The singlet oxygen photosensitizing efficiencies of the ortho-, meta-, and para-substituted free-base
and zinc (II) tetraarylporphyrins have been investigated on the basis of the heavy atom effect, electronegativity, and
spatial orbital overlap. Both the endocyclic and exocyclic heavy atom effects have been exploited to induce various
degrees of spin-orbit coupling. The ground state and the lowest singlet excited electronic state of the porphyrins have been
studied using optical absorption and fluorescence emission spectroscopic techniques. The experimental data suggest that
implementation of the halogen substituent effect to enhance the photosensitizing properties of a chromophore from the
excited triplet state should be done cautiously, as it may have negative implications.