Endothermic energy transfer

The energy of the triplet state of the sensitizer ( Sens ) must be greater than that of the reactant. If this condition is not met, the energy transfer becomes endothermic and cannot compete with other transformations of Sens. ... 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Endothermic energy transferred from surroundings to system... 

Similarly, Ervin and co-workers have measured acidities of organic molecules by measuring the energy for endothermic proton transfer reactions between acids and anionic bases." " Alternatively, it is possible to use competitive CID of proton-bound dimer ions." Nominally, these are relative approaches for measuring acidities, as the measured acidities depend on the properties of the reference acids or bases. However, it is usually possible to select references with very accurately known acidities (such as HE, HCN, or HCl), such that the accuracy of the final measurement depends predominantly on the accuracy of the threshold energy determination. 

At present, waste heat exhausted from the ICE is removed with any efficient radiator system through direct apparent heat exchanging. On the contrary, organic chemical hydrides can recuperate the chemical energy of endothermic reaction heat during exhausted heat removal. Heat transfers accompanying the phase change of evaporation and condensation of aromatic products and unconverted reactants will certainly facilitate the removal of heat from the ICE parts, with adoption of any new radiator system compelled. 

C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E. Thompson, and S.R. Forrest, Endothermic energy transfer a mechanism for generating very efficient high-energy phosphorescent emission in organic materials, Appl. Phys. Lett., 79 2062-2064 (2001). 

If AEt is about 1000 cm-1 (2.86 kcal), then ke is faster than k.e by a factor of 100. Thus the axiom that triplet energy transfer is efficient only when it is exothermic by at least 3 kcal arises because an energy gap of this size is needed to eliminate back transfer. In fact, forward energy transfer is moderately efficient even when it is slightly endothermic. (Note the rate values for 1,5- and 1,8-dinitronaphthalene where transfer is about 0.3 kcal endothermic.)... 

All the olefins involved in these studies are simple olefins to which energy transfer from ketones should be endothermic. Although the details of the mechanism of isomerization are still a matter of some debate,5,71 it is generally agreed that isomerization takes place by addition of the sensitizer to the olefin to form a new intermediate which may be of a biradical nature. Thus if energy transfer is not favored, there is another mechanism by which ketone sensitizers can induce olefin isomerization, and the observed quantum yields and photostationary states may differ sharply from those predicted by the energy transfer mechanism. 

In order to describe the energy transfer process in the condensed phase, several additional assumptions are applied to the above equations(1) no endothermic or exothermic reaction is involved within the condensed phase (below the burning surface), (2) the luminous flame zone does not contribute to the conductive heat... 

Photophysical energy transfer between the initially populated dye excited state (which is itself incapable of initiation) and other chromophores which, if populated, would yield free radical initiating species, is generally disfavored energetically. Nonclassical endothermic energy transfer processes are required. Alternatively, other processes which avoid typical energy transfer restrictions must be devised. Exciplex mediated electron transfer processes represent one such alternative. 

As it becomes clear from Fig. 15, a diazene-like conformation can be induced through energy transfer of about 300 kJ/mol. It should thus be possible to induce this conformational change photochemically. The diazene-like conformation corresponds to a double-bonded N2 moiety with two lone pairs, which can play an important role as hydrogen atom acceptors in the reduction process. This appears to be most important for a kinetic activation of the first endothermic reduction step. 

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