FULL PAPER DOI: 10.1002/ejoc.200901460 Synthesis and Photophysical Properties of 3,5-Bis(oxopyridinyl)- and 3,5-Bis(pyridinyloxy)-Substituted Boron-Dipyrromethenes Tamanna K. Khan, [a] M. Rajeswara Rao, [a] and M. Ravikanth* [a] Keywords: Boron / Nitrogen heterocycles / Dyes/pigments / Fluorescence / Fluorescent probes / Photophysics / Nucleophilic substitution Nucleophilic substitution reactions of 2-, 3- and 4-hydroxy- pyridines with 3,5-dibromo meso-aryl and meso-furyl boron- dipyrromethenes (BODIPYs) resulted in the formation of the corresponding 3,5-bis(oxopyridinyl)-BODIPYs and 3,5-bis- (pyridinyloxy)-BODIPYs in decent yields. The effect of a pyr- idone versus an oxypyridine at the 3- and 5-positions on the spectral, electrochemical and photophysical properties were studied as a function of solvent. The 3,5-bis(oxopyridinyl)- Introduction 2-, 3- and 4-Hydroxypyridines exhibit keto-enol tautomeriz- ation between the pyridone and hydroxypyridine form and exhibit interesting chemistry due to the presence of two, different, reactive, nucleophilic centres. [1] The hydroxypyr- idines would undergo nucleophilic substitution reactions by reacting either through the phenolic oxygen or the pyridine nitrogen, depending on the substrate and reaction condi- Scheme 1. Structures of 1–6. [a] Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India E-mail: ravikanth@chem.iitb.ac.in Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.200901460. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2010, 2314–2323 2314 BODIPYs exhibit broad, red-shifted absorption and emission bands, decreased quantum yields and lifetimes, displayed large Stokes shifts and easier reductions than did the 3,5- bis(pyridinyloxy)-BODIPYs. The differences in the properties of these two classes of BODIPY dyes are attributed to the extension of π-delocalization associated with the electron-de- ficient nature of the pyridone groups. tions. Boron-dipyrromethene (BODIPY) dyes are interest- ing fluorescent dyes with valuable properties such as high chemical and photostability and relatively high absorption coefficients and fluorescence quantum yields. [2] Further- more, BODIPY dyes can be optically excited with visible light, and their absorption and emission properties can be fine tuned by introducing appropriate substituents onto the BODIPY framework. [3] The substituents can be introduced onto any position of the BODIPY framework, although the electronic properties of BODIPYs are more significantly al- tered upon introducing substituents directly at the pyrrole C atoms than they are by adding substituents onto the meso-aryl group. The meso-aryl group and the BODIPY chromophore interact weakly since the two moieties are al-