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-s-indacene (BODIPY) dyes, recently (Angew. Chem., Int. Ed. 2011, 50, 11937) proposed as heavy atom free photosensitizers for O2(1Δg) 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.