Light-Harvesting in Porous Crystalline Compositions: Where We Stand

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Light-Harvesting in Porous Crystalline Compositions: Where We Stand Towards Robust Metal-Organic Frameworks Jierui Yu, Xinlin Li, and Pravas Deria ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05673 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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ACS Sustainable Chemistry & Engineering

Light-Harvesting in Porous Crystalline Compositions: Where We Stand Towards Robust Metal-Organic Frameworks Jierui Yu, Xinlin Li, and Pravas Deria* Department of Chemistry and Biochemistry, Southern Illinois University, 1245 Lincoln Drive, Carbondale, Illinois 62901, United State. KEYWORDS. Self-assembled chromophores; Energy transfer; excited-states; photon-energy conversion. *Corresponding author: Pravas Deria, [email protected] ABSTRACT: Molecular assemblies in metal-organic frameworks (MOFs) have the potential to be considered as functional artificial light-harvesting (LH) system with features similar to the natural LH machinery. With large photon absorptivity, the frameworks provide novel energy-transfer pathways enabling long-distance energy migration both in singlet and triplet manifolds. Furthermore, considering the eventual energy conversion utility, various strategies have been explored to achieve long-lived excited states and integration of photochemical ‘reaction-centers’. Understanding the excited state properties, such as exciton size, delocalization length, and dynamics within MOF based compositions as a function of their underlying topological-net can help to implement new strategies with improved LH utility. This paper summarizes the various unique photophysical process and elucidates how their structural parameters play a critical role in defining the photophysical processes within MOF structures with a particular emphasis on the robust Zr6IV -oxo node derived frameworks as future compositions.

Artificial Light-Harvesting Complexes Meeting the ever-growing energy demand has become a top priority for humankind and have stimulated scientists to find sustainable resource with least carbon-footprint.1 In this regard, artificial solar energy conversion remains the key target, since it can convert photon energy to electricity or transportable fuel, playing a role as phototrophic organisms in the ecosystem. Solar energy conversion process in the natural LH machinery involves multi-step energy cascade processes, where the first step is photon absorption within the antenna assembly of chlorophyll pigments (Figure 1). The energy is then directionally transferred to the reaction center for charge separation and carrier migration to enter in various dark-phase chemical reactions to generate bio-molecules. While delocalized, long-lived excitons define the core interest in the development of artificial light energy conversion system, delivering the photon energy to the conversion module, i.e. the reaction center, defines the other key step within a LH system. For the development of artificial LH system, various mechanistically different energy transfer (EnT; also defined as ET) processes have been studied both in singlet and triplet manifolds. Energy transfer within the singlet manifold involves fluorescence resonance energy transfer (FRET), where the EnT rate constant, according to the Förster model, depends in the donor-acceptor electronic coupling and the overlap integral between the donor fluorescence and acceptor absorption profile shapes. Alternatively, a Dexter-type triplet-triplet energy transfer (T-T EnT or TTET) process involves electron exchange that requires the donor and the acceptor species to be in close proximity. Even though the singlet excitons are short-lived, Förster EnT is very efficient and can lead to a long exciton migration.

On the contrary, the triplet excitons are long-lived but require the donor-acceptor arranged in extremely close proximity. An optimum strategy is required for appropriate positioning of the chromophores to achieve an efficient energy transfer process. Thus, investigations that help delineation of strategy to extend exciton lifetime and migration length are of primary interests in this perspective article.

Figure 1 Pigment assembly in the photosynthetic antenna complexes: LH2 and LH1. While the LH2 and LH1 components are responsible for the photon energy harvesting the reaction center (RC) is responsible for photochemical processes that include charge separation and carrier migration to distant enzymatic chemical reactions.

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Unique photophysical features of chromophore assembly in the natural photosynthetic LH antenna have inspired the development of various porphyrin and other chromophore based assemblies.2-4 In this regard, co-facially oriented, and ethynylenebridged macromolecular systems are important as these structures highlight the different degree of electronic as well as transition-dipole interaction, offering strategies to control the optical and potentiometric band-gaps in solution based experiments.5-8 It is important to note that these classic macromolecular systems incorporate malicious excited-state decay process in their naturally aggregated solid compositions during various device fabrications. Thus, constructing a precisely arranged chromophore assembly like that in LH complexes may prove a route to control their photophysical properties such as exciton size and extent of their delocalization as well as their dynamics simply by tuning the interchromophoric distance and orientation.9 In this aspect, MOFs are ideal hybrid material, constructed from organic struts or linkers, which can be chromophores, interconnected through metal nodes. The resulting arrays of framework structures provide extensive design flexibility in terms of pore size, shape, and dimensions, which, in turn, dictate the interchromophoric orientation.10-11 The crystallinity of MOFs introduces rigidity providing a predictable, yet precise molecular location of the chromophore linkers compared to any other known assembly-technique.12 Furthermore, MOFs can be functionalized post-synthetically though various modular chemistry which can introduce complementary chemical entities relevant for various photo-induced processes. In this per-

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spective, we summarize recent progress based on the understanding of excited-state properties and energy transfer processes within MOFs and underline the focus on the robust system for future implementation in the light-energy conversion systems.

