Diradicals fluorescence

Andrews and Day observed fluorescence from methylene-1-pyrazolines. They also showed that energy transfer from the lowest excited singlet state to dienes (piperylene) occurs with moderate efficiency. The decomposition products from methylene pyrazolines have been postulated to be the resonance stabilized diradicals,... 

The effect of oxygen on cyclic 1,3-diradicals shows that conformation can affect the triplet state lifetime ST Time resolved resonance Raman spectroscopy has been used to examine triplet states produced from different isomers of p-carotene. A theoretical study has also been reported on the a-cleavage of the triplet states of symmetric and non-symmetric ketones S mechanism for triplet state relaxation of aromatic molecules has been used to explain experimental data for substituted benzenes. The decay kinetics of triplet-triplet fluorescence in the mesitylene biradical (two sub-levels) have been measured between 10 and 77K in Shpolski matrices triplet state of dimesityl... 

According to this mechanism, the absorption of light produces an excited singlet state of the monomer which may either fluoresce [Eq. (6.67)] or be converted to an excited (and long-lived) triplet state [Eq. (6.68)]. The latter may be regarded as a diradical, that is, CH2-C(H)X. Attack on the monomer by this diradical ultimately yields two monoradicals [Eqs. (6.69) and (6.70)], which, in turn, initiate polymerization. For both types of direct photoinitiation the rates are proportional to the light intensity and to the extinction coefficient of the monomer (see later). 

Picosecond fluorescence spectroscopy has allowed direct observation of a hydrocarbon singlet 1,3-diradical from (77b),and detailed descriptions of studies on the decomposition of (77b) have been published in full. ° Flash vacuum pyrolysis of (80) and of (81) and further details of the thermal and photochemical decomposition of (82) ° have been described. 

As a starting point for the mechanistic discussion, it is useful to review the structural features of the excited states. The first singlet excited state of butadiene. Si, can be approximated as the i >2->--LUMO tt-tt transition. The lack of fluorescence from this excited state indicates that a very facile path exists for nonradiative energy transfer. The S2 state has doubly excited character and relaxes to a structure with ionic character that can rotate at the pyramidal carbon but not at the allyl fragment. The minimum energy of the Tj state corresponds to the allyl-methylene diradical with a nearly 90° twist and slight pyramidalization at the methylene carbon. In substituted systems, one or the other of the zwitterion structures (e.g., allyl cation versus allyl anion) may be favored. ... 

Valence isomerization of diazoalkanes into diazirines and vice versa is possible by photolysis, although it is always accompanied by dediazoniation. The photolysis of diazirines was investigated intensively in recent years (e.g., by O Gara and Dailey, 1992, and by Modarelli and Platz, 1993 see also the book of Michl and Bonacic-Koutecky, 1990). An electronically excited state is obtained that can, in principle, decay by at least four competitive pathways (Scheme 5-16) a) fluorescence, b) intersystem crossing with the production of triplet carbene, c) formation of an excited diradical followed by internal conversion and production of singlet carbene, and d)... 

These seemingly easily distinguished mechanisms are considered together as an alternative to the diradical mechanism for reasons which will become apparent. Electron transfer is particularly important in the more electron deficient, cyclic per-ester dioxetanones. Dioxetans in general do not respond to activators -fluorescent compounds of low ionisation potential (see p. 40) almost certainly because they are poorer oxidants than dioxetanones. There is however a hint that simple dioxetans, if sufficiently strained may accept electrons from an activator [36]. The light emitted from the cyclic compound (S) shows a linear dependence on DP A concentration, and the ionisation potential of several fluorescent compounds. However, true electron exchange luminescence cannot be... 

Strong quenching effects were observed by 02,1,3-pentadiene, tri-t-butyl phenol and tetramethylethylene (in increasing order of effectiveness). As DP A and 1,3-diphenyl isobenzofuran act as fluorescers, but not 9,10-dibromoan-thracene, the active species appears to be in the singlet state, although quantitative results have not been reported. It was concluded from the kinetic and spectroscopic data that dioxetanedione as well as the 1,4-diradical are present not only in the... 

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