Triplet energy, determination

For compounds that are very weakly phosphorescent or that phosphoresce at wavelengths out of the normal range of sensitivity of the spectrometer this method of triplet energy determination cannot be applied. For these compounds triplet energies can sometimes be determined by measuring their E-type or P-type delayed fluorescence. 

Scheme. 15.3 Schematic representation of the pulse radiolysis energy transfer teehnique applied to the characterisation of the triplet state (triplet energy determination)...

Phosphinidenes have either a singlet ground state with two lone pairs and an empty p-orbital on the phosphorus atom or a triplet ground state in which the phosphorus has instead one lone pair and two singly occupied p-orbitals. Not surprisingly, the electronic preference, i.e., the singlet-triplet energy gap (AEst) and thus the stabUity and reactivity of a phosphinidene, is determined by its substituent. 

A number of experimental techniques are available for the determination of triplet energy levels. Those most commonly employed are phosphorescence spectroscopy, phosphorescence excitation spectroscopy, singlet to triplet... 

Thus the triplet energy can be calculated from the singlet energy, determined from absorption and fluorescence measurements, and AE. 

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

Let us now return to the question of how E-type and P-type delayed fluorescence may be used to determine the triplet energy level. The efficiency of E-type delayed fluorescence is given by the following equation ... 

The lowest triplet energies of a number of organic molecules, determined by the various techniques described in this section, are presented in Tables 5.3-5.5. 

The first experimental determination of a singlet-triplet energy gap (Mst) for an organic nitrenium ion was made for the 1,3-dimethylbenzotriazolium ion . A powerful ligand, 4,5-bis(diphenylphosphinoyl)-l,2,3-triazole, has been synthesized and found to possess two different modes of chelation <00ACIE3321>. 

If several different triplet quenchers of different triplet energy are used and the efficiency of the photochemical reaction is determined, a value for the energy of the reactive Ti state of the reactant can be obtained. By considering the relative efficiency of quenching for each quencher and knowing its triplet energy, it will be found that the efficiency of... 

In contrast to the case of ZnCh-Ceo, no transient formation of Ceo" was detected at 1000 nm for any other dyad in Scheme 4b [65]. In each case, only the triplet-triplet absorption due to the chlorin or porphyrin moiety was observed due to the higher energy of the radical ion pair as compared to the triplet excited state as is expected from the redox potentials. Thus, the energy level of the radical ion pair in reference to the triplet energy of a component is an important factor in determining the lifetime of the radical ion pair. 

The Sc -promoted photoinduced electron transfer can be generally applied for formation of the radical cations of a variety of fullerene derivatives, which would otherwise be difficult to oxidize [135]. It has been shown that the electron-transfer oxidation reactivities of the triplet excited states of fullerenes are largely determined by the HOMO (highest occupied molecular orbital) energies of the fullerenes, whereas the triplet energies remain virtually the same among the fullerenes [135]. 

Phosphorescence measurements of eosin in ethanol place the 0-0 transition of 3Si 3S0 at 7150 A.98 Recent determinations of phosphorescence spectra of xanthene dyes in EPA at 77°K show these transitions to occur in the region of 7000 A 94,103 (Table I). The triplet energies of these dyes, 3Bi, are thus about 40 kcal/mole and the energy separations between the excited singlet and triplet states, AE = Elai — 3si, are about 12-13 kcal/mole. 

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