Layer-by-Layer Assembled Films of Perylene Diimide- and Squaraine

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Layer-by-Layer Assembled Films of Perylene Diimide- and Squaraine-Containing Metal-Organic Framework-like Materials: Solar Energy Capture and Directional Energy Transfer Hea Jung Park, Monica C. So, DAVID J GOSZTOLA, Gary P. Wiederrecht, Jonathan D. Emery, Alex B. F. Martinson, Süleyman Er, Christopher E. Wilmer, Nicolaas A. Vermeulen, Alán Aspuru-Guzik, J. Fraser Stoddart, Omar K. Farha, and Joseph T. Hupp ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03307 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

Layer-by-Layer Assembled Films of Perylene Diimide- and Squaraine-Containing Metal-Organic Framework-like Materials: Solar Energy Capture and Directional Energy Transfer Hea Jung Park,a,b†§ Monica C. So,a,c,‡§ David Gosztola,d Gary P. Wiederrecht,d Jonathan D. Emery,e Alex B. F. Martinson,e Süleyman Er,f,g Christopher E. Wilmer,f,h Nicolaas A. Vermeulen,a Alán Aspuru-Guzik,f J. Fraser Stoddart,a Omar K. Farha,a,i,* and Joseph T. Huppa,* a

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States

b

Pusan National University, Chemistry Institute for Functional Materials, Pusan, KR 609-735

c

Department of Chemistry and Biochemistry, California State University, Chico, 400 W. First Street, Chico, CA 95973 d

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, United States

e

Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, United States

ACS Paragon Plus Environment

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Department of Chemistryand Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, United States

g

Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, The Netherlands

h

Department of Chemicaland Petroleum Engineering, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, PA 15261, United States

i

Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia

ABSTRACT

We demonstrate that thin films of metal-organic framework (MOF)-like materials, containing two perylenediimides (PDICl4, PDIOPh2) and a squaraine dye (S1), can be fabricated by layerby-layer assembly (LbL). Interestingly, these LbL films absorb across the visible light region (400-750 nm) and facilitate directional energy transfer. Due to the high spectral overlap and oriented transition dipole moments of the donor (PDICl4 and PDIOPh2) and acceptor (S1) components, directional long-range energy transfer from the bluest to reddest absorber was successfully demonstrated in the multicomponent MOF-like films. These findings have significant implications for the development of solar energy conversion devices based on MOFs.

KEYWORDS


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Layer-by-Layer, multi-zone MOF-like film, energy cascade, directional energy transfer, exciton migration


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In nature, photosynthesis is initiated by the formation of molecular excited states (excitons) in light-harvesting pigments, absorbing different slices of light of the visible spectrum.1 This is followed by directional migration of the excitons towards a reaction center. Metal-organic frameworks (MOFs) are highly ordered structures that, in principle, can perform similar steps. MOFs contain inorganic nodes connected to organic linkers.2 Due to their modularity, nanoscale porosity, and enormous chemical variety, MOFs have been explored for a broad range of potential chemical applications including sensing, catalysis, gas storage, and gas separations. Due to their high chromophore density and structural tunability, MOFs are attractive compounds for systematic studies of energy transfer.3,4 The energy transfer properties of photo-excited MOFs have been investigated in recent studies.5-8 Lee et al. showed that energy transfer occurs in bulk MOF crystals containing both bodipy and porphyrins units as chromophores with high spectral overlap.9 Upon excitation of the bodipy antennae, exclusive emission occurs from the porphyrins. Following the work of Lee, Son et al. investigated the directionality of energy transfer in porphyrin-based MOF crystals.10 By adding acetylenic moieties along the bipyridyl axis of the porphyrin, the conjugation length was increased. This resulted in changes in the electronic properties, namely increased oscillator strength and reduced Stokes shift. These yielded a larger spectral overlap integral for the Q-band absorption and emission, resulting in faster energy transfer along the axis of the primary transition dipole moment. As a consequence, excitons within the MOF containing the asymmetric porphyrin migrate about 15 timesfurther than in an analogous MOF assembled with higher-symmetry chromophores. So et al. advanced the work by demonstrating energy transfer in thin films of porphyrin based MOFs that sensitized a red-absorbing squaraine dye.11 By using a


