Excited singlet and triplet state

Fluorescence and phosphorescence spectra of poly(propynoic acid)(FPA), polyphenylene (PP), and DPAcN show that the difference of energies between the lower excited singlet and triplet states, as observed in the case of PP (583 nm) and DPAcN (528 nm), is considerably greater than that of poly(propynoic acid) (270—300 nm) which besides transitions may undergo rr - transitions. PCSs showing only... 

Petke JD, Maggiora GM, Christoffersen RE (1992) Ab-initio configuration interaction and random phase approximation caclulations of the excited singlet and triplet states of uracil and cytosine. J Phys Chem 96 6992... 

Molecular orbital calculations on ethylene indicate that the lowest energy excited singlet and triplet states have a twisted geometry.(2) This geometry helps minimize electron-electron repulsion. Figure 9.1 gives the calculated... 

PHOTOCHEMICAL KINETICS CONCENTRATIONS, RATES, YIELDS, AND QUANTUM YIELDS For a molecule A undergoing light absorption and reaction in its lowest excited singlet state to form a product P, we can write the following hypothetical mechanism, where A and Af are the lowest excited singlet and triplet states, respectively ... 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

This method can be successfully applied to any aromatic hydrocarbon whose lowest excited singlet and triplet states are lower in energy than the substrate... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

To question 3 (deactivation of the excited stabilizer molecule in its excited singlet and triplet state) ... 

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

Here A, lA, and3A represent anthracene in the ground state, the first excited singlet state and first excited triplet state, respectively. In addition, I represents the onium salt initiation, while Rs and Rt correspond to the reactive centers formed by reaction of the onium salt with the excited singlet and triplet state anthracene, respectively. 

The Excited Singlet and Triplet States of Tyrosine and Tyrosinate... 

Figure 4. Estimated transition energies of low-lying excited singlet and triplet states in the 5.35A MgP-P dimer. S1-S4 and T1-T9 correspond to localized states on the indicated monomers. 85 and Tio correspond...

The lowest excited states of paramagnetic metal complexes are described by configuration interactions of the porphyrin (7T,tt ) excited singlet and triplet states and the "porphyrin-to-metal" or metal-to-porphyrin charge-transfer excited states (35,36). Thus T (phosphorescence) emission of paramagnetic metal complexes decays... 

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. 

Note The symbols S0, S, and T, represent the ground electronic state and first excited singlet and triplet states, respectively, of BaP. 02( Ag) is 22.5 kcal mol-1 above the ground state of 02, while for BaP the S, and T, states are 71 and 42 kcal mol-1 above the S0 ground state, respectively. 

B. Quenching of fluorescence by a08. The oxygen quenching of fluorescence of aromatic hydrocarbons both in solution and in vapour phase is in general diffusion-controlled. 1 he rate constants for the 08-qucnching of excited singlet and triplet states are given in Table 8.2... 

Rate constants for Oj-quenching of excited singlet and triplet state molecules. 

At very short times, very little motion of reactants has occurred so that little, if any, reaction will have taken place. But the manner of creation of the mixture of A and B reactants should be considered. A very simple means of preparing a reaction mixture is by photolysis. For instance, consider a solution of anthracene and carbon tetrabromide. Photostimulation of anthracene with an extremely short duration light pulse produces excited singlet (and triplet) states. The carbon tetrabromide quenches the excited singlet state fluorescence very efficiently. Just before the photostimulation event, the quencher (i.e. B) is randomly distributed throughout the system volume and for a short time after photostimulation, it remains randomly distributed. With the exception of the location where the fluorophor A is, there is no preferred location of the quencher B. No... 

The lowest-energy excited singlet and triplet states are called S, and Th respectively. In general, Tj has lower energy than S,. In formaldehyde, the transition n —>, it (T1) requires... 

An important problem concerns the nature of the first excited state actually involved in the photodimerization reaction (the lowest excited singlet or triplet states). The simple 77-HMO method does not distinguish between the excited singlet and triplet state. Its results concern the first excited state and cannot be correlated precisely with the different multiplicities. For such a correlation, the more refined approximations of the molecular orbital method, which eliminate spin degeneracy, must be used. 

Another less-common form of unimolecular decay is also temperature dependent and results in the phenomenon known as is-type delayed fluorescence.183 For example, the lowest excited singlet and triplet states of eosin are quite close together in energy, so that excited... 

Various mechanisms for the photoinduced geometrical isomerization of 9-styrylanthracenes have been proposed recently [90-93]. From steady-state and time-resolved fluorescence measurements on the parent 9-styrylanth-racenes 68a/69a and their formyl and 4-phenylsulfonyl derivatives (681/691 68j/69j), it has been concluded that the geometrical isomerization is an adiabatic process which occurs both in the excited singlet and triplet states [90]. Thus, for the conversion of the parent cis compound 68a into 69a, 85%... 

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