Triplet xanthone

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. 

A larger MIE of Ge was found in the photolysis of dimethyldiphenylgermane (Ph2Me2Ge) sensitised by triplet xanthone ( Xn ) in Brij 35 micelles [8]. 

Excited triplet xanthone was initially employed as a probe molecule to study the dynamics of surfactant aggregated to polyelectrolytes [186]. Triplet-triplet absorption spectra were obtained 40 ns and 1 ps after the laser pulse, and a blue shift was observed. These two spectra were ascribed respectively to xanthone within the macroassembly and in the aqueous phase. Quenching experiments were carried out where the probe residing in the aqueous phase was quenched, whereas that in the self-assembly was protected. The xanthone dissociation rate constant from the aggregates was 1.1 x 10 s" [186]. 

Quenching studies were also employed to determine the dissociation rate constant of triplet xanthone from CD cavities. These apparent rate constants were obtained by measuring the decay of triplet xanthone at high concentrations of cupric ions. The values for kp were respectively (9 2) x 10 s" and (15 2) X 10 s for P- and y-CDs [185]. The higher value obtained in the quenching experiments for y-CD indicated that the quencher had access to the probe within the CD cavity. Unfortunately, no detailed analysis employing Eq. (25) was performed and, for this reason, a comprehensive comparison between the two methods is not possible. 

Although the effect of alcohol addition on the ground state equilibrium constant for p-CD was opposite to that observed for y-CD, a decrease of the dissociation rate constant for triplet xanthone was also observed. The slow down of the dissociation process has been tentatively attributed to the preferential solvation of the CD cavity entrances by alcohols [190]. 

Frequently, co-solvents are added to aqueous cyclodextrin solutions in most cases, these co-solvents are employed to solubilize the probes. In Section IV.C, the decrease of the exit rate constant of triplet xanthone from CDs with the addition of alcohols was described. This effect was also apparent when studying the dynamics of 1-halonaphthalenes with P-CD in the presence of acetonitrile [141]. When the nitrite ion was used as quencher, the association rate constants decreased in the presence of the organic solvent while the dissociation rate constant increased (Table 21). The main rationalization to explain the change in mobility properties was that acetonitrile was small enough to coinclude inside the cavity a small amount of acetonitrile could preferentially solvate the entrances of the CD thereby leading to a different environment for the probe. 

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. 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

Photolysis of DAX in a methylcyclohexane glass at 77 K creates a metastable species detected by its optical absorption spectrum (Table 4). This solution does not exhibit an epr spectrum characteristic of a triplet carbene, nor is one observed when DAX is irradiated in Fluorolube (where triplet carbenes are generally more stable). Warming the frozen solution causes the reaction of the metastable transient and the formation of dimeric xanthone azine. 

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

The remaining compounds possess it,it emitting triplets in all media used (Group III). The members of this class are 1,2-diphenylacetone, o-methoxy-acetophenone, xanthone, and 2 -acetonaphthone. This group is characterized by a long-lived phosphorescence in all solvents and by a decrease in the energy of the emitting triplet upon increased solvent polarity. 

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 this case, only a thermal pathway for the formation of the cyclooctatetraene is proposed, in agreement with many other publications (for references, see Sec. II and Table 3). Tinnemans and Neckers [62], however, describe the ring opening of the ortho adduct from methyl phenylpropiolate and benzene as well as the reverse reaction as photochemical processes. The formation of this ortho adduct can also be accomplished in a xanthone-sensitized photoreaction [63], and in that case, the authors consider the ring opening as a thermal or a triplet-sensitized reaction and the reverse reaction as one proceeding via the singlet. 

The changes in the triplet-triplet absorption spectra of thioxanthone in solvents of different polarity facilitate a study of its behavior as a guest molecule in [1-cyclodextrins. In particular, the effects of structural changes to the host molecule on the complexation efficiency and dynamics have been found to be very similar to those of xanthone <2001PCB2122>. 

Figure 5. Triplet-triplet absorption of xanthone at three pH values (a) pH =7-0, (b) pH = 2-8, (c) pH = 2-1 (Ireland and Wyatt, 1972).

Figure 6. Relative optical densities of triplet-triplet absorption of protonated and unprotonated xanthone as a function of acidity O, at 530 nm (protonated form) ,

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... 

Quenching experiments (anthracene) as well as sensitisation (xanthone) show that in that reaction a triplet is involved 78b>. This is not clear in the photolysis of A, which leads also to a cyclopropene derivative B 78c>. 

Attempts to sensitize the rearrangement of [79a] to [80a] with xanthone (ET = 74 kcal/mole) under conditions where the sensitizer absorbed essentially all of the incident radiation resulted in no observable chemical change (35). To determine if the cyclobutanone was receiving triplet energy from the sensitizer, the direct and sensitized photochemistry of syn- and anti-2-sec-butylidenecyclobutanones [81] and [82] were investigated (35). Under direct irradiation (313 nm), the isomeric acetals [83] and [84] were produced quantitatively ( = 0.1-0.2) yet under sensitized conditions (e.g., with xanthone, acetophenone, benzophenone, or triphenylene),... 

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. ... 

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 . 

Figure 5.32. Energy-level diagram of anthracene (A) and xanthone (X). Double lines denote the three different triplet pairs for which TTA processes are indicated asterisks mark the delayed fluorescence (DF) resulting from hetero-TTA (by permission from Nickel and Roden, 1982).

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