Energy Transfer in Metal-Organic Frameworks Energy transfer is a key phenomenon especially in the composition of in vivo phototherapy, solar cells, and photocatalysis.13-17 Photo-excited chromophores can transfer energy to neighboring chromophores via a Förster or Dexter process and the energy transfer cascade can be terminated by an emission or charge separation followed by thermal recombination or chemical transformation (Scheme 1). Since MOFs can be considered as ensemble of molecular chromophores, like those in the natural LH complexes, unique EnT process can be expected in MOFs,18 Lin’s group provided the insight of LH and photocatalysis within MOFs and argued that MOFs should be considered as scaffolds for EnT owing to their well-defined crystalline structures and controllable chromophoric distances. 19 Few papers were then published to discuss the EnT process and where the exciton locates.20-21 Later, Shustova and coworkers emphasized the utilization of EnT within MOFs towards improved EnT and sensing capability.22

Singlet-to-Singlet EnT in MOFs Energy transfer within the singlet manifold can be described by Förster mechanism that involves FRET. In 2010, Hupp’s group, for the first time, reported FRET-EnT in ZnII-paddlewheel-based bodipy-porphyrin MOF, namely BOP MOF.23

Scheme 1 Energy diagram showing typical energy migration processes from photo-excited chromophore assemblies in MOFs (denoted as D) to the conversion site (RC module) consisting of EnT (denoted as A) or CT quencher. Examples of corresponding linkers and complementary molecular species are noted (see text for abbreviations).


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With a considerable overlap integral and good electronic coupling facilitated by a moderate bodipy-porphyrin interlinker distance (~1.5 nm), BOP MOF shows emission from the porphyrin moiety (650-710 nm) upon 543 nm excitation of the bodipy unit. The complete quenching of the bodipy emission at 560-615 nm highlight efficient strut-to-strut FRET process (Figure 2).

Figure 2 (a) Crystal structure of BOP MOF; b) Emissions of control BOB (without porphyrin) and BOP MOFs (λex = 520 nm). Reproduced with permission from ref. 23.23 Copyright 2011, American Chemical Society.

Later, Hupp and coworkers studied another coveted phenomenon of LH system: directional energy transfer within the crystalline framework. This study highlights the efficiency and maximum length that a singlet exciton migrates along the various direction (Figure 3).24 The authors adopted a classic strategy of breaking the electronic symmetry of an otherwise highly symmetric porphyrin linker by extending the conjugation along x-axis through ethynylene linkage.25-27 Two ZnII-paddle-wheelbased MOFs were synthesized, with 1,2,4,5-tetrakis(4-carboxyphenyl)-benzene (TCPB), and two porphyrin pillars, [5,15-dipyridyl-10,20-bis(pentafluorophenyl)]-porphyrin (F-H2P) and [5,15-di(4-pyridylacetyl)-10,20-diphenyl]porphyrin (DA-H2P). The fluorescence quenching experiment indicates that only a small fraction of ferrocene quencher can drastically reduce the emission due to a rapid exciton migration within the framework. Moreover, the average numbers of chromophore-to-chromophore hops by singlet excitons are estimated to be 2000 and 8 in DA-MOF and F-MOF, respectively. Such big difference on the number of hops between these two structures arises from the different extent of reduced electronic symmetry and enhanced linear conjugation in DA-H2P by incorporating an acetylene unit between tetrapyrrole core and each of the two pyridyl groups (Figure 3b). With the help of computed electronic coupling constants between the neighboring DA-ZnP linkers, the authors extracted efficient exciton hopping rates along various directions (highlighted with distances; Figure 3): for example, in DA-MOF, exciton hopping along the AB direction accounts for a staggering 55% of total hops (Figure 3b) enabling the exciton to migrate ~38 nm along AB, whereas the AE direction accounts for ~ 21% of total hops enabling an impressive 58 nm of exciton migration. In contrast, the F-MOF shows only ca 3.5 nm of maximum exciton displacement. The substantial distances and sizable anisotropy of singlet exciton migration revealed by this work show the importance of MOFs composites as an antenna-type light-harvesting system.

Figure 3 Structures, absorption spectra (blue) and emission spectra (red) of (a) F-ZnP and (b) DA-ZnP. Reproduced with permission from ref. 24.24 Copyright 2013, American Chemical Society.

Shustova and co-workers reported a MOF based system mimicking highly efficient EGFP-cyt b562 based EnT system.28 The 4-hydroxybenzylidene imidazolinone (HBI) based chromophores are non-emissive outside the β-barrel protective capsule. The authors first reported that the HBI derived chromophores are highly emissive within the rigid scaffold of Zn-based MOFs. Furthermore, highly efficient FRET was achieved between the HBI incorporated porphyrin-based MOF (as a b562 mimic). For this, a coordinative (HBI-derived linker) and a simple, diffusive non-coordinative incorporation approaches were adopted (Figure 4). The non-coordinative approach allowed the authors to vary the electronic structure of the HBI core widely to achieve high overlap integral with the porphyrin-based linker (as acceptor) resulting in an impressive 72% energy transfer from the HBI to porphyrin.

Figure 4 (a) EGFP-cyt b562 motif highlighting the encapsulated emissive HBI unit within the β-barrel protein and (b) two bio-inspired MOF-derived designs that incorporate the chromophore into the MOFs pores; see text. Reproduced with permission from ref. 28.28 Copyright 2016, American Chemical Society.

