Cobalt porphyrin covalently bonded to organo modified silica xerogels

Abstract

Some of the fine physicochemical properties displayed by porphyrin solutions can be preserved when these species are trapped within inorganic oxide pore networks, such as silica. A successful outcome is related to inhibiting the interaction between the macrocycle and SiOH surface groups. Here, when porphyrins are chemically bonded to the walls of a silica network through molecular bridges arising from functionalized alkoxides alone or combined with monomer precursors (i.e. lactams, diamines, etc.), their spectroscopic properties can be kept similar to those shown in solution. This latest outcome consists in bonding porphyrinic species to the pore walls of an organo-modified silica networks. In the present work, a strategy has been designed to uphold these useful properties by covalently bonding cobalt porphyrin molecules inside a SiO2 pore network where SiOH surface groups are exchanged by alkyl groups proceeding from organo-substituted alkoxides. In these hybrid systems, the electronic transitions are well preserved when SiOH surface groups are exchanged by groups such as methyl, ethyl, vinyl, etc. This separation action prevents flocculation among macrocyclic molecules (even if a change in polarity occurs inside the pores) due to the adsorption of bridging alkyl groups. A proper selection of both macrocyclic molecules and bridging molecules, in conjunction with a proper choice of the synthesis conditions, lead to the attainment of hybrid solid systems in which the spectroscopic properties are not only preserved but can even be adjusted by selecting suitable sizes and identities of the bridging alkyl groups.