Towards T-ray spectroscopy of retinal isomers: A review of methods and modelling

Abstract

The terahertz gap lies between the infrared and millimeter regions of the electromagnetic spectrum. Terahertz (THz) waves, or T-rays, bridge the gap between electronics and photonics, have novel properties and interact uniquely with many materials. Various rotational, vibrational and translational modes of molecules are within the THz range (0.1–10 THz). Since these modes are unique to a particular molecule it is possible to obtain a ‘THz fingerprint’ allowing for the identification of chemical substances. Astronomers and chemists have already utilised the THz region to characterise and identify small organic molecules. Terahertz spectroscopy allows not only for exploration of molecular structures but also for molecular dynamics. One difficulty in performing THz spectroscopy is that the data can be noisy and thus difficult to interpret. An a priori knowledge of the expected THz spectra allows improved experiments to be performed. Ab initio molecular orbital theory is a helpful tool in providing a great deal of information about intermolecular structure, interactions and dynamics. Recently, we have begun to investigate whether THz is a useful part of the spectrum for studying the photoisomerization of the retinal chromophore by using molecular modelling and vibrational mode calculations. The vibrational character of very low frequency modes can provide insight into molecular dynamics since they potentially relate to the torsional coordinates relevant to the actual isomerization itself. To date, very few studies on the retinal molecule and its isomers based on THz technology have been carried out. Initial experiments using THz Time-Domain Spectroscopy (THz–TDS) have shown that it is possible to distinguish between the different isomeric forms of retinal, indicating that the THz modality may be useful for studying very low frequencies and associated mechanics. In order to motivate further THz experiments on retinal, this paper reviews: (i) the status of the retinal chromophore, (ii) the work in the area uptil now, and (iii) the background of THz spectroscopy—furthermore we review and perform ab initio calculations for all-trans and 9-cis retinal in the THz regime.