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S₀, S₁, and T₁ 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 O₂(¹Δ_g) generation, were studied by multireference quantum chemical approaches. S₀→S₁ 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, S₁ 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 S₁, strongly correlated with facile ¹O₂ production, was presumed to occur via S₁→T₁ intersystem crossing (ISC) of the sensitizer. Further confirmation of this relation was provided by newly …