Exciton Coupling and Conformational Changes Impacting the Excited State Properties of Metal Organic Frameworks

• Main Authors: Andreas Windischbacher, Luca Steiner, Ritesh Haldar, Christof Wöll, Egbert Zojer, Anne-Marie Kelterer
• Format: Article
• Language: English
• Published: MDPI AG 2020-09-01
• Series: Molecules
• Subjects: metal organic frameworks; SURMOF; absorption; emission; time-dependent density functional theory; aggregation
• Online Access: https://www.mdpi.com/1420-3049/25/18/4230

In recent years, the photophysical properties of crystalline metal-organic frameworks (MOFs) have become increasingly relevant for their potential application in light-emitting devices, photovoltaics, nonlinear optics and sensing. The availability of high-quality experimental data for such systems makes them ideally suited for a validation of quantum mechanical simulations, aiming at an in-depth atomistic understanding of photophysical phenomena. Here we present a computational DFT study of the absorption and emission characteristics of a Zn-based surface-anchored metal-organic framework (Zn-SURMOF-2) containing anthracenedibenzoic acid (ADB) as linker. Combining band-structure and cluster-based simulations on ADB chromophores in various conformations and aggregation states, we are able to provide a detailed explanation of the experimentally observed photophysical properties of Zn-ADB SURMOF-2: The unexpected (weak) red-shift of the absorption maxima upon incorporating ADB chromophores into SURMOF-2 can be explained by a combination of excitonic coupling effects with conformational changes of the chromophores already in their ground state. As far as the unusually large red-shift of the emission of Zn-ADB SURMOF-2 is concerned, based on our simulations, we attribute it to a modification of the exciton coupling compared to conventional H-aggregates, which results from a relative slip of the centers of neighboring chromophores upon incorporation in Zn-ADB SURMOF-2.