The Low-temperature Photochemistry of s-cis Acyclic 1,3-Dienes¶

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Abstract
In this article we have examined the very low‐temperature photochemistry of three acyclic 1,3‐dienes. We have used high‐temperature deposition techniques combined with matrix isolation to create samples enriched with the thermally meta‐stable s‐cis form. This technique has allowed us to examine the separate photochemistry of the s‐cis and s‐trans conformers. Our results suggest the presence and the absence of barriers on the excited‐state surface. In particular, we have found that the electrocyclic closure and s‐cis–s‐trans photochemical isomerization stops at 15 K for 2,3‐dimethyl‐1,3‐butadiene‐d10. The closure occurs at higher temperatures in solution but is slowed by a deuterium isotope effect. The s‐trans conformer of EE‐2,4‐hexadiene shows almost no photoreactivity in a matrix under 254 nm irradiation, but the s‐cis conformer is rapidly converted to ZE‐2,4‐hexadiene (ZE‐HXD). The photoreactivity of ZE‐HXD is similar in that there is a relatively quick conversion of the s‐cis conformer under these conditions, with only a very slow conversion of the s‐trans to photoproducts.