The One-Electron Transfer Induced Photocycloreversion of Stilbazolium Cyclodimers

Abstract

Abstract The irradiation of iodide salts of stereo isomeric stilbazolium cyclodimers in their CT bands (ca. 355 nm) resulted in a one-electron transfer induced photocycloreversion, which led to the formation of stilbazolium ions; the quantum yields (Φ) being highly sensitive to their structures, i.e., the Φ for syn-Head-to-Head (syn-HH) was found to be 0.35, for syn-Head-to-Tail (syn-HT) 0.0026, and for anti-Head-to-Head (anti-HH) 0.00095, respectively. This reaction shows a distinct contrast to photocycloreversion by irradiation at 255 nm, which resulted in a highly efficient, but nonselective, splitting, the Φ’s being in the range of 0.3—0.8. In the photosensitized cycloreversion of methyl sulfate salts of dimers using 9,10-dimethoxyanthracene (DMA) as a sensitizer, the quantum efficiencies show a structural dependence similar to the above case of irradiation at their CT bands. Cosensitization with DMA and 1-cyanonaphthalene as an electron mediator resulted in relative quantum yields showing a close coincidence to the yields in the irradiation of the CT band, i.e., 254 : 2.6 : 1 for syn-HH, syn-HT, and anti-HH, respectively. This implies that the one-electron transfer mechanism can be initiated both by CT band irradiation as well as DMA sensitization. It can be concluded that the structural dependence of the quantum efficiencies is attributed to the through-bond interaction of two pyridinium rings, which facilitates the splitting of the C–C bond in the cyclobutane ring.