Fluorescence optimization of chlorophyll covalently bonded to mesoporous silica synthesized by the sol–gel method

• New experimental methodology is set for bonding tetrapyrroles to natural analogs.
• Chlorophyll covalent bonding to SBA-15 or organo-modified silica is achieved.
• Chlorophyll properties depend on the organic chain bonding it to a pore network.
• Stable and fluorescent chlorophyll can be trapped within pore systems.

Both chlorophyll and the heme group of hemoglobin possess a central tetrapyrrolic structure formed by four pyrroles bonded to methine groups. It is important to stress that natural and synthetic tetrapyrroles involve interesting properties; and sometimes, in order to deploy them, it is convenient to trap these macrocycles within pore networks. Due to their nature, these molecules cannot be inserted by diffusion inside pores and the classical impregnation method can just render lowly concentrated and heterogeneous materials. To overcome this difficulty our research group has been working in the development of sol–gel methodologies for effectively inserting synthetic tetrapyrrolic species inside inorganic pore networks. In this way, it has been found that the average pore size and specific surface area of the trapping substrate depend on the cation identity and structure of the macrocyclic complex. Yet, the interactions of the captured species with the M-OH surface groups resting at the pore walls affect the efficient display of the properties of these species. The physicochemical properties of the captured macrocycle remain similar to those observed in solution by situating the species far from the M-OH groups or substituting these by alkyl or aryl groups. Here, we present the first results of the application of the above methodology for the covalent bonding of chlorophyll a in stable and monomeric form inside translucent organo modified silica xerogels and mesoporous SBA-15 substrates. In these systems, chlorophyll remains fixed to the silica pore surface while its UV absorption and fluorescence spectra remain similar to those displayed in solution. The optimization of the synthesis of materials based on the trapping of natural chlorophyll could be a crucial step in the development of novel photoreactive, optical, catalytic, sensoring, and medical devices.

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