Douhal and coworkers investigated how the interlayer distance in a series of aluminum-based 2D MOFs (as host) can tune the EnT within the Nile Red (NR) guest dye incorporated by adsorption (Figure 5).29 The ligands in these 2D MOFs that point towards the interlayer space is varied with different length of alkyl chains. The NR dyes manifest EnT catachrestic between identical chromophores with an emission peak around 660 nm. The difference in the length of the alkyl chains at the interlayer space used to control the distance between MOF layers, which, in turn, introduced a variation in the extent of inter-



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chromophoric interaction between the incorporated NR molecules. Within this system, EnT between NR molecules was enhanced in NR@Al-ITQ-EB, with a short ethyl chain causing a tighter arrangement between 2D MOF layers than the NR@AlITQ-HB (with heptyl chain).29

Figure 6 The LbL assemblies of porphyrins -based linkers with varying interlayer distance. a) the pristine LbL assembly with 4,4’bipyridine pillar (interlayer distance 14.7 Å); b) the collapsed 2D MOF after SALE with pyridine, resulting in a dipole-to-dipole distance of 14.3 Å, and 16.8 Å. c) DABCO pillar-based de novo porphyrin array with a reduced dipole-to-dipole distance of 9.9 Å, studied in another work. Reproduced with permission from ref. 30 and ref. 31.30-31 Copyright 2016 and 2018, American Chemical Society.

Figure 5 Structure of Nile-red (NR) dye and the NR adsorped AlITQ-HB and Al-ITQ-EB frameworks. Reproduced with permission from ref. 29.29 Copyright 2018, American Chemical Society.

For the framework compositions, it is important to understand how the co-facial orientation and the interchromophoric distance affect the EnT efficiency. Hupp and co-workers published two articles based on films constructed via a controlled chromophoric arrangement with layer-by-layer (LbL) assemblies. In these ZnII-paddle-wheel-based MOFs, tetratopic tetrakis(4-carboxyphenyl)porphyrin (TCPP) linkers are arranged in a parallel face-to-face orientation with 4,4’-bipyridine pillars to provide an interlayer distance of 14.7 Å. To reduce the interlayer distance, the authors replaced the 4,4’-bipyridine -pillars with monodentate pyridine through the post-synthesis process called solvent assisted linker exchange (SALE). This replacement caused collapsed layers with the interlayer distance of only 6.9 Å, where the neighboring layers shift laterally relieving the parallel construction in the parent MOF to a skewed orientation in such way that the chromophore to chromophore distance (also the distance between their corresponding transition-dipoles) become 14.3 Å and 16.8 Å in between neighboring and alternating layers, respectively (Figure 6).30 Since the distance between two closest transition-dipoles remains essentially unchanged, (i.e. 14.7 Å) the spectroscopic parameters did not alter. With the help of TCPP(Pd) as a structurally compatible acceptor at the final layer, the exciton migration length was determined to be ~9 in the 4,4’-bipyridine system (before SALE) and ~11 in the pyridine system (after SALE). In contrast, the author examined a new system where the distance between the two nearest neighbor transition-dipoles was shortened (9.9 Å), with the DABCO-based pillar.31 The spectroscopic parameters showed a significant difference with an approximate exciton migration length of 26 layers. These results indicated that the distance between the chromophore transition-dipoles is more critical for EnT than the interlayer distance within a framework structure.

C. Wang et al. also suggested a possible long-distance FRET derived exciton hopping, termed as jumping beyond nearest neighbors (JBNN) beside a well-adopted step-by-step nearestneighbor hopping (NNH) mechanism. the authors constructed two ZnII-based MOFs with truxene linker as the LH-antenna and post-synthetically incorporated different amount of Coumarin 343 probe representing a reaction center(Figure 7).32 The intensity ratio of the probe (coumarin) emission over the linker for samples containing different amounts of the probe molecule does not fit the profile predicted with a NNH model. The NNH model with the added long-distance energy transfer term; the JBNN model fits the experimental data accounting for ~67% of EnT commences through long-distance hopping.

Figure 7 a) Schematic diagram showing two distinct pathways for exciton migration within the chromophore network in MOF; b) Fraction of JBNN (jump beyond nearest neighbor) in the overall energy transfer rates as a function of normalized Förster distances RLL/rL (linker-to-linker) and RLD/rL (linker-to-dye) for dye doping level of 0.5%. Reproduced with permission from ref. 32.32 Copyright 2016, American Chemical Society.

Porphyrin-based systems have been investigated for various unique features, including the photochemical production of singlet oxygen (1O2) for other chemical transformations and phototherapy. Lee and co-workers reported a mixed linker csq topology ZrIV-based MOF, TCPP@NU-1000 (Figure 8).33 In these hybrid structures, a dramatic decrease in the 1,3,6,8tetrakis(p-benzoate)pyrene (TBAPy) linker emission (at 470 nm) was observed with clear emission from the free base TCPP linker at 670 nm. The steady-state and time-resolved emission data indicated that the TBAPy linker served as singlet energy donor to populate the S1 state of TCPP linker which then through inter-system crossing (ISC) produces triplet excited (T1) state for photochemical transformation producing singlet oxygen. Their design took advantage of the high quantum yield


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of H4TBAPy, and high ISC efficiency of TCPP to build up the efficient EnT system.

Figure 8 Scheme showing LH and photo-driven singlet oxygen generation in a mixed-ligand MOF featuring of deprotonated H4TCPP and H4TBAPy linkers. Reproduced with permission from ref. 33.33 Copyright 2017, American Chemical Society.

H.-C. Zhou and coworkers have reported an interesting targeted phototherapy achieved with a Zr6IV -oxo based, UiO-66 derived MOF, that can generate singlet oxygen on demand through a controlled EnT process. Here, the energy transduction pathway for the photochemical singlet oxygen generation was controlled by the close-open forms of a molecular photo-switch: a UV-reactive 1,2-bis(5-4-carboxyphenyl)-2-methylthien-3yl)cyclopent-1-ene (BCDTE) co-linker (Figure 9a).34 The BCDTE linker upon UV irradiation yields a ring-closed variant that possesses low-lying S1 state which can be efficiently populated by the excited TCPP. This ‘off mode’ energy transduction route suppresses the ISC of the singlet TCPP to populate its triplet states and thus no 1O2 is formed (the purple route; Fig. 9d). On the contrary, BCDTE linker undergoes a ring-opening process upon visible light irradiation: this form of the BCDTE has S1 state energy that is higher than the S1 state of TCPP and thus cannot be populated by the later. As a result, the excited TCPP will undergo ISC process to generate 1O2 (Figure 9).

