Figure 5.

Energy diagram illustrating photophysical processes leading to phosphorescence emission and quenching by oxygen (O₂) and other diffusing quenchers (Q_i). Absorption of a photon by an Oxyphor molecule, based on a Pt or Pd porphyrin,³⁰ is followed by ultrafast S₁→T₁ intersystem crossing (isc) (k_isc=10¹¹-10¹³ s⁻¹), yielding the triplet state (T₁) with nearly unity efficiency. The subsequent T₁→S₀ phosphorescence is characterized by the rate constant k_p, the inverse of the phosphorescence lifetime (1/τ₀). The rate equation (1) can be integrated under quasi-stationary approximation, resulting in a single-exponential decay of the phosphorescence intensity I(t) (2), where the rate constant k is defined by the Stern-Volmer relationship (3).