Electronic Communication in Closely Connected BODIPY-Based Bichromophores

A small series of closely spaced, bichromophoric boron dipyrromethene (BODIPY) derivatives has been examined by optical spectroscopy and compared to the corresponding mononuclear dyes. The compounds vary according to the site of attachment and also by the nature of alkyl or aryl substituents incorporated into the dipyrrin backbone. Excitonic coupling splits the lowest-energy absorption transition in each case, but to highly variable degrees. There are also marked changes in the fluorescence quantum yields across the series but much less variation in the excited-state lifetimes. After comparing different models, it is concluded that the ideal dipole approximation gives a crude qualitative representation of the observed splitting of the absorption transition, but the extended dipole approach is not applicable to these systems. Agreement is substantially improved by employing a model that takes into account the dihedral angle between the planes of the two dipyrrin units. The large variation in radiative rate constants, and those for the accompanying nonradiative processes, is accountable in terms of electronic coupling and/or intensity borrowing between the two excitonic states. In all cases, the dihedral angle between the two BODIPY units plays a key role.