Triplet recombination of radical ion pairs: CIDNP effects and DFT calculations on 1,2-dicyanoethylene.

Radical ion pairs generated by electron transfer from photo-excited aromatic hydrocarbons to maleo- and fumaronitrile (cis- and trans-1,2-dicyanoethylene, 1) undergo back electron transfer from singlet and triplet pairs. The pair energies relative to the reactant ground states and to the triplet state, respectively, determine the competition between the recombination pathways. Cross sections through the potential surfaces of the radical anion and the triplet state of 1 have been examined by density functional theory calculations.

Radical cations and triplet states of 1,2-disubstituted cyclopropanes: comparison of potential surfaces.

Radical ion pairs generated by photo-induced electron transfer from 1,2-disubstituted cyclopropanes to various acceptors undergo return electron transfer in pairs of singlet and triplet multiplicity. The pair energies relative to the reactant ground states and to accessible triplet states, respectively, determine the competition between the recombination pathways. The potential surfaces of the radical cations and triplet states of 1,2-diphenyl-, 1, and 1,2-dimethylcyclopropane, 2, have been examined by density functional theory calculations.

A tribute to Stanislao Cannizzaro, chemical informationist and photochemist.

Stanislao Cannizzaro is known widely for the Cannizzaro reaction, the "disproportionation" of benzaldehyde upon reaction with alkali, for his approach to teaching chemistry, "Sunto di un corso di filosofia chimica", which he presented at the Karlsruhe Congress of 1860, and for his work on the photochemistry of santonin. In Cannizzaro's laboratory two research associates, Giacomo Ciamician and Paul Silber, and a senior colleague, Emanuele Paternó, became acquainted with the basic methods of sunlight-inducd photochemistry.

Biradicals by triplet recombination of radical ion pairs.

Radical ion pairs generated by photoinduced electron transfer may undergo return electron transfer (RET) in pairs of singlet or triplet multiplicity. RET efficiencies are determined by the free energy of RET and the topologies of the potential surfaces of parent molecule, radical ion and triplet state. If radical ion geometries are different from the corresponding triplet states, RET occurs either with cleavage ("dissociative" RET; 1,2-diphenylcyclopropane radical cations) or formation of C-C bonds ("associative" RET; norbornadiene radical cation).

Photoinduced electron transfer in ionic liquids: use of 2,4,6-triphenylthiapyrylium as a photosensitizer probe.

The photochemistry of 2,4,6-triphenylthiapyrylium, TPTP+, in an ionic liquid, bmim-PF6, and in zeolite Y has been investigated and compared. Fluorescence spectroscopy was used to characterize the singlet excited state and to demonstrate singlet quenching by dicyclopentadiene, DCP, as an electron donor. Time-resolved laser spectroscopy documents generation of the triplet excited state, 3TPTP+, and reduction by DCP, generating the corresponding radical, TPTP*, and radical ion, DCP+*.

Incorporation of 2-arylhexa-1,5-diene into pentasil zeolite: A distorted 1-arylcyclohexane-1,4-diyl radical cation at room temperature.

Incorporation into a redox-active pentasil zeolite [(Na,H)-ZSM-5] converted 2-arylhexa-1,5-dienes (9a-c; aryl = phenyl, tolyl, anisyl) into 1-arylcyclohexane-1,4-diyl radical cations, 10a-c*+. The ESR spectra of 10a-c*+ (six lines, g = 2.0026; a = 9.

Divergent oxidative rearrangements in solution and in a zeolite: distal vs proximal bond cleavage of methylenecyclopropanes.

Irradiation of 9,10-dicyanoanthracene (DCA) or p-chloranil in the presence of E-1-benzylidene-2-phenylcyclopropane (E-5) in CH(2)Cl(2) causes E-5 to undergo methylenecyclopropane rearrangement. An adduct, Z-7, between DCA and 5 firmly supports the involvement of a bifunctional trimethylenemethane radical cation. In contrast, incorporation of E-5 into HZSM-5 produces trans,trans-1,4-diphenyl-1,3-butadiene radical cation sequestered in the HZSM-5 interior, tt-8(.

Photoreactions of trans-1-o-Hydroxyphenyl-2-phenylcyclopropane.

The photochemistry of trans-1-o-hydroxyphenyl-2-phenylcyclopropane, trans-1, was studied under a variety of experimental conditions. Direct irradiation through quartz in cyclohexane gave rise mainly to ring-expanded products, 2-phenyl-3,4-dihydro-2H-benzopyran, 2, 2-benzyl-2,3-dihydrobenzofuran, 3, and 1-o-hydroxyphenylindan, 4. The major products, 2 and 3, are rationalized by intramolecular proton transfer.

Electron Transfer Photochemistry of Homochrysanthemol: Intramolecular Nucleophilic Attack on the Cyclopropane Ring.

The electron-transfer photochemistry of homochrysanthemol, 1, resulted exclusively in intramolecular "substitution" at the quaternary cyclopropane carbon, generating the five-membered cyclic ethers, 2 and 4. The alternative "addition" to the terminal carbon of the double bond, which would result in seven-membered cyclic ethers, 3 and 5, was not observed. Apparently, the five-membered transition state leading to 2 and 4 is significantly favored over the seven-membered one required for formation of 3 and 5.