Synthesis of Novel Organoboron Chromophores and Their Photophysical Properties Evaluation

Boron dipyrromethenes (BODIPYs) are widely known for their excellent optical properties such as high fluorescence quantum yield (Φf), sharp absorption and emission spectra, as well as high photo- and chemical stability. The later property also allows to perform an easy functionalization of BODIPY core through a large variety of chemical transformations. Despite the huge advancements that were made in the field of BODIPY dyes, their general properties are not yet ideal with the major drawback being the relatively small Stokes shifts (400-600 cm-1, in most cases). This generally leads to the loss of fluorescence in the solid state due to self-quenching. In 2002, Brower and Lightner described new, highly fluorescent (Φf ≥ 0.8) chromophores containing a 3H,5H-dipyrrolo[1,2-c:2′,1′-f]pyrimidin-3,5-dione core to which they attached the generic name “xanthoglows”. These compounds are characterized by strong structural resemblance to boron dipyrromethenes. More importantly, xanthoglows possess rather large Stokes shifts in addition to high fluorescence quantum yields (Φf). Typically, xanthoglows are prepared by the treatment of dipyrrinones with an excess of N,N′-carbonyldiimidazole (CDI) to assemble the N,N′-carbonyl-bridged dipyrrinone core in nearly quantitative yield. The required dipyrrinones could be conveniently prepared by a base-catalyzed aldol condensation between a 2-formylpyrrole and a 3-pyrrolin-2-one. It is important to stress that despite the high generality, this strategy only allows access to dipyrrinones and xanthoglows that are unsubstituted at their central carbon (5-position). Very recently, our group has developed a novel efficient approach towards a novel class of dipyrrinones utilizing oxidative transformations of BODIPY dyes. Treating the BODIPY derivative with copper(II) salt in the presence of air led to the formation of the 3-oxido dye. The subsequent in situ treatment with aqueous HCl produced 5-aryldipyrrinone. The resulting novel 5-aryldipyrrinone could be easily carbonylated using CDI in the presence of DBU. In this way, a new fluorescent xanthoglow derivative bearing an aryl group at 5-position could be made in a good yield of 80%. Performing a focused study on the oxidative transformation of BODIPYs and on the subsequent functionalization of the resulting xanthoglow derivatives will lead to production of a series of novel interesting fluorescent probes and sensors. Furthermore, we believe that the discovered methodologies could be useful for the fields of molecular rotors and organic light-emitting devices.