Use of linker for preparation of missing solid-state photoproducts: Head-to-head photodimerization of anthracene-9-propionic acid in its crystalline double salt with cyclohexane-l,2-diamine

In order to realize the uncommon head-to-head photodimerization of 9-substituted anthracenes rather than the common head-to-tail photodimerization, a regiochemical control of the reaction through use of supramolecular linkers has been attempted in the solid state. Thus, crystalline double salts are prepared from anthracene-9-propionic acid (9-AP) and 1,2-diamines such as ethylenediamine (en), racemic or optically active (R,R)- or (S,S)-trans-cyclohexane-1,2-diamine (t-chxn), and cis-cyclohexane-1,2-diamine (c-chxn), and these salts are irradiated in the solid state. The 1,2-diamine is used as a linker molecule to connect two molecules of 9-AP by formation; of a double salt. Like the previously published head-to-head photodimerization of (E)-cinnamic acids, the head-to-head photodimer of 9-AP is successfully produced from solid-state photolysis of the double salts of gauche 1,2-diamines, i.e. (R,R)- and (S,S)-t-chxn and c-chxn. The crystal structure for the low-photoreactivity double salt with (+/-)-t-chxn has been successfully solved.