The photoisomerization of cis,trans-1,2-dideuterio-1,4-diphenyl-1,3-butadiene in solution. No bicycle-pedal.

cis,trans-1,2-Dideuterio-1,4-diphenyl-1,3-butadiene (ct-DPBd2) was synthesized and its cis-trans photoisomerization in cyclohexane-d12 (C6D12) at room temperature was monitored by 1H NMR spectroscopy. The results reveal formation of only trans,trans-1,2-dideuterio-1,4-diphenyl-1,3-butadiene (tt-DPBd2). The failure to detect formation of trans,cis-1,2-dideuterio-1,4-diphenyl-1,3-butadiene (tc-DPBd2) eliminates the possibility that an identity bicycle pedal process contributes to inefficiency in the cis-trans photoisomerization of cis,trans-1,4-diphenyl-1,3-butadiene (ct-DPB).

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-Htd2) addresses this issue. Degassed cyclohexane- d12 (C6D12) and CD3CN solutions were irradiated at 254 nm in quartz NMR tubes, and the progress of the reactions was followed by 1H 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.

Lumisterol to Tachysterol Photoisomerization in EPA Glass at 77 K. A Comparative Study.

We present a comparative study of the photoisomerizations of lumisterol (Lumi), previtamin (Pre), and provitamin D3 (Pro) to tachysterol (Tachy) at 77 K in EPA (5:5:2 ether, isopentane, and ethanol by volume) glass. Fluorescence, fluorescence excitation, and UV spectra, measured in the course of these reactions, were analyzed using singular value decomposition with self-modeling (SVD-SM). This represents an extension of previous work that led to the conclusion that in the EPA glass Pre exists as an s-cis,s-cis-conformer (cZc-Pre) which gives, exclusively, an unstable s-cis,s-cis-conformer of Tachy (cEc-Tachy) and Pro gives mainly the tEc-Tachy, that corresponds to a stable s-trans,s-cis-conformer. ( Redwood , C. ; et al. J. Phys. Chem. Lett. 2013 , 4 , 716 - 721 . ) The surprising result was that the major Pre photoproduct from Pro also has a tZc-Pre conformation instead of the expected cZc-Pre conformation. Accordingly, the Pre to Tachy cis-trans photoisomerization proceeds via a conformer specific one-bond-twist (OBT) process as proposed by Havinga ( Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 718 - 719 . Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 994 - 1004 . Maessen , P. A. Ph.D. Thesis, State University at Leiden, Leiden, The Netherlands, 1983. ). The role of the EPA glass in controlling conformer distributions and reaction outcomes is further explored by the extension of the studies to Lumi, whose structure differs substantially from that of its stereoisomer, Pro. Initially, the light-induced conrotatory ring openings of Pro and Lumi are expected to give cZc-Pre conformers that differ in the relative orientation of the double bond dihedral angles that define the chiral axis of the triene moiety: (-)cZ(-)c-Pre and (+)cZ(+)c-Pre, respectively. In the case of Pro, much of the cZc-Pre proceeds to tZc-Pre, the precursor of tEc-Tachy. In contrast, we show that under the same conditions most cZc-Pre formed from Lumi retains the cZc-conformation and isomerizes to cEc-Tachy. cZc-Pre from Lumi was not detected by fluorescence, but UV absorption measurements establish its formation as an essential intermediate to Tachy. Aided by theoretical calculations of conformer UV and CD spectra, we conclude that fluorescent thermodynamic Pre and nonfluorescent Pre from Lumi are both (+)cZ(+)c-Pre conformers. They differ in the orientation of the OH in the A ring, pseudoequatorial in the former and pseudoaxial in the latter. The most likely major photochemical sequences starting from Pre and Lumi are (+)cZ(+)c-Pre-eq-OH → (+)cE(+)c-Tachy-eq-OH and Lumi → (+)cZ(+)c-Pre-ax-OH → (+)cE(+)c-Tachy-eq-OH.

Photochemistry of the 1,4-Diphenyl-1,3-butadienes in Ethanol. Trapping Conical Intersections.

We report photoisomerization and photoaddition quantum yields in ethanol starting from each 1,4-diphenyl-1,3-butadiene, DPB, isomer. Despite the fact that the trans,trans isomer, tt-DPB, has a significant fluorescence quantum yield and lifetime, whereas no fluorescence is observed from the cis isomers, ether formation occurs with similar efficiency from tt-DPB and ct-DPB and less efficiently from cc-DPB. Photoaddition is about 72 times slower than photoisomerization to the ct- and tt-DPB isomers starting from tt- and ct-DPB, respectively. The results are consistent with addition of alcohol to the common zwitterionic trans-phenallyl cation/benzyl anion intermediate that leads to photoisomerization through a conical intersection. Ether formation from cc-DPB tracks inefficient formation of tt-DPB indicating that the small bicycle pedal cc-DPB → tt-DPB component proceeds stepwise through the same zwitterionic trans-phenallyl cation/benzyl anion intermediate. Previous results concerning the addition of methanol to the stilbenes are similarly interpreted. In contrast to trans-stilbene, the fluorescence and photoisomerization quantum yields of tt-DPB are inconsistent with the assumption of strict complementarity between radiative and torsional relaxation channels of tt-DPB in alcohols.

Competing adiabatic and nonadiabatic pathways in the cis-trans photoisomerization of cis-1,2-di(1-methyl-2-naphthyl)ethene. A zwitterionic twisted intermediate.

Identical fluorescence lifetimes and spectra of cis- and trans-1,2-di(1-methyl-2-naphthyl)ethene reveal an adiabatic cis → trans photoisomerization pathway that accounts for a significant fraction of observed cis → trans photoisomerization quantum yields. The fluorescence quantum yields of both isomers decrease as the solvent is changed from methylcyclohexane to the more polarizable toluene or to the more polar acetonitrile and there is a corresponding increase in the photoisomerization quantum yield in the trans → cis direction. The pronounced solvent dependence of the contribution of the adiabatic pathway to cis → trans photoisomerization--methylcyclohexane (70%), toluene (45%), acetonitrile (31%)--is consistent with the participation of a zwitterionic twisted intermediate, (1)p*, on the singlet excited state surface which is stabilized as the polarizability and/or polarity of the solvent is increased. Solvent stabilization of (1)p* favours the nonadiabatic photoisomerization pathways of both isomers and diminishes the cis → trans adiabatic pathway.

Bicycle pedal photoisomerization of 1-phenyl-4-(4-pyridyl)-1,3-butadienes in glassy isopentane at 77 K.

In glassy isopentane at 77 K, 1-phenyl-4-(4-pyridyl)-cis-1,cis-3-butadiene (cc-PPyB) and 1-phenyl-4-(4-pyridyl)-cis-1,trans-3-butadiene (ct-PPyB) can undergo simultaneous two-bond photoisomerization. Under the same conditions, 1-phenyl-4-(4-pyridyl)-trans-1,cis-3-butadiene (tc-PPyB) gives tt-PPyB, the ultimate photoproduct in all cases.