Superradiance and Directional Exciton Migration in Metal–Organic Frameworks

Abstract

Crystalline metal−organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multilinker molecular excitons display super-radiancea hallmark of coupled oscillators seen in LHCswith radiative rate constant (k rad ) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely, PCN-222(Zn), NU-1000, and SIU-100, with similar topology but varying linkers suggest that the size of such molecular excitons depends on the electronic symmetry of the linker. This multilinker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. The understandings will provide design principles, especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.

Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal–organic framework assemblies

“…Additionally, 222's triangular motif with cofacial conformation translating along the lateral direction of the crystallographic c-axis further improves excited state delocalisation and hopping rates, 50 due to head-to-tail coupling. 51 Steady-state luminescence measurements were conducted towards further understanding of photoinduced processes in our systems. Luminescence experiments on a 0.05 mM solution of 2-Zn in MeCN showed two prominent emission bands at λ = 649 and 714 nm upon excitation at either 415 or 515 nm (Soret or Q band, respectively) (Figures 5a-b), matching literature reports ascribed to the de-excitation of the S 1 states.…”

“…9,19 Alternatively, in absence of a Re catalyst in direct vicinity, directional exciton migration proceeds within the MOF structure. 19 As an exciton can visit 100+ linkers in its lifetime, 19,51 this energy funnelling is highly delocalised and provides a long-range, antennae-like catalytic channel (Figure 6d). The minute, deactivated CO evolution when using full irradiation is noteworthy considering that Soret and Q bands are concomitantly excited.…”

Connecting the Dots for Fundamental Understanding of Structure-Photophysics-Property Relationships of COFs, MOFs, and Perovskites using a Multiparticle Holstein Formalism

“…As in organic materials, UV-vis absorption and emission processes in hybrid materials are governed by delocalized excitations. [129][130][131][132][133]300 However, unlike organic materials where Frenkel and CT excitons are the predominant form of excitations, photoexcitation of hybrid materials result in the formation of Frenkel and Wannier-Mott excitons associated with the organic and inorganic components, respectively. 301 Frenkel excitons are bound electron-hole pairs localized on a single unit cell, while Wannier-Mott excitons are characterized by a much larger Bohr radius and consist of an electron and a hole that can be tens of unit cells or molecules apart.…”

“…Excited state processes of technologically relevant organic and hybrid materials such as polymers, COFs, MOFs, and perovskites are mainly governed by the quantum mechanical behavior of mobile quasiparticles, such as Frenkel, charge-transfer, and Wannier-Mott excitons, polarons and bipolarons, trions, and the interaction of these quasiparticles with intra-and inter-molecular vibrational modes. [129][130][131][132][133][134][135][136][137][138][139][140][141][142][143][144] These vibronic effects, which arise from the coupling of electronic transitions with the nuclear vibrational motion, play a central role in determining the photophysical response of organic 145,[145][146][147] and hybrid materials [148][149][150][151][152][153][154] as well as excited state dynamics in chemical and biological systems. [155][156][157][158][159][160][161][161][162][163] It is therefore not surprising that any progress in the design of novel materials warrants a robust theoretical methodology that can provide an in-depth understanding of how the complicated interplay between photons, excitons, polarons, bipolarons, phonons, and spins influences the photoinduced excited state processes of a wide range of materials.…”

Metal–Organic-Framework-Based Photo-electrochemical Cells for Solar Fuel Generation

“…Recently, a research field aimed at understanding the fundamental photophysical properties of photoactive MOFs has emerged, toward their possible incorporation in artificial photosynthesis systems. − As an outcome, there is a rapidly growing interest in studying MOFs for photoelectrochemical cells (PECs) to generate solar fuels . PECs capable of converting light to chemical energy, are considered as highly promising, economically feasible approach for producing alternative fuel sources.…”