Tris(3-hydroxypropyl)methyl as a stable linker for porphyrin monolayer on silicate glass

To develop a stable organic-inorganic interface, we investigated kinetics of acid hydrolysis of monolayers of various porphyrins prepared by the condensation reaction of hydroxyporphyrin with silanol groups on the surface of silicate glass. The half-lives of the monolayers attached through a 5-hydroxypentyl linker, a tris(hydroxymethyl)methyl linker, and a tris(3-hydroxypropyl)methyl linker in 1 M HCl at 50 °C were 182 min, 27 min and 460 min, respectively, indicating that tris(3-hydroxypropyl)methyl appended porphyrin formed a stable monolayer. To understand the stability of the monolayers, we performed molecular modeling studies. Molecular orbital calculations indicated that the flexible trimethylene spacer facilitates multiple bonding of porphyrin to silicate surface, while the tris(hydroxymethyl)methyl linker was rigid and only one OH group can form silyl ester linkage. The remaining OH groups may function as a catalytic group in acid hydrolysis. Attachment of long alkyl chains to porphyrin also improved the hydrolytic stability of the monolayer, where the hydrolysis rate constant was ten times smaller than that of porphyrin without long alkyl groups.