Crossing, intersystem enhancement

Another possibility is deactivation through increased intersystem crossing,(1) which would occur via enhanced spin—orbit coupling.(28) If intersystem crossing is enhanced, then the phosphorescence quantum yield of a... 

The present authors wish to point out that near degeneracy of zero-order potential surfaces or zero-order crossings can enhance intersystem crossing. This has previously been pointed out,39,56,186 but here we wish to investigate the effect in further detail. We assume that there is a vibrational coordinate Q such that the spin-free, zero-order 1B2u curve intersects a spin-free triplet curve as indicated in Figure 9. The nonadiabatic inter-... 

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Once produced, 3CAR can easily return to the ground state dissipating the energy as heat or it can be quenched physically via enhanced intersystem crossing by ground state oxygen, Scheme... 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Such an enhancement of the fluorescence quantum yield can be explained in terms of the relative locations of the singlet n-n and n-n states. In the absence of cation the lowest excited states has n-n character, which results in an efficient intersystem crossing to the triplet state and consequently a low fluorescence quantum yield. In the presence of cation, which strongly interacts with the lone pair of the carbonyl group, the n-n state is likely to be shifted to higher energy so that the lowest excited state becomes n-n. An outstanding selectivity of Na+ versus K+ was found the ratio of the stability constants is 1300 in a mixture of ethanol and water (60 40 v/v). 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

The intersystem crossing process has opposite effects on the yields of fluorescence and phosphorescence since it depletes the singlet state and populates the triplet state. It is commonly known that heavy ions, such as iodide and bromide, increase intersystem crossing by spin-orbit coupling.(1617) For proteins, fluorescence can be quenched as phosphorescence yield is enhanced. 8,19) However, although the phosphorescence yield is increased, the lifetime is decreased. This effect arises because spin-orbit coupling, which increases the intersystem crossing rate from 5, to Tt, also increases the conversion rate from T, to S0. 

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

Heavy atom enhancement of intersystem crossing has been used to determine the mechanism of acridine photoreduction in ethanol.115 It was found that addition of sodium iodide decreased the fluorescence intensity and the rate of disappearance of acridine to the same extent, confirming that the singlet state is responsible for photoreduction. From the increase in triplet state absorption upon addition of iodide it was found that Of for acridine was 0.76. Thus the short singlet lifetime (0.8 nsec) of acridine is due to rapid inter-system crossing to unreactive triplet states. 

---