A general framework for non-exponential delayed fluorescence and phosphorescence decay analysis, illustrated on Protoporphyrin IX

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 (pO₂), as its delayed fluorescence (DF) is quenched by oxygen, creating a direct relationship between the DF lifetime and the pO₂. A limitation of both techniques is the ignorance of the PpIX concentration in tissues when the pO₂ is measured or during PDT. 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. 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 Hε[PpIX] < 1 μJ. cm^-3, H:excitation radiant exposure per pulse, ε: molar extinction coefficient at excitation wavelength) while P-type (Triplet Triplet Annihilation) DF took place at higher excited states concentrations (Hε[PpIX] > 10 μJ. 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.

Understanding delayed fluorescence and triplet decays of Protoporphyrin IX under hypoxic conditions

Photosensitizers of singlet oxygen exhibit three main types of reverse intersystem-crossing (RISC): thermally activated, triplet-triplet annihilation, and singlet oxygen feedback. RISC can be followed by delayed fluorescence (DF) emission, which can provide important information about the excited state dynamics in the studied system. An excellent model example is a widely used clinical photosensitizer Protoporphyrin IX, which manifests all three mentioned types of RISC and DF. Here, we estimated rate constants of individual RISC and DF processes in Protoporphyrin IX in dimethylformamide, and we showed how these affect triplet decays and DF signals under diverse experimental conditions, such as a varying oxygen concentration or excitation intensity. This provided a basis for a general discussion on guidelines for a more precise analysis of long-lived signals. Furthermore, it has been found that PpIX photoproducts and potential transient excited complexes introduce a new overlapping delayed luminescence spectral band with a distinct lifetime. These findings are important for design of more accurate biological oxygen sensors and assays based on DF and triplet lifetime.