Bethe-Salpeter Study of the Optical Absorption of trans and cis Azobenzene-Functionalized Metal-Organic Frameworks using Molecular and Periodic Models

The optical absorption spectra of the azobenzene-functionalized metal-organic framework, PCN-123, are calculated in cis and trans configurations using the Bethe-Salpeter equation (BSE) formalism and the GW approximation using periodic and non-periodic models. In the visible, near-UV and mid-UV region the optical excitations in the MOF are associated with the azobenzene functionalities and this results in spectral features similar to the case of the gas phase azobenzene and the azo-functionalized ligand. The most noticeable difference is the significantly more intense S₁ band for cis in the MOF as compared to the free molecules which points to a faster and more complete cis→trans isomerization in the framework, with strong implications for the design of MOFs with high photoconversion efficiencies. Consistent with these findings, all the molecular models employed to represent the MOF, including the smallest, are found to yield a reasonable description of the low energy optical spectra (between 2 and 5 eV) of the periodic framework, with the exception of the stronger S₁ band of cis in the MOF, a feature that we attribute to a limitation of the fragment model to correctly represent the wavefunction of the extended framework.