Thiocyanation of 3-substituted and 3,5-disubstituted BODIPYs and its application for the synthesis of new fluorescent sensors

Interest in BODIPYs (acronym of boron dipyrromethene) has skyrocketed in recent decades, mainly due to their favourable photophysical properties and the wide range of functionalization methods reported for these organic fluorescent dyes. In this context, a simple and straightforward method for the direct thiocyanation of 1,3,5,7-tetramethy-BODIPYs using ammonium thiocyanate and oxone was recently reported as an alternative for the preparation of thiocyanated and thioalkylated BODIPYs. Herein, we performed the thiocyanation of 3-substituted and 3,5-disubstituted BODIPY dyes, which were synthesized from the nucleophilic substitution of halogenated precursors with morpholine, propanethiol and sodium methoxide. There was a direct relation between the electron-donating character of the substituent and the yields of the thiocyanated BODIPYs, which gives support to a mechanism based on the electrophilic substitution by a thiocyanogen species formed in situ. Spectroscopic and photophysical characterization of these new fluorophores was performed and included bidimensional NMR, UV/vis absorption, fluorescence emission and fluorescence quantum yields. The photophysical properties are highly dependable on the structural features of each dye. While 3-morpholino-8-phenyl BODIPYs are virtually non-fluorescent, the fluorescence quantum yields of 3-(4-methoxybenzylamino)-8-methyl BODIPYs were close to 0.9. The thiocyanation of BODIPYs can result in interesting photophysical shifts that can be explored in the fine-tuning of fluorescent sensors. We also report the results of a preliminary qualitative analysis that indicates interesting bathochromic or hypsochromic shifts on the absorption and fluorescence emission spectra when some of the highly emissive dyes were treated with strong acid. (AU)