Benzophenone intersystem crossing rates

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... 

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. 

Wong and Wan (136a) have suggested application of photo-CIDEP to the ISC study of triplet benzophenone. They pointed out that while PMDR methods normally study the dynamics of the ISC is zero field and in solid state, the photo-CIDEP study is potentially useful to estimate the intersystem crossing relative rates at a constant external magnetic field and in the liquid state. 

A FBrster-type transfer mechanism (48) was proposed to occur. Batochromic shifts and hyperchromicity were observed In the absorption spectra of systems [10b,1,3] and to the largest extent In [10b,2] (44,45). On specific excitation of the benzophenone chromophore, triplet transfer to the naphtalene moiety occurs with 100 percent efficiency. The rate of transfer is greater than lO s l g d not measurably dependent on chain length. Since the ratio of Intersystem crossing in benzophenone is of the order of (49), energy transfer will occur, after inter-... 

Spectroscopic Properties. The phosphorescence quantum yields of the initiators at 77 K in 2-propanol are compared in Table IV. These were obtained upon excitation at the absorption maximum of 290 nm for all the compounds. Both the absorption and phosphorescence spectra of the initiators were similar to those of benzophenone. The quantum yields on the otherhand were variable. The phosphorescence emission spectra of all the compounds had maxima at 414, 443 and 474 nm respectively. No fluorescence was observed which is typical of aromatic ketones indicating a high rate of intersystem crossing. 

Aloi se, S. Ruckebusch, C. Blanchet, L. Rehault, J. Buntinx, G. Huvenne, J.-P. /. Phys. Chem. A 2008,112,224 used subpicosecond time-resolved absorption spectroscopy to study intersystem crossing in benzophenone. They identified an intermediate species (IS) in the process, and it was tentatively identified as a vibrationally excited ("hot") Ti state. The rate constant for Si to IS was calculated to be 1.54 x lO s , while that for IS to Tj was found to be 1 x lO s. ... 

Due to a fast and efficient intersystem crossing process (cf. Table 3.2) most ketones perform mainly triplet photochemistry [25,73], In Table 3.6, the photoreactivity of triplet-excited acetone, biacetyl, and benzophenone is compared with the triplet-excited azoaUcane DBH-T. The data for both chromophores follow similar trends. Namely, dienes and amines quench with quite high rate constants, while ethers and aromatic compounds react rather inefficiently. 

The above mechanism has been inferred from a quantitative study of the photoreduction of benzophenone by benzhydrol in benzene solution (Moore et al., 1961 Hammond et al, 1961a). It was shown that, as predicted by the mechanism, the quantum yield of the photoreduction, 4>b, obeyed the rate law given in Eq. (55) where 4>is, the quantum yield of intersystem crossing for benzophenone, could be taken as unity (see also Table IV). 

---