1,3,5-Hexatrienes excited singlet states

Theoretical and time-resolved spectroscopic studies of triene photochemistry The dynamics of relaxation of the excited singlet states of E- and Z-l,3,5-hexatriene (HT) have recently been studied in the gas phase and in solution. In the gas phase, population of the 21 / state of the Z-isomer by internal conversion from the spectroscopic 11B state has been estimated to occur with a lifetime Tig of about 20 fs, while the lifetime of the 21 / state has been determined to be T2A =730 fs47. The lifetime of the latter in ethanol solution has been determined by Fuss and coworkers to be T2a = 470 fs52. A similar 21A lifetime has been reported for -l,3,5-hexatriene in cyclohexane and acetonitrile solution by Ohta and coworkers48. 

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. 

Pressure Dependence. That the precursor for reaction (10) is identical whether the reactant is 1,3-cyclohexadiene or 1,3,5-hexatriene is best illustrated by the Stern-Volmer plot for hydrogen formation (Fig. 3). The data for both molecules fit the same straight line. It is unlikely that (rans-1,3,5-hexatriene would give the same electronically excited singlet state as m-hexatriene or cyclohexadiene since the short life of this state would be unfavorable for rotational processes. But it is... 

The hydrogen migration reaction to give 1,2,4-hexatriene is observed in the gas phase as well as in solution. It is not quenched by an increase in pressure, nor is it observed during pyrolysis of 1,3,5-hexatriene. It has hence been taken to be a reaction which proceeds from the electronically excited singlet state. Based on exactly the same reasoning, the process... 

Theoretical and time-resolved spectroscopic studies of triene photochemistry The dynamics of relaxation of the excited singlet states of E- and Z-l,3,5-hexatriene... 

Evidence has been produced " which suggests that cycloprop[f>]indoles and other products from the photolysis of ethyl 2-cyano-l,2-dihydroquinoline-l-carboxylates, e.g. (110), arise not, as was assumed previously, via an aza-hexatriene intermediate but via the excited singlet state of (110) or a dipolar species of form (111). In contrast, the long-assumed (but recently doubted) intermediacy of 1,2-dihydroquinolines in the Doebner-von Miller quinoline synthesis has been confirmed. The isolable dihydroquinolines are produced in high yield on allowing the arylamine and acetaldehyde to stand at room temperature for 24 hours. 

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. 

On the other hand, the 1,3,5-hexatriene was postulated to be formed from the singlet excited state. On the basis of electron-scavenging experiments it was concluded that the... 

We will now discuss an application of the strategy described above to the radiationless decay and competitive photoproduct formation process in the cyclohexadiene (CHD)/hexatriene (HT) system. A theoretical study of the first singlet excited state (2Aj) of cZc-hexa-l,3,5-triene (cZc-HT)... 

Hilinski, E.F., McGowan, W.M., Sears, D.F., Jr., and Saltiel, J., Evolutions of singlet excited-state absorption and fluorescence of all-frflns-l,6-diphenyl-l,3,5-hexatriene in the picosecond time domain,/. Phys. Chem., 100, 3308-3311, 1996. 

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