Lowest singlet transition

Absorptions in the UV spectra of thiiranes are observed around 260 nm ( - other transitions are reported in the vacuum UV spectrum, and the calculated lowest singlet transition energies correspond to n - oxirane groups behave as electron withdrawing substituents when attached to aromatic rings as indicated by the UV spectra of 2-arylthiiranes. 

The UV spectrum has been measured in hexane and cyclohexane other solvents also seem to have been used. In the 305-343 nm region, the UV spectrum strongly resembles that of benzo[c]thiophene.2 The lack of solvent dependence together with a mirror relationship to its fluorescence spectrum (in isopentane-methylcyclohexane 3 1 at 77°K) is indicative of the 7t,7r -character of the lowest singlet transition. ... 

FIGURE 7.6. Hypothetical orbital energy diagram for the lowest singlet transition in (a) a radical cation (b) a radical anion. 

These selection rules are affected by molecular vibrations, since vibrations distort the symmetry of a molecule in both electronic states. Therefore, an otherwise forbidden transition may be (weakly) allowed. An example is found in the lowest singlet-singlet absorption in benzene at 260 nm. Finally, the Franck-Condon principle restricts the nature of allowed transitions. A large number of calculated Franck-Condon factors are now available for diatomic molecules. 

Fig. 2 Jablonski energy level diagram illustrating possible transitions, where solid lines represent absorption processes and dotted lines represent scattering processes. Key A, IR absorption B, near-IR absorption of an overtone C, Rayleigh scattering D, Stokes Raman transition and E, anti-Stokes Raman transition. S0 is the singlet ground state, S, the lowest singlet excited state, and v represents vibrational energy levels within each electronic state.

Octatetraene is the shortest unsubstituted polyene that exhibits fluorescence. The X kg —> 2 kg transition is clearly seen in one- and two-photon absorption spectra and the 2 kg is unambiguously identified to be the lowest singlet state1. 

As shown in Fig. 10, the lA2 state is predicted to be the lowest singlet state of 4a.55,57 The CASSCF vibrational analysis reveals one imaginary frequency for this state, corresponding to an out-of-plane a2 vibration, which leads to the cyclic allene 3a.57 Thus,1 A2-4a is a transition state for the enantiomerization of 3a. The analogous process of internal rotation about the C-C bonds in allene is also predicted to occur via an open-shell1A2 transition state.72... 

The magnitude of 1kisc is governed by El-Sayed s selection rules that is, the rate of intersystem crossing from the lowest singlet state to the triplet manifold is relatively large when the transition involves a change of orbital type. For example ... 

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