Synthesis and luminescence properties of biphenyl-type firefly luciferin analogs with a new, near-infrared light-emitting bioluminophore

New firefly luciferin analogs of the 4,4′-substituted biphenyl-type were synthesized. One analog with a 4′-dimethylamino group possessed bioluminescence activity, emitting near-infrared biological window light at 675 nm suitable for deep-site bioimaging of living animals. The chemiluminescence light-emission maximum of the corresponding methyl ester of the bioluminescence active analog was 500 nm, implying that biphenyl and thiazolinone rings in the light emitter might be placed in a coplanar conformation at the polar luciferase active site.