Identification of excimer delayed fluorescence by Protoporphyrin IX: A novel access to local chromophore concentration?

Protoporphyrin IX (PpIX) is a molecule produced in the mitochondria following the administration of its approved precursor, aminolevulinic acid (ALA). Strong light absorber at different wavelengths in the visible range, PpIX is extensively used as a photosensitizer (PS) for Photodynamic Therapy (PDT). PpIX is also an ideal molecular probe for the quantification of the tissue oxygen partial pressure (pO(2)), as its delayed fluorescence (DF) is quenched by oxygen, creating a direct relationship between the DF lifetime and the pO(2). A limitation of both techniques is the ignorance of the PpIX concentration in tissues when the pO(2) is measured or during PDT.& nbsp;In this study, the prompt (PF) and delayed fluorescence of PpIX dissolved in DiMethylFormamide (DMF) were acquired, in absence of oxygen, at different PpIX concentrations.& nbsp;Measurements of the PpIX emission for different excitation energies and temperatures, as well as spectral considerations led to the conclusion that E-type (thermal) DF was the dominant DF mechanism at low PpIX excited states concentrations (density of absorbed energy H epsilon[PpIX] < 1 mu J. cm(-3), H:excitation radiant exposure per pulse, epsilon: molar extinction coefficient at excitation wavelength) while P-type (Triplet Triplet Annihilation) DF took place at higher excited states concentrations (H epsilon[PpIX] > 10 mu J. cm(-3)). The gradual development of a strong, red-shifted structureless DF peak at 670 nm, invisible in the PF and absorption spectra, strongly points towards the first observation of PpIX excimer DF (EDF). It appears that, similarly to other aromatic molecules, PpIX excimers can be formed either by the encounter of two molecules in the first excited triplet state T1, or by the reaction of an excited singlet S1 with a triplet T1. Excimer DF could be beneficially used to determine the local concentration of PpIX, as the initial DF intensity ratio I0670/I0630 is linearly correlated with the local PpIX concentration, and thus rises up to the challenge of PpIX based pO2 measurement and PDT. This work could also pave the way for a fine comprehension of the production, diffusion and catabolization of PpIX in biological tissues.