Tuning the Performance\nof Metalloporphyrin-Based Pressure-Sensitive\nPaints through the Nature of the Central Metal Ion

Since the 1980s, pressure-sensitive paint (PSP) has been\nused as\nan optical pressure sensor for measuring surface pressure on aircraft\nmodels in wind tunnels. Typically, PSPs have utilized platinum(II)-5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)-porphyrin\ndue to its high pressure sensitivity, phosphorescence lifetime of\n∼50 μs, reasonable quantum yield of emission, and resistance\nto photo-oxidation. This work investigates the photophysics and electronic\nstructure of metal complexes of 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)-porphyrin,\nnamely, Zn(II), Pd(II), and Ir(III), as potentially improved luminophores\nfor polymer-based PSPs. The metal ion was found to preferentially\nstabilize the a<sub>2u</sub> MO of the porphyrin with increasing electronegativity,\nthus blue-shifting absorption/emission maxima and increasing Q-band\nintensity. The lifetime and quantum yield of emission increased and\ndecreased, respectively, in the order of Pt(II) to Ir(III) to Pd(II),\nprimarily due to the heavy atom effect. The increase in phosphorescence\nlifetimes resulted in the pressure sensitivity of the PSPs increasing\nin the order of Pt(II) to Ir(III) to Pd(II). However, the temperature\nsensitivity at pressures >70 kPa also increased with increasing\nphosphorescence\nlifetime. Overall, this work identified that the central metal ion\nof porphyrin luminophores can be used to tailor the resulting lifetime\nof the luminophore and therefore heavily influences the pressure and\ntemperature sensitivity of polymer PSP formulations. This new insight\ninto luminophore design can be used to optimize PSPs for a desired\napplication.