The facile activation of cis- and trans-1,2-diphenylbenzocyclobutene (DBC) either by charge-trans... more The facile activation of cis- and trans-1,2-diphenylbenzocyclobutene (DBC) either by charge-transfer irradiation of the electron donor-acceptor complex with tetracyanoethylene or by chloranil photosensitization leads to a series of rapid [4 + 2] cycloadditions. The role of the cation radical DBC+. as the reactive intermediate which undergoes a stereospecific, conrotatory cycloreversion is delineated, especially with regard to “contact” and “solvent-separated” ion pairs. Such cycloadditions induced by electron transfer are discussed in the context of the thermal valence tautomerization of DBC previously established by Huisgen, Quinkert, and co-workers. Elektron-Transfer-Photochemie von Benzocyclobutenen. – Stereospezifische elektrocyclische Reaktionen ihrer Kation-Radikale Die leichte Aktivierung von cis- und trans-1,2-Diphenylbenzocyclobuten (DBC) entweder durch Bestrahlung der Charge-transfer-Verbindung mit Tetracyanethylen oder durch Photosensibilisierung mit Chloranil fuhrt zu einer Serie schneller [4 + 2]-Cycloadditionen. Die Rolle des Kationradikals DBC+. als reaktives Zwischenprodukt, das eine stereospezifische conrotatorische Cycloreversion eingeht, wird vorwiegend uber „Kontakt”- oder „Losungsmittel-getrennte” Ionen-Paare erklart. Solche Cycloadditionen, die durch einen Elektronentransfer eingeleitet werden, werden im Zusammenhang mit einer thermischen Valenztautomerie von DBC diskutiert, wie sie fruher von Huisgen, Quinkert und Mitarbeitern begrundet wurden.
In the 20th century, photochemistry blossomed from a poorly defined to a highly sophisticated sci... more In the 20th century, photochemistry blossomed from a poorly defined to a highly sophisticated science. Early breakthroughs in exploratory photochemistry and the underlying physical principles led to new diverse, yet inter-related areas of research. The alluring goal of asymmetric synthesis with circularly polarized light proved elusive. The discovery of the electron brought a gradual awakening to the idea of electron transfer. Time-resolved spectroscopy developed from ms to fs resolution. The field of photosynthesis progressed from an interest in function and structure of photosynthetic pigments to the isolation and structure elucidation of photosynthetic reaction centers (rhodobacter sphaeroides), to the detailed kinetics of sequential electron-transfer steps in natural and synthetic light-harvesting systems.
Return electron transfer (RET) in radical ion pairs may populate the reagent ground states or, in... more Return electron transfer (RET) in radical ion pairs may populate the reagent ground states or, in the case of triplet pairs, one reagent triplet state. The efficiency of triplet RET is governed by the free energies of singlet and triplet RET and by the topologies of the potential surfaces of parent molecules, radical ions, and accessible triplet states or biradicals. RET in triplet radical ion pairs is exemplified for two distinct relationships between the three potential surfaces.
Proceedings of the National Academy of Sciences, 1975
The reactions of photo-excited chlorophylls and pheophytins with quinones have been investigated ... more The reactions of photo-excited chlorophylls and pheophytins with quinones have been investigated by nuclear magnetic resonance techniques. In slightly alkaline solutions the quinone signals showed line broadening which is explained by degenerate electron transfer between semiquinone radical ions and their benzoquinone parents. In neutral solutions, chemically induced dynamic nuclear polarization was observed which is ascribed to the pair, chlorophyll cation-semiquinone anion. A kinetic analysis of the dependence of these effects upon the quinone concentration suggests that only the reactions of triplet-chlorophyll with the quinones give rise to chemically induced dynamic nuclear polarization effects.
Cleavage of Thymine Dimers Sensitized by Quinones. Chemically Induced Dynamic Nuclear Polarizatio... more Cleavage of Thymine Dimers Sensitized by Quinones. Chemically Induced Dynamic Nuclear Polarization in Radical Ions Sir: The photosensitized cleavage of thymine photo-dimers is being investigated in several laboratories' in order to provide guidelines for understanding the ...
