Tuning the Performance of Metalloporphyrin-Based Pressure-Sensitive Paints through the Nature of the Central Metal Ion

Since the 1980s, pressure-sensitive paint (PSP) has been used as an optical pressure sensor for measuring surface pressure on aircraft models in wind tunnels. Typically, PSPs have utilized platinum(II)-5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)-porphyrin due to its high pressure sensitivity, phosphorescence lifetime of ∼50 μs, reasonable quantum yield of emission, and resistance to photo-oxidation. This work investigates the photophysics and electronic structure of metal complexes of 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)-porphyrin, namely, Zn(II), Pd(II), and Ir(III), as potentially improved luminophores for polymer-based PSPs. The metal ion was found to preferentially stabilize the a_2u MO of the porphyrin with increasing electronegativity, thus blue-shifting absorption/emission maxima and increasing Q-band intensity. The lifetime and quantum yield of emission increased and decreased, respectively, in the order of Pt(II) to Ir(III) to Pd(II), primarily due to the heavy atom effect. The increase in phosphorescence lifetimes resulted in the pressure sensitivity of the PSPs increasing in the order of Pt(II) to Ir(III) to Pd(II). However, the temperature sensitivity at pressures >70 kPa also increased with increasing phosphorescence lifetime. Overall, this work identified that the central metal ion of porphyrin luminophores can be used to tailor the resulting lifetime of the luminophore and therefore heavily influences the pressure and temperature sensitivity of polymer PSP formulations. This new insight into luminophore design can be used to optimize PSPs for a desired application.