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MOF film with a thickness matching that of the exciton propagation length, essentially all excitons created in the porphyrinic MOF proved transferable to the red-absorbing dye. In addition to porphyrins, other attractive molecular conduits for exciton transport include various perylenediimides (PDIs). PDI-based compounds have historically served as appealing charge acceptors in solar energy conversion devices, due to their large electron affinities and high electron mobilities.12 Substituents at the perylene bay positions (see Figure 1) can significantly affect the electronic properties, such as redox potentials,13 fluorescence, and absorption. From a synthetic standpoint, the bay positions of the PDIs are easy to modify. This feature facilitates tuning of the optical properties. To obtain chromophoric arrays, MOF-like materials14 were prepared as surface-supported films. Recently, solvothermal synthesis15,16 and electrophoretic deposition17,18 have been used to create micron thick MOF films. If MOFs are to be integrated effectively into solar energy conversion devices, however, a key requirement is that the absorption path-length match the exciton propagation length. Consequently, interfacing MOFs directly with surfaces via an assembly method that enables precise control of film thickness is clearly desirable. One such method is layer-by-layer (LbL) assembly, also termed liquid-phase epitaxy.2,11,19-22 MOF film formation entails repetitive, sequential exposure of a functionalized support to molecular and ionic building blocks. Key advantages of LbL assembly include the ability to: a) control MOF film thickness (average thickness) with close to molecular-scale precision,23 b) control crystal orientation with respect to the supporting platform, c) systematically vary the chemical composition of a given MOF-like film by changing the choice of building blocks as the repetitive assembly process progresses,24,25 and d) make effective use of much less concentrated solutions of organic reactants than typically required for solvothermal synthesis (thereby permitting even


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poorly soluble building blocks to be used). Notably, the LbL approach lends itself well to automation, thereby facilitating synthesis reproducibility. The generality and simplicity of LbL methods should allow for their exploitation for the preparation of a wide variety of molecular devices.26,27 Since the linkers in MOFs are readily tunable, they can be selected to favor long-range energy transfer (FRET, Förster Resonance Energy Transfer)-either via a single step or by multiple rapid short steps. Given the high spectral overlap between the emission of the donor and the absorption of the acceptor, the exciton can migrate from the bluest to reddest absorber. If effectively optimized, this should result in essentially exclusive emission from the red-most absorbing organic linker in the MOF. By positioning chromophores within a distance matching the exciton propagation length, essentially complete energy transfer occurs. If the exciton needs to move farther, however, energy transfer is inefficient. Therefore, MOFs are viable candidates for investigating the nuances of long-range energy transfer. Here we describe long-range energy transfer within MOF-like films across three types of chromophores (two PDIs and one squaraine dye (S1)), arranged in an energy cascade manner. We previously reported the LbL synthesis of oriented thin-films of porous coordination polymers containing porphyrins.2, 5 As shown in Figure 1a,28 films studied here contain: 1,2,4,5tetrakis(4-carboxyphenyl)benzene (L1) units as tetratopic linkers that coordinate pairs of Zn(II) ions in paddlewheel fashion, and either (or both) N,N’-di(4-pyridyl)-1,7-di(3,5-di-tertbutylphenoxy)-3,4,9,10-perylenetetracarboxylic diimide (PDIOPh2) and N,N’-di(4-pyridyl)1,6,7,12-tetrachloro-3,4,9,10-perylenetetracarboxylic diimide (PDICl4) linkers that serve as Zn(II)-ligating spacers/pillars between L1-defined layers. Using automated LbL assembly, thin films containing PDICl4/PDIOPh2 were grown on 3-aminopropyltrimethoxysilane (3-APTMS)


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functionalized quartz slides at 45oC with 10 µM solution for PDICl4 and at 50oC with 25 µM solution for PDIOPh2.

Figure 1. (a) Schematic diagram for sequential deposition of the MOF-like film via LbL assembly. Over N cycles, the thin MOF-like film is formed on silicon platform functionalized with 3-APTMS. Introduction of Zn(II), followed by L1, and then PDICl4 or PDIOPh2. Note that the bay positions of the PDI are functionalized with either chlorine or 3,5-di-t-butylphenoxyl groups.(b) Prepared thin MOF-like films of film A, film B, film C, film D, and film D+S1. In poly-chromatic samples, each zone is defined by the incorporation of one type of pillaring chromophore. The order in which the zones were deposited is PDICl4, followed by PDIOPh2. To monitor the thicknesses of MOF-like films with nanometer precision, we used profilometry.29 The film growth rates were 0.7-1.2 layers/cycle. These were calculated from the measured


