Wide-range IR spectra of diarylethene derivatives and their simulation using the Density Functional Theory 

Diarylethenes (DAEs), promising photochromic molecular switches, undergo pericyclic reactions upon ultraviolet or visible light illumination. For this reason, most studies on DAEs employ UV-vis spectroscopies. However, also their infrared (IR) spectra are valuable in particular, for understanding the vibrational dynamics which accompanies the relevant photoreactions. An accurate assignment of IR bands to molecular modes can be achieved through a comparison between experimental and computed theoretical spectra. Even though more sophisticated computational methods are available, the density functional theory (DFT) is usually employed for this task, because of its modest cost and versatility. Here, we have tested the ability of several DFT functionals to reproduce the wide-range, 400-3200 cm −1 , IR spectra of open and closed isomers of four representative DAE molecules. We find that global and range-separated, corrected for anharmonicity by scaling factors, hybrid DFT functionals are able to reproduce the IR spectra of DAEs accurately, especially in the low-frequency and fingerprint ranges. Instead of the most commonly used B3LYP functional, we recommend the use of dispersion-corrected PBE0 functional, which at the same cost as B3LYP, offers a more accurate and reliable performance, with a mean absolute error of just under 10 cm −1 and a mean signed error of 0.2 cm −1 .