Triplet ground state solvent effects

Such solvent stabilization effects on the reactivity of singlet carbenes in equilibrium with their triplet ground states have also been observed experimentally in other arylcarbene derivatives. ... 

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Heavy Atom Effects. By virtue of their ability to enhance spin-orbit coupling, heavy atoms promote both radiative and nonradiative spin forbidden processes.164 Thus heavy atom solvents have been used to increase the extinction coefficient of ground state to triplet absorption and thereby render these transitions visible.165... 

The relative linearity of a Stern-Volmer plot of 1/< > versus 1/[A] for the photoreaction of cyclohexenone supports this mechanism and suggests that only one excited state is involved (30). The [2+2] addition of the triplet to a ground state molecule is not concerted, and there is a large solvent polarity effect on the regioselectivity for the head-to-head (HH) versus head-to-tail (HT) photodimers, with the more polar head-to-head isomer being favored in polar media. The polarity effect is attributed to the large difference in the dipole moments of the transition states leading to the products. 

The crossing oj between the reactant s ground state PEC and that of the radical product, allows the establishment of the following structure-efficiency relationships (i) an increase in coupling decreases of triplet reactions when but will have no effect when > E (ii) in a reaction series, the formation of less stable radicals or a reduction of will lower E and may increase because will increase relative to E (iii) in a reaction series, the reduction of d caused by transition state stabilization lowers E and may reduce (iv) does not depend on deuteration (v) singlet states are less efficient that triplet states (vi) of Type II reactions may increase with solvent polarity. 

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