Comparative Study of the Photophysical Properties of Nonplanar Tetraphenylporphyrin and Octaethylporphyrin Diacids

The photophysical properties of the lowest excited singlet states, S 1 (π,π*), of two porphyrin diacids have been investigated. The diacids are H 4 TPP 2+ and H 4 OEP 2+, the diprotonated forms of free base tetraphenylporphyrin (H 2 TPP) and octaethylporphyrin (H 2 OEP), respectively. Both diacids exhibit perturbed static and dynamic characteristics relative to the parent neutral complexes in solution at room temperature. These properties include enhanced yields of S 1 → S 0 radiationless deactivation (internal conversion), which increase from ∼0.1 for H 2 TPP and H 2 OEP to 0.4 for H 4 OEP 2+ and 0.6 for H 4 TPP 2+ . The fluorescence lifetimes of both diacids are strongly temperature dependent, with an activation enthalpy of ∼1400 cm -1 for S 1 -state deactivation. The enhanced nonradiative decays and many other photophysical consequences of diacid formation are attributed primarily to nonplanar macrocycle distortions. Both H 4 TPP 2+ and H 4 OEP 2+ have been shown previously by X-ray crystallography to adopt saddle-shaped conformations, and the magnitudes of the perturbed properties for the two diacids in solution correlate with the extent of the deviations from planarity in the crystals. A model is proposed to explain the nonradiative decay behavior of the porphyrin diacids that is relevant to nonplanar porphyrins in general. The model includes the existence of decay funnels on the S 1 (π,π*)-state energy surface that are separated from the equilibrium conformation and other minima by activation barriers. It is suggested that these funnels involve configurations at which the potential-energy surfaces of the ground and excited states approach more closely than at the equilibrium excited-state structure(s) from which steady-state fluorescence occurs. Possible contributions to the relevant nuclear coordinates are discussed.