Solvent and Substituent Effects on the Redox Potentials of Several Substituted Tetraphenylporphyrins

Abstract

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...

Hot Electron Transfer from CdTe Quantum Dot (QD) to Porphyrin and Ultrafast Electron Transfer from Porphyrin to CdTe QD in CdTe QD–Tetrakis(4-carboxyphenyl)porphyrin Nanocomposites

“…Furthermore, the reduction potential of TCPP was found to be 1.01 eV, which was measured by Ransdell employing cyclic voltammetry in dimethyl sulfoxide. 73 This observed reduction potential value leads to prediction of the LUMO energy of TCPP to be approximately −3.0 eV 74 which is very close to our calculated LUMO energy. Hence photoinduced electron transfer from the excited singlet state of TCPP should be the reason for quenching of fluorescence as well as the singlet lifetime of TCPP in CdTe QD−TCPP NC systems as observed in time-resolved photoluminescence studies upon excitation at 630 nm.…”

“…Inclusion of the role of solvent in the TD-DFT calculation could narrow down this discrepancy by adjusting the HOMO and LUMO energy levels without altering the exergonic nature of the electron transfer from the Q x state of TCPP. Furthermore, the reduction potential of TCPP was found to be 1.01 eV, which was measured by Ransdell employing cyclic voltammetry in dimethyl sulfoxide . This observed reduction potential value leads to prediction of the LUMO energy of TCPP to be approximately −3.0 eV which is very close to our calculated LUMO energy.…”