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The photochemical reaction of a pyrimidine and a ketone occurs either as a Paternò-Büchi (PB) reaction or as energy transfer (ET) from the triplet ketone to the pyrimidine. It is rare for the two types of reactions to occur concurrently, and their competitive mechanism remains unknown. In this work, two classes of products, regioisomeric oxetane(s) ( 2, 3) from a PB reaction and three isomeric dimers of 5-fluoro-1,3-dimethyl uracil (FDMU) ( 4- 6) from a photosensitized dimerization of FDMU, are obtained through the UV irradiation of FDMU with various benzophenones (BPs). The ratio of the two products (oxetanes to dimers) reveals that the two competitive reactions depend strongly on the triplet energy levels ( ET) of the BPs. The BPs with higher ET values lead to higher proportions of dimers, whereas those with lower ET values give higher proportions of oxetane(s), with the generation of just two regioisomeric oxetanes for the BP with the lowest ET of the eight BPs investigated. The ratio of the two oxetanes ( 2: 3) decreases with the BP ET value. The competitive mechanism for the two types of photochemical reactions is demonstrated through quenching experiments and investigation of temperature effects. Kinetic analysis shows that the rate constants of the two [2+2] photocycloadditions are comparable. Furthermore, in combination with the results of previous studies, we have gained insight into the dependence of the photochemical type and the regioselectivity in the PB reaction on the triplet energy gaps (Δ E) between the pyrimidines and ketones. For ketones with higher ET values than the pyrimidines, the photochemical reaction is a photosensitized dimerization of the pyrimidine. In the opposite case, a PB reaction occurs, and the lower the ET of the ketones, the lower the ratio of oxetanes ( 2: 3). When the ET of values of the ketones are close to those of the pyrimidines, the two reactions occur concurrently, and the higher the ET of the ketones, the higher the proportion of the dimers. The ratio of oxetanes ( 2: 3) decreases with the ET value of the BPs. [ABSTRACT FROM AUTHOR]

A new type of fluorescent probes for thiophenols, 6HQM-DNP and 7HQM-DNP, containing 6- or 7-hydroxy quinonlinium as fluorophore and 2,4-dinitrophenoxy (DNP) as nucleophilic recognition unit were constructed. As ethers, these non-fluorescent probe molecules can release the corresponding fluorescent quinolinium (6HQM and 7HQM) through aromatic nucleophilic substitution (SNAr) by thiolate anions from thiophenols. The sensing reaction is highly sensitive (detection limit of 8 n M for 7HQM-DNP) and highly selective to thiophenols over aliphatic thiols and other nucleophiles under neutral conditions (pH 7.3). The probes respond rapidly to thiophenols, with second-order rate constants k=45 M−1 s−1 for 7HQM-DNP and 24 M−1 s−1 for 6HQM-DNP. Furthermore, the selective detection of thiophenols in living cells by 7HQM-DNP was demonstrated by confocal fluorescence imaging. In addition, these quinolinium salts show excellent chemical and thermal stability. In conclusion, this type of probes may find use in the detection of thiophenols in environmental samples and biosystems. [ABSTRACT FROM AUTHOR]

The title compound, C21H20N4O5S·C6H6, is a by-product of the synthesis of tazobactam. The crystal structure is stabilized by inter­molecular C—H⋯O hydrogen bonds. [ABSTRACT FROM AUTHOR]

In the title salt, C(11)H(13)N(2) (+)·C(6)H(2)N(3)O(7) (-), the dihedral angles between the benzene ring in the cation and the imidazolium ring and the benzene ring of the picrate anion are 113.7 (2) and 116.3 (2)°, respectively. The imidazolium ring is nearly parallel to the benzene ring of the picrate anion, the dihedral angle between the planes being 2.6 (1)°. The nitro groups in the picrate anions are disordered (occupancy ratio 0.54:0.46). The crystal packing is stabilized by weak C-H⋯O inter-actions between the cation-anion pairs.