N21,N22‐carbonyl‐bridged biliverdin. red‐blue color change effected by conformation

An N₂₁,N₂₂‐carbonyl‐bridged mesobiliverdin, prepared in high yield by reaction of the unbridged parent (λ_max 639 nm, ? 15,700, chloroform) with 1,1′‐carbonyldiimidazole and 1,8‐diazabicyclo5.4.0]undec‐7‐ene, gave magenta‐colored solutions in chloroform that absorb strongly in the visible spectrum (λ_max 534 nm, ? 27,700) and shifted to bright blue (λ_max 669 nm, ? 35,300) upon addition of trifluoroacetic acid.