Boron dipyrromethene dyes: a rational avenue for sensing and light emitting devices

Boron dipyrromethene dyes bearing nitro, amino, isocyanate and isothiocyanate functions were readily prepared under mild conditions. Various combinations allow to produce urea, diurea, thiourea, dithiourea in the 3, 4 and 5-substitution positions of the appended phenyl group. Condensation of the 3,4-substituted diamino derivative with 1,10-phenanthroline-5,6-dione and 6-formyl-2-methylpyridine allow to prepare dipyridophenazine and indole derivatives. The 3,5-dinitro-substituted indacene dye was characterized by an X-ray molecular structure showing a pronounced tilt angle of the dinitrophenyl group relative to the indacene core (approximately 84 degrees) whereas one nitro groups is basically coplanar with the phenyl ring and the second titled by approximately 21 degrees. The optical properties of these dyes reveals on/off switching of the fluorescence from the nitro to the amino compounds and further to the urea likely understood in the framework of an photoinduced electron transfer process.