Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP] +· reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP] +· formed by charge exchange with M +, which is also present in the spectrum. Negative ion spectra are dominated by [TPP] −· formed by charge exchange with M +, which is also present in the spectrum. Negative ion spectra are dominated by [TPP] −·, which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP] +· with M and by a second pathway under plasma ignition conditions through reaction of TPP with M +. In negative ion mode, metallation is only observed under plasma ignition conditions through reaction, of [TPP] −· with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.