A series of 11 different boron−dipyrromethene (BODIPY) dimers is carefully examined by means of ab initio and
Tamm−Dancoff approximated density functional theory methods. Vertical and 0−0 excitation energies along with the
tetraradical character of these dimers are determined. Possible application of a series of linked dimers for photodynamic therapy (PDT) was investigated through computing their excitation energies, spin−orbit coupling matrix elements, and singlet−triplet energy gaps. Finally through a systematic investigation of a series of 36 different BODIPY and aza-BODIPY dimers, a new class of near-IR heavy atom free photosensitizers for PDT action is introduced.