α-Methylstilbene and the Duality of Mechanism in the Quenching of Stilbene Triplets by Molecular Oxygen†

Visible Light-Induced Regioselective E to Z Isomerization of Polarized 1,3-Dienes: Experimental and Theoretical Insights

The stereocontrol of E → Z isomerization on a (1E,3E)-diene, instead of a simple alkene, can be more challenging due to the increased number of isomerization possibilities. Herein, we report visible light-mediated regioselective (1E,3E → 1E,3Z) isomerization of (1E,3E)-diene. The reaction conditions are mild and easy to apply and can be applied to a wide range of substances, with an excellent yield and selectivity (90:10). It is evident from the crystal structures that the cause of regioselectivity for the isomerization of 1,3-diene may not be limited to 1,3-allylic strains; CH-π interaction may also play a vital role. Computational studies suggest that this regioselective photoisomerization is a thermodynamically feasible process and requires the accumulation of spin density on the modified double bond for the transformation to occur.

α-Methylstilbene Isomers: Relationship of Structure to Photophysics and Photochemistry

Significant differences in the photochemical and photophysical behavior of trans-α-methylstilbene and trans-stilbene have been attributed to structural changes caused by the steric requirements of the methyl group. We present here the X-ray structures of cis- and trans-α-methylstilbene (c- and t-MeSt). This is the first X-ray structure of a cis-stilbene. Despite the pronounced departure from phenyl group coplanarity, the solid-state packing of t-MeSt resembles that of trans-stilbene in that both exhibit disorder with a bicycle pedal structural relationship, dynamic in t-St but static in t-MeSt. We compare the X-ray structures with calculated structures. We also compare our steady state and transient photochemical and spectroscopic results with predictions in a recent theoretical paper that anticipated some of our experiments. Deviations from planarity imposed by the methyl substitution account for the shorter lifetimes of the trans excited states. The rapid torsional relaxation of ¹t-MeSt* to the twisted intermediate ¹p*, k_tp = 2.9 × 10¹² s^-1, observed using fs transient absorption spectroscopy, explains the sharp decrease in the fluorescence quantum yield of t-MeSt. We correct misconceptions that have appeared in the literature concerning the shape of the stilbene potential energy surface in S₁. The nonplanarity due to methyl substitution leads to chirality issues that are relevant in biological molecules such as the protonated Schiff bases of retinal in the opsins.

Synthetic feasibility of oxygen-driven photoisomerizations of alkenes and polyenes

This review describes O₂-dependent photoreactions for possible routes to double-bond isomerizations. E,Z-isomerizations triggered by O₂ and visible light are a new area of potential synthetic interest. The reaction involves the reversible addition of O₂ to form a peroxy intermediate with oxygen evolution and partial regeneration of the compound as its isomer. Targeting of O₂-dependent photoisomerizations also relates to a practical use of visible light, for example the improved light penetration depth for visible as opposed to UV photons in batch sensitized reactions. This review is intended to draw a link between visible-light formation of a peroxy intermediate and its dark degradation with O₂ release for unsaturated compound isomerization. This review should be of interest both to photochemists and synthetic organic chemists, as it ties together mechanistic and synthetic work, drawing attention to an overlooked subject.

Temperature dependence of the 1,6-diphenyl-1,3,5-hexatriene triplet lifetime in solution and theoretical evaluation of triplet conformer interconversion

The 1,6-diphenyl-1,3,5-hexatriene (DPH) triplet decay rate constant in degassed benzene is strongly temperature-dependent. Photostationary states for the fluorenone-sensitized photoisomerization of DPH in degassed methylcyclohexane (MCH) favor the all-trans isomer and become richer in the cis isomers as the temperature is increased. The results are interpreted in terms of competing decays from equilibrated triplet conformers on the basis of the experimentally derived DPH triplet potential energy surface [Saltiel, J.; Crowder, J. M.; Wang, S. J. Am. Chem. Soc. 1999, 121, 895 902; 5352]. Calculated geometries and energy differences [B3LYP/6-311+G(d,p)] are consistent with the experimental model.