Monitoring cis-to-trans isomerization of azobenzene using terahertz time-domain spectroscopy.

We present terahertz time-domain spectroscopy (THz-TDS) to explore the conformational dynamics of thermally induced and photoinduced isomerization of azobenzene. The essence of the method is that isomerization of azobenzene proceeds via large structural changes in the molecule, while the THz response is sensitive to these changes. We experimentally demonstrate that the THz spectra of azobenzene show remarkable variations upon heating and irradiation, and as such quantitatively recorded and identified THz spectroscopy can be used to monitor the isomerization process. Specifically, the measured THz spectra clearly reveal that the rate of thermal-isomerization from cis-to-trans in non-polar solvents is faster than that in polar solvents, and an about 6-fold acceleration of the rate could be achieved when Au NPs were introduced as a catalyst into azobenzenes. Moreover, we provide evidence that the temperature and Au NP catalyst do not have an obvious influence on the photoinduced isomerization of azobenzene. The presented example illustrates the power of the THz-TDS method to open up a novel avenue for exploring molecular dynamics.