Mid-infrared recognition of the reversible photoswitching of fulgides

IR absorption spectra of E- and C-isomers of several photochromic fulgide compounds were examined. As a characteristic feature, the spectra of all C-isomers show peaks that are completely missing in the spectra of the corresponding E-isomers. Photoisomerization reactions of fulgides can, therefore, be monitored and their isomerization state can be detected by measuring the IR absorption at the characteristic wavenumbers. It is verified that mid-IR irradiation using tunable IR diode lasers does not induce any photoreaction. Thus, for the application of fulgides as optical recording media or photoswitches, narrowband mid-IR absorption measurement provides a method for entirely non-destructive readout.