Singlet-triplet transitions nitrogen

Numerous photochemical transformations of pyridazines and their N-oxides have been reported by several groups. The lowest singlet-triplet transition observed for pyridazine in rigid-glass solution was later revised, and the intersystem crossing quantum yield was determined. Photolysis of pyridazine in the gas phase gives nitrogen and vinylacetylene as main products. ... 

In practice, handling Sq- transitions is often simpler (though less satisfactory), since absorption spectra are not normally available and the maxima of the B and BH+ phosphorescence bands must then be used in the Forster cycle. However Sq-Tj absorption spectra can sometimes be obtained, especially if perturbation methods can be used to enhance the singlet to triplet transition probability. For example, Grabowska and Pakula (1966) induced Sq-Tj absorption in a series of nitrogen-containing heterocyclic compounds by the oxygen perturbation method of Evans (1957). Hence, for these compounds, by combination of absorption and phosphorescence spectral results, the 0-0 transitions could be located more accurately. 

In the case of PN the origin of the absorption band at 350 nm is the excitation of an electron from the lone pair orbital on nitrogen (nz) to the singly occupied nitrogen 2p orbital that lies in the molecular plane (py). In the case of PN two transitions (To -> T3, To T4) contribute to the absorption band around 300 nm, one of which is the same in nature as in the case of PN. It is reasonable to assume that the same situation will be found with the substituted phenylnitrenes. Therefore, we can predict that the influence of the substituents on the maxima of the intensive absorption bands of substituted singlet and triplet phenylnitrenes will be similar. Indeed, Fig. 5... 

Fig. 19.4. Stimulated emission depletion (STED) microscopy reveals densely packed charged nitrogen vacancy (NV) color centers in a diamond crystal, (a) State diagram of NV centers in diamond (see inserted sketch) showing the triplet ground ( A) and fluorescent state ( E) along with a dark singlet state ( E) and the transitions of excitation (Exc), emission (Em), and stimulated emission (STED). (b) The steep decline in fluorescence with increasing intensity /sted shows that the STED-beam is able to switch off the centers almost in a digital-like fashion. This nearly rectangular ...

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