Benzophenone intersystem crossing

These reactions usually occur via the triplet excited state T,. The intersystem crossing of the initially formed singlet excited state is so fast (fc 10 s ) that reactions of the S state are usually not observed. The reaction of benzophenone has been particularly closeh studied. Some of the facts that have been established in support of the general mechanisir. outlined above are as follows ... 

The sensitizer in our experiments is benzophenone (BP) which reacts as shown in Scheme 2. UV light of 300 to 400 nm is absorbed and excites the aromatic ketone group to a singlet state which by intersystem crossing (ISC) reverts to a triplet state, abstracts a... 

Figure 5.6 State diagram for benzophenone. Here intersystem crossing involves a change in orbital type...

Intersystem crossing to the benzophenone triplet then occurs ... 

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). 

The photohydrolysis of 2-fluoro-4-nitroanisole to 2-methoxy-5-nitrophenole is sensitized by benzophenone and completely quenched by sodium sorbate The excited state multiplicity in photoaminations has also been studied. Photolysis of mNA in liquid ammonia yields m-nitroaniline. If the amination is carried out in a large excess of benzophenone, 2-methoxy-4-nitroaniline is formed instead and thus an excited singlet state as reacting species is envisaged in the unsensitized photoamination loo.ioi). it may well be that uptake of the nucleophile present in high concentration successfully competes with intersystem crossing. 

An alternate method can be used to obtain separate from the intersystem crossing quantum yield. In this case, we will also use benzophenone (BP) and naphthalene (N) as our example. Now one of our samples will contain both compounds, and we will choose the concentration of naphthalene so that it will quench at least 98% of the benzophenone triplets. This will require knowledge of the... 

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... 

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... 

Figure 3.34 The compensation between the changes of spin and orbital angular momenta can lead to allowed intersystem crossing (isc). In benzophenone crossing from Sfn-n ) to T2(7r-7r ) takes only approximately 20ps...

The time-scale of molecular vibrations is of the order of 10 13 s, just outside the ps range. Internal conversions and in particular vibrational cascades therefore fall into the femtosecond (10-15s) time-scale. However, the spin-forbidden processes of intersystem crossing take place in times of a few ps to several ns. The case of benzophenone is a good example of the compensation between spin and orbital angular momentum. The rise of the triplet state absorption shows that intersystem crossing is completed within some 20 ps. 

Conversion to the lower energy triplet (T,) by spin inversion (intersystem crossing) although formally forbidden this can occur with very high efficiency when the energy difference between the two states is small. It is most notable in carbonyl and aromatic compounds (e.g. intersystem crossing occurs with 100 per cent efficiency in the case of benzophenone). 

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. 

For most carbonyl compounds, we expect to have two near-lying excited states in the triplet manifold, which are either n-n or tc-tc in character. The n-7T states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an n-7t triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the 71-71 state, largely centered on the aromatic part of the molecule, as in the case of p-methoxyacetophenone, the reactivity decreases significantly (8,9). With the nature of lignin and the nature of the model we have chosen, we are mostly interested in molecules which have this type of behavior. 

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