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The general difference observed between the photophysical behavior of 2-phenylbenzazoles and parent unsubstituted benzazole is explained through an exchange of the energy order of the two lower excited singlet states of both compounds. The energy variation takes place because S1 -excited 2-phenylbenzazoles suffer a subnanosecond conformational change towards planarity by torsion around the phenyl-heterocycle bond. This torsion is in accordance with theoretical calculations, with ultraviolet (UV)-visible and fluorescence data, and with results obtained on rotationally-constrained 2-phenylindoles. These changes, as well as the usually high fluorescence quantum yields and small Stokes shifts exhibited by unsubstituted benzazoles, make 2-phenylbenzazoles suitable dye lasers for the UV-zone of the spectrum. [ABSTRACT FROM AUTHOR]

Therapeutic application of many drugs is often hampered by poor or denied access to intracellular targets. A case in point is miltefosine (MT), an orally active antiparasitic drug, which becomes ineffective when parasites develop dysfunctional uptake systems. We report here the synthesis of a fluorescent BODIPY-embedding MT analogue with appropriate thiol functionalization allowing linkage to the cell-penetrating Tat(48-60) peptide through disulfide or thioether linkages. The resulting constructs are efficiently internalized into the otherwise MT-invulnerable R40 Leishmania strain, resulting in fast parasite killing, and hence successful avoidance of the resistance. In the disulfide-linked conjugate, an additional fluoro tag on the Tat moiety allows to monitor its reductive cleavage within the cytoplasm. Terminally differentiated cells such as peritoneal macrophages, impervious to MT unless infected by Leishmania, can uptake the drug in its Tat-conjugated form. The results afford proof-of-principle for using CPP vectors to avert drug resistance in parasites, and/or for tackling leishmaniasis by modulating macrophage uptake. [ABSTRACT FROM AUTHOR]

The spectroscopic properties in water solution of the different prototropic forms of the strongly fluorescent hemiacetal 4,9-dihydroxy-1,2-dihydro-4,11a-methanooxocino[4,5-b]benzofuran-5(4H)-one (1a, monardine), the aza analogue 4,9-dihydroxy-3,4-dihydro-1H-4,11a-methanobenzofuro[2,3-d]azocin-5(2H)-one (2a, azamonardine) and the respective 2-carboxyl derivatives (1b, 2b) have been studied by experimental and quantum-chemical methods. Monardine and carboxymonardine are the major products of new fluorogenic, room-temperature reactions of hydroxytyrosol or salvianic acid in aqueous solution, respectively, and present unique photophysical properties. Near neutral pH (pKa = 7.2) monardine switches from a weakly emitting, UV-absorbing (382 nm) neutral species to a VIS-absorbing (426 nm), blue emitting (464 nm) anion form, with a fluorescence quantum yield ΦF = 1 and single-exponential decay τF = 2.74 ns. This binary-like spectroscopic change from the neutral to the anionic form was interpreted based on time-dependent density functional theory (TDDFT) calculations as due to (i) the reversal of (n,π*) and (π,π*) lowest-lying singlet excited states, and (ii) a change in the triplet-state distribution accompanying monardine ionization which may abolish de-excitation via intersystem crossing. A similar fluorogenic reaction takes place with catecholamines such as dopamine and DOPA, to yield fluorescent azocines 2a and 2b which, depending on pH, may be present as cationic, neutral or anionic species. TDDFT computations of these forms were also carried out to assign the corresponding excitation transitions and emission properties. Besides the analytical interest of the fluorogenic reactions, the photochemical stability and biocompatibility of the bright–dark pH-controlled molecular switches 1a and 1b may facilitate novel labels and probes to be developed for superresolution fluorescence microscopy. [ABSTRACT FROM AUTHOR]

The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Δg)] and, to a lesser extent, superoxide radical anion (O2•−)—both species photogenerated from triplet excited Rf, 3Rf*—participate in the photodegradation. At pH 7, UR is slowly degraded through an O2•−-mediated mechanism, whereas Rf disappears through its reaction with O2(1Δg) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species—a back electron transfer that also produces O2•−—and the elimination from the medium of O2(1Δg) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Δg) were in the range 5 × 105 M−1 s−1 (at pH 7) to 1.3 × 108 M−1 s−1 (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Δg)-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Δg)-reactive quenching with UR was observed. [ABSTRACT FROM AUTHOR]