H. J. C. Jacobs

Effects of wavelength and conformation on the photochemistry of vitamin D and related conjugated trienes

Abstract Irradiation experiments under various conditions establish that the photoreactions of conjugated trienes are governed by the ground state conformational equilibrium and the NEER principle, as well as by the wavelength dependent relaxation behaviour of the excited species.

Wavelength effect on the photochemical reactions of (Z)-2,5-dimethyl-1,3,5-hexatriene : selective excitation of rotamers

Abstract The UV absorption spectrum of Z-2,5-dimethyl-1,3,5-hexatriene 1 is typical of a cZt triene, but the long wavelength tail Indicates the presence of the (minor) cZc rotamer. The sudden change In the photoreactivity of 1 around 290 nm is in agreement with selective excitation of these rotamers. No indication is obtained for an intrinsic excitation energy dependence of the photoreactivity of the Individual rotamers.

Synthesis and fluorescence of a configurationally locked Z-hexatriene : 1,2-divinylcyclopentene

Abstract The syntheses of the configurationally locked Z-hexatriene, 1,2-divinylcyclopentene, and its terminally tetradeuterated isotopomer are described, and some photochemical and photophysical properties are reported. The trienes turn out to be photostable under oxygen-free conditions. In contrast to “unlocked” hexatrienes, both compounds fluoresce (room temperature, methanol). The fluorescence is structured and is the nearly perfect mirror image of the absorption band, with a small Stokes shift. Therefore the emission might originate in the 1 1 B 1 state. This is the first observation of fluorescence of simple hexatrienes in solution. Apparently torsional motion around the central (originally double) bond in the excited singlet state is an important radiationless deactivation pathway in “unlocked” hexatrienes; if it is obstructed fluorescence becomes a passable deactivation pathway.

Deuterium isotope effects in the photochemistry of 2,5-dimethyl- and 2,5-di-tert-butyl-1,3,5-hexatrienes

Abstract Deuterium isotope effects on the efficiencies of light-induced rearrangements of 2,5-dimethyl- and 2,5-di- tert -butylhexatrienes are reported. The 4π electrocyclizations show an inverse secondary isotope effect, in agreement with the existence of the theoretically predicted barrier on the potential energy surface of the excited state. In the formation of allylcyclopropene derivatives isotope effects provide evidence for a mechanism involving a biradical intermediate. The surprisingly large decrease in the efficiency of E — Z isomerizations in E - and Z -2,5-di- tert -butyl-1,3,5-hexatrienes after deuteration of the central double bond is ascribed to a reduced frequency of the hydrogen out-of-plane deformation which initiates the reaction.

Photochemistry of 2,5-dimethyl- and 2,5-di-tert-butyl-1,3,5-hexatrienes. Conformation and reactivity. A quantitative study

Abstract The quantum yields of isomerization upon direct irradiation at 254 nm of 2,5-di- tert -butyl- and 2,5-dimethyl-1,3,5-hexatrienes have been determined using a novel simulation method for the time evolution of the composition of the reaction mixture under irradiation. Two factors controlling the efficiency of photochemical rearrangements in 1,3,5-hexatrienes are identified: for certain bond-forming processes particular conformations are required, as dictated by the non-equilibration of excited rotamers principles; the efficiencies of E – Z interconversion and allylcyclopropene formation from E -trienes are enhanced by non-planarity.

Photochemistry of planar E-hexatrienes: an AM1 study

Abstract The photochemical formation of an allylcyclopropene derivative from E-hexatriene is studied with the AM1 semiempirical method. Two possible reaction mechanisms, with a differing sequence of [1,2]-hydrogen migration and [1,3]-bond formation, are compared. In this study a new phenomenon emerged: when three bonds of the triene are twisted by 90°, a conical intersection between ground and excited state occurs, through which efficient radiationless decay and product formation can take place. The reaction mechanism in which [1,3]-bond formation is the primary step is the more efficient of the two mechanisms studied, but decay via the conical intersection is somewhat more facile.