Excitation of crystalline all-trans retinal under pressure

Polycrystalline all-trans retinal (ATR) has been excited at 647.1 nm under pressure up to 9 GPa at room temperature and the resulting Raman spectrum has been measured. The large majority of Raman bands in the 700-1700 cm-1 range have positive pressure coefficients, i.e. bonds stiffen upon compression. The Raman spectrum is recovered after the compression-decompression cycle indicating that the molecular unit does not undergo an irreversible transformation under our experimental conditions. Model density functional (DF) calculations with the B3-LYP exchange-correlation functional and the 6-31G* basis set have been performed on 1-cis,3-trans,5-trans,7-trans,9-trans,1,5,9-trimethyldecapentaenal (TMDPL) mimicking the polyenic moiety of ATR. By shortening the single and double CC bonds of TMDPL up to 0.0015 Å it was possible to fit the frequency dependence on pressure of the most intense polyenic bands with single and double bond compressibilities ?0.00292 and 0.00117 Å/GPa, respectively. The trans -> cis isomerization in the crystal state under pressure and in the presence of the 647.1 nm excitation light is suggested by the Raman spectrum in the fingerprint region. A simplified model is proposed for the occurrence of this process in the crystal phase.