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thicknesses of films, the number of cycles deposited, and the height of one layer of PDI and associated node (i.e. 26 Å). The high spectral overlap of the fluorescence of PDICl4 and absorption of PDIOPh2 in solution state suggests that PDIOPh2 can be excited by dipolar coupling to proximal, photo-excited PDICl4 (Figure S1). S1 can be excited from PDIOPh2 through FRET. S1 (Figure 2a) is an ideal far-red absorber in this multichromophoric system because it has high spectral overlap with PDIOPh2 and a high molar absorption coefficient (2.7x105 M-1cm-1). Table S1 summarizes photophysical data for PDICl4, PDIOPh2, and S1 in solution. Absorption coefficients of PDICl4 and PDIOPh2-based MOF-like materials were calculated (Figure S12). Thirty cycles of PDICl4 zone (56 nm) were deposited on 3-APTMS functionalized quartz, followed by 10 cycles of PDIOPh2 zone (20nm). S1 (2 nm) was deposited on top of the film by soaking in 200 µM MeOH solution for 50 minutes (Figure 1b and S9). Each component zone was deposited with different thickness considering different molar absorption coefficients (Table S1) of PDIs and S1. When the three components were deposited on a quartz substrate (film A), we observed that the absorption from L1, PDI struts and S1 (Figure 2b) spanned 350-750 nm visible region. Film A showed absorption at 450-550 nm for PDICl4, 500-650 nm for PDIOPh2 and 600-750 nm for S1 (Figure S2). Notably, the calculated absorption spectra (Figure S12) showed a similar trend in absorption spectra when the PDI components are incorporated in the solid state. Individual PDICl4 and PDIOPh2 films showed more red shifted emission maximum at 572 and 603 nm than that of solution states (Figure S4-S5 and Table S2).


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Figure 2. (a) Molecular structure of S1 and (b) absorption spectrum of film A containing PDICl4, PDIOPh2 , and S1. FRET is maximized when the donor and acceptor transition dipoles are oriented parallel to each other. Ideally, the long axis and therefore, the transition dipole moment, of each of the PDIs in a given MOF-like film will be aligned with each of the other PDIs. Since thin films of preferentially oriented chromophores will exhibit polarization-dependent absorbance, we performed variable-polarization measurements to deduce the alignment of LbL-assembled 60 cycles of PDIOPh2 MOF-like films, film B (Figure 3). Upon illumination at normal incidence (0°), the absorbance at 550nm showed no periodic change with polarization angle, suggesting no preferential x vs. y orientation for the primary dipole of PDIOPh2, as expected. In contrast, when the substrate was tilted 43° with respect to the incident beam, to allow a substantial fraction (~0.5) of the electric field vector to oscillate perpendicular to the substrate (z-direction), there was a periodic dependence on polarization angle. As expected, the absorption was maximized,


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when the polarization was aligned (0° and 180°) with the substrate normal. This observation was consistent with the overall dipole moment of PDIOPh2 oriented normal to the substrate. These results confirmed the formation of an aligned PDI structure on the quartz surface assembled by LbL.

Figure 3. Film B shows polarization angle dependent absorption intensity when monitored at 550nm, the maximum absorption wavelength of PDIOPh2. For films used to investigate incomplete FRET, the thickness of each PDI zone was approximately 20 nm (film C). When excited at 525 nm (absorption maximum for PDICl4), Film C shows emission at 572 nm, 603 nm, and 684 nm, attributable to PDICl4, PDIOPh2 and S1, respectively (Figure S6). The observation of emission at 684 nm implies energy transfer from photo-excited PDICl4 to PDIOPh2 and finally to S1. Nevertheless, the observation of weak fluorescence also at 603 nm suggestslessthan 100% efficiency in propagating excitons through the PDIOPh2 zone. Similarly, the observation of emission at 572 nm points to inefficiencies in propagating excitons across the entirety of the PDICl4 zone. (An alternative explanation based on


