Exploring the Triplet State of Chlorophyll a in Mesoporous Silica Matrices by Time-Resolved EPR

Chlorophyll is a pivotal molecule in photosynthesis due to its ability to absorb solar light and start the photochemical process. The chlorophyll triplet state is easily populated from the light-induced singlet excited state via Intersystem Crossing (ISC) and can react with oxygen generating singlet oxygen, posing a threat to the stability of chlorophyll, but also an opportunity for photodynamic therapy. Here, we focus on the study of the photo-physical properties of chlorophyll a and WSCP (Water-Soluble Chlorophyll Protein, a protein binding the pigment), which have been adsorbed in mesoporous silica matrices. We adopt SBA-15, a silica matrix with well-ordered hexagonal structure with pores of 70 Å of diameter. The triplet formed upon illumination in these systems is studied by time-resolved EPR spectroscopy. Analysis of the EPR spectra shows that the triplet state is very sensitive to the inclusion in the pores of SBA-15 modifying its spin polarization. Chl a in SBA-15 loses its central metal ion, while its structure remains unchanged if the mesoporous silica is previously soaked into a basic solution before Chl a adsorption, as revealed by its zero-field splitting parameters of the triplet state. WSCP is readily included in the silica matrix, but its embedded pigments are no longer protected by the protein core.