Oxygen-Varying Correlated Fluorescence for Determining the Stern-Volmer Constant of Porphyrin

The achievement of the Stern-Volmer constant (K_SV) of porphyrin, a parameter to describe the ability of energy transfer to oxygen, enables us to understand and engineer the functionalities of porphyrin. In this study, we propose a new method to obtain K_SV by steady fluorescence based on the population correlation between the excited triplet state (T₁) and the singlet excited state (S₁). Nonlinear fluorescence intensity of porphyrin [hematoporphyrin monomethyl ether (HMME)] as a function of pump power is observed, and this nonlinearity is oxygen-concentration-dependent. These observations originated from the population correlation, where more intense pump power introduces a stronger population correlation. Most importantly, we have found that the oxygen content can influence this correlation via the quenching of T₁ based on its sequent change of fluorescence intensity; K_SV ∼ 28 kPa^-1 can be obtained theoretically for HMME. Our investigation not only offers a promising method of achieving K_SV conveniently by steady fluorescence but also enlightens the advances of regulable fluorescence correlation.