Orthogonal Topochemical Reactions in a Single Crystal Mediated by Distinct Physical Stimuli

Single crystals capable of undergoing two distinct chemical transformations are rare, and achieving two distinct transformations controlled by two different stimuli is unprecedented. Here, we report an M:BPE co‐crystal formed through complementary hydrogen bonding between the carboxylic acid group of M and the pyridyl group of BPE , further reinforced by C─H···O and C─H···N interactions. Monomer M contains azide and alkyne groups that undergo a heat‐driven topochemical click reaction, while BPE features an olefin that enables light‐driven topochemical [2+2] dimerization. Upon heating, M in the co‐crystal undergoes a regiospecific click reaction in a single‐crystal‐to‐single‐crystal manner to yield a 1,4‐triazolyl‐linked cyclic dimer. In contrast, photoirradiation drives the BPE units in the co‐crystal to undergo [2+2] photodimerization, yielding the rctt isomer. This stimulus‐dependent behavior represents the first example of orthogonal reactivity within a single crystal mediated by different external stimuli.