Phantom triplet

Before we cite another experiment that supports the idea that isomerization occurs from the phantom triplet state, it will be helpful to complete our set of stilbene reactions and construct an experimental potential energy diagram. 

These reactions, while not very important for high-energy donors in the absence of azulene, do help define what the potential energy diagrams should look like, that is, the fram-stilbene triplet is a discrete intermediate while the cij-stilbene triplet is a high vibrational form of the phantom triplet. Herkstroeter and McClure(13> have been able to observe the fro/w-stilbene triplet via flash photolysis using low-temperature glasses however, no similar triplet was observed for cfr-stilbene. Only when they studied cfr-stilbene-like... 

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

In addition, 20-30% of the phantom singlets intersystem cross to the phantom triplet state ... 

The photosensitized isomerization of ethyler.es and stilbenes has been studied in detail by Hammond and his group and it has been found that in each case both the cis-trans and the trans-cis isomerizations occur with equal efficiency. A common (tc, jt ) triplet state is assumed to be the intermediate in each case. This triplet state is referred to as the perpendicular triplet state and was termed a phantom triplet state by Hammond. The energy of the perpendicular triplet state is expected to be low since in this configuration the overlap between the re and rc orbitals is minimized. The potential energy of S0, Tx and Sx states as a function of twist angle is given in Figure 7.7... 

The exact mechanism of nonvertical energy transfer is still unclear. Originally Hammond thought of it as occurring directly, during a prolonged encounter between donor and acceptor.23 Now, however, isotope effects and other evidence favor the view that the nonspectroscopic, or phantom, triplet is formed... 

When reaction occurs from the triplet manifold, the further problem arises of whether the reactive states are the spectroscopic triplets, 3cis and 3trans, or whether reaction occurs from a single phantom triplet. Direct irradiation of the S0— T1 absorption bands of cis- and trans-1,2-dichloroethene gives the same (trans/cis) quantum yield for each isomer, indicating that the phantom... 

When excitation is brought about by triplet energy transfer and if isomerization occurs from the phantom triplet, then... 

Patemo-Buchi reactions, 238-255 Pauli s exclusion principle, 19,24,31,40 Perturbation, external heavy atom, 145 Perturbation theory time dependent, 53 Phantom triplet, 229 Phase-shift method, 309, 311 a-phosphorescence, 129 -phosphorescence, 157 Phosphorescence... 

E-azobenzene) and about 192 and 142 kJ moH (Z-azobenzene). Monti et al. located triplet states at 146 (E-azobenzene) and 121 kJ mol (Z-azobenzene). From their kinetic results, they inferred that the azobenzene acceptor should be twisted (phantom triplet) when accepting the energy and calculated... 

Stationary state ratio for the sensitized isomerization of the a-methylstilbenes. This result is consistent with the phantom triplet picture but it means that the... 

The absence of reversible excitation transfer involving cis triplets and appropriate donors as well as the absence of quenching of cis triplets by azulene were established and require the inclusion of a rapid isomerization of the cis triplet to either the trans triplet, Eq. (17), or the nonspectroscopic phantom triplet. It should be pointed out that two considerations make involvement of the latter triplet attractive. The first is that it allows easy... 

---