Metalation of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin in silver colloids studied via time dependence of surface-enhanced resonance Raman spectra

Abstract

In SERS-active systems, when a free base porphyrin is adsorbed at a roughened metal surface, the metal ion may be incorporated into the porphyrin core. We have investigated the metalation kinetics of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (TMPyP), reflected in the time evolution of its SERRS spectra. To establish proper metalation Raman markers, resonance Raman spectra of the free base and chemically Ag metalated TMPyP forms were measured under different conditions. The increase in intensity of the 395 cm-1 line, the decrease in intensity of the 329 cm-1 line, the reduction of the 1337+1360 cm-1 doublet to one strong line at 1340 cm-1 and the vanishing of the 1140 cm-1 medium line (the last exceptionally for 514.5 nm excitation) were found to be suitable markers for quantitative analysis. All other spectral changes related to metalation may coincide with effects caused by other variations of the porphyrin state. Time evolution of the porphyrin metalation was studied in various Ag colloid systems, with or without phosphate anions, citrate and/or Triton X-100. Time-dependent SERRS spectra of TMPyP were recorded within periods from minutes to several hours. Factor analysis of the SERRS spectra proved that the spectral changes were induced by only one phenomenon (metalation). The SERRS spectra were decomposed into the spectrum of the free base TMPyP and that of its Ag metalated form, and the metalation kinetics were determined by means of the time dependence of the metalated form portion in the SERRS spectrum. The results show a remarkable reliance on the state of the metal surface. © 1998 John Wiley & Sons, Ltd.