Dying to find new chromophores
New dyes that have sharp absorptions and fluorescence emission bands in the red or near infrared as well high molar absorption coefficients and high fluorescence quantum yields could be used widely optical engineering, analytical chemistry, biological imaging and sensors, as well as in materials science.
Writing in Chemistry - An Asian Journal, Wim Dehaen and colleagues at the Catholic University Leuven, University of Mons, Belgium, and colleagues at Lanzhou University, China, describe the preparation of difluoroboron dipyrromethene (BODIPY)-based dyes with increasing conformational rigidity that have absorption in the visible region of the spectrum.
Boron times ahead
Formally, the BODIPY core is 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene in which a dipyrromethene is complexed with a disubstituted boron atom, usually a BF2 unit. Due to instability of the required unsubstituted dipyrromethene precursor, the unsubstituted BODIPY dye was not synthesised until 2009 at which point three independent groups published different approaches to the compound.
According to Dehaen, a substantial number of BODIPY dye analogues have been produced since the 1960s, mainly by extending the conjugation of the system using aryl substituents. These various substituted BODIPY dyes show red shifts of more than 100 nanometres but have only low to moderate fluorescence quantum yields because of the non-radiative decay arising from their non-rigid structure. Recently, however, chemists have turned to alkenyl, and even more recently alkynyl substituents that produce large red shifts and have rigidity because of their bonding, a triple bond in the latter case, that boosts their quantum yields.
Researchers have even managed to nudge BODIPY dyes into the near-infrared by functionalizing the peripheral aromatic rings core using heterocycles. Unfortunately, these materials require complex multi-step syntheses to produce the fused-ring pyrrole starting materials; moreover, they restricted to symmetrical frameworks for synthetic reasons.
BODIPY dyes
Now, Dehaen and colleagues have synthesised two BODIPY dyes from a conformationally unconstrained indacene using a relatively simple intramolecular benzofuran formation reaction catalysed by palladium. The team explains that these new dyes have restricted rotation of their phenoxy moieties, and thus absorb and fluoresce more intensely at longer wavelengths relative to their unrestricted analogues. It is the reduction in conformational flexibility that also endow them with markedly higher fluorescence quantum yields. The team has now carried out quantum mechanics calculations on their compounds to underpin the experimental findings. The calculations were, in turn, corroborated by X-ray diffraction data that revealed a progressive increase in the planarity of the chromophore associated with increasing conformational rigidity.
The team adds that they have also examined the effect of the molecular structure on the visible absorption and fluorescence emission properties of the dyes as a function of solvent. To do this they used a new, generalized treatment of the solvent effect reported in 2009 by Javier Catalan of the Autonomic University of Madrid. (J Phys Chem B, 2009, 113, 5951-5960; http://pubs.acs.org/doi/abs/10.1021/jp8095727).
The Catalan approach considers dipolarity (SdP, a new scale), polarizability (SP), acidity (SA), and basicity (SB) of the solvent. Dehaen and colleagues found that applying this assessment revealed solvent polarizability to be the main factor in accounting for the small solvent-dependent shifts of the visible absorption and fluorescence emission bands of their dyes.
The next step will be to exploit this practically simple synthesis for developing other BODIPY-based dyes that could be even more structural rigid and have still higher quantum yields.
"In the near future, conjugation of appropriate molecular recognition moieties to the BODIPY scaffold could lead to highly selective fluorescing (bio)sensors for diverse substrates including cations, anions or neutral molecules," Dehaen told SpectroscopyNOW. "After the necessary functionalisations have been carried out to improve the solubility of the dyes in water, this research might lead to applications in biological environment." He adds that, "The use of BODIPY dyes with extended conjugation has been described in several proof of principle applications, but has been hampered up to now by their tedious synthesis. E.g. BODIPYs might form part of dye sensitized solar cells."
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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