In this paper, the photochromic reaction of the 1,2-bis(5′-ethoxy-2′-(2″-pyridyl) thiazolyl) compound (named DE) was studied by ultraviolet–visible absorption and various Raman spectroscopies associated with density functional theory calculations. To explain the growth of the visible absorption spectrum when the compound is irradiated with ultraviolet light, we suggest the existence of several conformations of the colored form. We also studied the vibrational spectroscopic properties of DE in different conditions such as powder, thin solid film, or in gold nanorods colloidal solutions. This compound is found photochromic in all these conditions. The theoretical Raman spectra of the open and closed forms reproduce fairly well the experimental data and help the complete assignment of the observed bands. Copyright © 2013 John Wiley & Sons, Ltd.