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inefficient FRET across the PDICl4/PDIOPh2 interface can be ruled out based on experiments described below.) Returning to the observation of emission from S1 based on remote excitation of PDICl4, given the high concentration of chromophores in each zone, we assume that energy transfer from PDICl4 from to S1 is mediated by an extended series of nearest-neighbor energy transfer steps, i.e. PDICl4/PDICl4, PDICl4/PDIOPh2, and PDIOPh2/PDIOPh2, but not PDIOPh2/PDICl4. Given the possibility of intra-zone energy transfer in both the forward and reverse direction (i.e. diffusive-type migration of excitons), as well as laterally, the total number of energy-transfer steps entailed in sensitizing the emission of S1 via excitation of PDICl4 will substantially exceed the number of chromophores defining a unidirectional pathway between PDICl4 and S1. For films used to study FRET between component zones, first a PDICl4 zone with a thickness of 8 nm was deposited. Next, a PDIOPh2 zone of 11 nm was added on the top of the PDICl4 zone (film D). When excited at 525 nm (absorption maximum of individual PDICl4 film), this film showed emission at 603 nm (PDIOPh2) but no emission at 575 nm (PDICl4) (Figure S7). These results indicate that the excited-state energy of PDICl4 is transferred essentially quantitatively to the PDIOPh2 zone. We next examined film D+S1 in Figure 4, which includes S1 as the reddest absorber and final emitter. This film emits exclusively at 684 nm (characteristic of S1) when excited at 525 nm (absorption maximum of PDICl4) inFigure 4. Emission from PDICl4 (575 nm) and PDIOPh2 (603 nm) was not observed. The observation of emission of S1 indicates that excitons residing initially on PDICl4are efficiently transferred via the intermediacy of several layers of PDIOPh2. Thus, efficient energy cascading, as well as intra-zone energy transfer occurs, over approximately 20 nm maximum distance shown in Figure 4. As shown in Figure S6, however,


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when the maximum transmission distance is extended to ca. 40 nm, leakage in the form of fluorescence from PDICl4 or PDIOPh2 can occur. Nevertheless, the combined results show that aligned, multicomponent LbL films can act as antenna-type light harvesters, with absorbed energy being directionally delivered to a terminal dye layer.

Figure4. The solid-state fluorescence spectrum of the film shows exclusive emission from squaraine upon excitation of PDICl4, indicating that the exciton migrated through PDICl4 and PDIOPh2 and became trapped by S1. Multicomponent MOF-like films constitute attractive platforms for absorbing light across the entire visible light region and slightly beyond (350-750 nm here, based on functionalized perylene-diimide- and squaraine-type chromophores). Automated layer-by-layer assembly permits the films to be grown in oriented fashion and with close to molecular-scale control over zone thickness and overall film thickness. For multicomponent MOF-like films of approximately 20 nm average thickness, excitons can be transported essentially quantitatively from the bluest chromophores, through chromophores of intermediate color, to a terminal layer of red


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chromophores. For similar films of approximately 40 nm average thickness, however, detectable light leakage (PDI fluorescence) occurs and transport is less than quantitative. The observed panchromatic absorption and cascade-type transport and delivery of molecular excitons, over tens of nanometers, to the film exterior points to applications in light-to-electrical or light-to-chemical energy conversion. It clearly would be desirable, however, to increase the number of MOF units over which excitons can be efficiently transported and, thereby, increase the fraction of incident photons that are usefully harvested. One limitation of the PDI chromophores, as used here, is that the terminal pyridyl groups are only poorly electronically coupled to the perylene-like core where the exciton is formed. As such, the single step, peryleneto-perylene (edge-to-edge) exciton transfer distance, in the direction normal to the film support, is roughly 12 Å. In its simplest form, Förster theory promises that single-step energy transfer rates will vary as r-6, where r is energy-donor/energy-acceptor separation distance. An improved MOF film design would employ chromophores that yield much shorter donor/acceptor separation distances. Efforts along this line are currently underway in our lab. Another means of boosting the fraction of incident photons absorbed by a film of a given thickness would be to decrease the MOF void volume. We intentionally included sizable MOF voids and apertures in our films, anticipating that these would be needed for solvent and electrolyte permeation-which, in turn, would likely be necessary for film-based charge transport if films were used as the light-harvesting components of liquid-junction solar cells. Recent work by Liu and co-workers,30 however, shows that compact MOF films can function as an effectively for charge transport, even when deployed in liquid-junction solar cells (i.e. photoelectrochemical cells). Reduction of void volume, therefore, is also an element of our continuing work to design


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and assemble multi-chromophore MOF-like films suitable for solar energy conversion applications. ASSOCIATED CONTENT Supporting

Information.

