One-pot efficient synthesis of dimeric, trimeric, and tetrameric BODIPY dyes for panchromatic absorptionw

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes have received an upsurge of interest because they possess a variety of distinctive and desirable properties such as high absorption coefficients, narrow absorption and emission bands, good stability, excellent emission quantum yields, and have promising applications in supramolecular fluorescent gels, solar cells, sensing and imaging. A series of BODIPY-based oligomers, polymers and dendrimers have been reported. These oligomeric and polymeric BODIPY dyes display significant red shifts in absorption and emission, and possess broad absorption bands compared with their monomeric forms. Polymeric BODIPY dyes have been shown to have promising applications in solar cells. However, in most previous reports, multistep organic reactions were required, employing several protecting groups and reactive intermediates, in order to prepare oligomeric and polymeric BODIPY dyes. These tedious procedures limit practical use of these new BODIPY dyes. It is highly desirable to develop a facile, efficient and cost-effective method to prepare oligomeric BODIPY dyes for potential solar cell and other applications. In this communication, we report efficient synthesis of dimeric, trimeric and tetrameric BODIPY dyes with a formyl capping end group through a one-pot Knoevenagel self-condensation reaction of b-formyl BODIPY dye (Scheme 1). The formyl capping end group of these new BODIPY dyes can be further functionalized with a variety of groups, e.g. a cyanoacrylic acid electron acceptor group, through the Knoevenagel reaction. These BODIPY dyes display panchromatic absorption from visible to partial near-infrared regions. A variety of aryl groups can be introduced to BODIPY monomers at the meso-position to enhance solubility of dimeric, trimeric and tetrameric BODIPY dyes in organic solvents (Scheme 2). We employed tri(ethylene glycol)methyl ether side chains instead of n-alkyl chains to facilitate separation of dimeric, trimeric and tetrameric compounds. Meso-arylsubstituted BODIPY dye (5) was prepared by reacting the formyl benzene derivatives (3) with an excess of 2,4-dimethylpyrrole under acid catalysis, followed by oxidation with 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and treatment with BF3-etherate in the presence of N,N-diisopropylethylamine (DIEA) (Scheme 2). b-Formyl BODIPY dye (6) was prepared from BODIPY dye 5 via the Vilsmeier–Haack reaction. Self-condensation of b-formyl BODIPY dye 6 for 3 h afforded dimeric, trimeric and tetrameric BODIPY dyes (Scheme 2). Prolonged reaction times beyond 3 h resulted in formation of oligomeric and polymeric BODIPY dyes with higher molecular weights. Chemical structures of the new monomeric, dimeric, trimeric and tetrameric BODIPY dyes were confirmed by NMR spectra, and high-resolution mass spectroscopy.z The H NMR spectrum of monomeric BODIPY dye shows four different singlet peaks at 2.79, 2.56, 1.72 and 1.50 ppm corresponding to four different methyl protons at 5,3,7,1-positions, respectively, and one single peak at 6.13 ppm corresponding to the proton at 2-position (Fig. 1). After the one-pot Knoevenagel self-condensation reaction of monomeric BODIPY dye, the singlet peak corresponding to the proton at the 2-position of the dimeric BODIPY dye shifts downfield from