Characterization of Bodipy Dimers Formed in a Molecularly Confined Environment

Recently, Johansson and co-workers provided the first direct evidence for the existence of nonfluorescent bodipy (4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene) H dimers in double-labeled proteins and fluorescent J dimers in labeled lipid vesicles (Bergstrom, F.; et al. J. Am. Chem. Soc. 2002, 124, 196), allowing for the calculation of many of the properties of the dimers. Herein, we report on the use of molecular confinement within a sodium silicate derived glass to provide a highly reproducible system wherein nonfluorescent bodipy H dimers can be formed from the free probe essentially quantitatively without any interference from higher-order aggregates or fluorescent J dimers. The formation of the H dimer followed an unexpected first order kinetic process. On the basis of analysis of the fluorescence anisotropy of the entrapped monomer, it was concluded that the H-dimer formation was promoted by adsorption of monomers onto the silica surface (rate limiting step), followed by rapid dimerization. Using exciton coupling theory, it was determined that the H dimer consisted of two strongly coupled monomers that were stacked in a parallel orientation with a distance of 7.6 Å between the monomer units. The transition dipole moment of the monomer was determined to be 26.6 × 10-30 C m (8.1 D), the emission quantum yield of the H dimer was found to be close to zero, and the Förster distance for energy transfer between the monomer and H dimer was calculated to be 56 ± 2 Å. All of these values are in excellent agreement with those determined by Johansson et al.