Intramolecular photochemical electron transfer. Part 5.—Solvent dependence of electron transfer in a porphyrin–amide–quinone molecule

Abstract

The compound PAQ, which consists of a tetra-arylporphine attached to methyl-p-benzoquinone via a single amide linkage, exhibits light-induced intramolecular electron transfer from the porphyrin excited-singlet state to the quinone at a rate which is strongly solvent-dependent. The rate constants are found to correlate well with the semiclassical Marcus theory of electron transfer, provided that the solvent effect on both the Gibbs energy change, AGO, for the electron-transfer reaction 'P*AQ -+ P'+AQ'-and the Marcus reorganisation energy, A, are considered. The AGO values are obtained from direct measurement of redox potentials in each solvent with various work-term corrections for Coulombic interaction in P'+AQ'-, and 1 is calculated from the optical and dielectric properties of each solvent. For fourteen solvents, reasonable agreement with Marcus theory is found using this approach on uncorrected AGO values and those corrected with a solventdependent work term ; a solvent-independent correction is not successful. For two solvent mixtures (acetonitrile-benzonitrile and acetonitrilechloroform), excellent agreement with Marcus theory is found using uncorrected AGO values and those corrected with a solvent-dependent work term. We have found that Weller's method for calculating A G O in various solvents from a single measurement in a reference solvent gives a poor correlation with Marcus theory, primarily because of a poor prediction of the solvent dependence of AGO.

Quenching of zinc tetraphenylporphine by oxygen and by 1,4-benzoquinone in nitrile solvents: An optoacoustic spectroscopy studyDedicated to Professor Frank Wilkinson on the occasion of his retirement.

“…( 17) are very similar within the series, with extreme values differing only by 5%. Indeed very similar E ± values for the photoinduced electron transfer reactions in Zn porphyrin-bridge-BQ and H 2 TPP-bridge-BQ triads in various nitriles were measured by Leland et al 72 and by Schmidt et al 73 The values of DV str calculated from eqn. (15) (see Table 3) indicate a volume contraction across the solvent series, and vary from À 12.5 ml mol À1 in acetonitrile to À 29.5 ml mol À1 in valeronitrile.…”

Photoinduced Electron Transfer in Folic Acid Investigated by Ultrafast Infrared Spectroscopy

“…We have modeled the rate of ET, k et , derived from the TRIR measurements using the well-known semiclassical Marcus theory of ET as follows: 48–50 $$k_{\text{et}}=\frac{2\mathrm{\pi }{H}^2}{h\sqrt{4\mathrm{\pi }\mathrm{\lambda }(r)k_{\mathrm{normalB}}T}}{\mathrm{normale}}^{-{(\mathrm{\Delta }G+\mathrm{\lambda }(r))}^2/4\mathrm{\lambda }(r)k_{\mathrm{normalB}}T}$$ $$\mathrm{\Delta }G=-h\mathrm{\upsilon }+(E_{\text{ox}}^{\mathrm{normalD}}-E_{\text{red}}^{\mathrm{normalA}})-\frac{{\mathrm{normale}}^2}{4\mathrm{\pi }{\mathrm{normal\epsilon }}_0{\mathrm{normal\epsilon }}_{\mathrm{normals}}r}$$ $$\mathrm{\lambda }(r)=\frac{{\mathrm{normale}}^2}{4\mathrm{\pi }{\mathrm{normal\epsilon }}_0}(\frac{1}{{\mathrm{normal\epsilon }}_{\text{op}}}-\frac{1}{{\mathrm{normal\epsilon }}_{\mathrm{normals}}})(\frac{1}{2r_{\mathrm{normalA}}}+\frac{1}{2r_{\mathrm{normalD}}}-\frac{1}{r})$$ H is the electronic coupling energy of the donor and acceptor, k B is Boltzmann’s constant (J K −1 ), T is the temperature (300 K), h is Planck’s constant (6.626076 × 10 −34 J s), and G is the Gibbs free energy change for the ET. The Gibbs free energy change depends on the reduction potential of the acceptor ( E red ), the oxidation potential of the donor ( E ox ), the electronic charge ( e ), the vacuum permittivity (ε 0 ), and the static dielectric constant of the solvent (water, ε s = 77.56).…”

Long-Lived Triplet State Charge Separation in Novel Piperidine-Bridged Donor−Acceptor Systems

“…Even though the bridges in linked systems occupy part of the space available to the solvent in nonbridged ion pairs, it has been found that eq 1 can be applied successfully, at least if the bridges are arranged in an extended fashion as in 3a and 3b. 3,9, [46][47][48][49][50] For the D and A species under study, the one-electron oxidation and reduction potentials (E ox (D) and E red (A)) in acetonitrile are known (see Table 1). To obtain an estimate of the CT-state energies in different solvents, we set the ionic radii r at 4.1 Å, which experience has shown to be a reasonable value for small aromatic systems.…”