The facile activation of cis- and trans-1,2-diphenylbenzocyclobutene (DBC) either by charge-trans... more The facile activation of cis- and trans-1,2-diphenylbenzocyclobutene (DBC) either by charge-transfer irradiation of the electron donor-acceptor complex with tetracyanoethylene or by chloranil photosensitization leads to a series of rapid [4 + 2] cycloadditions. The role of the cation radical DBC+. as the reactive intermediate which undergoes a stereospecific, conrotatory cycloreversion is delineated, especially with regard to “contact” and “solvent-separated” ion pairs. Such cycloadditions induced by electron transfer are discussed in the context of the thermal valence tautomerization of DBC previously established by Huisgen, Quinkert, and co-workers. Elektron-Transfer-Photochemie von Benzocyclobutenen. – Stereospezifische elektrocyclische Reaktionen ihrer Kation-Radikale Die leichte Aktivierung von cis- und trans-1,2-Diphenylbenzocyclobuten (DBC) entweder durch Bestrahlung der Charge-transfer-Verbindung mit Tetracyanethylen oder durch Photosensibilisierung mit Chloranil fuhrt zu einer Serie schneller [4 + 2]-Cycloadditionen. Die Rolle des Kationradikals DBC+. als reaktives Zwischenprodukt, das eine stereospezifische conrotatorische Cycloreversion eingeht, wird vorwiegend uber „Kontakt”- oder „Losungsmittel-getrennte” Ionen-Paare erklart. Solche Cycloadditionen, die durch einen Elektronentransfer eingeleitet werden, werden im Zusammenhang mit einer thermischen Valenztautomerie von DBC diskutiert, wie sie fruher von Huisgen, Quinkert und Mitarbeitern begrundet wurden.
In the 20th century, photochemistry blossomed from a poorly defined to a highly sophisticated sci... more In the 20th century, photochemistry blossomed from a poorly defined to a highly sophisticated science. Early breakthroughs in exploratory photochemistry and the underlying physical principles led to new diverse, yet inter-related areas of research. The alluring goal of asymmetric synthesis with circularly polarized light proved elusive. The discovery of the electron brought a gradual awakening to the idea of electron transfer. Time-resolved spectroscopy developed from ms to fs resolution. The field of photosynthesis progressed from an interest in function and structure of photosynthetic pigments to the isolation and structure elucidation of photosynthetic reaction centers (rhodobacter sphaeroides), to the detailed kinetics of sequential electron-transfer steps in natural and synthetic light-harvesting systems.
Return electron transfer (RET) in radical ion pairs may populate the reagent ground states or, in... more Return electron transfer (RET) in radical ion pairs may populate the reagent ground states or, in the case of triplet pairs, one reagent triplet state. The efficiency of triplet RET is governed by the free energies of singlet and triplet RET and by the topologies of the potential surfaces of parent molecules, radical ions, and accessible triplet states or biradicals. RET in triplet radical ion pairs is exemplified for two distinct relationships between the three potential surfaces.
Proceedings of the National Academy of Sciences, 1975
The reactions of photo-excited chlorophylls and pheophytins with quinones have been investigated ... more The reactions of photo-excited chlorophylls and pheophytins with quinones have been investigated by nuclear magnetic resonance techniques. In slightly alkaline solutions the quinone signals showed line broadening which is explained by degenerate electron transfer between semiquinone radical ions and their benzoquinone parents. In neutral solutions, chemically induced dynamic nuclear polarization was observed which is ascribed to the pair, chlorophyll cation-semiquinone anion. A kinetic analysis of the dependence of these effects upon the quinone concentration suggests that only the reactions of triplet-chlorophyll with the quinones give rise to chemically induced dynamic nuclear polarization effects.
Cleavage of Thymine Dimers Sensitized by Quinones. Chemically Induced Dynamic Nuclear Polarizatio... more Cleavage of Thymine Dimers Sensitized by Quinones. Chemically Induced Dynamic Nuclear Polarization in Radical Ions Sir: The photosensitized cleavage of thymine photo-dimers is being investigated in several laboratories' in order to provide guidelines for understanding the ...
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