and biological applications of two novel fluorescent 9-phenylethynylpyronin analogues containing either carbon or silicon at the position 10 are reported. Both fluorescent probes exhibited a relatively strong fluorescence in methanol and phosphate buffer saline in the near-infrared region (705-738 nm) upon irradiation of either of their absorption maxima in the blue and red regions. The compounds showed high selectivity toward mitochondria in myeloma cells in vivo and allowed their visualization in a favored tissue-transparent window, which makes them promising NIR fluorescent tags for applications in bioimaging.

To date, most known molecules that release carbon monoxide by the action of light are based on carbonyl complexes of metals. However, they suffer from several disadvantages, so the focus of this perspective is on photoactivatable metal-free CO precursors. The development and design of these systems from the starting point of deep-UV-absorbing hydrophobic molecules and leading to hydrophilic biocompatible visible-light-absorbing CO-releasing molecules (photoCORMs) is described with mechanistic details for several structural motifs. The possibilities of this development are not exhausted and here we discuss the design of new biologically interesting candidates.

A clean bifurcation between two important photochemical reactions through competition of a triplet state Type II H-abstraction reaction with a photo-Favorskii rearrangement for (o/p)-hydroxy-o-methylphenacyl esters that depends on the water content of the solvent has been established. The switch from the anhydrous Type II pathway that yields indanones to the aqueousdependent pathway producing benzofuranones occurs abruptly at low water concentrations (~8%). The surprisingly clean yields suggest that such reactions are synthetically promising.An intramolecular Type II reaction of alkylphenones, especially 2-methylacetophenoes, produces high yields of photoenols that have been shown to be valuable reactive intermediates. 1 When a leaving group (X) is present in the α-position of 2-alkylacetophenones (1), longer-lived (E)-photoenols liberate HX to give indanones 2 (2 ; Scheme 1) that can be employed as precursors to elaborate products used in synthetic methodology. 3 Likewise, the photo-Favorskii rearrangement of α-substituted phydroxyacetophenones (3) to p-hydroxyphenylacetic acid (4), discovered more recently, has enjoyed success in a variety of applications because of its rapid release of nucleofuges (Scheme 1). 4 Both reactions have exhibited very good photochemical efficiencies and are relatively free of side reactions that produce complicated product mixtures. The similarity of the chromophores, the common triplet state origin for reaction, and the extensive understanding of the mechanistic photochemistry 5 have prompted us to explore the intersection of the photochemical pathways to uncover which parameters control the pathway followed by a chromophore common to both. It is a rare occurrence in organic photochemistry when one can completely and cleanly divert the reaction or rearrangement pathway from a single product type to an alternative * parkbs@dongguk.edu; givensr@ku.edu; klan@sci.muni.cz. Supporting Information Available:Experimental details and characterization data for products are provided. This material is available free of charge via the Internet at http://pubs.acs.org. 3a Since it is well known that electron donating hydroxy and methoxy groups suppress Type II reactivity 6 and nearly as well known that the photo-Favorskii rearrangement has a strong affinity for water, 4k,7 we assumed that these two reaction pathways might be separated by the water content of the solvent media. In this work, the analogs of 4-hydroxyphenacyl derivatives with good leaving groups necessary for the photo-Favorskii process were fitted with 2-methyl substituent for efficient Type II hydrogen abstraction reaction to compete with the photo-Favorskii rearrangement. As an extension, a similar design, in which the 2-hydroxyphenacyl derivatives were fitted with a 6-methyl group, was also examined. NIH Public Access Synthesis of Phenacyl Esters4-Hydroxy-2-methyl (9a-c) and 2-hydroxy-4,6-dimethylphenacyl (14) esters, and the corresponding benzyl (10) and methyl (13) ethers were prepared by α-bromination of acetophen...

A novel class of pyronin analogues, which undergoes a photochemically induced cleavage of the C-C bond in the presence of water in both solution and on a silica gel surface upon direct irradiation with visible light, is reported. The reaction course can be monitored by characteristic fluorescence of both the starting compound and the final product. This system could find useful applications in the field of photoremovable protecting groups or caged fluorophores.