Micro Raman Investigation of the Photodimerization Reaction of 9-Cyanoanthracene in the Solid State

Abstract

The classic photodimerization reaction of 9-cyanoanthracene in the solid state has been studied by means of Raman microscopy, analyzing the time evolution both of the molecular and of the lattice phonon spectra from the reactant to the product. This has allowed us to verify that the formation of the dimer crystal structure always takes place with some delay with respect to the onset of the chemical reaction, as already observed in another crystal to crystal photoreaction. 1 This evidence points to a phenomenon of reconstructive phase transition. 2 The kinetic analysis of the spectroscopic data confirms the previous hypothesis of an autocatalytic mechanism for the process.

Solid-state photodimerization of 9-methyl-anthracene

“…c) regions. The latter spectra were found to differ especially in the range of 1350–1650 cm −1 , where some typical bands of the aromatic anthracene backbone disappear after dimerization, similarly to what was observed in the photodimerization of 9‐cyanoanthracene . The bands with maxima at 1411, 1488, 1563 and 1627 cm −1 for 9MA, which correspond to C–C and/or ring deformation modes, all of A 1 symmetry in the 9‐substituted anthracene molecules, are all relevant to our analysis.…”

“…Furthermore, confocality permits us to follow the progress of the reaction in situ and with a spatial resolution below 1 μm, with the possibility to discriminate reacted and unreacted domains in the same specimen on the very same spot. We found, alike previously studied systems, [12,15] that molecular changes are first produced in the reactant lattice, which leads, after a time delay, to the modifications of the unit cell. Depending upon the system studied and as a result of the complex interplay of energetics and dynamics of the specific reaction studied, the extent of the delay of both changes may considerably vary.…”

“…The topochemical nature makes the process here studied dramatically faster than those previously reported, and the photoproduct can be in fact obtained within minutes. Our strong feeling is that reaction rate and reaction yield are two non‐separable factors in solid state reactions, which both drive the outcome of the reaction.…”

“…This picture represents a key point in explaining the reaction mechanism in the solid state in all its stages. The observed time mismatch between chemical and lattice transformation can be used for a modeling of the system, and further experiments are needed to rationalize the different kinetic behaviors of molecular and lattice transformations …”

“…This kind of packing leads to short intermolecular face-to-face contacts which facilitate the dimerization driven by light. Among the systems recently revisited, we mention 9-anthracene-carboxylic acid, [14] 9-cyano-anthracene [15] and 9-methyl-anthracene (9MA). [16] The latter one is the subject of the present study, as the recent results on this photodimerization prompted our spectroscopic investigation.…”