Exploitation of the Z-scan technique as a method to optically probe pK(a) in organic materials: application to porphyrin derivatives

We demonstrate the use of the Z-scan technique as a method to measure a che mical parameter, the pK(a) value, that characterizes the equilibrium consta nt in acid-base reactions. The measurements were performed with picosecond pulses at 532 nm in Fe(III)-meso-tetrakis(4-N-methyl-pyridyl) porphyrin. Th e results were compared with linear absorption measurements and electron pa ramagnetic resonance, the usual techniques employed to determine pK(a) valu es in aqueous solutions, and showed excellent agreement. (C) 2000 Optical S ociety of America. OCIS codes: 190.3270, 190.4400, 160.4670, 160.4760.