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 a2u 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.