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

The photophysical properties of the lowest excited singlet states, S₁(π,π*), of two porphyrin diacids have been investigated. The diacids are H₄TPP²⁺ and H₄OEP²⁺, the diprotonated forms of free base tetraphenylporphyrin (H₂TPP) and octaethylporphyrin (H₂OEP), 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 S₁ → S₀ radiationless deactivation (internal conversion), which increase from ∼0.1 for H₂TPP and H₂OEP to 0.4 for H₄OEP²⁺ and 0.6 for H₄TPP²⁺. The fluorescence lifetimes of both diacids are strongly temperature dependent, with an activation enthalpy of ∼1400 cm^-1 for S₁-state deactivation. The enhanced nonradiative decays and many other photophysical consequences of diacid formation are attributed primarily to nonplanar macrocycle distortions. Both H₄TPP²⁺ and H₄OEP²⁺ 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 S₁-(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.