In vitro determination of the non radiative relaxation time of triplet state in protoporphyrin IX

In this work we used Photoacoustic Spectroscopy (PAS) to determine in vitro the non radiative relaxation time (NRRT) of protoporphyrin IX (PpIX) triplet state. In order to obtain this characteristic time, we have used PpIX disodium salt (DS) solutions of 25 % HCl. From each solution it was obtained its optical absorption spectrum, by using PAS, ranging from 300 to 800 nm. In the maximum absorption peak, at 410nm, we obtain the photoacoustic (PA) signal, amplitude and phase, as a function of the light modulation frequency, from 17 to 80 Hz. From the PA signal phase it was possible to determine, for each solution, the NRRT by using the Rosencwaig-Gesho theory, modified to include the effect of a finite nonradiative deexitation time. A NRRT average value, obtained from these solutions, is reported and compared with some NRRT of triplet states reported in the literature for molecules with the same tetrapyrrolic structure.