Heavy Atom Free Singlet Oxygen Generation: Doubly Substituted Configurations Dominate S₁ States of Bis-BODIPYs

S₀, S₁, and T₁ states of various orthogonal 8,8′ and 8,2′-bis-boradiaza-<i>s</i>-indacene (BODIPY) dyes, recently (<i>Angew. Chem., Int. Ed.</i> <b>2011</b>, <i>50</i>, 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 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 π-extension, as DS singlet states are destabilized. Decreased exchange coupling in π-extended cases appears to be responsible for this destabilization. Comparisons with iodine incorporated bis-BODIPYs suggest that the dynamics of ¹O₂ generation via DS S₁ 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 bis-BODIPYs with DS S₁ states, modifications in the chromophore cores are shown to be promising in fine-tuning the excitation characteristics