Photodimerization of Crystalline 9-Anthracenecarboxylic Acid: A Nontopotactic Autocatalytic Transformation

This work discusses the photoreaction of crystalline β-9-anthracenecarboxylic acid (C15H10O2, 9AC). The thermally reversible [4 + 4] photodimerization of crystalline 9AC was investigated with electronic and vibrational spectroscopy and with photocrystallographic techniques. The photocrystallographic studies and their interpretation lead to a complex picture of the mechanism of this photoreaction. The increase of the peak widths during photoreaction is interpreted as a disorder increase in the crystalline lattice. Quantum chemical calculations suggest that this disorder is caused by the formation of various product configurations during photoreaction. At least three possible configurations have been found. Different tautomeric sites lead to an additional disorder parameter adding another complexity in the observed photodimerization reaction. For the overall photodimerization a maximum conversion rate of 75% has been determined. Application of the Johnson, Mehl, Avrami, and Kolmogorov (JMAK) model leads to an extraordinary Avrami parameter suggesting unusual transformation kinetics of the bulk during photoreaction. The unusual Avrami parameter is discussed as an autocatalytic step during photoreaction through an excimer state. It is created by one photoexcited 9AC molecule and a ground state 9AC molecule. Taking into account this additional kinetical pathway, out of statistical reasons additional lattice disordering mechanisms are possible.