Optical spectroscopic characteristics and TD-DFT calculations of new pyrrolo[1,2-b]pyridazine derivatives

Five new pyrrolo[1,2-b]pyridazine derivatives: 5,6-dicarbomethoxy-2,7-dimethylpyrrolo[1,2-b]pyridazine (I), 5,6-dicarbomethoxy-7-methyl-2-phenylpyrrolo[1,2-b]pyridazine (II), 5,6-dicarbomethoxy-7-ethyl-2-phenylpyrrolo[1,2-b]pyridazine (III), 5-carboethoxy-7-methyl-2-phenylpyrrolo[1,2-b]pyridazine (IV) and 7-ethyl-2-phenylpyrrolo[1,2-b]pyridazine (V), with aryl or methyl substituents in pyridazinic ring and with alkyl, aryl and/or carbometoxy in the pyrrolic ring have been investigated by spectroscopic methods and electronic structure calculations. UV–vis absorption together with steady-state and time-resolved fluorescence measurements have been conducted in cyclohexane, n-hexane (as inert solvents), and in solid state to evidence comparatively the effect of the structure of the compounds on the absorption and fluorescence properties. All five compounds have an intense fluorescence with high quantum yields. The intense fluorescence is evidenced also in solid state. The electronic structure calculations have been performed in the framework of density functional theory (DFT) and time dependent DFT (TD-DFT) in order to elucidate the differences observed in absorption spectra as an effect of the substituents.