Understanding the Photophysical Properties of Pd and Pt Transition-Metal Isocorroles: A Theoretical Investigation.

The main photophysical properties of recently synthesized Pd- and Pt-isocorroles, characterized and proposed as potential photosensitizers in photodynamic therapy, have been investigated and rationalized using density functional theory and its time-dependent formulation. Both the 5 and 10-(2-pyrrolyl)-5,10,15-tris(4-methylphenyl) isomers have been considered. In addition to determining the minimum energy structures of the ground and excited states, we have calculated the excitation energies, spin-orbit coupling constants, nonradiative intersystem crossing rate constants and two photons cross section. Also, the type I PDT mechanisms have been considered. Results show that the photoactivity of both Pd and Pt complexes mainly depends on their singlet-triplet energy gap, which is necessary to promote the O2 3 Σ g - → 1 Δ g ${{}^{{\rm { 3}}}{{\rm { \Sigma{}}}}_{{\rm { g}}}^{{\rm { -}}}\to {}^{1}{\Delta }_{g}}$ excitation. The study of the photophysical properties of the free ligands indicates that this trend is common for both the free and Pt- and Pd-isocorroles. The production of superoxide ions (O2 .(-)) is possible only for the 10-(2-pyrrolyl)-5,10,15-tris(4-methylphenyl) isomers.