Figure 9 a) Photoisomerization of BCDTE; b) chemical structure of TCPP; c) UV/Vis spectra of BCDTE (30 μM) and TCPP (2 μM); Inset photographs of BCDTE solutions in its ring-opened (left) and ring-closed (right) forms; d) proposed scheme of photo-switch action on the control of 1O2 generation via competitive EnT pathways depends. Reproduced with permission from ref. 34.34 Copyright 2016, Wiley-VCH.

Considering inter-linker EnT is a key process within any LH system, the EnT rate constant and the exciton lifetime will dictate the maximum length an exciton can migrate. Successful long-range exciton migration will define the efficiency of harvested energy that can be delivered to the conversion site. For the Förster-based EnT, the electronic coupling plays a significant role. Since the electronic coupling can be tuned by the interchromophoric distance and orientation, the efficiency of the EnT can be controlled by the underlying MOFs topologies. In this regard, we have interrogated a series of chemically identical but topologically different ZrIV-based MOFs, namely, NU902 and MOF-525.35 These frameworks were built with tetratopic TCPP linker interconnected by optoelectronically inert Zr6IV -oxo based nodes(Figure 10a). The emission decay profiles for the free-base (Figure 10b) and zincII-metallated TCPP linkers show a trend of linker > MOF-525 > NU-902 due to a varying extent of interchromophoric interaction. Mimicking the reaction center of the LH complex, a node installed ferrocene (Fc) quenches the porphyrin emission via a competitive CT reaction. Varying degree of Fc loading at the NU-902 indicates (Figure 10c) that a saturation quenching can be reached at a Fc/TCPP ratio significantly lower than 1. Based on this amplified emission quenching experiments, we found that the exciton can visit ~170 chromophores undertaking an impressive 7000 hops with a hopping time constant of 0.65 ps for NU-902(H2).24 Thus, this work suggested that with the help of topological control, exciton dynamics can be tuned in MOFs.



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Figure 10 a) Crystal structures of NU-902 and MOF-525. Transient emission decay profiles for b) free-base samples; c) Amplified fluorescence quenching data for free-base NU-902 MOF at various Fc/por doping ratio. Reproduced with permission from ref. 35.35 Copyright 2016, Royal Society of Chemistry.

Triplet-to-Triplet EnT in MOFs One of the first studies on the energy transfer process in framework compositions was carried out by W. Lin’s group, with a ZnII-node-based MOF involving {Ru[4,4′-(HO2C)2bpy]2bpy}2+ linker. The Dexter-type linker-to-linker TTET was established with samples that were doped with a varying amount of Os-based linker {Os[4,4′-(HO2C)2-bpy]2bpy}2 (Figure 11a).11 In this mixed linker system, Ru-bpy linker act as triplet energy donor (3MLCT→S0 = 620 nm) and the Os(bpy) as the acceptor (3MLCT→S0 = 710 nm). With a low doping of Os(bpy), the authors showed a delayed rise of the 710 nm luminescent where for the sample with higher Os(bpy) loading the 710 nm rose faster with a corresponding quicker decay of Ru(bpy) at 620 nm (Figure 11b). This data suggested a Ru(bpy)-to-Ru(bpy) triplet exciton hopping is responsible to populate the Os(bpy) trap sites (delayed rise at 710 nm), where increasing the Os(bpy) concentration will need a shorter Ru-Ru hopping to cause a faster rise of the Os(bpy) luminescent. This work evinced the efficient energy migration and long-distance transfer in MOF system, giving a representative example for studying energy transfer dynamics in MOFs.

Figure 11 a) Crystal structure of LRuZn MOF; b) transient emission decay profiles for LRuZn with different doping ratio of Os(bpy) probed for Ru(bpy) at 620 nm and Os(bpy) at 710 nm, respectively. Reproduced with permission from ref. 11.11 Copyright 2010, American Chemical Society.

Figure 12 a) Room-temperature phosphorescence spectra of Cd– Eu/Tb/Gd-CPs under 310 nm excitation; b) time-resolved emission decays at 615 nm for Cd–Eu-CP and at 545 nm for Cd–Tb-CP under ambient conditions; c) cluster structure and linker of Cd-MOF; d) Cd–Eu/Tb-CP powder samples pictures at different time intervals during and after the UV excitation (254 nm) under ambient conditions; e) energy level diagram and the energy transfer process. Reproduced with permission from ref. 36.36 Copyright 2017, Royal Society of Chemistry.

Triplet-triplet-annihilation (TTA) within MOFs have also been investigated to realize photon energy upconversion (UC) processes. H.-C. Zhou and coworkers reported a Zr6IV -oxo based mixed ligand MOF, where an excited Pd-TCPP based T1 state transfers its energy to populate the T1 state of the diphenyl anthracene derived linker, DCDPA.39 By virtue of the framework positioning, two adjacent 3DCDPA undergo a TTA-UC process generating a singlet 1DCDPA, which then relaxes to ground state by emitting a relatively higher energy photon (Figure 13).

