Enhancing Dynamic Spectral Diffusion in Metal–Organic Frameworks through Defect Engineering

Abstract

The crystal packing of organic chromophores has a profound impact on their photophysical properties. Molecular crystal engineering is generally incapable of producing precisely spaced arrays of molecules for use in photovoltaics, light-emitting diodes, and sensors. A promising alternative strategy is the incorporation of chromophores into crystalline metal−organic frameworks (MOFs), leading to matrix coordination-induced emission (MCIE) upon confinement. However, it remains unclear how the precise arrangement of chromophores and defects dictates photophysical properties in these systems, limiting the rational design of well-defined photoluminescent materials. Herein, we report new, robust Zr-based MOFs constructed from the linker tetrakis(4-carboxyphenyl)ethylene (TCPE 4− ) that exhibit an unexpected structural transition in combination with a prominent shift from green to blue photoluminescence (PL) as a function of the amount of acid modulator (benzoic, formic, or acetic acid) used during synthesis. Time-resolved PL (TRPL) measurements provide full spectral information and reveal that the observed hypsochromic shift arises due to a higher concentration of linker substitution defects at higher modulator concentrations, leading to broader excitation transfer-induced spectral diffusion. Spectral diffusion of this type has not been reported in a MOF to date, and its observation provides structural information that is otherwise unobtainable using traditional crystallographic techniques. Our findings suggest that defects have a profound impact on the photophysical properties of MOFs and that their presence can be readily tuned to modify energy transfer processes within these materials.

Enhancing the Electrochemiluminescence of Porphyrin via Crystalline Networks of Metal–Organic Frameworks for Sensitive Detection of Cardiac Troponin I

“…The major emission peaks (λ ex = 415 nm) of the PCN-224 fluorescence spectra were consistent with TCPP without any significant differences (Figures C and S11A). All PCN-224 provided similar PL emission intensity without dependence on sample crystallinity and morphology (Figures C and S11A). This result was consistent with previous research studies, which proved that size and shape had little effect on PL properties .…”

Trapping of Photoluminescent Kinetic Intermediates During High-Concentration Synthesis of Non-Emissive Metal-Organic Frameworks

“…Unlike common single-wavelength techniques such as timecorrelated single photon counting, this approach permits sensitive, direct detection of the full PL spectral evolution over the decay lifetime. 67 The strong emission of H4dobdc in solution is quenched in the solid state (Figure 5). Intermolecular interactions evidently prevent the ESIPT mechanism, and rapid non-radiative decay dominates in solid H4dobdc.…”

Simplifying the Synthesis of Metal–Organic Frameworks

“…7 For example, a recently reported Zr-MOF from our lab, CORN-MOF-5 (CORN = Cornell University), requires 200 equiv of formic acid during its synthesis to form high-quality, phase-pure material. 21 Given the breadth of Zr-MOFs that have been synthesized, 18 many modulators have been tested that differ significantly in pK a and electronic and steric effects. 7 Few guiding principles exist for choosing the optimal modulator for a given Zr-MOF, contributing to the tendency among practitioners to simply add more of an unoptimized acid modulator to improve the quality of a material rather than identify a better modulator that can be employed in smaller amounts.…”

“…A substantial excess (1–500 equiv relative to the linker) of modulator is often included to maximize the crystallite size or other material properties of Zr-MOFs; the modulator is then generally discarded with the supernatant after a single use . For example, a recently reported Zr-MOF from our lab, CORN-MOF-5 (CORN = Cornell University), requires 200 equiv of formic acid during its synthesis to form high-quality, phase-pure material . Given the breadth of Zr-MOFs that have been synthesized, many modulators have been tested that differ significantly in p K a and electronic and steric effects .…”