Solid state double [2+2] photochemical reactions in the co-crystal forms of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one: establishing mechanism using single crystal X-ray, UV and 1H NMR

Double [2 + 2] photochemical reaction of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one (1P) was observed with 100% yield in four co-crystal forms of 1P with the hydrogen bonding template molecule phloroglucinol (PG) or 5-methoxy resorcinol (MR). The crystal structure of compound 1P does not have the required geometry to produce the double [2 + 2] reaction product, but it gives a single [2 + 2] reaction product with 15% yield upon time restricted irradiation. All the reactions in co-crystals of 1P resulted in the stereospecific exo-exo tetrapyridyl tricyclo[6.2.0.0 (3,6)]-decane. H-1 NMR and the UV-vis spectroscopic studies clearly established a stepwise mechanism for this reaction through the formation of monocyclobutane intermediate. One co-crystal form of 1P with PG and another co-crystal form of 1P with resorcinol were found to be unreactive due to different reasons. Furthermore the mechanochemical grinding products of 1P with RN or MR have exhibited similar structural and reactivity correlations with those crystallized from solution.