The T-T EnT was often investigated with the help of rareearth metals. D. Yan et al. reported a T-T phosphorescence energy transfer for long-lived afterglow MOFs,36 where EuIII and TbIII were doped in a Cd-based MOF. Here, the Cd-based MOF with m-benzene dicarboxylate linker served as an antenna to sensitize the lanthanide ions through its triplet state following an ISC of the initial photo-induced CP-based singlet excited states (Figure 12). Long-lived emission of 10.5 ms and 57 ms were achieved from EuIII and TbIII dopants, respectively. The authors attributed their unprecedented long-lived emission to the efficient TTET based sensitization achieved through high ISC efficiency of the CdII-MOF antenna system compared to any ZnII based MOF matrix. Mudring and co-workers, and Mustafa and co-workers also investigated, separately, the linker-toguest T-T EnT, with Ln-doped MIL-78 (Gd) and Eu-doped MIL-78 (Re) MOF systems, respectively.37-38


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Figure 13 a) Chemical structure of MOF linkers and metalloporphyrin dopant; b) energy scheme for TTA-UC process; c) proposed TTA-UC system constructed based on a pillar-layered MOF, and its spatial distribution of chromophores. Reproduced with permission from ref. 39.39 Copyright 2018, American Chemical Society.

avoided triplet exciton generation by incorporating triplet quenchers to avoid destruction by the singlet oxygen. For the in vitro functional LH modules, the triplet excitons are attractive due to their prolong lifetime. However, such design requires a high concentration of the photo-active species with near perfect ISC quantum yield. Besides, the dense arrangement of the donor and the acceptor species may need to be protected within the cumbersome inert atmosphere capsule. Thus, for functional material development, systems working in their singlet manifold may be a wiser route if the excited states properties can be manipulated. Thus, understanding the excited state properties, such as exciton delocalization and size, and their corresponding dynamics is important. Furthermore, strategies to prolong singlet exciton lifetime, control over ISC are extremely important. In a recent report, we showed for the first time the delocalized nature of the singlet excitons in MOFs. Studied with two chemically identical Zr6IV -oxo MOFs, NU-901 and NU-1000,41 we mapped the transition density matrix (TDM) from the TDDFT computed data on variously selected model clusters (Figure 15, panel b). The TDM mapping elucidates some unprecedented correlation between the hole and the electron of various excited states: for example, the S1, S2, and Sn states are delocalized over multiple chromophore linkers, in these cases, TBAPy. Furthermore, highly disperse off-diagonal densities in the TDM mapping of the respective Sn states indicated a larger and weakly bound exciton, which may be beneficial for dissociation. Few other critical findings of this work highlight that interchromophoric interaction is responsible for generating (a) polar excited states, which can be optically active depending on the topology of the network and (b) dark excited state (zeronegligible S0 → D1 oscillator strength) that lie below the optically active S1 states. These excited states can significantly contribute to the excited-state dynamics of the framework assembled chromophores.

TTA-UC not only provides a tool for bio-imaging, as shown by Zhou work above, but can enable a pan-chromatic low-energy light harvesting system. Morris and co-workers have recently reported TTA-UC system constructed from various anthracene dicarboxylates and a ZrIV-based node. Exciting the surface-bound (Pd)porphyrin sensitizer (λex = 532 nm) facilitates T-T EnT from 3(Pd)porphyrin to the anthracene dicarboxylate linker, resulting in ~46% TTA-UC efficiency (λprobe = 450 ± 25 nm; Figure 14). Figure 15 a) Structure of the NU-901. Contour plots of TD-DFT calculated TDMs corresponding to the optically active (b) S1 and c) Sn excited states for the NU-901 model compound showed in panel (a) (green highlighted). These plots depict e-h correlation during an optical excitation; thus the diagonal and off-diagonal densities represent the excitonic wave function (center of mass) over the structure (the axes are labeled with TBAPy units ) and the size of exciton; Reproduced with permission from ref. 41.41 Copyright 2018, American Chemical Society.

Figure 14 a) Structures of 9,10-MOF and 9,10-ADCA linker along with (Pd)MP sensitizer; (b) Excitation power dependence of UC emission intensity (λex = 532 nm; λprobe = 450 ± 25 nm). Reproduced with permission from ref. 40.40 Copyright 2018, Royal Society of Chemistry.

Excited-State Properties of MOFs All the energy transduction pathways in the natural LH systems operate only in their singlet manifold. Nature has carefully

Excimer formation in MOFs: a strategy towards long-lived signet excited state It was found that the emissive excited-state population for the MOF assembled chromophores relaxes faster relative to their monomeric form in homogeneous solution due to the involvement of various non-radiative decay pathways (such as dark state, vide supra) in the solid state.35, 42 Delineating functional LH system thus requires strategy to achieve long-lived singlet excited states. Due to high chromophore concentration, MOFs offers an ideal platform to generate various long-lived



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excited-state complex formation. One such excited state complex is known as excimer, which readily forms in MOFs. Douhal and coworkers have reported excimer formation in Zr6IV -oxo MOFs constructed with 2,6-naphthalene dicarboxylate linker (Zr-NDC) (Figure 16).43 Based on the time-constants obtained from the transient emission decay profiles, the authors found a solvent dependent excimer formation kinetics: in DMF solvent, the Zr-NDC MOF exhibited an excimer formation in 280 ps and in DCM solvent the formation is over 840 ps with ~12 ns lifetime.

