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Porphyrin Photophysics

Porphyrins can truly be referred to as the energy collector of life, for chlorophyll allows plants to harvest and store solar energy and this is the source of almost all free energy on earth.  Iron hemes are the active ingredient in hemoglobin in blood, iron hemes are also the functional part of the liver enzymes, and even the methanogenic bacteria in the stomach of most  mammals (including humans) contain an unusual nickel porphyrin.  Nature has evolved a quite remarkable molecule – some evolutionary scientists even believe that porphyrins came before the biotic era.  (See David Mauzerall’s recent review “Evolution of Porphyrins” in Clin. Derm. 1998.)

The commercial uses of porphyrins and metalloporphyrins include: (I)photodynamic therapeutics such as Photofrin to fight viral infections and cancer; (II)commercial oxidation catalysts to make fine chemicals; (III)components of printing inks and toners; (IV)protective coatings.  In addition to these current uses, there have been several hundreds of patents issued in the past few years for the use of porphyrins in molecular electronics, catalysts, inks, and other new materials.

Recently it has been noted that the nature and extent of distortion from planarity in porphyrins has profound effects on their chemical and photophysical properties

In collaboration with Dewey Holten, Kevin Smith, John Shelnutt, and Jack Fajer we are looking at the complex photodynamics of distorted Nickel porphyrins.

The recent realization that most, if not all, porphyrins found in nature are at least somewhat distorted from the planar geometry we normally think of them.  A consortium of investigators literally from coast to coast have teamed up to investigate the photophyical and chemical consequences of these distortions.  The model that is emerging is that these distortion play a vital role in the pysical properties of the macrocycle such that subtle peterbations in the geometry or the local environment have dramatic consequences on the reactivity of the porphyrin.  This is well illustrated by nickel(II)dodecaphenylporphyrin and by meso-nickel(II)tetra-tert-butylporphyrin.

The excited-state relaxation dynamics of NiT(t-Bu)P exhibit a dependence on solvent dielectric and temperature whose magnitude is unprecedented for porphyrinic systems.  The time constant for excited-state decay and ground-state recovery at room temperature spans several picoseconds in highly polar solvents to tens of nanoseconds in nonpolar media.  In both types of solvent, the lifetimes increase to several microseconds at liquid-nitrogen temperature.  The dramatic dependence of the excited-state deactivation on environmental factors is most readily understood if the photoexcited molecule has dipolar character.  This character could be derived from photoinduced conversion to an excited state that adopts an asymmetric structure, possesses inherent electronic asymmetry, or both.  The dielectric properties of the solvent contribute to both the energy of the dipolar excited state as well as the extent to which the medium reorganizes around the excited state relative to the nonpolar electronic ground state.  Temperature affects these properties as well as the population distribution in the low-frequency out-of-plane motions of the macrocycle.  These motions in turn contribute to the barriers that must be crossed during the excited state relaxation process and perhaps also to the mixing of the (d,d) and CT excited states.

NiDPP

It seems clear that both the solvent and temperature dependence of the excited-state relaxation of NiT(t-Bu)P and NiT(Ad)P ultimately derive from the profound effects that static and dynamic nonplanar distortions have on the (photo)physical and chemical properties of porphyrinic chromophores.  The wide range of excited-state kinetic behavior and other chemical properties accessible through conformational “tuning” may well mimic the properties of porphyrinic prosthetic groups, which are increasingly found to be nonplanar in the crystal structures of tetrapyrrole-protein complexes.  Furthermore, the work reported here on NiT(t-Bu)P may provide additional clues into the interactions between a tetrapyrrole cofactor and the surrounding protein.  For example, the recognition that certain types of nonplanar distortion of the porphyrin macrocycle impart dipolar character to the molecule (albiet small) may be of consequence for the transient dynamics in hemoglobin and myoglobin.  In particular, the transient dipolar character of the domed heme intermediates known to form in these systems could contribute to the protein response elicited by the ligand binding/release events at the active site.   In addition to the potential relevance to tetrapyrrole-protein complexes, the dependence of the excited-state dynamics of NiT(t-Bu)P on solvent dielectrics might also find use in molecular-switching applications.  In general, the accessibility of a wide range of excited-state properties in nonplanar porphyrins may afford simple avenues into building blocks for molecular optoelectronics devices.

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