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Kinetics of Solid State Photodimerization of 1,4-Dimethyl-2-pyridinone in its Molecular Compound

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posted on 15.07.2010 by Deng-Ke Cao, Thekku Veedu Sreevidya, Mark Botoshansky, Gilad Golden, Jason Brown Benedict, Menahem Kaftory
The [4 + 4] photodimerization of 1,4-dimethyl-2-pyridinone (A) in its molecular compound with 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) was investigated by irradiation of a single crystal of the compound. The conversion to the product in a single-crystal to single-crystal transformation enabled the determination of its crystal structure after each exposure cycle of 10−20 min with a pulsed 355 nm laser light. The unit cell changes as a function of the conversion were monitored and showed monotonic changes. The kinetics of the reaction was studied at two different temperatures (230 and 280 K) and revealed a sigmoidal behavior that could be explained by the JMAK model for crystal growth with a mechanism that is intermediate between random distribution of products in the crystal and the existence of growing nuclei. The Arrhenius plot provided calculated activation energy of 32.5 kJ/mol.