A ground-state dimer (denoted D-I) exhibiting a strong absorption maximum at 477 nm (epsilon = 97 000 M(-1)cm(-1)) can form between adjacent BODIPY groups attached to mutant forms of the protein, plasminogen activator inhibitor type 1 (PAI-1). No fluorescence from excited D-i was detected. A locally high concentration of BODIPY groups was also achieved by doping lipid phases (micelles, vesicles) with BODIPY-labeled lipids. In addition to an absorption band located at about 480 nm, a new weak absorption band is also observed at ca. 570 nm. Both bands are ascribed to the formation of BODIPY dinners of different conformation (D-I and D-II), Contrary to D-I in PAI-1, the D-II aggregates absorbing at 570 nm are emitting light observed as a broad band centered at about 630 nm. The integrated absorption band of D-I is about twice that of the monomer, which is compatible with exciton coupling within a dimer. The Forster radius of electronic energy transfer between a BODIPY excited monomer and the ground-state climer (D-I) is 57+/-2 Angstrom. A simple model of exciton coupling suggests that in D-I two BODIPY groups are stacked on top of each other in a sandwich-like configuration with parallel electronic transition dipoles, For D-II the model suggests that the S-0 --> S-1 transition dipoles are collinear. An explanation for the previously reported (J. Am. Chem. Soc, 1994, 116, 7801) exceptional light spectroscopic properties of BODIPY is also presented. These are ascribed to the extraordinary electric properties of the BODIPY chromophore. First, changes of the permanent electric dipole moment (Deltamu approximate to -0.05 D) and polarizability (-26 x 10(-40) C m(2) V-1) between the ground and the first excited states are small. Second, the S-0 <----> S-1 electronic transition dipole moments are perpendicular to Delta(μ) over right arrow.