Benzophenone fluorescence

From these equations one can approximate kf for benzophenone to be 5 x 10s sec-1. This, however, is the expected rate constant for fluorescence, which should be in competition with radiationless deactivation of the excited state kd. In actuality no fluorescence is observed for benzophenone although the fluorescence techniques are sensitive enough to detect fluorescence occurring with a quantum yield as low as 0/ = 0.001. Therefore kd must be at least 1000 times greater than kf We have... 

Quantum yields and lifetimes of emission (fluorescence) as well as other principal rates of deactivation have been measured on 2-hydroxy benzophenone and 2-hydroxyphenyl benzotriazole derivatives. Polymerizable UV screening agents have been prepared and copolymerized with acrylics in order to obtain transparent films containing nonfugitive UV screening agents. Preliminary results of studies of photodegradation on these copolymers are also reported here. 

Many aromatic aldehydes and ketones (e.g. benzophenone, anthrone, 1- and 2-naphthaldehyde) have a low-lying n-n excited state and thus exhibit low fluorescence quantum yields, as explained above. The dominant de-excitation pathway is intersystem crossing (whose efficiency has been found to be close to 1 for benzophenone). 

Logarithmic plots of the fluorescence decays for the lowest excited singlet state of H2TPP, H2OEP, and their deuterium analogues were described by single exponential forms. Fluorescence lifetimes of all compounds were obtained by deconvolution and fluorescence quantum yields were determined relative to rhodamine 640 in ethanol (32). Triplet quantum yields (0r) were determined relative to benzophenone in benzene by using the transient absorption (33). Quantum yields and lifetimes determined are summarized in "Table II". 

Indeed it has been shown154 that Aric, the rate of intersystem crossing, is greater than 1010 sec-1 for benzophenone and presumably for other aromatic ketones155 however, in many compounds it is only about 107 sec-1.156,157 Thus if their reaction rates with Sx are close to diffusion controlled, certain substrates can interfere with intersystem crossing. If the sensitizer fluoresces, singlet interactions can be detected by a decrease in the intensity and lifetime of the emission. The interaction may or may not lead to chemical reaction in the substrate, but kinetic complications will arise in... 

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. 

A study has been made of the relative efficiencies with which various transition metal chelates quench triplet benzophenone.194 The chelates vary widely in efficiency, and no generalizations can be drawn except that in some cases triplet energy transfer to a coupled metal-ligand triplet energy level probably accounts for at least part of the quenching. Rare earth ions can quench excited triplets by energy transfer, since, as discussed earlier, sensitized fluorescence of the metal ion results. 

Polystyrene light stabilization has been achieved with a variety of ulfravioler absorbers including the benzophenones, benzotriazoles. and salicylates. While yellowing of polystyrene occurs in many applications, it is particularly noticeable in diffusers used with fluorescent lights. This problem has been effectively solved by using ultraviolet light absorbers. In this instance, superior stabilization is achieved when the ultraviolet absorber is used in conjunction wirh specific antioxidants. 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

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. 

A small value of AEst facilitates intersystem crossing. We expect singlet state to be fast depleted along this pathway if the lowest excited state is of (n, n ) type. This pathway is further promoted due to the fact that Tnjt > by a factor of ten, due to the forbidden character of n -> n transition. Fluorescence with decreased rate constant for emission cannot compete efficiently with intersystem crossing. This explains the absence of room temperature emission in heterocyclics like benzophenone, acetophenone, quinoline, acridines, etc. They phosphoresce at low temperatures only. 

Hie assay was done directly on 0.2 ml of plasma by treating with alkaline potassium permanganate to form benzophenone, extracting with heptane and shaking the heptane layer with sulfuric acid. The fluorescence of the reaction product was measured with excitation at 360 nm and emission at 440 nm. Detection of 1 ug phenytoin per ml of plasma was reported. 

Abrahamsson et al37 described a fluori-metric microdetermination of phenytoin in plasma. The phenytoin is extracted from acidified plasma and oxidized to benzophenone which is then condensed with N -methylnicotin-amide. The resulting product is strongly fluorescent with excitation at 440 nm and emission at 500 nm. 

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... 

Sometimes the fluorescence and phosphorescence spectra of a compound in solution overlap. They may be separated as follows. If a suitable triplet energy acceptor is added, this will quench the phosphorescence, leaving the fluorescence unaffected, while a suitable triplet donor will sensitize the phosphorescence in the absence of any fluorescence. Back-strom and Sandros have analyzed the total luminescence spectra of biacetyl, benzil, and anisil in this way, using pyrene as the triplet acceptor and benzophenone as triplet donor.3... 

Apart from the low-resolution vapor spectrum reported by Glass, Robertson and Merritt 46>, no other study of the second system is available. It is therefore not possible to discuss line shapes in the vapor-phase spectrum in relation to the decay rates of the two-component fluorescence. More information is avEiilable for the crystal, but in the condensed phase the decay rate of the intermediate pressure-dependent fluorescence may have been considerably increased, as is the case for benzophenone. 

Some years ago, Schaap and co-workers developed a method by which compounds that do not quench the fluorescence of the singlet excited DCA sensitizer may nevertheless be rapidly oxidized [102, 108, 109, 160-162]. For example, epoxides 69a-d, unreactive under standard DCA-sensitized conditions [107,163], can be readily converted into the corresponding ozonides 70a-c, in high yields, by use of a non-light-absorbing aromatic hydrocarbon, i.e., biphenyl (BP) as a cosensitizer in conjunction with DCA. Variable amounts of carbonyl compounds, such as benzophenone from 69 a, b, benzaldehyde from 69 b, c, d are... 

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