Effects of the structure of entrapped substituted porphyrins on the textural characteristics of silica networks

Abstract

The outstanding photoluminescent properties of porphyrin molecules in liquid solution were extended to solid media when these species were chemically trapped inside translucent, monolithic SiO 2 pore networks. The entrapment was made in situ through the sol–gel technique by adding to the relevant macrocycle solution a functionalized silicon alkoxide, water, HCl, and (if required) a chemical agent that inhibited the aggregation or degradation of the porphyrin compound. The success in attaining solid materials that encapsulate and immobilize individual porphyrin molecules was not only based on the identity but also on the selection of the appropriate molar concentrations of the above reactants. It was found that the cavity sizes in which porphyrin molecules were encapsulated depended on the structure of the macrocyclic species as well as on the type of alkoxide used to structuralize the silica network. In principle, the sizes of the cavities containing the porphyrin molecules could be modulated in four possible ways: (i) by the physical insertion of the macrocycle species in the pore network; (ii) by linking porphyrin molecules endowed with specific end groups of given lengths to the pore walls; (iii) by using functionalized silicon alkoxides that create linking chains extending from the pore walls toward the macrocycle species (possibility not treated here); (iv) by affording the porphyrin and silicon alkoxide molecules with suitable functional groups that established a long covalent linking between the macrocycle and the pore walls. In these ways, the average cavity width usually varied from 2.0 to 3. 6 nm but this interval could even be extended from 3.6 to 9.4 nm. The whole picture of porphyrin encapsulation in porous media involved: (i) the creation and modulation of assorted cavity sizes, (ii) the modification of the polarity of the pore surface by the attachment of functionalizing molecules, and (iii) the permanence of the luminescent properties of porphyrin molecules when these are covalently encapsulated inside silica substrates.