Understanding Molecular Excited States at the Metal-Molecule Interface via Transition Density Matrix AnalysisA Case Study of Azobenzene Thiols on Gold.

Noble metal nanoparticles are rapidly gaining popularity as novel catalytic platforms to influence chemical reactions in various ways. Despite an increasingly large number of studies, many key processes especially at the metal-molecule interface are fundamentally still not fully understood up to this day. Throughout this work, we present a systematic study targeting the molecular excited states of covalently linked azobenzenes (ABs) on gold via a combination of time-dependent density functional theory and tight-binding calculations with transition density matrix analysis. We find that the optically bright ππ* state is strongly resonant to close-lying local gold excitations, which leads to splitting and redshift of the ππ* state and increased UV/vis absorption around the ππ* excitation energy. We show that these findings hold true across different AB isomers and demonstrate how a systematic variation of the AB-gold distance leads to a gradual localization of the ππ* excitation. Moreover, we discuss how a direct electronic interaction between the AB and the gold surface leads to the formation of delocalized hybrid states and investigate the exciton formation of AB dimers at the metal interface. Our results are carefully verified across a wide range of computational parameters including a large number of different density functionals, basis sets, and gold clusters of varying forms and sizes. The presented workflow is easily applicable to other functional molecules on metal surfaces to further broaden the understanding of substrate-surface interactions at the interface.