Computational Modeling of Optically Switchable Azobenzenes

Azobenzenes have a unique ability to undergo reversible isomerization between two geometrical isomer forms (the straight trans and the bent cis) when irradiated with the correct wavelength of UV or visible light. These systems are of interest because of their numerous potential applications as reversible photoswitchable absorbers. The azobenzenes can also be used as terminal groups in nickel dithiolene metal complexes, which have applications as photoswitchable absorbers in the near infrared region. To reduce the need to synthesize a large number of materials in order to establish structureproperty relationships, computational chemistry was used to model the absorption spectra of these materials as a function of molecular structure. Time-dependent density functional theory (TDDFT) was used in this study to model the absorption spectra of azobenzene derivatives, and the process was refined to produce results closer to observed spectra than previous research. An alternative method, Zerner’s intermediate neglect of differential overlap (ZINDO), was also investigated and the results were compared with those obtained with TDDFT. The TDDFT approach was found to give consistently better agreement with experimental data than the semiempirical ZINDO approach. Introduction Azobenzenes are noted for their ability to undergo photochemically induced isomerization. When irradiated with the correct wavelength of UV light, the rod-like trans isomer transforms into the bent cis isomer. Likewise, when irradiated with the correct wavelength of visible light, the bent cis isomer transforms into the rod-like trans isomer (Figure 1). Because of this rapid and reversible isomerization between cisazobenzene and trans-azobenzene, azobenzene derivatives can change the optical