Near perfect head-to-head selectivity on the supramolecular photocyclodimerisation of 2-anthracenecarboxylate with self-organised gemini surfactant bilayers

International audience; Silicificated self-organised crystalline bilayers of gemini surfactants were used as reaction templates for supramolecular regioselective [4+4] photocyclodimerisation of 2-anthracenecarboxylate in water. Near perfect head-to-head cyclodimers selectivity, up to 97% at room temperature was achieved with accelerated dimerisation. Ever since the first report in 1867, 1-3 the [4+4] photocyclodimerisation of anthracene and its derivatives has attracted much attention due to the unique structure and properties of the photocyclodimers obtained, and has been employed indeed as a key motif in energy storage and photoreversible devices. 4-8 However, controlling regio-and enantioselectivities of the photocyclodimerisation of anthracene derivatives, such as 2anthracenecarboxylate (AC) (Scheme 1), is a challenge, which has driven researchers to the use of supramolecular approach, 9,10 using cyclodextrins, 11-14 proteins, 15,16 molecular templates, 17-21 molecular cages, 20-22 twisted self-associated templates, 18 organogels, 23 and helical metal nanostructures 24 as hosts and templates. Most of the reported systems, however, require the multi-step synthesis of host/template, the modification of guest substrate and/or the irradiation in organic media at low temperature. From the Sustainable Development Goals (SDGs) point of view, a readily accessible, spontaneously self-organisable, operative under ambient conditions, yet biodegradable host/template is advocated, while a three-dimensionally well-defined, appropriately functionalised, structurally robust host/template is essential for directing the stereochemical course of photoreaction to desired product(s). Upon photoirradiation in aqueous media, AC form antiand syn-isomeric head-to-tail (HT) cyclodimers (ACD1 and ACD 2) and head-to-head (HH) cyclodimers (ACD 3 and ACD 4) in a 4:1 ratio (Table 1, entry 1) for steric and electrostatic reasons. Possessing two functional groups on the same side of molecule, the HH cyclodimers are structurally attractive as building blocks for functional devices and materials. 25,26 Wolff et al. succeeded to increase the HH/HT ratio of the photo-cyclodimerisation of 9-substituted anthracenes by executing the reaction in cationic and anionic micelles to gain modestly enhanced HH/HT ratios of 0.62-0.76, 27,28 for which the flexible micelle shell would be responsible for the limited value of HH/HT ratios.