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 πpy→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.