Synthesis, Characterization, Photoinduced Isomerization, and Spectroscopic Properties of Vinyl-1,8-naphthyridine Derivatives and Their Copper(I) Complexes

A series of 1,8-naphthyridine derivatives containing vinyl, 2-(2-acetylamino-pyridine-6-ethylene)-4-methyl-7-acetylamino-1,8-naphthyridine (L¹), 2-(2-acetylamino-pyridine-6-ethylene)-1,8-naphthyridine (L²), 2-(2-acetylamino-pyridinyl-6-ethylene)-4-methyl-7-hydroxyl-1,8-naphthyridine (L³), 2-(2-diacetylamino-pyridinyl-3-ethylene)-7-diacetylamino-1,8-naphthyridine (L⁴), and 7-(2-diacetylamino-pyridinyl-3-ethylene)-4′-acetyl-pyrrolo[1′,5′-a]-1,8-naphthyridine (L⁵), as well as complexes [CuL¹(PCy₃)](BF₄)₂ (1) (PCy₃ = tricyclohexylphosphine), [Cu₂L¹(PPh₃)₄](BF₄)₂ (2) (PPh₃ = triphenylphosphine), [Cu₂L¹(dppm)](BF₄)₂ (3) (dppm = bis(diphenylphosphino)methane), and [Cu₂(L¹)(dcpm)][BF₄]₂ (4) (dcpm = bis(dicyclohexylphosphino)methane, were synthesized. All these compounds, except for L¹ and L², were characterized by single crystal X-ray diffraction analysis, and a comprehensive study of their spectroscopic properties involving experimental theoretical studies is presented. We found an intramolecular 1,3-hydrogen transfer during the formation of L³ and L⁴, which in the case of the latter plays an important role in the 1,5-dipolar cyclization of L⁵. The spectral changes that originate from an intramolecular charge transfer (ICT) in the form of a π_py→π*_napy transition can be tuned through acid/base-controlled switching for L¹−L³. A photoinduced isomerization for L¹−L³, 1, and 2 having flexible structures was observed under 365 nm light irradiation. Quantum chemical calculations revealed that the dinuclear complexes with structural asymmetry exhibit different metal-to-ligand charge-transfer transitions.