Figure 17 a) DFT-optimized structures of dimeric model compounds consisting of two most closely positioned TBAPy linkers in MOFs; b) transient emission decay profiles for the MOF samples collected in low dielectric media; the black profile represents the benchmark free H4TBAPy linker in DMF solvent (λex = 405 nm). (c) Representative femtosecond transient absorption spectra for NU-901 (λex = 400 nm; parallel pump/probe polarization) highlighting the S1 → Sn and broad excimer-based transitions. Reproduced with permission from ref. 41 and ref. 42.41-42 Copyright 2017, American Chemical Society. Figure 16 Energy cascade scheme for the photo-excited Zr-NDC MOF (λex = 355 nm) in DMF (green) or DCM (red) suspensions. Reproduced with permission from ref. 43.43 Copyright 2016, Royal Society of Chemistry.

Given that the excimer formation intimately depends on the chromophore concentrations and interchromophoric distance, we have shown with a series of Zr6IV -oxo pyrene (TBAPy) MOFs (Figure 17)42 that the extent of excimer formation within MOFs is a function of their underlying topology that assembles the TBAPy linker with a center-to-center distance varying from 8.8-11 Å (Figure 17a). Analysis of the time-resolved fluorescence decay profiles (Figure 17b) indicates that the extent of the excimer formation and lifetime increases with a decrease in the interchromophoric distance and favorable for those frameworks that arrange the chromophores in a parallel orientation. Thus, an efficient (85%) formation of long-lived (6.3 ns) excimer was observed in the ROD-7 with the shortest interchromophoric distance.42 For these TBAPy based MOFs, the excimer formation was ultrafast owing to the proximity of the chromophore; in a later study we have shown the ultrafast spectral signature of the excimer formed in the MOF; with the emergence of a broad (580−1100 nm; Figure 17c) induced absorption originating from a fast excimer formation (τ ≃ 2 ps).41

𝐙𝐫𝟔𝐈𝐕 -oxo Derived MOFs: ideal platforms for artificial LH and energy conversion system Among the various MOF systems described above and many others, frameworks that are constructed from ZrIV ion based nodes or secondary building units (SBUs)35, 44-57 are hydrolytically and mechanically robust, and amenable to solvation in a wide range of dielectric media, including aqueous acid, which is generally required for photo and electrocatalytic production of solar fuels.58-61 Such stability stems from the strong ZrIV-carboxylate bond.62 The diamagnetic (3d0) electronic configuration of the ZrIV ions and the large electrochemical window provided by the Zr6IV -oxo SBUs make Zr-MOFs attractive for spectroscopic studies.43, 47, 49, 63-64 It is a common notion that being a d0 metal ion, the Zr6IV -oxo node would possess similar electronic properties seen in the well-known TiIV-oxo systems. Studies have found that the unique electronic energy level alignment between the Zr6IV -oxo node and the π-conjugated linkers are different than that of a TiIV -based system, where the low lying TiIV-centered LUMO can participate in ligand-to-metal charge transfer (LMCT) process or alters electronic properties of the linker.65-66 Gascon and coworkers, based on their DFT computations and EPR and transient absorption spectroscopic data on NH2MIL-125(Ti), NH2-UiO-66(Zr) MOF samples, reported that the band alignment in the NH2-MIL-125(Ti) promote a long-lived (9 ns) photoexcited LMCT state.65 In contrast, ZrIV ions with low binding energy do not promote overlap with the organic linker in the UiO materials and lead a linker-centered, shortlived excited-state (Figure 18).


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ACS Sustainable Chemistry & Engineering find a TiIV SBU based system will perform better for the photoor photoelectrochemical HER reaction. Matsuoka and coworkers reported a Pt/Ti-MOF-NH2 hybrid system, where the TiIV BDC-NH2 derived MIL-125 MOF transfer the photo-induced reducing electron to the platinum (HER catalysts) to achieve 11.7 μmol/h hydrogen production efficiency (Figure 20). In contrast, a corresponding Pt/Zr-MOF-NH2 (in a UiO framework) could not produce hydrogen due to an inefficient LMCT (no ESR signal corresponding to ZrIII was detected).69

Figure 18 (a) LUMOs for model nodes and (b) EPR spectra of dark (black) and UV-illuminated (red) MOFs: NH2-MIL-125(Ti) (left), and NH2-UiO-66(Zr) (right). Reproduced with permission from ref. 65.65 Copyright 2016, Nature Publish Group.

Theoretical studies that examined the electronic properties of the Zr6IV -oxo based MOFs in comparison with other metal SBUs also suggested that zirconium –SBUs can be considered optically ‘inert’ in the UV-vis region. Wu et al. conducted a series of computational studies based on UiO-66 type framework with different metal nodes.66 Density-of states (DOS) plots for these systems indicated that zirconium 4d band are ~2 eV wider than a large band-gap benzene-dicarboxylate (BDC) linker and thus are not suitable for photocatalysis via a ZrIII formation through LMCT of photo-excited BDC (Figure 19).

Figure 19 Total (black) and projected (red and blue) DOS of UiO66(M) with M = Zr, Hf, Th, Ti, U, and Ce. Only the most stable isotope of U is shown. Reproduced with permission from ref. 66.66 Copyright 2018, American Chemical Society.

Figure 20 (a) Diffuse reflectance UV−vis (DRUV−vis) absorption spectra of TiIV -based MOFs (inset: spectra of free linkers); (b) wavelength dependence of the hydrogen evolution rate for Pt/TiMOF-NH2 measured on the marked wavelength of incident light (1-6) relative to the DRUV-vis spectrum of Ti-MOF-NH2 (a). Reproduced with permission from ref. 69.69 Copyright 2012, American Chemical Society.

