Heavy atom free singlet oxygen generation: doubly substituted configurations dominate S1 states of Bis-BODIPYs
Author
Duman, S.
Cakmak, Y.
Kolemen, S.
Akkaya, E. U.
Dede, Y.
Date
2012Source Title
Journal of Organic Chemistry
Print ISSN
0022-3263
Electronic ISSN
1520-6904
Publisher
American Chemical Society
Volume
77
Issue
10
Pages
4516 - 4527
Language
English
Type
ArticleItem Usage Stats
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Abstract
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(1
Δg) 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 bisBODIPYs 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 1
O2 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 1
O2
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 bisBODIPYs with DS S1 states, modifications in the chromophore cores are shown to be promising in fine-tuning the excitation
characteristics.