Phosphorescence of benzophenone

Direct evidence for triplet-triplet transfer has been provided by sensitizing both the isomerization of cis- or ira/w-olefins,28 38 79,80 and the dimerization of some cyclic olefins81-83 with carbonyl compounds. Furthermore, the phosphorescence of some carbonyl compounds can be quenched by olefins (for example, acetone with 2-pentene30 and phenyl-cyclopropyl ketone with 2-methyl-2-butene37). On the other hand, the phosphorescence of benzophenone is not quenched by 2-methyl-2-butene37 nor is the photoreduction of benzophenone quenched efficiently by cyclohexene (Table II). 

Triplet-triplet energy transfer was reported to occur from benzophenone-capped (3-CD (97) to 1-bromonaphtahlene at 77 K [84], In this case, the quenching efficiency of phosphorescence of benzophenone donor is 39% and the apparent efficiency of energy transfer was estimated to be 60%. 

After adsorption of benzophenone on Ti/Al binary oxides, the systems were UV-irradiated, and benzhydrol and benzpinacol were detected as major products of photolysis. Their yields increased with UV-irradiation time, whereas the intensity of the phosphorescence of benzophenone decreased. Furthermore, the yield of benzhydrol depends strongly on the Ti/Al ratio. Benzpinacol is the only product observed in the photolysis of benzophenone adsorbed on Si02 and porous vycor glass... 

Upon adsorption of benzophenone on oxides with strongly acidic properties, the phosphorescence spectrum exhibits a structureless band with a Atnax at about 490 nm in addition to the normal phosphorescence of benzophenone. The A max of the excitation spectrum of this band was observed at approximately 380 nm, and its intensity increased in the order of the aluminosilicate, H-mordenite, and HY zeolite. In the spectrum of HY zeolite containing benzophenone, only one structureless phosphorescence band could be observed. A similar phosphorescence band could be observed for benzophenone dissolved in CHCI3, which also involves dry HCl. We can therefore assign phosphorescence at about 490 nm to the protonated form ofbenzophenone. These findings correspond with studies of the photoluminescence of benzophenone dissolved in various concentrated acidic solutions (277). Consequently, since the presence of a phosphorescence spectrum at about 490 nm with benzophenone adsorbed on the aluminosilicate, H-mordenite, or HY zeolite is associated with the presence of the protonated form of benzophenone, the data indicate the existence of proton-donor centers on these oxides with acid strengths < for benzophenone (about 5.6) (216). On HY zeolite, almost all the adsorbed benzophenone changes into protonated benzophenone. On aluminosilicate surfaces, the relative intensities of the phosphorescence spectra attributed to the protonated and unprotonated forms are approximately the same. 

The mechanism of the quenching of triplet benzophenenone by both electron and hydrogen donors involves change transfer effects". Diffuse reflectance laser flash photolysis studies of the reactions of triplet benzophenone with hydrogen atom donors on surfaces have also been reported". A dual phosphorescence of benzophenone at 77K in HjO/EtOH in glasses indicates that in this environment a... 

Let us illustrate this chemistry with an example. Benzophenone is a well-known photosensitizer, which will phosphoresce at low temperature (77 K) after excitation in the range 360-370 run. This phosphorescence of benzophenone is quenched by adding a polyaromatic hydrocarbon (PAH) such as naphthalene phosphorescence is then observed from this species even though it has no absorption band around 360-370 nm. Such observations clearly describe the activation at wavelengths otherwise transparent for a given medium. This sequence of processes can be described as follows ... 

Matsui K., Momose F. Luminescence properties of tris(2,2 -bipyridine)ruthenium(II) in sol-gel systems of Si02. Chem. Mater. 1997 9 2588-2591 Matsui K., Nozawa K. Molecular probing for the microenvironment ofphotonics materials prepared by the sol-gel process. Bull. Chem. Soc. Jpn. 1997 70 2331-2335 Matsui K., Yamamoto T., Goto T., Nozawa K., Bessho K. Fluorescence and infrared spectra of phenyl-modified silica gels prepared by the sol-gel process. J. Chem. Soc. Jpn. 1998 489-494 Matsui K., Nozawa K., Yoshida T. Phosphorescence of benzophenone in sol-gel silica. Bull. Chem. Soc. Jpn. 1999 72 591-596... 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

Fig. 23 Entropy effects on intramolecular reactions of polymethylene chains. Plot of 9AS (e.u.) against number of single bonds for (O) nucleophilic substitutions at saturated carbon ( ) electron-exchange reactions (A) quenching of benzophenone phosphorescence. The straight line has intercept +30 e.u. and slope —4.0 e.u. per rotor. The right-hand ordinate reports the purely entropic EM s calculated as exp(0AS /J )...

