Oxidation and reduction potentials have been measured by voltammetry for a series of substituted tetraphenylporphyrins in DMSO and aqueous DMSO solutions. Hammett substitutent effect correlations effectively describe the positions of the first and second reduction waves; more electron-withdrawing substituents shift the reduction waves to more positive potentials with reaction constants (p') of +60 and +57 mV, respectively. The oxidation potentials show a similar trend (p' = +63 mV); however, a sharp break is observed for highly electron-donating substituents such as alkoxy and amino (p' = +390 mV). We p r o p that these latter porphyrins undergo oxidation at the benzene ring rather than the porphyrin ring. Solvent effects are correlated by the ET(30) solvent parameter, with the more highly polar solvent (more aqueous DMSO) shifting the reduction potentials to more positive values. IatroductionPorphyrins are commonly used as redox catalysts in photochemical and electrochemical processes. Recent comprehensive reviews of porphyrin and metalloporphyrin ph~tochemistry~-~ and e l e~t r o c h e m i s~~ are available. A particularly valuable feature is the relative ease of making small structural changes that allow fine tuning of many of the porphyrin properties. We have been developing a polymeric porphyrin system for artificial photosynthesis, in which the crucial electron transport characteristics are governed by small differences in redox potentials caused by para substituents on tetraphenylporphyrin (TPP).68 In the study reported here, we have measured the oxidation and reduction potentials of a series of para-substituted freebase TPPs (Figure l) to verify that the redox potentials are predictable. We have focusad on thoae substituents that would be found in amiddnked and ester-linked porphyrins (amines, phenols, amides, esters, and carboxylic acids), since these substituents are commonly found not only in our work but in numerous caw of linked porphyrins used for electron-transfer ~tudies.~ In addition, we have studied the effect of aqueous solutions in modifying the redox potentials measured in a nonaqueous solvent. Experimental SectionMiteria. Tetraphenylporphyrin (TPP) and tetrakis(4-aminopheny1)porphyrin (TAPP) were purchased from MidCentury Chemicals (Posen, IL). Tetrakis(4-methoxypheny1)-porphyrin (TMPP) was purchased from Aldrich Chemical (Milwaukee, WI). Tetrakis(ecarboxypheny1)porphyrin (TCPP) was purchased from Porphyrin Products (Logan, UT). Tetrakis(4-hydroxypheny1)porphyrin (THPP) was prepared by demethylation of TMPP using pyridinium hydrochloride.1° Tetrakis(4-cyanopheny1)porphyrin (TCNPP) and tetrakis[4-(carboxymethyl)phenyl]porphyrin (TCMPP) were prepared from pyrrole and the corresponding substituted benzaldehyde according to the Adler method." Tetrakis(4-(N-(4'-methylphenyl)-carboxamid0)phenyl)porphyrin (TTCPP) was prepared from TCPP by treatment with refluxing thionyl chloride, removal of excess thionyl chloride by vacuum distillation, and treatment of the porphyrin acid chloride (TCCPP) with ...
Some specific conclusions are summarized. 1.The usefulness of linear free energy relationships in correlating variations in redox potentials with changes in substituent was confirmed with two exceptions. Two of the porphyrins were shown to undergo a different electrochemical oxidation mechanism than the remaining porphyrins, and another porphyrin was shown to be more difficult to reduce than predicted on the basis of its substituent constant. 2.Solvent effects, here investigated as the effect of added water on the reduction potential of tetraaminophenylporphyrin in DMSO, were demonstrated to correlate with the Dimroth-Reichardt solvent parameter, E T , determined experimentally for each water-DMSO mix. 2. LIST OF TABLES 3.4. 5.6. 7.8. 9.10. 11.12. can be used to drive the photochemical splitting of water into hydrogen gas and oxygen gas. This is illustrated in Figure 1, where S is an absorbing chromophore, A is an electron acceptor, and 0 is an electron donor. Figure 3), the resultant straight line can be written as: LIST OF FIGURESwhere p is the slope of the plot and A is the Y-intercept, which is equal to log k h '. As before, k h ' is the value of k h for the unsubstituted ethyl benzoate. This notation is used instead of that of Hammett, who used Ki 0 and k h 0 in their place, in order to avoid confusion when correlating EO values.Hammett then proposed choosing one system as the reference K or k, and recommended using Ki for substituted benzoic acids since at the time those constants were readily available and easily measured with good precision. A general relationship between a given rate or equilibrium constant k and the reference system Ki can be described using the equation below: [ 2] Hammett then went on to define C1 (x), the substituent constant for substituent X, as below:log KjK' Ionization Figure 3. Comparison of hydrolysis rates of esters with ionization constant~of acids for m-and pbenzoic acid derivatives.[ 3 ]Substituting equation [3] into equation [2] relating the generic k to Ki for benzoic acids yields the following equation:or,[5 ] Given the existence of a quantitative correlation, the fact that it is linear in the logarithms can be no mere accident; for the linear relationship between the logarithms of the constants is equivalent to a relationship between the quantities -RT In k, which are the free energies of reaction or of activation. From the definition of an equilibrium constant, we know that~Go = -RTeln K =.-nFEo. From this it follows that:In K = (nF/RT)eEO or converting to base 10, log K = (nF/2.303RT)eEO [7] [ 8 ] 8Expressing equation [8] in the form of equation [4] yields the following equation: [9 ] where EO' is EO for unsubstituted tetraphenylporphyrin. Model Porphryins For Thin-Film MembranesThe assembly of molecular components into photochemical molecular devices is the goal of a new field of research. 11Many useful applications are possible, such as photochemical synthesis, photochromism, and photodecomposition. Applications such as these may not require an organiz...
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