Information

on

general

materials,

synthesis,

fabrication,

instrumentation, spectroscopic and structural characterization, and computational details are available here. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Present Addresses †Department of Chemistry, Pusan National University, Busan 46241, South Korea ‡Department of Chemistry & Biochemistry, California State University, Chico, 400 W. 1st Street, Chico, CA 95929-0210, United States Author Contributions §These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge support from the U.S. Dept. of Energy, Office of Science, Office of Basic Energy Sciences (grant no. DE-FG02-87ER13808), Joint Center of Excellence in


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Integrated Nano-Systems (JCIN) at King Abdulaziz City for Science and Technology (KACST), and Northwestern University. H.J.P. was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2013R1A6A3A03020498). M.C.S. acknowledges support from the Department of Defense through the National Defense Science & Engineering Graduate (NDSEG) Fellowship Program. Use of the Center of Nanoscale Materials is funded by the U.S. Dept. of Energy, Office of Science, Office of Basic Energy Sciences through contract no. DEAC02- 06CH11357. A. A.-G., S. E., and C. E. W. acknowledge support from the Center for Excitonics and Energy Frontier Research Center funded by the U.S. Department of Energy under award DE-SC0001088. Computations were run on the Harvard University’s Odyssey cluster, supported by the Research Computing Group of the FAS Division of Science. REFERENCES (1) McDermott,G.; Prince, S.; Freer,A.; Hawthornthwaite-Lawless, A. M.; Papiz,M. Z.; Cogdell, R. J.; Isaacs, N. W. Crystal Structure of an Integral Membrane Light-Harvesting Complex from Photosynthetic Bacteria. Nature. 1995, 374, 517-521. (2) So,M. C.; Beyzavi,M. H.; Sawhney,R.; Shekhah,O.; Eddaoudi,M.; Al-Juaid,S. S.; Hupp, J. T.; Farha,O. K. Post-assembly Transformations of Porphyrin-Containing Metal–Organic Framework (MOF) Films Fabricated via Automated Layer-by-Layer Coordination. Chem. Commun. 2015, 51, 85-88. (3) So,M. C.; Wiederrecht,G. P.; Mondloch,J. E.; Hupp, J. T.; Farha,O. K. Metal–Organic Framework Materials for Light-Harvesting and Energy Transfer.Chem. Commun. 2015, 51, 3501-3510.


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(4) Williams, Derek E.; Shustova, Natalia B.Metal–Organic Frameworks as a Versatile Tool To Study and Model Energy Transfer Processes.Chem. Eur. J. 2015, 21, 15474-15479. (5) Maza, W. A.; Haring, A. J.; Ahrenholtz, S. R.; Epley, C. C.; Lin, S. Y.; Morris, A. J. Ruthenium(II)-Polypyridyl Zirconium(IV) Metal-Organic Frameworks as a New Class of Sensitized Solar Cells. Chem. Sci. 2016, 7, 719–727. (6) Mondal, S. S.; Bhunia, A.; Attallah, A. G.; Matthes, P. R.; Kelling, A.; Schilde, U.; MüllerBuschbaum, K.; Krause-Rehberg, R.; Janiak, C.; Holdt, H.-J. Study of the Discrepancies between Crystallographic Porosity and Guest Access into Cadmium-Imidazolate Frameworks and Tunable Luminescence Properties by Incorporation of Lanthanides. Chem. Eur. J. 2016, 22, 6905-6913. (7) Williams,D. E.; Rietman,J. A.; Maier,J. M.; Tan,R.; Greytak,A. B.; Smith,M. D.; Krause,J. A.; Shustova, N. B. Energy Transfer on Demand: Photoswitch-Directed Behavior of Metal– Porphyrin Frameworks. J. Am. Chem. Soc. 2014, 136, 11886−11889. (8) Dolgopolova,E. A.; Williams,D. E.; Greytak,A. B.; Rice,A. M.; Smith,M. D.; Krause,J. A.; Shustova, N. B.A Bio-inspired Approach for Chromophore Communication: Ligand-toLigand and Host-to-Guest Energy Transfer in HybridCrystalline Scaffolds. Angew. Chem. Int. Ed. 2015, 54, 13639-13643. (9) Lee,C. Y.; Farha,O. K.; Hong,B. J.; Sarjeant,A. A.; Nguyen, S. T.; Hupp,J. T. LightHarvesting Metal–Organic Frameworks (MOFs): Efficient Strut-to-Strut Energy Transfer in Bodipy and Porphyrin-Based MOFs. J. Am. Chem. Soc. 2011, 133, 15858-15861.


