Triplet, energy levels, determination

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... 

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 triplet energy of thianaphthene 1,1-dioxide was determined by two indirect methods. The first involved the use of several sensitizers of decreasing triplet energy. The results summarized in Table 1 indicate that triplet lies between 53 and 49 kcal mol-1. The second method is more precise and involves the use of thianaphthene 1,1-dioxide as a sensitizer to establish a photostationary state of the a—methylstilbenes. The composition of the photostationary state of a-methylstilbene has been determined as a function of the triplet energy level of the sensitizer. The results indicate a triplet energy for thianaphthene 1,1-dioxide of 50 1 kcal mol-1. Quantum yields of the photodimerization of thianaphthene 1,1-dioxide were determined in benzene as a function of concentration. Oisc is 0.18. The product distribution as a function of solvent polarity demonstrates the ratio of the head-to-head to head-to-tail dimer (HH/HT) increases with the polarity of the solvents. This is consistent with preferential solvatation of the head-to-head transitions state. 

Table 3 Triplet energy levels of different compounds determined in EPA matrix at 77 K [25]...

At 245 GHz and at low temperatures (< 50 K), the differences between the populations of triplet energy levels have a perceptible influence on the intensities of the Ams = 1 lines. These differences result essentially from the dominating electron Zeeman term. The outermost peaks in Fig. 4.24, corresponding to maximum zfs have an intensity ratio opposite to that in Fig. 4.15, and therefore D < 0, as confirmed by the spectral simulations in the figure. The value of J was not determined in this case, however. 

The triplet-state energy level of oxytetracycline, the excitation maximum (412 nm), lifetimes of Eu-OxTc (58 p.s) and Eu-OxTc-Cit (158 p.s), were determined. A 25-fold luminescence enhancement at 615 nm occurs upon addition of citrate within a short 5-min incubation time at neutral pH. It s accompanied by a threefold increase of the luminescence decay time. The optimal conditions for determination of OxTc are equal concentrations of Eu(III) and citrate (C = T lO mol-E ), pH 7.2. Eor determination of citrate, the optimal conditions concentrations of Eu(HI) and OxTc are 1 0,5 (Cg = MO Huol-E-i, = 5-10-HuohE-i) at pH 7.2. 

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. 

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