Fluorescence of Diimidazo[1,2-a:2′,1′-c]quinoxalinium Salts Under Various Conditions

Bathochromic Shift of Fluorescence Peak in Dipyrrolo[1,2-a:2',1'-c]quinoxaline by Introducing Each of Electron-Donating and Electron-Withdrawing Substituent

Development of organic fluorophore is an important theme. Especially, the fluorophores with longer fluorescence peaks are useful to biological probes. One of the methods to change the fluorescence peak is the introduction of substituents. However, opposing characteristics of the substituents lead to different changes in the fluorescence peaks. Furthermore, the introduction of the substituent also affects their electric properties. Thus, if the materials were developed with the substituent effect on the optical and electric properties separately, it will be useful to design the functional materials related to both optical and electric properties. Herein, we investigated the substituent effect of dipyrrolo[1,2-a:2',1'-c]quinoxalines on fluorescence properties. We synthesized the compounds bearing electron-donating or electron-withdrawing substituents at the benzene ring on dipyrrolo[1,2-a:2',1'-c]quinoxaline, which would have more direct influence on the optical properties. By introducing each substituent at the 6 position of dipyrrolo[1,2-a:2',1'-c]quinoxaline, the bathochromic shift was observed in the fluorescence spectra. In the case of fluorine substituent, the change of the fluorescence peak reached was about 19 nm. Using a TDDFT calculation, we explained the reason for such a substituent effect that large on the increment of LUMO energy or decrement of HOMO energy occurred by introducing electron-withdrawing or electron-donating substituents at the 6 position, respectively. The substituent effect on the change of orbital energies is typical although the different characteristics of substituents resulted in the similar tendency about the change of fluorescence peak. Furthermore, with the introduction of phenyl substituents at the 3 and 10 positions, we achieved 40-50 nm longer fluorescence peaks compared with that of the original dipyrrolo[1,2-a:2',1'-c]quinoxaline.

Allowing the direct interaction of N-aryl α-diimines with a high valent metal chloride: one-pot WCl6-promoted formation of quinoxalinium salts

The full potential of a high valent metal chloride as both a chlorinating and an oxidative agent was explored by allowing WCl₆ to react with N-(2,6-diisopropylphenyl) α-diimines, in CH₂Cl₂ at room temperature. These α-diimines underwent unprecedented conversion to quinoxalinium cations via intramolecular C-N coupling.

Substituent Effect at the C4-Position of 1,3a,6a-Triazapentalene

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.

Synthesis and Optical Properties of Imidazole- and Benzimidazole-Based Fused π-Conjugated Compounds: Influence of Substituent, Counteranion, and π-Conjugated System

Fused π-conjugated imidazolium chlorides having hydrogen (1-Cl), octyloxy (2-Cl), N,N-dibutylamino (3-Cl), trifluoromethyl (4-Cl), and cyano (5-Cl) groups substituted on the benzene ring at the 2-position of imidazole were prepared. Counteranion exchanges from chloride to bis(trifluoromethanesulfonyl)imidate (2-TFSI) and tetrafluoroborate (2-BF4) were performed. The optical properties of these compounds (absorption and emission wavelengths, fluorescence quantum yield, and solvatochromism) were influenced by both the substituent and anion character, which was investigated by theoretical calculations using the density functional theory (DFT) and symmetry-adapted cluster-configuration interaction (SAC-CI) methods. Fused π-conjugated benzimidazolium chlorides having N,N-dibutylamino (6-Cl) and cyano (7-Cl) groups were also prepared to observe the different solvatochromic shifts.

Synthesis of yellow and red fluorescent 1,3a,6a-triazapentalenes and the theoretical investigation of their optical properties

To expand the originally developed fluorescent 1,3a,6a-triazapentalenes as fluorescent labelling reagents, the fluorescence wavelength of the 1,3a,6a-triazapentalenes was extended to the red color region. Based on the noteworthy correlation of the fluorescence wavelength with the inductive effect of the 2-substituent, electron-deficient 2-(2-cyano-4-methoxycarbonylphenyl)-1,3a,6a-triazapentalene and 2-(2,6-dicyano-4-methoxycarbonylphenyl)-1,3a,6a-triazapentalene were synthesized. The former exhibited yellow fluorescence and the latter exhibited red fluorescence, and both compounds exhibited large Stokes shifts, and the 1,3a,6a-triazapentalene system enabled the same fluorescent chromophore to cover the entire region of visible wavelengths. The potential applications of the 1,3a,6a-triazapentalenes as fluorescent probes in the fields of the life sciences were investigated, and the 1,3a,6a-triazapentalene system was clearly proven to be useful as a fluorescent reagent for live cell imaging. Quantum chemical calculations were performed to investigate the optical properties of the 1,3a,6a-triazapentalenes. These calculations revealed that the excitation involves a significant charge-transfer from the 1,3a,6a-triazapentalene skeleton to the 2-substituent. The calculated absorption and fluorescence wavelengths showed a good correlation with the experimental ones, and thus the system could enable the theoretical design of substituents with the desired optical properties.

Copper(I)-catalyzed cycloaddition of 4-bromosydnones and alkynes for the regioselective synthesis of 1,4,5-trisubstituted pyrazoles

Copper-catalyzed cycloaddition of alkynes with 4-bromosydnones provides a convenient, mild, and regioselective method for the synthesis of a wide range of bromopyrazoles. The broad functional group tolerance of the cycloaddition reaction and further palladium-catalyzed cross-coupling reactions allowed the preparation of polyfunctionalized 1,4,5-pyrazoles that are otherwise difficult to obtain by conventional methods.