To determine if the ZrIV exert any electron withdrawing effect, we have synthesized benzoate capped Zr6IV -oxo cluster Zr6(OH)4O4(OOCR)12 (R=Phenyl; Figure 21a).70-71 The UVvis spectrum of the Zr6IV -oxo-benzoate (ZrO-BA) cluster is essentially the same as that of the benzoic acid in solution (λmax = 250 nm, like that reported by Matsuoka; Figure 20a-b69). Electron withdrawing ability of the ZrIV was assessed via core-level X-ray Photoelectron Spectroscopy (XPS)72-73 which produces atom-centered holes to reflect the electrostatic potential at specific atom sites.74-75 At first, we found that the XPS spectra of the Zr 3d5/2 and 3d3/2 (centered at 185.1 and 182.7 eV) remain unchanged in two TBAPy based NU-901 and NU-1000 MOF, as well as in the ZrO -BA clusters (Figure 21c). Likewise, the carbon 1s peaks (Figure 21d) for both the aromatic and carboxylate carbons remain unaltered in these MOF samples relative to the free H4TBAPy linker (in solid powder form). These XPS spectroscopic data unambiguously suggest that the installation of the large aromatic linkers through the ZrIV-carboxylate bonds does not alter the electronic energy of the Zr -ions nor the linker carbon atoms. Therefore, the Zr6IV -oxo node has no influence, whatsoever, on the electronic structure of the linker (and their assembly). Together, all these computational and spectroscopic evidence suggest that the Zr6IV -oxo based SBUs can be considered electro-optically inert and simply responsible for neutral framework construction where the photophysical properties relevant to LH can be studied without the involvement of the metal node.

Electrochemical experiments that examined the redox properties of the Zr6IV -oxo based system, such as NU-1000, suggest that the frontier molecular orbitals are linker centered and their energy remains unaltered in the framework compared to their free monomeric state studied in solution.67-68 Based on these computational and experimental findings, it is not surprising to



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(λi ~ 0.42 eV), these experimental data indicate that the solvent reorganization must be very high. This high solvent/environmental reorganization energy, even in low dielectric media, only be accounted by a water-like polar environment presented by the Zr6IV -oxo nodes.

Figure 21 a) Structure of ZrO-BA; note the coordination of one propanol (or its deprotonated anion) in lieu of a μ2η2 benzoate binding; b) PXRD of synthesized ZrO-BA cluster (red); simulated PXRD pattern for the panel (a) cluster (blue). XPS spectra for c) zirconium 3d5/2 and 3d3/2 and (d) carbon 1s in NU-901 (blue), NU1000 (red) MOFs compared to the ZrO-BA and H4TBAPy benchmark samples (black). Unpublished data; Crystal structure and the corresponding simulated PXRD plot for the ZrO-BA cluster were generated using CIF from ref. 71.71

Given that the polar hydroxyl and aqua ligands at the optoelectronically inert Zr6IV -oxo nodes are protruding to the framework pore-channels, it cannot be over-looked as a chemically inert moiety, especially for a CT process. For that, we examined the photo-induced charge-transfer process in a TBAPy –based MOF involving node anchored ferrocene moiety. Like porphyrin derived compositions, pyrene-based molecules have garnered great interest in organic optoelectronics owing to its unique photophysics such as high quantum yield. 76-78 For this, post-synthetically prepared Fc@NU-1000 samples were studied: the ferrocene moiety was fixed through Zr-carboxylate bonds via solvent assisted ligand incorporation (SALI)52-53, 79 using Fc-COOH. SALI positioned the incoming Fc-group in the close proximity of the Zr6IV -oxo node with a dTBAPy-Fc ~ 10Å (max Fc loading 1 Fc/node; Figure 22 a, b).80 In this arrangement the ferrocene experiences a polar microenvironment defined by the protruding hydroxyl/aqua ligands at the Zr6IV -oxo node. Based on fs-TA spectroscopic data a CT-complex [NU1000]• ‾/Fc+ was evinced which was prepared from a photo-excited [NU-1000]*/Fc species. The charge separation (CS) and thermal recombination time constants were determined to be ca 9 and 430 ps, respectively. Series of fluorescence quenching experiment in different dielectric media suggested that the CS rate remains essentially unchanged in the low-dielectric regime (εs = 1 - 9; Table 1). Such non-responsive kinetics indicate that this system possesses high total reorganization energy comparable to the CT driving force. Since the estimated ΔG0 (~1 eV) is significantly higher than the small internal reorganizational energy

Figure 22 a) Proposed structure of Fc@NU-1000; b) DFToptimized model revealing the node anchored position of Fc-COO in MOF; c) Photo-induced charge-transfer process shown with the potentiometric energy diagram in Fc@NU-1000 sample. Panel (a) also highlight two adjacent channels A and B where the arrows indicate possible co-functionalization sites. Reproduced with permission from ref. 80.80 Copyright 2018, American Chemical Society. Table 1. Charge transfer rate calculated from emission decay. Media

εs

ke × 1010 (s−1)

λs (eV)a

Air

1.0006

9.4

0

3-MePent

1.89

8.6

0

Toluene

2.38

7.5

0.041

2-MeTHF

6.97

7.4

0.576

CF3-Toluene

9.18

6.7

0.621

solvent reorganization energy (λs) for the bulk solvent was calculated by the Marcus relation.81 Reproduced with permission from ref. 80,80 Copyright 2018, American Chemical Society. aThe

