https://zenodo.org/record/1258475/files/article.pdf

Electron transfers in chemistry and biology

Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buckminsterfullerene, C(60), is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C(60) molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C(60) anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C(60), the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.

https://science.sciencemag.org/content/sci/258/5087/1474.full.pdf

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene.

Powerful first-order analysis of intraprotein electron transfer is developed from electron-transfer measurements both in biological and in chemical systems. A variation of 20 A in the distance between donors and acceptors in protein changes the electron-transfer rate by 10(12)-fold. Protein presents a uniform electronic barrier to electron tunnelling and a uniform nuclear characteristic frequency, properties similar to an organic glass. Selection of distance, free energy and reorganization energy are sufficient to define rate and directional specificity of biological electron transfer, meeting physiological requirements in diverse systems.

Nature of biological electron transfer

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this “integrated modular assembly” approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principleslight absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled elec...

Chemical approaches to artificial photosynthesis

Intramolecular long-distance electron transfer (EI) has been actively studied in recent years in order to test existing theories in a quantitative way and to provide the necessary constants for predicting ET rates from simple structural parameters. Theoretical predictions of an "inverted region," where increasing the driving force of the reaction will decrease its rate, have begun to be experimentally confirmed. A predicted nonlinear dependence of ET rates on the polarity of the solvent has also been confirmed. This work has implications for the design of efficient photochemical charge-separation devices. Other studies have been directed toward determining the distance dependence of ET reactions. Model studies on different series of compounds give similar distance dependences. When different stereochemical structures are compared, it becomes apparent that geometrical factors must be taken into account. Finally, the mechanism of coupling between donor and acceptor in weakly interacting systems has become of major importance. The theoretical and experimental evidence favors a model in which coupling is provided by the interaction with the orbitals of the intervening molecular fragments, although more experimental evidence is needed.

https://science.sciencemag.org/content/sci/240/4851/440.full.pdf

Intramolecular Long-Distance Electron Transfer in Organic Molecules

Rate constants for both radical ion pair formation and recombination in porphyrin - quinone donor-acceptor molecules are reported. The ion pair formation and recombination depend on the exothermicity of the respective electron-transfer reaction in a manner previously reported. The question as to what fraction of energy change is due to relaxation processes involving the solvent vs. the porphyrin - quinone molecules has not been answered by these studies. However, the measured reorganization energy was found to be approximately solvent independent. It was observed that in these molecules the donor-acceptor distance is restricted, and the highly exothermic charge recombination reactions do not produce electronically excited states to the donor or acceptor. 11 references, 2 figures.

Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules

High-quantum-yield long-lived charge separation in a photosynthetic reaction center model

Osseous sarcoidosis can masquerade as metastatic prostate cancer. Awareness of this is helpful in the staging of prostate cancer in a patient with sarcoidosis. 

https://doi.org/10.1016/j.eucr.2022.102255

E. Bradley Pewitt

Papers

Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules

High-quantum-yield long-lived charge separation in a photosynthetic reaction center model

Careful consideration of sarcoidosis in diagnosis and staging of prostate cancer: A case report