Kinetics of solid state thermal monomerization of 9-cyanoanthracene photodimer (9CNAD) and photodimerization of 9-cyanoanthracene (9CNA)

Abstract

The kinetics of the thermal monomerization of 9-cyanoanthracene dimer (9CNAD) has been followed both dynamically and isothermally by differential scanning calorimetry (DSC) and isothermally by spectrophotometry. The reaction is found to be similar to many other solid state decomposition reactions and exhibits an induction period corresponding to surface nucleation, an acceleratory region reflecting the formation of the product at the surface of nucleii and a deceleratory region where the reaction centres interact. These DSC experiments yield activation parameters which are different from previously published data. The more reliable isothermal study by spectrophotometry confirms that the reaction occurs in three stages; an induction period, a zero order process with an activation enthalpy of 98 kJ mol^–1 followed by a first-order process with an activation enthalpy of 68 kJ mol^–1. The involvement of an intermediate radical is postulated.

By following the build up of luminescence which develops as a result of the photodimerization of 9-cyanoanthracene single crystals with time and temperature it has been possible to obtain an activation enthalpy for this reaction. The value of 22 kJ mol^–1 is identified with the energy required to bring a pair of 9-cyanoanthracene molecules into a favoured “incipient dimer” orientation to form a chemically bonded species and relates to a diffusion controlled process.