Exciton Chirality. (A) Origins of and (B) Applications from Strongly Fluorescent Dipyrrinone Chromophores

Summary.

(A) The origin of exciton interaction and examples of its application to organic stereochemistry are presented. (B) N,N′-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. N,N′-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield (φ_F) ∼0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester of an analog with benzoic acid replacing propionic, N,N′-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (φ_F=0.68, λ_em=493 nm, λ_ex=422 nm in CHCl₃) and UV-Vis absorption (ɛ∼21000 at 424 nm) in organic solvents. Its diester with (1S,2S)-cyclohexanediol is fluorescent and exhibits exciton circular dichroism (Δɛ=+15 dm³·mol⁻¹·cm⁻¹, λ=432 nm; Δɛ=−4 dm³·mol⁻¹ cm⁻¹, λ=380 nm) that correlates with the Exciton Chirality Rule.