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(10) Son,H.-J.; Jin,S.; Patwardhan,S.; Wezenberg,S. J.; Jeong,N. C.; So,M.; Wilmer,C. E.; Sarjeant,A. A.; Schatz,G. C.; Snurr,R. Q.; Farha,O. K.; Wiederrecht, G. P.; Hupp,J. T. LightHarvesting and Ultrafast Energy Migration in Porphyrin-Based Metal–Organic Frameworks. J. Am. Chem. Soc. 2012, 135, 862-869. (11) So,M. C.; Jin,S.; Son,H.-J.; Wiederrecht,G. P.; Farha, O. K.; Hupp,J. T. Layer-by-Layer Fabrication of Oriented Porous Thin Films Based on Porphyrin-Containing Metal–Organic Frameworks.J. Am. Chem. Soc. 2013, 135, 15698-15701. (12) Breeze,A.; Salomon,A.; Ginley,D.; Gregg,B.; Tillmann, H.; Hörhold,H.-H. PolymerPerylene Diimide Heterojunction Solar Cells. Appl. Phys. Lett. 2002, 81, 3085-3087. (13) Chao,C.-C.; Leung,M.-k.; Su,Y. O.; Chiu,K.-Y.; Lin,T.-H.; Shieh, S.-J.; Lin,S.-C. Photophysical and Electrochemical Properties of 1,7-Diaryl-Substituted Perylene Diimides. J. Org. Chem. 2005, 70, 4323-4331. (14) Although the thin-film materials described here clearly are preferentially oriented and clearly are coordination polymers, we have not observed diffraction from the films and, therefore, have not shown that ordering rises to the level of crystallinity. In our experience, specifically with pillared-paddlewheel type compounds, conventional X-ray diffraction yields evidence for crystallinity only when films are significantly thicker (ca. three-fold thicker) than those grown here. This behavior can be contrasted with that for films of the iconic MOF material, HKUST-1. The lack of discernible diffraction is also consistent with the tendency of bulk pillared-paddlewheel MOFs, featuring lengthy pillars, to display only weak diffraction unless special measures are taken in evacuating the MOF pores of synthesis solvent. See, for example: "Supercritical Processing as a Route to High Internal Surface


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Areas and Permanent Microporosity in Metal-Organic Framework Materials," Nelson, A. P.; Farha, O. K.; Mulfort, K. L.; Hupp, J. T. J. Am. Chem. Soc., 2009, 131, 458-460. In the absence of evidence here for film crystallinity, the materials can be more precisely described as “MOF-like.” (15) Ahrenholtz,S. R.; Epley, C. C.; Morris,A. J. Solvothermal Preparation of an Electrocatalytic Metalloporphyrin MOF Thin Film and its Redox Hopping Charge-Transfer Mechanism. J. Am. Chem. Soc. 2014, 136, 2464-2472. (16) Kung,C.-W.; Wang,T. C.; Mondloch,J. E.; Fairen-Jimenez,D.; Gardner,D. M.; Bury,W.; Klingsporn,J. M.; Barnes,J. C.; Van Duyne, R.; Stoddart,J. F. Metal–Organic Framework Thin Films Composed of Free-Standing Acicular Nanorods Exhibiting Reversible Electrochromism. Chem. Mater. 2013, 25, 5012-5017. (17) Hod,I.; Bury,W.; Karlin,D. M.; Deria,P.; Kung,C. W.; Katz,M. J.; So,M.; Klahr,B.; Jin, D.; Chung,Y. W. Directed Growth of Electroactive Metal-Organic Framework Thin Films Using Electrophoretic Deposition. Adv. Mater.2014, 26, 6295-6300. (18) Hwang,Y.; Sohn,H.; Phan,A.; Yaghi,O. M.; Candler,R. N. Dielectrophoresis-Assembled Zeolitic Imidazolate Framework Nanoparticle-Coupled Resonators for Highly Sensitive and Selective Gas Detection. Nano Lett. 2013, 13, 5271−5276. (19) Shekhah,O.; Wang,H.; Kowarik,S.; Schreiber,F.; Paulus,M.; Tolan,M.; Sternemann,C.; Evers,F.; Zacher,D.; Fischer, R. A.;

Wöll,C. Step-by-Step Route for the Synthesis of

Metal−Organic Frameworks. J. Am. Chem. Soc. 2007, 129, 15118-15119.


