Comparative study of photophysical properties of isomeric tetrapyridyl- and tetra-(N-hexadecylpyridiniumyl) porphyrins

Photophysical properties (absorption spectra, fluorescence spectra, lifetim es and quantum yields) were studied for the three isomers of tetrapyridylpo rphyrins (TPyPs) in CHCl3 and tetra-(N-hexadecylpyridiniumyl) porphyrins (T C(16)PyPs) in CHCl3, CH3OH and Triton X-100 micelle solution. While the gro und-state and excited-state properties are very similar for the three isome rs of neutral TPyP, significant differences exist among the isomers of amph iphilic TC16PyP. Amphiphilic porphyrins bearing hydrophilic pyridinium and hydrophobic long-chain alkyl groups are characterized by reduced fluorescen ce quantum yields, biexponential fluorescence decay, and appreciable aggreg ation in nonpolar solvents. The observed biexponential fluorescence decay o f amphiphilic porphyrins can be interpreted in terms of mixing of the S-1 a nd a close-lying CT state in which an electron is transferred from the porp hyrin core to the electron-deficient pyridinium group. (C) 1999 Elsevier Sc ience B.V. All rights reserved.