Higher triplet state

Intersystem crossing from Si to and higher triplet states has also been reported in some aromatic molecules Li, R. Lim, E.C. Chem. Phys., 1972, 57, 605 Sharf, B. Silbey, R. 

In aromatic hydrocarbons, short-lived ](it, it ) is the lowest excited state and energy gap between (n, it ) and 8(it, it ) states is large. Both these factors are conducive to fluorescence emission and in general aromatic hydrocarbons are good fluorescer. Sometimes, the prediction may not come true if a higher triplet state T2 is available near the St state such as in anthracene. In such cases, fluorescence and phosphorescence both are observed at low temperatures in suitable solvent medium specially when S, and T, are states of different symmetry type. Some data correlating AEst and 4[Pg.148]

Table I shows that in either dioxane or acetonitrile the quantum yield for degradation of I, is unaffected by the presence of 0.1 M of triplet quencher, either sorbic acid, naphthalene or cyclohexadiene. In ethanol, triplet quenchers reduce < >d from 0.34 to 0.14. Quantum yields for intersystem crossing, as determined by a laser opto-acoustic technique ( ), were 0.36 in ethanol and 0.59 in dioxane. These results agree with our earlier report (3), and indicate that significant reactivity occurs from St of I in protic solvents, and that reaction occurs exclusively from Sx in aprotic solvents. While triplet quenching experiments cannot rigorously exclude participation by short-lived higher triplet states, Palm et al (9) have obtained conclusive evidence from CIDNP experiments for singlet-state participation in a series of aryloxy-acetophenones. Note that the triplet state of I is formed in aprotic solvents, and that in deaerated solutions at room temperature it decays by first-order kinetics with a lifetime of 200 ns (3). Remarkably, despite having lifetimes about 100 times longer than other, differently-substituted, aryloxyacetophenones (the longer lifetimes may...

It is obvious that in both 1,3-cyclohexadiene and 1,3,5-hexatriene there is a large energy gap which separates the excited singlet state from the lowest and even the next higher triplet states. Experimentally, the crossover of the singlet excited state to the triplet excited states has not been observed. 

Carbonyl groups are most significant in triplet state photochemistry. Spectroscopic studies include time resolved ESR studies of enols of -hydroxybenzaldehyde 2 energy transfer from acetophenones to 9, 1 0-d ibr omoa n t hr a ce ne (S.,) which involves higher triplet states of the donor283, two photon excitation of substituted triplets of acetophenone , transient resonance Raman spectra of deuterated benzophenones , and diffuse reflectance of... 

S can be one of the higher triplet states T , such as is the case for anthracene (cf. Example 5.3), and that in molecules with lone pairs of electrons either T, or T may be of the same type as S,. This is illustrated in Figure 5.13, where various possibilities for the eneiigy order of the and (n,jt )... 

Zero-field splitting in both the T and the Tg states of m-xylylene (19), one of the few very large molecules for which such information is available experimentally for a higher triplet state, was examined by Havlas and Michl using both if ... 

Figure 1. A simplified energy level diagram for the excitation by photons hv) of a molecule. Sq, S, and S2 represent singlet electronic states of the molecule. T, and indicate the first and higher triplet states of the molecule respectively. The molecule can relax back to the ground state from either S, or T, radiatively or nonradiatively. and k ...

Figure 4.6 Diagram of excited states (ground state So, first excited singlet state S], lowest triplet state T, and higher triplet states T ) and transition processes between these sates fora phthalocyanine with a closed-shell central metal. Reprinted with permission from [178).

The zero-order eigenfunction for, say, the lowest triplet state is written as and Greek letters are employed for the higher triplet states the singlet-state zero-order functions are written as Then the 0, and... 

Triplet states cannot be produced from the ground state by direct absorption of a photon this transition is forbidden. The lowest excited triplet state is rather produced by the lowest singlet state in a radiationless transition (intersystem crossing). Higher triplet states are exclusively formed from the lowest triplet state by absorption of new photons. Deactivation of a triplet state is similar to that for singlet states but the transition occurs with phosphorescence instead of with fluorescence. 

Triplet-Triplet Absorption. A molecule in a triplet excited state may absorb a photon to give a higher triplet state. Triplet-triplet absorption spectroscopy can be an important technique for detecting triplet excited states. 

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