1.3.5- Hexatriene triplet state

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

Prom the results presented in Ref. [132] the absence of observed phosphorescence in the short polyenes is understood as a combination of small T — So transition probability and vibrational quenching. Because of the similarity of results for all triplet state quantities of the ethylene, butadiene and hexatriene molecules investigated here, one can propose that these arguments also hold as explanation for the lack of phosphorescence in the longer polyenes [132]. 

It is not possible to separate the photochemistry of 1,3-cyclohexadiene from that of 1,3,5-hexatriene, its major product, since the latter absorbs radiation in the same region of the spectrum and even more intensely to revert to 1,3-cyclohexadiene. The ir- - w absorption of 1,3,5-hexatriene shows considerable vibrational structure. The radiative lifetimes for the singlet states of both 1,3-cyclohexadiene and 1,3,5-hexatriene can be calculated to be less than 10 sec. The lowest triplet state of trans-1,3,5-hexatriene has been placed at 47 kcal./mole, and a second triplet state may lie at approximately 14 kcal./mole above that. These values were also obtained by direct singlet — triplet excitation. 

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. 

It has been known for some time that hexatrienes can be formed by reaction of excited benzene molecules with solvent in a frozen glass (262-264). Some details of the mechanism of this process have been deduced by Simons and Smith (90). They propose that intermolecular vibronic interaction with solvent is a necessary but not sufficient requirement for production of hexatrienes in viscous media. The reaction is associated with radiationless decay of the triplet state rather than a bimole-cular reaction of triplet benzene with neighboring solvent molecules. Since radiationless decay constitutes intersystem crossing, the product of the reaction is also in the ground state. It is stressed by the authors that hexatrienes are not produced in fluid media or by nonphosphorescent benzene derivatives in which there is a rapid competing decay of the triplet state. 

The efficient Z/E-lsomerization in the triplet state contrasts with the low quantum yields for Z/E-isomerization of singlet hexatrlene. Evidently Isomerization as well as Intersystem crossing of Si-hexatriene are inefficient processes. 

---