Xanthone fluorescence

For this technique to be applicable, one or both forms of an acid-base pair must show a T—T absorption. If, as in the case of xanthone (see Fig. 5), T—T absorption spectra are obtained from both forms, the observed optical densities must be corrected for any overlap in a manner analogous to that used for fluorescence inten-... 

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... 

Fig. 1. Chemical structure of aflatoxins. (a) The B-type aflatoxins are characterized hy a cyclopeutane E-ring. These compoimds have a hlue fluorescence under long-wavelength ultraviolet hght (h) The G-type aflatoxins have a xanthone ring in place of the cyclopentane, (c) Aflatoxins of the B2 and G2 type have a saturated bis-furanyl ring. Only the bis-firran is shown, (d) Aflatoxins of the Bi and Gi type have a hydrated his-furanyl structure.

Delayed fluorescence from a very-short-lived upper excited singlet state populated by hetero-TTA has been observed for the first time using the system A = anthracene and X = xanthone (Nickel and Roden, 1982). An energy-level diagram for this system is shown in Figure 5.32, and the corrected spectrum of the delayed fluorescence of anthracene and xanthone in trichlorotrifluoroe-thane is depicted in Figure 5.33. The band at 36,000-40,000 cm has been assigned to the delayed fluorescence of anthracene produced by Tf +... 

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).

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 . 

An unusual example of delayed fluorescence exemplifying El Sayed s rules (Section 2.1.6) was recently reported for the triplet sensitizer xanthone,127 which undergoes ultrafast ISC within 1 ps. Delayed fluorescence with a lifetime of 700 ps was observed in aqueous solution. Temperature-dependent steady-state and time-resolved fluorescence experiments indicate that the T2(n,it ) state, which is primarily accessed by ISC from Si(ji,ji ), is nearly isoenergetic with the Sj state. The delayed fluorescence is attributed to reverse ISC from T2(n,it ), in competition with internal conversion to Tl(7I,7l ). 

Heinz, B., Schmidt, B., Root, C., Satzger, H., Milota, F., Fierz, B., Kiefhaber, T., Zinth, W., Gilch, P, On the Unusual Fluorescence Properties of Xanthone in Water, Phys. Chem. Chem. Phys. 2006, 8, 3432 3439. 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

Loyola University of Chicago), found that Hammond s estimate is accurate, and the fluorescent lifetime of 2 and 40 ate 180 ps. Triplet-sensitized reactions of 2-pyridones have not been extensively studied. However Sharp and Hammond found no evidence for 2-pyri-done photoproducts when xanthone is used as a photosensitizer. Using tethered pyridones, Nakamura found [2+2] and [4+2] cycloaddition products with benzophenone sensitization (see Figure 14). This is consistent with 2-pyrone photochemistry,but the generality of these results await further experimentation. 

On the other hand, a variety of aromatic ketones have proved to be reactive. Polycyclic aromatic ketones such as anthrone, fluorenone, and xanthone all undergo successful Ru catalyzed copolymerization with 1,3-divinyltetramethyl-disiloxane [12], see Figure 8. It has also been possible to incorporate phenanthrene units, which strongly absorb in ultraviolet and fluoresce, into an fl/l(carbosilane/siloxane) by copolymerization of 2-acetylphenanthrene and 1,3-divinyltetramethyldisiloxane [13]. Initially, the molecular weight of these materials was quite low [12]. 

The European mushrooms of the genus Cortinarius (Corti-nariaceae) characteristically contain the xanthone dermoxanthone (9-115) and its methyl ester that were found in the stem of the surprise webcap C. semisanguineus. These xanthones are responsible for the bright yellow fluorescence of the mushroom under UV light. 

The photochemistry of xanthone (29) in the presence of CD [86] and added alcohols [86,87] was studied by absorption, fluorescence and laser-flash photolysis. The addition of CD to a 29 water solution caused a blue-shift of the fluorescence spectrum and a substantial decrease of its intensity this allowed the determination of the ground-state association equilibrium constant (Ki — 36, 1000 and 200 for a-, fi- and y-CD, respectively) [86]. The intensity decrease at equal [CD] was smaller in the presence of alcohols which suggested the formation of a ternary complex with equilibrium constant K4. With / -CD and hydroxypropyl-j8-CD (UP-fi-CD) K4complexation strength was observed. In the presence of HP-/5-CD no K4 dependence on the alcohol nature was observed while, in the presence of /5-CD, K4 increased in the order 1-butanol < cyclohexanol < cyclopentanol < 1-pent-anol < tert-butanol < 2-butanol. 

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