Calorimetric, spectroscopic and structural studies of the monomerization of crystalline 9-cyanoanthracene photodimer (9 CNAD)

Abstract

The thermal monomerization of 9-cyanoanthracene photodimer (9 CNAD) crystals has been studied using differential scanning calorimetric (d.s.c.), luminescence and X-ray diffraction techniques. The early stages of monomerization (up to ≈ 3% decomposition) includes a homogeneous topochemical process with the monomer produced within the dimer host in the form of centrosymmetric pairs having different degrees of overlap. Such pairs give rise to different luminescence characteristics and the excimeric emission at 490 nm is confirmed as emanating from one of these pairs.

The first exothermic peak which appears in the dynamic decomposition calorigrams of the more crystalline 9 CNAD samples at ≈ 425 K is due to the heterogeneous topotactic growth of monomer 9-cyanoanthracene (9 CNA) inside the dimer host crystal. This peak is absent for finely powdered material. At later stages an amorphous phase of 9 CNA forms and both crystalline and amorphous phases subsequently melt yielding separate endotherms at 452 and 453.5 K. Both the dynamic and isothermal decomposition calorigrams are found to be highly dependent on the nature of the 9 CNAD samples and contain information about at least one crystallization process.

The unit cell parameters for the dimer have been evaluated together with those of the developing monomer phase. They differ in only the stack (c) axis.

The dimer undergoes monomerization when irradiated by X-rays confirming our initial views about the involvement of a biradical intermediate during the course of monomerization.