Triplet states energy decomposition

Azoalkanes are very efficient quenchers of both acetone and biacetyl triplets,160 and undergo decomposition from their excited triplet states. This decomposition can be sensitized very efficiently by triplets whose energies are as low as 42 kcal, much below the spectroscopic triplet levels of the azo compounds.461... 

The low energy mechanism is thought to involve the formation of a long-lived singlet intermediate followed by an intersystem crossing to a triplet state and decomposition into products, viz. 

When pyrrole is irradiated, only decomposition products were obtained. Theoretical data can fit this statement (Fig. 6). In fact, the direct irradiation populates the excited singlet state, which can be converted into the Dewar pyrrole or into the corresponding triplet state. Clearly, the intersystem crossing to the triplet state allows the system to reach the lowest energy state. The excited triplet state can give the biradical intermediate, and this intermediate can give either the decomposition... 

The results of this type of experiment show that CH2 itself is usually formed as a singlet species, that can decay to the triplet state, which consequently has a lower energy (MO calculations and experimental determinations show that the difference in energy between singlet and triplet CH2 is 8-10kcal mol or 33-42 kJ moP ). However, it is possible to prepare triplet CH2 directly by a photosensitized decomposition of diazomethane.Methylene (CH2) is so reactive " that it generally reacts as the singlet before it has a chance to decay to the triplet state. 

MBPT calculated barriers are too high, and gave a revised range of 13.7-20.9 kcal mol with the lower values more likely. Triplet states with intersystem crossing could also lead to other lower energy decomposition mechanisms <93AG(E)230>. 

The lifetime of the triplet excited molecule with energy 1 eV below the Si energy is probably extremely short the triplet excitations are suggested to take place to dissociative states [90]. Such conclusion can be obtained from the biphotonic sensitization experiments mentioned in connection with the energy of the triplet states (Sec. 2.1) if the decomposition had not been fast enough (t < 10 sec) due to the back transfer of energy, the sensitization could not have been observed [29]. 

H atom elimination is of lower importance from this state, 81 82 IC in the mechanism suggested by Wichramaaratchi et al. is practically identical with 8i 8x used in the terminology of Orlandi s group. The authors of Refs. 55, 83, 121, and 122 all agree that there is a dissociative triplet state, T , which can easily be populated by ISC from 81. We show the potential energy curve of this state (T ) as it has been suggested by Orlandi et al. [83,121]. This state gives only radical decomposition products. In order to explain the results of the biphotonic sensitization experiments (Sec. 2.1) another triplet state —1 eV below 81 (Ti) is also needed. Most probably, this triplet is also dissociative. 

A nonvertical type of energy transfer is undoubtedly responsible for these sensitized decompositions, since no low-lying spectroscopic triplet states are known for peroxides. The presence of the carbonyl groups seems necessary, since apparently only acyl peroxide bonds are broken directly by energy transfer. It is conceivable that there are low-lying, repulsive triplet states of these peroxides. 

---