Excitation of benzophenone in solid solution at 77K with light of wavelength 366nm produces phosphorescence. As naphthalene is added, the... 

Data obtained from the photoreduction of benzophenone in isopropanol indicates that the quantum yield (for the formation of acetone) is nearly constant using irradiation at several different wavelengths between 366 and 254 mp..16 The quantum yield for phosphorescence emission from the carbonyl n,ir triplet state is independent of exciting wavelength.22 Furthermore, the singlet excitation spectrum for the phosphorescence emission has been found to be very similar (if not identical) to the absorption spectrum.23 There are other examples where irradiation in the region of a given transition has ultimately led to a triplet of a different type.24... 

The room temperature phosphorescence spectra of benzophenone and its substituted analogues show a weak band about 1600 cm -1 higher in frequency than the first phosphorescence band. Because the temperature dependence of its intensity corresponds to an activation energy approximately equal to the... 

Benzophenone was found to possess the criteria for a suitable donor or sensitizer For this molecule, the intersystem crossing JD 3D occurs with unit efficiency (tfisc = 1). Under the experimental setup, no phosphorescence was observed in absence of benzophenone. After... 

Triplet—triplet energy transfer from benzophenone to phenanthrene in polymethylmethacrylate at 77 and 298 K was studied by steady-state phosphorescence depolarisation techniques [182], They were unable to see any clear evidence for the orientational dependence of the transfer probability [eqn. (92)]. This may be due to the relative magnitude of the phosphorescence lifetime of benzophenone ( 5 ms) and the much shorter rotational relaxation time of benzophenone implied by the observation by Rice and Kenney-Wallace [250] that coumarin-2 and pyrene have rotational times of < 1 ns, and rhodamine 6G of 5.7 ns in polymethyl methacrylate at room temperature. Indeed, the latter system of rhodamine 6G in polymethyl methacrylate could provide an interesting donor (to rose bengal or some such acceptor) where the rotational time is comparable with the fluorescence time and hence to the dipole—dipole energy transfer time. In this case, the definition of R0 in eqn. (77) is incorrect, since k cannot now be averaged over all orientations. 

Figure 7.28 Phosphorescence spectrum of benzophenone in acetonitrile at room temperature.

Figure 13.4 (a) Absorption spectra of benzophenone (ethanol, 20°C) and naphthalene (ethanol + methanol, — 180°C). (b) Phosphorescence emission spectra at — 190°C in ether + ethanol, under steady irradiation at 3660 A. Benzophenone, 2 x 10 2 M benzophenone + naphthalene, 2 x 10-2 M and 3.2 x 10-1 M, respectively concentration of pure naphthalene (solid line) not known. From A. Terenin and V. Ermolaev, Trans. Faraday Soc., 52, 1042 (1956). Reproduced by permission of the Faraday Society. 

Compound 50 showed a maximum absorption peak at 300 nm. After irradiation at 300 nm, they showed the fluorescence emissions reported in Table 6. In the presence of benzophenone, the authors observed also the phosphorescence spectra and the observed emissions are collected in Table 6. 

The low temperature phosphorescence spectrum of benzophenone (see Figure 1) has a well-resolved structure, in which the splitting corresponds to the carbonyl vibrational frequencies. From this structure, one can determine that the triplet energy of benzophenone is approximately 69 keal/mol. (70)... 

Figure 1 Phosphorescence spectra of benzophenone, a-guaiacoxy-acetoveratrone (I), and acetoveratrone (II) in ethanol glass at 77K.

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... 

Phosphorescence measurements of benzophenone adsorbed on Ti/Al binary oxides show the presence of its protonated form in addition to benzophenone hydrogen-bonded to surface OH groups (Nishiguchi... 

Fig. 48. Phosphorescence spectrum (a) and its excitation spectra (b) of benzophenone at 4.2 K. Dashed line, solution in CX b continuous line, adsorbed on Si02 [reproduced with permission from Fenin et al 216). ...

The phosphorescence spectra of benzophenone dissolved in CCU and adsorbed on MgO, 7-AI2O3, and Si02 (all of which have a similar phosphorescence character) include four separable maxima (Table V) (216). The presence of these maxima, which are attributed to the electronic transitions from the excited state to lower vibrational levels of the ground electronic... 

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