Abstract EN:

Carbon capture and sequestration is emerging as an important technologyfor mitigation of climate change. Novel porous materials acting as solidstate adsorbents for CO2 gas have shown promising use of energy efficientcarbon capture using temperature and pressure swing operation forcapture. Metal-organic frameworks (MOFs), crystalline hybrid porousmaterials made up of organic linkers and metal nodes, constitute animportant family of candidate materials for carbon capture. MOFsfunctionalized with azobenzene, a widely studied photoswitch capable ofchanging its geometry from trans to cis configurations and vice-versaupon UV-Vis irradiation, are capable of capturing carbon with lightstimulated photoswitching process. The use of light as a stimulus, inthis case, can reduce greatly the energy costs of the carbon capture.Modeling atomistically the azobenzene functionalized PCN-123 MOF withinthe density functional theory (DFT), the correct ground state geometryof the photoswitch in cis and trans configuration is determined bycalculating potential energy surfaces along relevant degrees of freedom.This immediately reveals the microscopic mechanism of the carboncapture to be the blocking and unblocking of the metal-node, a prominentadsorption site of CO2, by the cis and trans configurations ofazobenzene. To achieve high efficiency of gas capture using such MOFs,the photoswitching of the azobenzene must be controlled by using lightstimuli of appropriate wavelengths yielding higher fractions of eitherisomers. A detailed understanding of the optical absorption of theazobenzene photoswitches is indispensable for the same. State-of-the-artmany-body perturbation theory (MBPT) methods, namely GW andBethe-Salpeter equation (BSE), are used to accurately compute andcharacterize the optical spectra of solvated azobenzene derivatives andazobenzene functionalized MOF. In case of solvated azobenzene molecules,the GW/BSE approach is combined with a non-equilibrium embedding schemeto account for the dielectric screening by the solvents. The problem ofthe dependence of the BSE spectra on DFT exchange-correlation functionalis addressed by tuning the PBE global hybrid functional such that theDFT and GW ionization potentials (IP) are matched to the fulfillment ofthe DFT ionization potential (IP) theorem. The IP-tuning coupled withnon-equilibrium embedding at BSE/GW yields very accurate excitationenergies and cis-trans band separations of a range of azobenzenederivatives. The BSE/GW spectra of PCN-123 MOF with cis or transconfigurations are calculated using both the periodic and molecularmodels using semilocal DFT starting point. In the low energy regime,visible, near UV and mid UV, optical excitations of the azobenzenefunctionalized MOF are associated with the azobenzene functionalization.In the low-energy regime, the BSE/GW spectra of the embedded molecularmodels of the MOF are in a reasonable agreement with thoseof periodic MOF paving way for computationally cheaper estimation of theoptical spectra of ligand-functionalized MOFs. A noticeable differencein the periodic and molecular spectra is the larger intensity of the S1excitation in the former which suggests a possibly more efficientphotoswitching of the azobenzene in the MOF than a standalone molecule.The use of MBPT methods also allows for a quantitative estimation of theelectron addition and removal energies for the MOFs. The parent MOF ofPCN-123, the MOF-5 was earlier considered as a semiconductor by severalcomputational and experimental studies. The GW/BSE calculations carriedout here suggest that it is a wide gap (8 eV) insulator with itsoptical excitations featuring strong excitonic bindings of more than 3eVs. Overall, the thesis sheds new light on the charged and neutralexcitations in azobenzene functionalized MOF suggesting new efficientand accurate computational strategies.