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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.

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Photoelectrochemical cells have been constructed by depositing monolayers of oriented covalently linked zinc/free base porphyrin heterodimers onto ∼30 nm nonporous layers of TiO 2 on ITO, deposited by metalorganic chemical vapor deposition (MO-CVD), and onto ∼100 nm porous, nanostructured TiO 2 layers, spincoated from a suspension of P25 (Degussa) on ITO. Fluorescence quenching of the dyes on both types of TiO 2 substrates is compared with that of dilute solutions of the dyes and with that of dye-coated, porous ZrO 2 (Degussa) substrates. By functionalizing one of the porphyrin dimer components with carboxylic substituents, which bind to the TiO 2 or ZrO 2 substrate surface, either the zinc porphyrin (ZnP) or the free base porphyrin (H 2 P) component of the dimer can be made to be in direct contact with the substrate. These dimer-substrate arrangements are denoted ZnP-H 2 P--TiO 2 (dimer 1) and H 2 P-ZnP--TiO 2 (dimer 2), respectively, where "--" denotes binding of the carboxyl-substituted porphyrin in the heterodimer to the substrate surface. In solution as well as deposited on ZrO 2 , in contact with the solvent without a redox couple, both types of dimers show efficient internal ZnP to H 2 P energy transfer. Deposited on TiO 2 , in the presence of the solvent, monolayers of both types of dimers show less efficient energy transfer than the dimers on ZrO 2 . For a ZnP-H 2 P--TiO 2 electrochemical cell the photocurrent action spectrum reproduces the absorption spectrum, i.e., contains contributions of both the ZnP and H 2 P moieties. By contrast, for H 2 P-ZnP--TiO 2 cells mainly the ZnP dimer component contributes to the photocurrent, demonstrating that in H 2 P-ZnP--TiO 2 cells electron transfer from the ZnP into the TiO 2 substrate is faster than energy transfer to the adjacent free base porphyrin. The photocurrent action spectrum of the ZnP-H 2 P--TiO 2 cell also demonstrates that energy transfer in monolayers of this dimer results in sensitization of the semiconductor substrate, since the spectral response of a cell is enhanced with respect to that of a cell with a monolayer of a monomeric sensitizer. These results are relevant for the construction of a solar cell containing a supramolecular, light-collecting antenna.