Effects of excitation power density on the Stern–Volmer constant measurement

The Stern-Volmer constant (KSV) is an important parameter to describe the capability of energy transfer to oxygen for porphyrin and its derivatives. By fitting Stern-Volmer equation, IP0/IP = 1 + KSV[O2], the KSV is generally determined through phosphorescence intensities (IP) under aerobic and oxygen-free conditions. In this work, the effect of excitation power density on the KSV measurement is theoretically analyzed and experimentally studied, using palladium octaethylporphyrin (PdOEP) as an example. The IP of PdOEP increased nonlinearly with excitation power density, and the power dependent slope of IP0/IP could be obtained. By way of the functional relationship between the slope of IP0/IP and power density, the real KSV of PdOEP was fitted to be 58 ± 2 kPa-1. The oxygen-dependent phosphorescence lifetimes (τP) and IP under a weak excitation power are also measured to calculate the real KSV, which verifies our analysis.