Photochemistry of the 1,6-Dideuterio-1,3,5-hexatrienes in Solution: Efficient Terminal Bond Photoisomerization in One-Bond-Twist and Bicycle Pedal Ways.

The report that the central bond photoisomerization of the 1,3,5-hexatrienes (Hts) is highly inefficient has encouraged theoreticians to seek conical intersections (CIs) at geometries that can explain rapid nonradiative return to the initially excited isomer. Because they are photochemically silent, torsional relaxations about the terminal double bonds of the Hts have not been evaluated as significant radiationless decay pathways. Study of the photoisomerization of trans,trans,trans- and trans,cis,trans-1,6-dideuterio-1,3,5-hexatrienes ( ttt- and tct-Ht) addresses this issue. Degassed cyclohexane- d (CD) and CDCN solutions were irradiated at 254 nm in quartz NMR tubes, and the progress of the reactions was followed by H NMR. Photoisomerization rates based on the integration of terminal hydrogen NMR peaks are in reasonable agreement with rates obtained by fitting pure isomer NMR spectra to the phase shift and baseline corrected experimental NMR spectra. The results show that terminal bond isomerization is highly efficient, especially when one considers that central bond isomerization is much more efficient than previously reported and is mainly observed together with terminal bond isomerization. A mechanism involving terminal one-bond-twist (OBT) in competition with a bicycle pedal (BP) process accounts for all terminal and most central bond photoisomerization. OBT central bond isomerization is a minor reaction that is observed primarily in the tct to ttt direction. Most surprising is the prominent role of the BP process in central bond photoisomerization. Proposed initially to account for photoisomerization in free volume constraining media, it is observed here in the absence of medium constraints.