Protonation effect on the nonlinear absorption, nonlinear refraction and optical limiting properties of tetraphenylporphyrin

Nonlinear optical properties of tetraphenylporphyrin(H_2TPP) and protonated tetraphenylporphyrin(H_4TPP^(2+)) in toluene were investigated by Z-scan technique using a nanosecond laser with 5 ns pulse at 532 nm.Results show that H_4TPP^(2+) exhibits weaker nonlinear refraction but enhanced reverse saturable absorption(RSA) and optical limiting performance in comparison with pristine H_2TPP.Since no nonlinear scattering is observed in H_4TPP^(2+) under low input fluence, and H_4TPP^(2+) exhibits weaker nonlinear scattering signals than H_2TPP under high input fluence, the enhancement of RSA and optical limiting performance can be attributed to the larger ratio of excited state absorption cross-section to that of the ground state of H_4TPP^(2+).H_4TPP^(2+) also exhibits superior optical limiting performance, even better than the benchmark RSA material C_(60).