Stilbene radical cations energies

Photoexcitation of aromatics in zeolites also led to cation radicals, as reported by lu and Thomas for pyrene and anthracene [128]. Ultraviolet photoexcitation of trani-stilbene in NaX formed the rran.y-stilbene radical cation (475 nm) and the zeolite-entrapped electron shows up as Na4 + (500 nm) [129]. Other organic species that form radical cations in zeolites upon high-energy UV excitation include 4-vinylanisole, tra .y-anethole, several styrenes and di(4-methoxyphenyl)ethylenes [130]. 

F re 14. Computed potential energy surfaces of isomerization of stilbene radical cations [147]. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

Irradiation of particles of semiconductors like Ti02 or CdS suspended in a solution of stilbene led to isomerization of its cis- to trans-isomer (29-30). The reaction seems to be initiated by electron transfer from cis-stilbene to the positive hole generated in the semiconductor by irradiation. The resulting cis radical cation (c ) is several kcal mol" higher in energy than a trans radical cation (t ) over the ground state trans isomer (31). Accordingly, the isomerization from c " to t " is exothermic. 

An increase in the cA-stilbene concentration favors the chain propagation and decreases the probability of termination when the DCNA anion-radicals react with the stilbene cation-radicals. A decrease in the irradiation intensity has a similar effect The chain propagation is the first-order process, whereas termination of the chains is the second-order process. A temperature rise accelerates the accumulation of the stilbene cation-radicals. In this system, the free energy of electron transfer is -53- —44 kJ moD (the cation-radical generation is in fact an endothermal process). If a polar solvent is substituted for a nonpolar one, the conversion of the cii-stilbene cation-radical into the trani-stilbene cation-radical deepens. Polar solvents break ion pairs, releasing free ion-radicals. The cA-stilbene cation-radicals isomerize more easily on being released. The stilbene cation-radical not shielded with a counterion has a more positive charge, and therefore, becomes stabilized in the... 

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

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