Ground-State versus Excited-State Interchromophoric Interaction: Topology Dependent Excimer Contribution in Metal–Organic Framework Photophysics

Abstract

Metal-organic frameworks (MOFs) define emerging materials with unique optoelectronic properties that stem from the highly organized chromophoric linkers within their frameworks. The extent of ground- and excited-state interchromophoric interaction among the π-conjugated macrocyclic linkers was studied within three tetraphenyl-pyrene (1,3,6,8-tetrakis(p-benzoic acid)pyrene; HTBAPy)-based MOFs: ROD-7 (In(OH)TBAPy, frz), NU-901 (scu), and NU-1000 (csq) via steady-state and time-resolved spectroscopic techniques. These experimental data along with computational results indicate that the extent of the interchromophoric interaction, leading to a reduced optical band gap, varies across the series of MOFs and is a function of the relative orientation of the TBAPy linkers determined by their respective framework topology. The trend in the S → S emission lifetime is consistent with their relative optical bandgap. Analyses of the transient emission decay profiles and time-resolved emission spectroscopic data, recorded in low dielectric media, reveal that a long-lived emissive excimer state appears ∼1850 ± 150 cm lower in energy relative to their corresponding S → S transitions. The emissive contribution from this excimer state, as well as its corresponding transition energy and time constants, are also found to be dependent on MOF identity. Such variation in properties are particularly influenced by the number density of the TBAPy linkers presented by the topology of a given MOF that are primed to form such an excited state complex. The present work shows how the specific arrangement of the linkers can play a key role in the photophysical properties of MOFs.

Unravelling Why and to What Extent the Topology of Similar Ce‐Based MOFs Conditions their Photodynamic: Relevance to Photocatalysis and Photonics

“…Nevertheless, comparable situations in other MOFs have been reported, where excimers emission was observed. [18,26,29,35] Note also that the green emission can be recorded upon excitation at both, the UV and visible absorption bands, which reinforces the explanation involving excimers ( Figure S3A, Supporting Information). The excitation spectra gating the green emission do not depend on the observation wavelength, but are different from the absorption one, indicating the involvement of efficient nonradiative events.…”

“…It has been reported that similar pyrene-core molecules in solution and having different aromatic substituents, only present single exponential decays of ≈0.5-2.3 ns, corresponding to the monomer emission, [33] and H 4 TBpY in dimethylformamide shows a single emission of 1.90 ns. [26] Hence, we assign the intermediate component (1.67 ns) to the emission of the monomer of H 4 TBpY. Previous studies have shown that the pyrene excimers are characterized by very long lifetimes, [36] so the longest lifetime observed here is probably not of excimers emission, but of a charge-transfer character species (Scheme 1 and Scheme S1A, Supporting Information).…”

“…One possibility is excimer formation upon interaction of the linkers at S 1 and S 0 states as it has been observed in other MOFs. [18,21,26,29,35] Examining the structure of Ce-NU-1000 MOF ( Figure 1A), the linkers forming the triangular holes may be close enough and in a favorable position to produce excimers. The distance (11 Å) and the angle between them (65°) are not ideal for an excimer formation.…”

“…[18,[22][23][24][25] Other studies have explored the dynamics of Zr-based MOFs having the same linkers and metal clusters but different crystalline phases, and thus different distances and angles between their neighboring linkers. [26][27][28][29] These reports have shown the key role of the structural parameters in the interchromophoric interactions that lead to the formation of excimers in a specific network, and to their absence in others even having the same composition, but different structure. From the point of view of theory, MOFs with different metals in the clusters have been examined to understand the relevance of the metal-linker orbitals interaction.…”

Enhanced Drug Photosafety by Interchromophoric Interaction Owing to Intramolecular Charge Separation

“…[17] Integration of the photoactive pyridylpyrimidine moietyi nto am ore complex structure strongly modifies its photobehavior and leads to an improved toxicological profile of IMT.H owever,i ti sn ot clear how the intramolecular interactionb etween the different chromophores of IMT occur.I nterchromophorici nteraction is an omnipotent process that is responsible for the electron, proton, energy, and charge transfersa mongo rganic light-emitting diodes (OLED), supramolecular frameworks, metal-organic frameworks,l ight-harvestings ystems, and biomolecules. [18][19][20][21][22] Never-theless, there are few examples that unveilt he nature of an interchromophoric interaction. In an attemptt ounderstand the unexplained photobehavior of IMT and the chromophorechromophore interactions amongi ts substructure molecules (Scheme 1), an investigationh as been conducted herein by using femto-to-nanosecond transient absorption, picosecond time-resolved resonance Raman spectroscopy (ps-TR 3 ), and nanosecondt ransient resonance Ramans pectroscopy.T o better understand the photosafety of IMT,D NA was used as a model substrate to explore the photochemical reactions between IMT and DNA.…”