Selective adsorption of biladien-ab-one and zinc biladien-ab-one to mesoporous silica

The adsorption of linear tetrapyrroles and porphyrins to mesoporous silicas with pore diameters of 2.3, 2.7, 4.0, and 7.0 nm from a benzene solution was investigated. As linear tetrapyrroles, 1,15,21,22-tetrahydro-19-benzoyl-5,10,15-triphenyl-15-hydroxybilin-1-one (biladien-ab-one, 1a), its zinc complexes (2 and 3), 1,15,21,22-tetrahydro-19-(4-hexyloxybenzoyl)-5,10,15-tri(4-hexyloxy)phenyl-15-hydroxybilin-1-one (C6-biladienone, 1b), 1,15,21,22-tetrahydro-19-(4-nonyloxybenzoyl)-5,10,15-tri(4-nonyloxy)phenyl-15-hydroxybilin-1-one (C9-biladienone, 1c), 1,15, 21,22-tetrahydro-19-(4-dodecyloxybenzoyl)-5,10,15-tri(4-dodecyloxy)phenyl-15-hydroxybilin-1-one (C12-biladienone, 1d), and 1,15,21,22-tetrahydro-19-(3,5-didodecyloxybenzoyl)-5,10,15-tri(3, 5-di-dodecyloxy)phenyl-15-hydroxybilin-1-one (1e) were employed. As porphyrins, 5,10,15,20-tetraphenylporphyrin (4a) and 5,10,15,20-tetra(4-methoxyphenyl)porphyrin (4b) were used. The isothermal adsorption curves were determined, showing that the amount of adsorption decreases in the order, 1a > 1b > 1c > 1d > 2, while neither 1e, 4a, nor 4b were adsorbed to mesoporous silicas. Mesoporous silica with the diameter of 4.0 nm adsorbed the largest amounts of biladienones. The adsorption was not observed in THF, acetone or ethanol. Neither 1c nor 1d was adsorbed to mesoporous silica with the pore diameter of 2.3 nm. These results indicate that the absorption is driven by polar interactions such as hydrogen bonding, and size exclusion effects are observed even for a molecule with flexible alkyl chains. The amount of adsorbed biladienone increased as the temperature was raised from 25 to 35 °C. The adsorption of biladien-ab-ones at 80 °C caused dehydration of biladien-ab-one to bilatrien-abc-one due to the acidity of the mesoporous silica.