Molecular assemblies are famous for their versatility. Chromophoric assemblies, which are reminiscent of natural light-harvesting systems, are treated as a promising material to either realize or conduct the study of energy transportation. Yet, molecular assemblies with different arrangements are neither easy to synthesize, nor well investigated. Metal-organic frameworks (MOFs) as emerging materials of hybrid molecular assembly are good platforms to conduct the study of energy conversion and transportation. Such applications require the fundamental understanding between their excited state properties and underlying topological-network, or spacial arrangement. Here, we report two chemically identical MOFs, NU-901 and NU-1000, which are both constituted by tetrabenzoate-pyrene (H4TBAPy), and ZrIV-oxo node, only with different topology, as the platform to conduct the interrogation of their excited states properties. The computational models are carefully investigated and constructed in such a way that only the most interacted chromophores are involved. In different MOFs, the excited states have differed in terms of anisotropies and delocalization. Spectroscopic techniques are also used to interrogate the impact of topological difference to their excited state electronic structures. TDDFT computed transition density matrices for selected models revealed that the optically relevant excited states are delocalized in NU-901 model, with a maximum exciton size of ~1.7 nm, while the corresponding excited states of NU-1000 are localized within the closest proximity triangle. The results also suggested the evolution of polar excitons: their oscillator strengths vary with the extent of interchromophoric interaction depending on their topological network. Femtosecond transient absorption (fs-TA) spectroscopic data of NU-901 highlight instantaneous spectral evolution of intense S1 → Sn transition which diminishes with the emergence of a broad induced absorption originating from a fast excimer formation. This fs-TA data reveals the direct observation of fast excimer formation in MOFs. This feature seen in NU-901 is clearly absent in NU-1000 and the H4TBAPy linker. Furthermore, transient and steady-state fluorescence data collected as a function of solvent dielectrics reveal that the emissive states in both MOF samples are essentially electronically non-polar. However, low-lying polar excited states may get involved in the excited-state decay processes in polar solvents.

What should be considered to control the electronic structure of excited states: Impact upon topological arrangement

2019 Fall National Meeting

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