Blue Fluorescence from BF₂ Complexes of N,O-Benzamide Ligands: Synthesis, Structure, and Photophysical Properties

Abstract

Small molecules having intense luminescence properties are required to promote biological and organic material applications. We prepared five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings and successfully converted them into three types of the difluoroboronated complexes, Py@BAs, as novel blue fluorophores. Py@BA having a pyridine moiety (2-Py@BA) showed no fluorescence in solution, whereas Py@BAs of pyridazine and pyrazine moieties (2,3-Py@BA and 2,5-Py@BA, respectively) emitted blue fluorescence with quantum yields of ca. 0.1. Transient absorption measurements using laser flash photolysis of the Py@BAs revealed the triplet formation of 2,3- and 2,5-Py@BAs, while little transient signal was observed for 2-Py@BA. Therefore, the deactivation processes from the lowest excited singlet state of fluorescent 2,3- and 2,5-Py@BAs consist of fluorescence and intersystem crossing to the triplet state while that of the nonfluorescent Py@BA is governed almost entirely by internal conversion to the ground state. Conversely, in the solid state, 2-Py@BA emitted intense fluorescence with a fluorescence quantum yield as high as 0.66, whereas 2,3- and 2,5-Py@BAs showed fluorescence with quantum yields of ca. 0.2. The crystal structure of 2-Py@BA took a herringbone packing motif, whereas those for 2,3- and 2,5-Py@BAs were two-dimensional sheetlike. On the basis of the difference in crystal structures, the emission mechanism in the solid state was discussed.

Organolithium-Mediated Postfunctionalization of Thiazolo[3,2-c][1,3,5,2]oxadiazaborinine Fluorescent Dyes

“…One of the least studied classes of the N,O-coordinating organoboron dyes is based on the oxadiazaborinine ring . Usually, the oxadiazaborinine dyes have been synthesized by complexation of boron trifluoride with amide of electron-poor 2-amino- N -heterocycles such as pyridine, pyrazine, , pyridazine, 1,8-naphthyridine, or 1,3,4-thiadiazole . Our conception is based on the incorporation of an electron-rich heterocyclic building block into the oxadiazaborinine structure to increase the chemical stability of these complexes.…”

Controlling Two-Photon Action Cross Section by Changing a Single Heteroatom Position in Fluorescent Dyes

“…To our knowledge, there is only one previous publication addressing the influence of the nitrogen atom position in isoelectronic BF 2 derivatives on properties of dyes. 32 However, that contribution considered three (out of five) nitrogen positions and, more importantly, those molecules were not photoisomerizable.…”

“…Moreover, the successful functionalization of BF-carrying compounds to optimize their photochemical properties has been regularly achieved, through substitution, extension of the conjugated path with either double or triple carbon–carbon bonds, planarization, benzoannulation, substitution of the fluorine atoms, replacement of an atom binding Lewis acid, etc. 16 , 17 Recently, Yamaji et al demonstrated that the fluorescence quantum yield in two diazine-based difluoroborates significantly differ from that of the corresponding pyridine derivative, 18 , 19 and we decided to adopt a similar strategy in our research to enhance fluorescence quantum yield, thus maximizing TPACS.…”

“…Our interest in this class of compounds stems from their excellent photophysical properties. − In general, fluoroborates exhibit an intense and sharp emission with high quantum yields, nanosecond lifetimes, and narrow absorption bands in the visible spectral region characterized by high extinction coefficients. − , In addition, they show a good stability in biological environmentsa feature that places them in a privileged spot as compared to many other molecular probe candidates. Moreover, the successful functionalization of BF-carrying compounds to optimize their photochemical properties has been regularly achieved, through substitution, extension of the conjugated path with either double or triple carbon–carbon bonds, planarization, benzoannulation, substitution of the fluorine atoms, replacement of an atom binding Lewis acid, etc. , Recently, Yamaji et al demonstrated that the fluorescence quantum yield in two diazine-based difluoroborates significantly differ from that of the corresponding pyridine derivative, , and we decided to adopt a similar strategy in our research to enhance fluorescence quantum yield, thus maximizing TPACS.…”

“…Nevertheless, we note that the heterocyclic part of the present dyes is well separated from the carbon–carbon double bond and no steric impact on the photoisomerization can be expected. To our knowledge, there is only one previous publication addressing the influence of the nitrogen atom position in isoelectronic BF 2 derivatives on properties of dyes . However, that contribution considered three (out of five) nitrogen positions and, more importantly, those molecules were not photoisomerizable.…”