Knowledge Bank

The photolysis of trans-cinnamic acid has been known for many years to yield the $dimera$. However the nature of the dimer is known to depend on the orientation of the monomers within an individual crystal of the starting material. Thus head-to-head and head-to-tail dimers are formed from the corresponding orientation of the monomers, while a third arrangement of the monomers will not dimerise. In the current work we have shown that Raman spectroscopy is an effective way to follow these photodimerisations; the spectra of the monomer and dimer show quite distinctive features. In particular the intense Raman band arising from the stretching vibration of the exo-cyclic double bond of the monomer is seen to decay upon photolysis. We have extended the work to a series of derivatives of cinnamic acid (2-carboxy, 2-chloro, 2-methoxy and 2-methyl). We have also studied co-crystallised mixtures of derivatives. In recent experiments we have monitored the photodimerisation of single crystals of these compounds. This has been done using a Raman microscope located at the University of Bordeaux. France and an infrared microscope at station 13-3 of the synchrotron ring at Daresbury, UK.