Laser active cyanopyrromethene–BF2 complexes

Treatment with a mixture of formic and hydrobromic acids converted ethyl 3,4-diethyl-5-methyl-pyrrole-2-carboxylate 7a to 3,3′,4,4′-tetraethyl-5,5′-dimethylpyrromethene hydrobromide 8a presumably via the condensation of α-unsubstituted and α-formylpyrrole intermediates 7c and 7e formed in situ. The corresponding 6-cyanohexaalkylpyrromethane 9a was obtained by the addition of hydrogen cyanide to the pyrromethene 8a and was oxidized with bromine to an unstable pyrromethene 10a , an intermediate converted to 1,2,6,7-tetraethyl-3,5-dimethyl-8-cyanopyrromethene–BF₂ complex 3 , (PM TEDC), λ_las (plastic) 613–639 nm, in a reaction with boron trifluoride etherate. Ethyl 3,4,5-trimethylpyrrole-2-carboxylate 7b was similarly converted to 1,2,3,5,6,7-hexamethyl-8-cyanopyrromethene–BF₂ complex 4 , (PM HMC), λ_las (plastic) 615–639 nm. Immediately after formation by a condensation between propionyl chloride and 2,4-dimethyl-3-cyanopyrrole 16 , unstable 3,3′,5,5′-tetramethyl-6-ethyl-4,4′-dicyanopyrromethene hydrochloride 17 was treated with boron trifluoride etherate to give 1,3,5,7-tetramethyl -2,6-dicyano-8-ethylpyrromethene–BF₂ complex 18 , λ_las (ethanol) 540–565 nm.