Xanthone triplet state

The results described above represent the first example of the FR mechanism (Scheme 1). Semiempirical calculations on this molecule showed that the intersystem crossing to the excited triplet state is favored The reaction cannot be sensitized by xanthone because the triplet state of 3,4-diphenyl-1,2,5-oxadiazole is lower than that of xanthone. The cleavage of the triplet state to the biradical is favored, considering the relative energy of this intermediate (Fig. 23) (OOOUPl). 

Product (41) was also obtained in the presence of xanthone as sensitizer. This product could be totally eliminated by the triplet quencher piperylene, indicating (41) to be derived from the diphenylethylene triplet state. 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

In the case of xanthone at least, this order is not only shown up in the Forster cycle estimates, but has been confirmed by observing the variation with pH of the optical densities of the triplet states of B and BH+ and comparing it with the fluorescence intensity behaviour (see Fig. 6). Confirmation that the pK order obtained using the Forster cycle is reliable in such cases is also found in a direct determination of p/ (Tj) of benzophenone by a laser technique the value derived is consistent with earlier phosphorescence observations (Rayner and Wyatt, 1974). Ledger and Porter (1972) observed a marked decrease in the phosphorescence intensity of benzophenone near pH 5, and the apparent discrepancy between this result and the p/ (Tj )-value of 1-5 is due to the very large difference in lifetimes of BH+(T,) and B(Tj). Since unprotonated benzophenone has a very shortlived St state [1/kj for the intersystem crossing alone in ethanol is 16 5 ps (Hochstrasser et al., 1974)], protonation in this state is unlikely. However, Forster cycle calculations indicate that the singlet state would be a weaker base than the triplet state. The realization that unprotonated benzaldehyde and acetophenone had Tj states of the... 

Triplet states of xanthone depend upon the composition of the solvent the presence of water enhances the yield of 2 un state Dual phosphorescence from 2-(2 -hydroxyphenyl) benzoxazole is due to keto-enol tautomerism and the kinetics multi-exponential decay is due to differences of environment . Triplet state properties and triplet state-oxygen interactions of the biologically interesting linear and angular furocoumarins are useful in view of possible clinical application . 

Generation and trapping of radical cations of a,co-diphenylpolyenes within the channels of pentasil zeolites provides an environment which allows these transient species to be spectroscopically characterized . Similarly, complexation of xanthone in cyclodextrin has made it possible for the triplet state of this molecule to be fully characterized . Association and dissociation processes are related to the dipoles developed in the complex and in solution. A unimodal Lorentzian lifetime distribution for 2-anilinonaphthalene-6-sulphonate B-cyclodextran inclusion complexes have been recovered by multifrequency phase-modulation fluorometry in the presence of the quenchers Cu, acrylamide, and I . Both the fluorescence and phosphorescence spectra of benzo[f]quinoline adsorbed on p-cyclodextrin/NaCl have been determined as a function of temperature . 

On irradiation in alcohols in the presence of a triplet sensitizer, e.g. xanthone, 2(5//)-thiophenone affords i ff-addition products [62] (Scheme 21). This behavior is identical to that of the corresponding furanones, which also undergo H-atom abstraction from the excited triplet state [63],... 

The excited triplet state of xanthone has a n,n configuration in most solvents which makes this excited state fairly unreactive. In addition, the maximum of the triplet-triplet absorption spectra is dependent on solvent polarity [184], In carbon tetrachloride, the maximum occurs at 655 nm, whereas in water, it is blue shifted to 580 nm [184,185]. The n,n nature of the excited triplet state of xanthone leads to a higher dipole moment for the excited triplet state when compared to the ground state. This is the impetus required for relocation. 

Phenyl cations are likewise generated by irradiation of the corresponding diazo-nium salts. In this case, no activating group is required, but the substituents play an important role. Thus, the cation is formed selectively in the desired triplet state with electron-withdrawing substituted derivatives, whereas in the other cases sensitization by benzophenone or xanthone is required [22]. 

Table 14 Equilibrium constants and association and dissociation rate constants for the ground-state and triplet excited state of xanthone (35) with CDs...

Sensitisation by a high-energy donor, such as triplet xanthone, can populate both the Qi and the D, states by energy transfer (Figure 10.4). This results in both the photosolvation reaction and phosphorescence emission. 

The excited states involved in the mechanisms of the photochemical transformations of the 2-alkylidenecyclo-butanones were elucidated without special difficulty (vide supra). These a, B-unsaturated ketones undergo only one of the reactions characteristic of cyclobutanones (i.e., ring expansion). In addition, the triplet energy of the enone chromophore is low enough that this excited state may be efficiently and selectively populated by standard carbonyl triplet sensitizers (e.g., acetophenone, xanthone, and benzophenone), thereby demonstrating that ring expansion occurs via the 2-alkylidenecyclobutanone state, while the isomerization process (i.e., [81]J[82]) occurs via T. ... 

BF, and SOgMe in (142), the cleavage is sensitized by xanthone but evidence is presented which indicates that for the former four compounds exciplex formation occurs rather than triplet energy transfer. A useful semi quantitative scale of photofugacities of the substituent groups in the excited singlet state is given. 

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