LAUSR

Abstract We report the nonlinear optical absorption and optical limiting studies of 5,15-di(pyridyl)-10,20- bis(pentafluorophenyl)porphyrin and its metal complexes with Cu(II) and Zn(II) in chlorobenzene under nanosecond regime. All the samples were found to exhibit significant reverse saturable absorption behaviour and interestingly, the freebase sample showed the strongest response, yielding a larger value for the nonlinear absorption coefficient than the metalated compounds. Detailed data analysis pointed out the possibility of unintended protonation occurring in the freebase sample when subjected to laser irradiation, which was later confirmed by means of a number of spectroscopical studies. This photo-induced protonation was identified as the primary reason behind the enhanced nonlinear optical (NLO) response of the freebase compound which we tried to explain from the perspective of structural variations occurred during protonation. Optical limiting efficiency was also found to be quite high for the protonated porphyrin sample with a limiting threshold of 1.27 J/cm2. Although protonation process heightens the NLO activities in the freebase porphyrin, extreme care must be taken when using protic solvents for the NLO study of porphyrins in solution-phase.