Triplet states, intersystem crossing

A further possibility is that molecules undergo a nonradiative transition from the excited singlet states to the triplet states (intersystem crossing). This is the origin of some loss mechanisms for dye lasers. 

Deactivation processes competing with fluorescence are mainly nonradiative deactivation to the S0 state (IC) and nonradiative transition to a triplet state (intersystem crossing, ISC). Photochemical products are often formed from this triplet state. Important photochemical reactions are the E—yZ isomerization of ethylene, the oxidation of pyrazoline to pyrazole, and the dimerization of cou-marins. 

Due to a very efficient singlet- to triplet-state intersystem crossing, the [2 + 2]-photocycloaddition chemistry of 2-quinolones can be initiated easily by direct excitation at 300-350 nm [102], The addition of a sensitizer is not required. The parent compound has been first employed in a [2 + 2]-photocycloaddition as early as 1968 [103]. With regard to regio- and stereoselectivity, 2-quinolone (107) behaves as expected, exhibiting a preference for HTproduct formation with electron-rich olefins, such as 1,1-dimethoxyethene (Scheme 6.39, DMA = N,N-dimethylacetamide). The highly efficient reaction delivers product 108 quantitatively [104], The preference for... 

Excited Triplet State, Intersystem Crossing, Phosphorescence, and Selection Rules... 

Skeletal motions can also induce transitions from the lowest excited singlet state to the lowest triplet state (intersystem crossing). Thus, changes in k may be due not only to internal conversion but also to Intersystem crossing. It should be noted that Injection can also take place from the triplet state, as discussed by Spltler et al. (29). 

The excited molecule can cross over to its triplet state (intersystem crossing), and from its triplet state, lose energy in the form of heat (internal conversion) and/or light phosphorescence, as it returns to its ground state. 

CHL + —> photosynthesis or CHL triplet-excited state intersystem crossing 3CHL + 3O2 CHL -F Ozor 3CHL + CAR CHL -F CAR CAR iCAR —F harmless decay O2 -F CAR CARO2 CAR can be regenerated or O2 + AA O2 photodynamic action... 

The study of the short-lived excited states is limited by the low concentrations in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates (Section 10.3) and laser sources (Section 10.2) have helped in the study of the triplet states of molecules. The triplet-triplet absorption, triplet lifetimes, intersystem crossing rates from triplet to ground singlet State, etc. can be effectively measured because of high concentrations of the... 

For the reactions described so far in this section, the ketone substrates have lowest excited states that are (n.ii ) in character aliphatic ketones may react by way of the singlet or the triplet state, and aryl ketones normally through the triplet because intersystem crossing is very efficient. The efficiency of photochemical hydrogen abstraction from compounds such as alcohols or ethers is very much lower if the ketone has a lowest (Ji,n triplet state, as does I - or 2-acetylnaphthalene (CmH-COMe). However, all aryl ketones, regardless of whether their lowest triplet state is fn,Jt l or (Jt.Ji ), react photochemically with amines to give photoreduction or photoaddition products. A different mechanism operates (4.38), that begins... 

It has been shown in the case of /i-diketonate complexes of lanthanides that the excited singlet state of the ligand goes to the ligand triplet by intersystem crossing and then energy is transferred from the ligand triplet to the metal ion manifold. Finally, the emission occurs from the excited state of the metal. 

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