Boosting Transport Distances for Molecular Excitons within Photoexcited Metal–Organic Framework Films
Source abstract
Abstract
Herein, we describe the fabrication of porphyrin-containing metal-organic framework thin films with 1,4-diazabicyclo[2.2.2]octane (DABCO) pillaring linkers and investigate exciton transport within the films. Steady-state emission spectroscopy indicates that the exciton can traverse up to 26 porphyrin layers when DABCO is used as a pillaring linker, whereas on average only 9-11 layers can be traversed when either 4,4'-bipyridine (a pillaring linker) or pyridine (a nonpillaring, layer-interdigitating ligand) is used. These results can be understood by taking into account the decreased separation distances between transition dipoles associated with chromophores (porphyrins) sited in adjacent layers. Shorter distances translate into faster Förster-type exciton hopping and, therefore, more hops within the few nanosecond lifetime of the porphyrin's singlet excited-state. The findings have favorable implications for the development of MOF-based photoelectrodes and photoelectrochemical energy-conversion devices.
Source: openalex_inverted_index · Quality: baseline
Research context
OpenAlex topics: Metal-Organic Frameworks: Synthesis and Applications · Magnetism in coordination complexes · Machine Learning in Materials Science
SciVX discovery tags are terminology-based. OpenAlex topics are source-provided classifications.
Affiliations
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
Funding
Basic Energy Sciences · Northwestern University
References (55)
Show source-provided references
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36. These numbers are slightly greater than what we reported previously because we are no longer assuming (incorrectly) that all nearby excitons, including short-lived excitons, are quantitatively quenchable by Pd-porphyrins (see text for details).
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38. Atomic force microscopy (AFM) measurements, which are inherently higher in spatial resolution than profilometry, show films to be considerably rougher (Figure S2) than indicated by profilometry. In our discussion of exciton-transport distances we neglect film roughness. Thus, the reported distances likely correspond to lower-limit estimates of the true transport distances. Work in progress is aimed at identifying and largely eliminating the chemical factors responsible for LbL MOF film roughness.
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42. In our previous report, measurements extended to only 1 ns—too short to reveal the presence of a low-amplitude 9 ns decay component (ref (35)).
43. At the same time, as noted above, a subpopulation of short-lived excitons displays a resistance to fluorescence quenching that appears insensitive to MOF film thickness, at least over the range of thicknesses probed. It is tempting to equate the relative intensities of P2-quenchable and nonquenchable emission with the population ratio of long-lived excitons to short-lived excitons and thereby gain insight into the density of putative aggregate or defect sites. Unfortunately, we lack the needed information about emission quantum yields for long- vs short-lived excitons as well as information about the number of chromophores over which short-lived excitons may be delocalized.
44. For quenching measurements, illumination is done from the Pd-porphyrin side of the film, and we assume that fluorescence is dominated by free-base porphyrins situated furthest from the Pd-porphyrin quenching layers. These constitute about 38% of the photoexcited porphyrins. The Soret peak absorbance for a 54-cycle film is ∼0.5 and corresponds to absorption of ∼68% of the incident photons, i.e., 100 × (1 – 10 – Abs) = 100 × (1 – 10 – 0.5). To remove 62% of the absorbed photons, an absorbance of 0.27 would be required. The thickness of a 54-cycle film is about 44 layers (i.e., two layers of Pd-porphyrin plus 42 layers of free-base porphyrin). The quantity (0.27/0.5) × 44 (= 24 layers) gives an estimate of the number of layers that are non-emissive and sited between the light source and the emissive layers (which number 20 layers (= 44 −24)). Of the 24 non-emissive layers, 22 contain free-base porphyrins. This quantity (22) can be equated with the average number of layers an exciton can traverse. If we instead assume that all 42 free-base porphyrin layers are equally illuminated, i.e., we neglect inner-filter effects, we conclude that the exciton traverses 26 layers (= 0.62 × 42 layers). Clearly, inner-filter effects need to be taken into account for films absorbing this strongly.
45. If we assume that 22 layers is a reasonable approximation for the root mean square displacement of the exciton characterized by a 9 ns lifetime, we find that the average number of chromophore-to-chromophore energy-transfer steps (exciton hopping steps) is ∼ 484 (= 222) and time required for each hop is ∼ 20 ps. Notably, this value is 3 to 4 times shorter than the lifetime of the quenching-resistant chromophore subpopulation discussed above. In turn, these observations imply that the exciton associated with the short fluorescence time is less mobile (i.e. is characterized by a substantially longer chromophore-to-chromophore hopping rate) than the exciton associated with 9 ns fluorescence.
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