The smallest and one of the brightest. Efficient preparation and optical description of the parent borondipyrromethene system.

Fully unsubstituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) 1 was prepared. Reaction of 8-thiomethylbodipy 2 with triethylsilane in the presence of a catalytic amount of Pd and a stoichiometric amount of copper(I) thienyl-2-carboxylate (CuTC) in THF at 55 degrees C gave compound 1 in nearly quantitative yield. This compound displays high quantum yields (up to 93%) in polar solvents including water. Its optical properties and crystal structure are discussed.