General procedure for the synthesis of 8-hydroxyquinoline-2-carbaldehyde from 8-hydroxyquinaldine: Preparation of 3,3'-(1E,1'E)-(propane-1,3-diylbis(aza-1-yl-1-ylidene))bis(methyl-1-ylidene-1-ylidene)diquinolin-8-ol (H2PBIQ). A mixture of 8-hydroxy-2-methylquinoline (4 g, 25 mmol), selenium dioxide (3.5 g, 31 mmol), water (3 mL), and 1,4-diethylene glycol (300 mL) was stirred for 24 h at 80 °C under nitrogen protection. After completion of the reaction, the mixture was cooled, filtered and the filtrate evaporated. The resulting solid was purified by silica gel column chromatography (unfolding solvent was petroleum ether: ethyl acetate, 95:5, v/v) to afford pure yellow needle-like crystals of 8-hydroxyquinoline-2-carbaldehyde. The yield was 90%; 1H NMR (400 MHz, CDCl3): δ10.25 (s, 1H), 8.35 (d, J=8.5 Hz, 1H), 8.18 (s, 1H), 8.09 (d, J=8.5 Hz, 1H), 7.66 (t, J=8.0 Hz, 1H), 7.46 (d, J=8.2 Hz, 1H), and 7.31 (d, J=7.7Hz, 1H).