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(20) Summerfield,A.; Cebula,I.; Schröder,M.; Beton,P.H. Nucleation and Early Stages of Layerby-Layer Growth of Metal Organic Frameworks on Surfaces. J. Phys. Chem. C 2015, 119, 23544−23551. (21) Wannapaiboon,S.; Tu,M.; Sumida,K.; Khaletskaya,K.; Furukawa,S.; Kitagawa.S.; Fischer,R.A. Hierarchical Structuring of Metal–Organic Framework Thin-Films on Quartz Crystal Microbalance (QCM) Substrates for Selective Adsorption Applications. J. Mater. Chem. A 2015, 3, 23385–23394. (22) Stavila,V.; Volponi,J.; Katzenmeyer,A.M.; Dixon,M.C.; Allendorf,M.D. Kinetics and Mechanism of Metal–Organic Framework Thin Film Growth: Systematic Investigation of HKUST-1 Deposition on QCM Electrodes. Chem. Sci. 2012, 3, 1531–1540. (23) In some cases multiple molecular layers are deposited in a single growth cycle. MOFs based on single, polytopic linkers, such as HKUST-1 (based on benzene-tricarboxylate) appear to be particularly prone to such growth. See, for example, refs. 20 and 22. Comparatively high linker concentrations (substantially higher than used here) also appear to favor growth of more than one structural repeat unit per LbL synthesis cycle. (24) Liu,B.; Ma,M.; Zacher,D.; Bétard,A.; Yusenko,K.; Metzler-Nolte,N.; Wöll,C.; Fischer,R. A. Chemistry of SURMOFs: Layer-Selective Installation of Functional Groups and Postsynthetic Covalent Modification Probed by Fluorescence Microscopy.J. Am. Chem. Soc. 2011, 133,1734–1737. (25) Zacher,D.; Yusenko,K.; Betard,A.; Henke,S.; Molon,M.; Ladnorg,T.; Shekhah,O.; Schuepbach,B.; Arcos,T.; Michael,K.; Meilikhov,M.; Winter,J.; Terfort,A.; Wöll,C.;


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Fischer,R. A. Liquid-Phase Epitaxy of Multicomponent Layer-Based Porous Coordination Polymer Thin Films of [M(L)(P)0.5] Type: Importance of Deposition Sequence on the Oriented Growth. Chem. Eur. J. 2011, 17, 1448-1455. (26) Gu,Z.-G.; Fu,W.-Q.; Wu,X.; Zhang,J. Liquid-phase Epitaxial Growth of a Homochiral MOF Thin Film on Poly(L-DOPA) Functionalized Substrate for Improved Enantiomer Separation. Chem. Commun. 2016, 52, 772-775. (27) Kung, C.-W.; Mondloch, J. E.; Wang, T. C.; Bury, W.; Hoffeditz, W.; Klahr, B. M.; Klet, R. C.; Farha, O. K.; Hupp, J. T. Metal–Organic Framework Thin Films as Platforms for Atomic Layer Deposition of Cobalt Ions To Enable Electrocatalytic Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 28223-28230. (28) The multi-layer structures shown in Figure 1a are idealized. We have previously found evidence for island growth that persisted through about ten LbL cycles; see ref. 11. Consistent with such behavior, profilometry scans indicate formation of films of uneven thickness. One consequence of neglecting complications due to island growth may be slight underestimates of film thicknesses relevant to exciton transport. Refs. 20-21, in particular, offer insightful descriptions and discussions of island growth in LbL MOF film syntheses. (29) “Nanometer precision” refers to local thickness. Reported values are averages over lengths of scans. Even considering averaging, the reported values are unlikely to be more precise than plus/minus one MOF structural repeat unit, i.e. 2 to 3 nm. (30) Liu,J.; Zhou,W.; Liu,J.; Howard,I.; Kilibarda,G.; Schlabach,S.; Coupry,D.; Addicoat,M.; Yoneda,S.; Tsutsui,Y.; Sakurai,T.; Seki,S.; Wang,Z.; Lindemann,P.; Redel,E.; Heine,T.;


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Wöll, C. Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?Angew. Chem. Int. Ed. 2015, 54,7441 –7445.


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Graphical Abstract


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We demonstrate that thin films of metal-organic framework-like materials, containing two perylene diimides (PDICl4, PDIOPh2) and a squaraine dye (S1), absorb across the visible light region and facilitate directional energy transfer. 35x26mm (600 x 600 DPI)


Layer-by-Layer Assembled Films of Perylene Diimide- and Squaraine

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