Intersystem crossing benzenes

El-Sayed 28> has reported on the phosphorescence spectrum of [2.2]paracyclophane. The emission differs both in wavelength ( 4700 A) and in duration (3.3 s) from that of benzene ( 3400 A, 6 s) hence a favorable intersystem crossing from the lowest singlet to the emitting triplet state was inferred. The emission spectrum also indicates that interactions take place between the two aromatic nuclei in the triplet state. 

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

The photoreactivity of o-methyl acetophenone 11 has been studied exten-sively it is somewhat different from 1 because the singlet excited ketone (Sik) in 11 intersystem crosses to its triplet state in less than quantitative yields, as observed for 1 (Scheme 8). Thus, Sik in 11 decays by both intramolecular H-atom abstraction to form exclusively photoenol Z-13 and intersystem crossing to Tik of 11. Haag et al. estimated that Tik of 11 has a lifetime of 10 ns in benzene and decays by intramolecular H-atom abstraction to form biradical 12. The maximum... 

The difference between Es and ET is about constant for the helicenes with five to nine rings (Table 22), while the kisc increase by a factor of about 3 I25). This may suggest the participation of a quasi degenerate triplet state in the intersystem crossing. This most likely will be the 3Lb state since a direct spin-orbit coupling between Lb and 3Lb is allowed in the helicenes because of their C2-symmetry. It is proposed 125) that the increase in kisc reflects the departure from coplanarity when the amount of benzene rings increases. 

After exclusion of a predissociation process responsible for the lifetime shortening in complexes of benzene with noble gases, we consider the external heavy-atom effect on the intersystems crossing rate as the origin of the lifetime shortening [42]. The strong decrease of the lifetime in the... 

Furthermore, we expect that intersystem crossing plays a minor role in the nonradiative relaxation of high Rydberg states in benzene. 

Intersystem crossover processes between singlet and triplet states are well known in photochemistry. Intersystem crossing processes have been of importance in interpreting benzene photochemistry. Anticipating that... 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

The addition of benzene to 2,3-dihydropyran was examined in the presence of cyclopropyl bromide [177] in an attempt to determine the nature of the excited state of benzene responsible for the formation of the ortho adduct [11,12], The absorption of the bromide at 254 nm was taken into account (90% of the radiation was absorbed by benzene) and a decrease of the rate of formation of 50% in the heavy-atom solvent was observed. Cyclopropyl bromide also quenched the fluorescence of benzene (kq = 3.82 X 10s L/mol 1 /s ). These data are interpreted as enhanced intersystem crossing of Si benzene and the necessary involvement of the Si state in formation of the ortho cycloadduct. 

---