Conformational and electronic effects of phenyl-ring fluorination on the photophysical properties of nonplanar dodecaarylporphyrins

Static and time-resolved optical measurements have been performed on a number of free-base and zinc dodecaarylporphyrins with varying degrees of fluorination of the peripheral aryl rings. These studies were performed in a variety of solvents of differing polarity and metal-ligating ability and at room and low temperatures. All of the compounds are deduced to be nonplanar based on their perturbed photophysical properties relative to planar analogues and on the X-ray data available for these molecules. The dodecaarylporphyrins studied generally separate into two classes based on their photophysical properties and the presence or absence of meso-pentafluorophenyl rings. The photophysical properties are clearly affected by the electron-withdrawing characteristics of these fluorinated phenyl rings, but structural effects derived from the interaction of the phenyl rings with each other, the macrocycle, and the solvent are also apparent. Many of the differences in properties among the molecules studied and the perturbed photophysical properties of nonplanar porphyrins in general are associated with the ability of these molecules to undergo photoinduced conformational changes. For the dodecaarylporphyrins, the perturbed properties are also a result of the ability of these compounds to access multiple conformations in the ground- and excited-electronic states. The studies demonstrate the strong linkage that nonplanar porphyrins have between their electronic properties, structural characteristics, conformational dynamics, porphyrin-solvent interactions, and photophysical behavior. These connections are far stronger than those exhibited by planar porphyrins and lead to detailed differences in properties among the compounds studied here as well as the photophysical properties of nonplanar porphyrins as a whole.