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For the fluorescence imaging of biologically active small compounds, the development of compact fluorophores that do not perturb bioactivity is required. Here we report a compact derivative of fluorescent 1,3a,6a-triazapentalenes, 2-isobutenylcarbonyl-1,3a,6a-triazapentalene (TAP-VK1), as a fluorescent labeling reagent. The reaction of TAP-VK1 with various aliphatic thiols proceeds smoothly to afford the corresponding 1,4-adducts in high yields, and nucleophiles other than thiols do not react. After the addition of thiol groups in dichloromethane, the emission maximum of TAP-VK1 shifts to a shorter wavelength and the fluorescence intensity is substantially increased. The utility of TAP-VK1 as a compact fluorescent labeling reagent is clearly demonstrated by the labeling of Captopril, which is a small molecular drug for hypertension. The successful imaging of Captopril, one of the most compact drugs, in this study demonstrates the usefulness of compact fluorophores for mechanistic studies.

Various 2,4-disubstituted-1,3a,6a-triazapentalenes possessing methyl and phenyl groups at the C4-position were synthesized. Fluorescence observation of the synthetic 4-methyl-and 4-phenyl-1,3a,6a-triazapentalenes revealed that the introduction of a substituent at the C4-position allowed a long-wavelength shift of the fluorescence maximum. Furthermore, the phenyl group at the C4-position was found to induce a substantial increase in the extinction coefficient value.Key words 1,3a,6a-triazapentalene; substituent effect; longer-wavelength shift; 4-methyl-1,3a,6a-triazapentalene; 4-phenyl-1,3a,6a-triazapentalene Fluorescent organic molecules are an important class of compounds in modern science and technology, and are widely used in biological imaging probes, sensors, lasers, and lightemitting devices.1-3) Thus, the development of useful fluorescent organic molecules is crucial for the advancement of many industries, and has been a subject of intensive research. [4][5][6][7][8][9][10][11][12][13][14][15] In particular, fluorescent organic molecules have received a great deal of attention in a pharmacology field, because the visualization of biologically active small compounds by introducing fluorophores is one of the most useful methods in mechanistic studies. [16][17][18][19][20][21][22] However, the introductions of fluorescent organic molecules sometimes induces a loss of biological activities because the physical properties of fluorescent molecules affect the biological activities of the original compound. Furthermore, highly fluorescent organic molecules are usually larger than small biologically active compounds, and the fluorescent labeling reagent disturbs the original interactions of the parent compounds with target proteins such as receptors. To overcome these problems, there is strong demand for the development of small fluorophores. We recently discovered that 1,3a,6a-triazapentalene 1 is a compact and highly fluorescent chromophore 23) (Fig. 1). Since the disclosure of the fluorescence properties of 1,3a,6a-triazapentalenes, the 1,3a,6a-triazapentalene system has attracted a great deal of interest as a novel fluorophores, and another group has also reported the alternative synthesis of 1,3a,6a-triazapentalenes.24) 1,3a,6a-Triazapentalenes exhibit various useful fluorescence properties such as intense fluorescence, a large Stokes shift, and a positive fluorescence solvatochromism. More interestingly, the fluorescence wavelengths of 1,3a,6a-triazapentalenes vary widely with 2-substituents. For example, the fluorescence maxima shifts to longer wavelengths as the inductive effect of the 2-substituents increases. On the basis of this effect, the fluorescence wavelength of 1,3a,6a-triazapentalenes has recently been extended to the red color region, and their molecules are smaller than the conventional yellow and red fluorescent molecules.25) The yellow derivatives were proven to be useful fluorescent labeling reagents. On the other hand, the introduction of an electron-donating substituent at the C5-position ...

From measurements of the phosphorescence spectrum and its decay at 77 K, the lowest triplet state of benzophenone (BP) at low concentrations (ca. 1 x mol dm-3) has been assigned to be of nn* character even in a strong hydrogen-bonding solvent such as 2,2,2-trifluoroethanol (TFE). At high sample concentrations (ca. 1 x mol dm-3), another nn*-type phosphorescence due to aggregated BP is observed. Addition of water (H,O or D,O, 1-4 vol.%) to BP-TFE solutions causes dual phosphorescence with lifetimes of 3.6 and 15.9 ms, but only the long-lived phosphorescence is observed in TFE-water (a5 vol.%): The nn*-type short-lived phosphorescences at low and high sample concentrations are attributed to free triplet BP (or BP hydrogen-bonded with TFE) and aggregated triplet BP, respectively, while the long-lived phosphorescence is attributed to a complex of triplet BP-water-TFE (1 : 1 : 1) or triplet BP-water (1 : 1) of mixed nn*-nn* character.

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