S-0, S-1, and T-1 states of various orthogonal 8,8' and 8,2'-bis-boradiaza-s-indacene (BODIPY) dyes, recently (Angew. Chem, Int. Ed. 2011, 50, 11937) proposed as heavy atom free photosensitizers for O-2((1)Delta(g)) generation, were studied by multireference quantum chemical approaches. S-0 -> S-1 excitation characteristics of certain bis-BODIPYs are shown to be drastically different than the parent BODIPY chromophore. Whereas a simple HOMO -> LUMO-type single substitution perfectly accounts for the BODIPY core, S-1 states of certain orthogonal bis-BODIPYs are described as linear combinations of doubly substituted (DS) configurations which overall yield four electrons in four singly occupied orbitals. Computed DS character of S-1, strongly correlated with facile O-1(2) production, was presumed to occur via S-1 -> T-1 intersystem crossing (ISC) of the sensitizer. Further confirmation of this relation was provided by newly synthesized BODIPY derivatives and comparison of spectroscopic properties of their dimers and monomers. Near-IR absorption, desired for potential photodynamic therapy applications, was not pursuable for bis-chromophores by the standard strategy of pi-extension, as DS singlet states are destabilized. Decreased exchange coupling in pi-extended cases appears to be responsible for this destabilization. Comparisons with iodine incorporated bis-BODIPYs suggest that the dynamics of O-1(2) generation via DS S-1 states are qualitatively different from that via ISC originating from heavy atom spin orbit coupling. Although red-shifting the absorption wavelength to enter the therapeutic window does not seem attainable for orthogonal BODIPYs with DS S-1 states, modifications in the chromophore cores are shown to be promising in fine-tuning the excitation characteristics.