Heavy atom free singlet oxygen generation: Doubly substituted configurations dominate S 1 states of bis-BODIPYs

摘要

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( 1Δ 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 1O 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 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 1O 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 bis-BODIPYs with DS S 1 states, modifications in the chromophore cores are shown to be promising in fine-tuning the excitation characteristics. © 2012 American Chemical Society.