Since the polar environment at the Zr6IV -oxo node suppresses the CT kinetics, we wanted to test if capping the hydroxyl/aqua ligands can prove to be beneficial. Towards this, we set to incorporate hydrophobic alkane chains at the node of Fc@NU-1000. As such, NU-1000 possess 8-connected Zr6IV oxo node, which leaves four binding sites (occupied by hydroxyl/aqua ligands) per SBU that can be decorated with carboxylate-based ligands through SALI.52-53, 79 These four binding sites protrude towards two adjacent hexagonal mesopores (channel A and B for the central SBU marked with a circle in Figure 22a). Co-functionalization of the pristine NU-1000 using mixed carboxylic acids Fc-COOH/Pr-COOH or FcCOOH/Me-COOH (in 1:3 ratio) resulted in only two alkyl-carboxylates and one Fc-COO incorporated per node. These two incoming alkyl-carboxylates were fixed at the available binding sites that are protruding towards the other channel (shown with the arrows in Figure 22a). Since the Fc-group sits right on top


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of the Zr6IV -oxo node with a limited rotational barrier, the closest binding site available in the same channel is sterically prohibited and remain unfunctionalized with any alkyl carboxylate, which leaves the Fc-moiety exposed to the hydroxyl/aqua ligands.53 Nevertheless, the CT kinetic experiments with the Bu/Fc@NU-1000 sample (Table 2) indicate a slight improvement compared to the Fc-only system.80 Thus, a node modification can alter the polar microenvironment to boost the CT kinetics. Table 2. Charge-transfer rate comparison between Fc@NU1000 and Bu&Fc@NU-1000.* Media

εs

Fc@NU-1000 ke × 1010 (s−1)

Bu/Fc@NU-1000 ke × 1010 (s−1)

3-MePentane

1.89

8.6

11.3

CF3-Toluene

9.18

8.6

9.9

*unpublished data

chemical functionalities via post-synthesis processes will play a big role in the manipulation of the chemical environment required for energy harvesting and conversion. Besides, TiIVbased SBU can be utilized when photochemical transformations are required. Based on the current literature, we expect to witness more unique works based on the robust, optoelectronically inert Zr6IV -oxo SBUs. On the other hand, most of the pioneering studies highlighted here are based on MOFs comprised of commonly accessible symmetric porphyrin (e.g. TCPP), pyrene (e.g. TBAPy), naphthalene, and anthracene derived linkers. Contemporary advancements that establish the structural correlation of the MOF excited-state properties will surely facilitate new tailor-made linker design. Overall, the advancement with the porous chromophore arrays has been impressive and moving forward with new strategies that will lead to the novel solid composition of matters for light harvesting application towards a sustainable future.

ACKNOWLEDGMENT

Conclusion and perspective Contemporary advancements on MOFs constructed from optoelectronically active species have laid the ground for delineating well-defined systems towards light harvesting utility. The precise arrangements of the chromophore-based linkers around the wide range of possible pore-geometry provide a viable strategy to incorporate unique photophysical features that are impossible to achieve in a homogeneous system in condensed phase as well as in their solid-phase aggregates. The exemplary investigations discussed here highlight the efficient light harvesting utility facilitated by the directional energy transfer over a long distance. Strategies that have been explored to achieve long distance singlet energy transfer include generation of long-lived excitons, tuning chromophore symmetry, and positioning to improve both overlap integral and transition dipole coupling. While the current examples of the systems that examined triplet energy transfer have relied on the close proximity and linker connectivity of the highly concentrated chromophores in MOF. Several examples highlight the successful integration of photochemical ‘reaction-centers’. On the other hand, manipulation of the photophysical properties such as new spectral evolution or generating long-lived excited states require a greater understanding of their excited state properties, such as exciton size, delocalization length, and dynamics as a function of their underlying topological-net. Such information will help devise new strategies to control exciton diffusion and the ease of dissociation for future design improved LH utility. So far studies that interrogated the structural impact on the MOF photophysics mainly relied on single framework based systems by varying the interchromophoric distance and orientation. In these regards, it is important to note that interpenetrated (catenated) systems will hold some key features to attain much shorter interchromophoric distant in a Jaggregated orientation similar to the natural LH system. We expect to witness such a unique system in the future. Finally, depending on the required targeted utility, delineation of the robust framework is needed. In this regard, MOFs constructed from carboxylate bound Zr6IV -oxo node represents a strong candidate. The large size of this optoelectronically inert node can impart large interchromophoric distance as well as suppress charge transfer kinetics due to its polar nature. However, the ability of such robust SBUs to incorporate various

We thank SIUC for the startup support.

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SYNOPSIS TOC Metal-organic frameworks are emerging platform for artificial LH assemblies with unique structure-dependent photophysical properties and energytransfer efficiency.

Biographies

Jierui Yu received his B.S. in Chemistry from Lanzhou University, China in 2014. He is currently pursuing his doctoral studies in Southern Illinois University Carbondale under the guidance of Prof. Pravas Deria. His dissertation research is focused on the study of metal-organic frameworks and their photophysical properties.

Xinlin Li is currently a Ph. D. student at Southern Illinois University Carbondale under the supervision of Prof. Pravas Deria. He obtained his B.S. in Chemistry (2014) and M.S. in Chemical Engineering (2017) at Lanzhou University, China. His current research interests include electrooptical properties of metal-organic frameworks and related porous compositions.

Pravas Deria is currently an assistant professor in the Chemistry & Biochemistry Department at Southern Illinois University Carbondale. He earned a B.Sc. degree from the University of Calcutta and a M. Sc. from Indian Institute of Technology Kanpur, India. He obtained his Ph.D. in Chemistry from the University of Pennsylvania, Philadelphia, under the direction of Prof. Michael J. Therien and pursued postdoctoral research with Prof. Joseph T. Hupp at Northwestern University. His current research mainly focuses on the rational design, synthesis, and characterization of metal-organic frameworks and related porous coordination polymers and investigation of their fundamental photophysical, photochemical, and potentiometric properties of for energy conversion applications.


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