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An "off-on" fluorescent nanoprobe for near-infrared multiphoton imaging of singlet oxygen has been developed. The nanoprobe comprises a naphthoxazole fluorescent unit and a singlet-oxygen-sensitive furan derivative attached to the surface of mesoporous silica nanoparticles. In solution, the fluorescence of the nanoprobe increases upon reaction with singlet oxygen both under one- and multiphoton excitation, with fluorescence enhancements up to 180-fold. The nanoprobe can be readily internalized by macrophage cells and is capable of imaging intracellular singlet oxygen under multiphoton excitation.

Singlet oxygen ( 1 O 2 ) is the first electronic excited state of molecular oxygen. Due to its non-radical and non-ionic character as well as its mild reactivity, 1 O 2 has a pivotal role in cell signaling processes at low concentration, yet it is cytotoxic at high concentrations. Quantifying the production of 1 O 2 , particularly in biological systems, is therefore essential for understanding and controlling its effects. 1 O 2 can be produced by chemical and biological reactions, yet its most common method of production is by photosensitization, whereby an initially photoexcited molecule transfers its acquired electronic energy to the dioxygen molecule. The efficiency of this process is characterized by the 1 O 2 production quantum yield, Φ Δ , which can be determined by directly monitoring its intrinsic weak near-infrared phosphorescence or indirectly by trapping it with a suitable acceptor, a process that can be monitored by common analytical techniques. Indirect methods are thus very popular, yet they may lead to severe errors if used incorrectly. Herein we describe the common aspects of indirect methods and propose a general step-by-step procedure for the determination of Φ Δ values. In addition, we identify the key experimental conditions that need to be controlled to obtain meaningful results.

Naphthoxazole derivatives are small heterocyclic compounds endowed with outstanding fluorescence properties. In this work, we report a detailed study of the intense white light fluorescence observed in naphthoxazole-quinoline dyads in solvent mixtures including at least a strong hydrogen bonding solvent. The same phenomenon was also studied in inclusion complexes naphthoxazole derivatives-sulfonated-βCD either in aqueous solution as well as in solid phase. A novel mechanism of white molecular fluorescence generation based on solvent-to-fluorophore proton transfer facilitated by ground state hydrogen bonding was characterized. The emission combines both, a blue charge transfer fluorescence emitted by the locally excited singlet state along with a red-shifted emission from a proton transfer complex., (© 2018 The American Society of Photobiology.)

In this study, we report the synthesis and the photochemical behavior of a series of new "click-on" fluorescent probes designed to detect singlet oxygen. They include a highly fluorescent chemical structure, an aryloxazole ring, linked to a furan moiety operating as singlet oxygen trap. Their activity depends on both the structure of the aryloxazole fluorophore and the electron-donating and electron-accepting properties of the substituents attached to the C-5 of the furan ring. All probes are selectively oxidized by singlet oxygen to give a single fluorescent product in methanol and produce negligible amounts of singlet oxygen themselves by self-sensitization. The most promising dyad, (E)-2-(2-(5-methylfuran-2-yl)vinyl)naphtho[1,2-d]oxazole, FN-6, shows outstanding reactivity and sensitivity: it traps singlet oxygen with a rate constant (5,8 ± 0.1) x 1(07) M-1 s-1 and its fluorescence increases by a factor of 500 upon reaction. Analysis of the dyads reactivity in terms of linear free energy relationships using the modified Swain and Lupton parameter F and the Fukui condensed function for the electrophilic attack, suggests that cycloaddition of singlet oxygen to the furan ring is partially concerted and possibly involves an exciplex with a "more open" structure than could be expected for a concerted cycloaddition., Competing Interests: The authors have declared that no competing interests exist.

The photophysical properties of 2-phenyl-naphtho[1,2-d][1,3]oxazole, 2(4-N,N-dimethylaminophenyl)naphtho[1,2-d][1,3]oxazole and 2(4-N,N-diphenylaminophenyl) naphtho[1,2-d][1,3]oxazole were studied in a series of solvents. UV-Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV-Vis absorption and fluorescence spectra in terms of the Lippert-Mataga equation shows a large increase in the excited-state dipole moment, which is also compatible with the formation of an intramolecular charge-transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1-3 ns as well as their high photostability., (© 2013 The American Society of Photobiology.)