Designing a visible light-mediated double photoswitch: a combination of biradical and azobenzene structural motifs that can be switched independently.

A new molecular switch is presented that combines both biradical and azobenzene motifs to perform visible light-induced constitutional and stereo-isomerisation within the same molecule. The insertion of isonitrile-functionalised azobenzenes into the four-membered biradical [˙P(μ-NTer)P˙] (1), yielding a phosphorus-centred cyclopentane-1,3-diyl (-4B and -5B), represents a straightforward method to generate the desired double switches (-4B and -5B) in excellent yields (>90%). The switching properties are demonstrated for the fluorinated species -5B and, interestingly, can occur either stepwise or simultaneously, depending on the order in which the sample is irradiated with red and/or green light. All possible isomerisation reactions, , housane formation in the phosphorus-centred cyclopentane-1,3-diyl fragment and / isomerisation at the azobenzene, can be switched by irradiation and the reaction products -5H, -5H and -5B (when performing the thermal reverse reaction in the dark) are identified using F{H} and P{H} NMR spectroscopy. Results from quantum chemical calculations contribute to the understanding and visualisation of the different isomers of each of the observed compounds (-5B, -5H, -5H, and -5B) caused by the unique structure of the double switches.