Triplet ground state polarization

The description of the chemisorption in terms of cycloaddition reactions is useful if it leads to reliable predictions of the reaction products for most reactions, a variety of products are possible, yet only one will result from a particular cycloaddition mechanism. Central to the applicability of such schemes is the notion that the silicon dimers contain a weak 7r bond responsible for the enforced concerted motion of the two electrons involved. However, in reality there is little evidence to support the presence of even a weak 7r bond within the dimers. While DFT calculations that enforce spin pairing depict the bond as a singlet biradical [32], spin-polarized calculations predict a triplet ground state for the unbuckled dimer [33] with no 7T character whatsoever. The decoupling of the two silicon electrons means that their motion is not likely to be concerted so that a [2+2] cycloaddition reaction becomes better represented as an independent [1 + 2+1] process, a notation that recognizes the independence of the silicon free radicals. This mechanism is also illustrated in Fig. 3. In practice such a reaction is unlikely to proceed in a concerted fashion, and a key signature for it would be the... 

Anodization of Si in HF under an applied magnetic field produces an enhancement of the PL efficiency at RT, accompanied by an enhanced porosity compared to PS samples prepared without an applied field. The degree of polarization of the emitted PL is reduced for field-assisted preparation [Na3]. At low temperatures (4.2 K), the Stokes shift and the decay time of the PL are found to be increased, if compared to PS formed under zero magnetic field. This has been interpreted as Zeeman splitting of the spin-triplet exciton states. It indicates that the ground state of the luminescing silicon crystallite is a triplet state [Kol3]. 

We can conclude from these thermodynamic considerations that it is possible to estimate the redox potentials of excited molecules, if we know the equilibrium redox potentials for the molecules in the ground state, as well for reduction as for oxidation, and add or subtract from these redox potentials the excitation energy AE of the lowest singlet or triplet state. For most dye molecules the reduction redox potential is experimentally more easily accessible than the oxidation redox potential. In such cases we have found that an estimation can be made by assuming that the ionisation energy of the dye molecule in crystalline state is similar to the ionisation energy in a polar solvent and gives an approximate value for the absolute redox potential. Such estimations are especially useful for a comparison of molecules with similar structure. 

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. 

---