Triplet state thermally populated

Attempts to detect a thermally populated triplet state ( A ) of 8 by ESR spectroscopy were unsuccessful. This was attributed to the high reactivity of the diradical, which presumably easily abstracts hydrogen atoms in hydrocarbon matrices (to form p-quinodimethane), even at very low temperatures. In this context, the triplet state of /7-phenylenebis(phenylmethylene) has been observed. Apparently, substitution of the carbenic hydrogens of 8 by phenyl groups confers sufficient stability (thermodynamic and perhaps kinetic) to the biradical, which allows its observation. According to variable-temperature ESR spectroscopy, the triplet state of /7-phenylenebis(phenylmethylene) is thermally populated and the singlet state lies 0.5-1 kcal/mol lower in energy [76-79]. 

In the case of the pseudo-me/o-isomer (m-[38]), signals due to a quintet state were not observed. Instead, a set of signals due to the thermally populated triplet Tj appeared at 25 K. The temperature dependence data were analysed in terms of an equation similar to (13) [3 exp —AEjRT) in place of 5 exp (-iAEIRT) in the numerator] to give AE j = 98cm" ( = 280 cal mol" ). The behaviour of p-[38] was more like o-[38], although the former was more reactive and did not allow a satisfactory temperature-dependence study. 

These observations can be explained as follows When the HOMO-LUMO energy gap is sufficiently narrow, the equilibrium between the singlet ground state and a thermally accessible triplet state prevails, and a high population of the triplet state occurs. Also, the paramagnetic dilution in-... 

A thermally populated triplet excited state of the triclinic form of (KS03)2N02 has been detected by e.p.r. and static susceptibility measurements. The crystal structure of HK2NO6S2 has been redetermined from three-dimensional neutron-diffraction data. The reaction of SO2 with molybdate ions in aqueous solution... 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

The isomerization of all-trans-[3-carotene under N2 atmosphere by photosensitization action of eight chlorophyll compounds naturally present in the extracts of green vegetables was investigated by illumination with fluorescent white light (3000 lux) at 12°C to minimize the thermal degradation (O Neil and Schwartz 1995). All chlorophylls showed similar isomeric distribution and efficiency, 9-d.s- 3-carotene is the main isomer formed. On the other hand, the illumination of all-fran.s-(3-car-otene without chlorophylls indicated that the main isomer formed was 13-d.s- 3-carotcnc, probably due to the population of a different triplet state manifold by direct photolysis. 

As stated earlier, in the state of thermal equilibrium at room temperature, dihydrogen (H2) contains 25.1% parahydrogen (nuclear singlet state) and 74.9% orthohydrogen (nuclear triplet state) [19]. This behavior reflects the three-fold degeneracy of the triplet state and the almost equal population of the energy levels, as demanded by the Maxwell-Boltzmann distribution. At lower temperatures, different ratios prevail (Fig. 12.5) due to the different symmetry of the singlet and the triplet state [19]. 

Several routes are possible to populate the triplet state. The triplet excited state can, in principle, be directly excited from the ground state, but a low extinction coefficient associated with the S0 to T, transition (reflected in the long lifetime) makes direct excitation an inefficient process for tryptophan. The triplet state can be thermally populated, but for tryptophan the large energy gap between the ground state and the triplet state makes this process unfavorable. Energy transfer from a higher state can also populate the... 

In several types of compounds, excited triplet levels lie very close to the ground states, as has been mentioned already. Such thermal population of low-lying triplet levels occurs frequently in inorganic complexes as well as with organic biradicals. Thermochromism of bianthrone has been explained as due to thermal conversion to a triplet state.129... 

ISC from the optically prepared singlet state populates one or two low-lying A" triplet states in a few hundreds of femtoseconds, see Sect. 3. Triplet states are initially populated hot, that is nonequilibrated both in terms of the molecular structure and the medium. Relaxation processes, which occur on the timescale of picoseconds to nanoseconds (depending on the medium), will be discussed in Sect. 5. Herein, we will deal with thermally equilibrated (relaxed) lowest triplet states and their theoretical as well as experimental characterization. 

Energy transfer populates the triplet state (3im state) of the azo compound. Then cis-trans isomerisation occurs leading to the cis-isomer, which is in most cases unstable and decomposes thermally at room temperature. 

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