Photophysics of mesoporphyrin IX in solution and confined in sol-gel-derived matrices

In this work we investigate both the stationary and transient spectroscopic properties of mesoporphyrin IX when incorporated into porous matrices produced using the sol-gel technique. These matrices were produced using both titanium and silicon containing precursors to form solid glassy monoliths, with the addition (in some samples) of DMF as a drying control chemical additive. Steady state and time-resolved fluorescence measurements were performed on the freshly gelled and finished matrices and comparison with solution studies made. The latter show a dependence of the mesoporphyrin absorption and emission spectra with refractive index and also with the ET(30) solvent scale. Anomalies are encountered with both aprotic and protic solvents, respectively, when using these scales, indicating interaction with nitrogens in the pyrrole ring. The effect of pH is also clear and the presence of the cationic and dicationic forms of the porphyrin observed via steady state and time-resolved measurements, both in solution and with the porphyrin incorporated within the matrix pore structure where an estimate of refractive index and ET(30) value were made. With time the porphyrin was found to aggregate and degrade within the matrix environment, although differences were encountered between the various matrix forms.