Femtochemistry uncovers the nature of electron transfer reactions

Ultrafast Dynamics of Photoinduced Electron Transfer in Bay-Aryl-Substituted Perylene Diimide Derivatives

Blends of donors and acceptors have been widely used in bulk-heterojunction solar cells to have exciton formation and charge separation by photoinduced electron transfer (PET). In this work, we have synthesized perylene diimide (PDI)-based materials having different aryl substituents at the bay positions (4-Anisyl-PDI, CBZ-N-Ph-PDI, and 4-Pyridyl-PDI) to understand the excited-state dynamics of electron transfer. The detailed photophysics was studied using steady-state as well as ultrafast dynamics of the excited states in different solvents. CBZ-N-Ph-PDI showed tremendous effects of the solvent on the electronic properties compared with the other two derivatives. The emission quantum yield of CBZ-N-Ph-PDI decreases drastically in dichloromethane and other polar solvents, indicating strong electron transfer. DFT calculations showed that in CBZ-N-Ph-PDI the HOMO is centered mostly on the N-phenylcarbazole and the LUMO is on the electron-poor PDI moieties. In addition, the energy levels of the HOMO and HOMO-1 in CBZ-N-Ph-PDI are estimated to be identical. The free energy change for charge separation (ΔG_CS) was calculated using electrochemical and photophysical data and found to be negative for CBZ-N-Ph-PDI. The ground- and excited-state dipole moment ratios suggest that the excited state of 4-Pyridyl-PDI (1.90) is less polar than that of 4-Anisyl-PDI (3.67), which provides an idea of the lower possibility of charge separation in 4-Anisyl-PDI and 4-Pyridyl-PDI. Ultrafast photodynamics studies of 4-Anisyl-PDI, CBZ-N-Ph-PDI, and 4-Pyridyl-PDI showed fast electron transfer only in CBZ-N-Ph-PDI and not in the other PDI derivatives. It was also observed that electron transfer is faster in DCM and THF than in toluene. Ultrafast dynamics studies showed the presence of an equilibrium between electron transfer and decay from the singlet excited state. Ultrafast studies also showed the features of the N-phenylcarbazole cation and PDI anion, further confirming the intramolecular electron transfer in CBZ-N-Ph-PDI.

Non-thiol reagents regulate ryanodine receptor function by redox interactions that modify reactive thiols

The Ca(2+) release channel (CRC) from sarcoplasmic reticulum (SR) is rich in thiol groups, and their oxidation/- reduction by thiol reagents activates/inhibits the CRC. Most channel regulators are not thiol reagents, and the mechanism of their action is illusive. Here the authors show that many channel activators act as electron acceptors, while many channel inhibitors act as electron donors in free radical reactions. The channel activator, caffeine, and the CRC inhibitor, tetracaine, are shown to interact competitively, which suggests that there exists a common site(s) on the CRC, that integrates the donor/acceptor effects of ligands. Moreover, channel activators shift the redox potential of reactive thiols on the ryanodine receptor (RyR) to more negative values and decrease the number of reactive thiols, while channel inhibitors shift the redox potential to more positive values and increase the number of reactive thiols. These observations suggest that the non-thiol channel modulators shift the thiol-disulfide balance within CRC by transiently exchanging electrons with the Ca(2+) release protein.