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)₂P˙] (1), yielding a phosphorus-centred cyclopentane-1,3-diyl (<i>E</i>-4B and <i>E</i>-5B), represents a straightforward method to generate the desired double switches (<i>E</i>-4B and <i>E</i>-5B) in excellent yields (>90%). The switching properties are demonstrated for the fluorinated species <i>E</i>-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, <i>i.e.</i>, housane formation in the phosphorus-centred cyclopentane-1,3-diyl fragment and <i>E</i>/<i>Z</i> isomerisation at the azobenzene, can be switched by irradiation and the reaction products <i>E</i>-5H, <i>Z</i>-5H and <i>Z</i>-5B (when performing the thermal reverse reaction in the dark) are identified using ¹⁹F{¹H} and ³¹P{¹H} NMR spectroscopy. Results from quantum chemical calculations contribute to the understanding and visualisation of the different isomers of each of the observed compounds (<i>E</i>-5B, <i>E</i>-5H, <i>Z</i>-5H, and <i>Z</i>-5B) caused by the unique structure of the double switches.