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Host-guest interactions derived multilayer perylene diimide thin film constructed on a scaffolding porphyrin monolayer Mengyuan Zhu, Gyan H Aryal, Nan Zhang, Hong Zhang, Xiaoye Su, Russell H. Schmehl, Janarthanan Jayawickramarajah, Xiu Liu, Jin Hu, and Jiang Wei Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504297w • Publication Date (Web): 13 Dec 2014 Downloaded from http://pubs.acs.org on December 19, 2014

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Host-guest interactions derived multilayer perylene diimide thin film constructed on a scaffolding porphyrin monolayer Mengyuan Zhua, Gyan H Aryala, Nan Zhanga, Hong Zhanga, Xiaoye Sua, Russell Schmehla, Xue Liub, Jin Hub, Jiang Weib and Janarthanan Jayawickramarajaha* a Department of Chemistry, Tulane University, 2015 Percival Stern Hall, New Orleans, Louisiana, 70118, United States; b Department of Department of Physics & Engineering Physics, Tulane University, 2001 Percival Stern Hall, New Orleans, Louisiana, 70118, United States

Keywords: adamantane; β-cyclodextrin; layer by layer assembly; perylene diimides; porphyrins; host-guest interactions

The development of methods to grow well-ordered chromophore thin films on solid substrates is of importance because such surface-associated arrays have potential applications in the generation of functional electronic and optical materials and devices. In this manuscript, we demonstrate a straightforward layer-by-layer (LBL) supramolecular deposition strategy to prepare numerous layers (up to 19) of functionalized perylene diimide (PDI) chromophores built upon a covalent scaffolding multivalent porphyrin monolayer. Our thin film formation strategy

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employs water as the immersion solvent and exploits the β-cyclodextrin-adamantane host-guest couple in addition to PDI based aromatic stacking. Within the resultant film the porphyrin scaffold is oriented close to parallel to the glass substrate whilst the PDI chromophores are aligned closer to the surface normal. Together, the porphyrin monolayer and the multi PDI layers exhibit a large absorption band-width in the visible spectrum. Importantly, because a selfassembly strategy was utilized, when a single monolayer of PDI is deposited on the porphyrin scaffolding layer, this PDI monolayer can be readily dis-assembled by washing with DMF leading to the regeneration of the porphyrin monolayer. The PDI thin film can subsequently be re-grown from the regenerated porphyrin surface. The reported LBL strategy will be of broad interest for researchers developing well-organized chromophoric films and materials due to its simplicity as well as the added advantage of being performed in sustainable and cost-effective aqueous media.

Introduction In order to develop the next generation of functional electronic and optical devices there is the need to precisely control the orientation, organization, and growth of molecules on surfaces.1 Indeed, a number of advanced fabrication methods have been developed to organize and pattern molecules at the nanometer scale and above. These techniques include atomic layer deposition,2 molecular beam epitaxy,3 chemical vapor deposition,4 and the Langmuir-Blodgett technique.5 However, many of these methods necessitate the use of carefully modulated workspaces, welltrained technicians, and instrumentation that are sophisticated and expensive. In contrast, Layer by Layer (LBL) assembly/fabrication is a straightforward and remarkably low-cost tool for constructing thin films with finely controlled molecular architecture and composition.6 LBL assembly is performed by simply depositing—in an alternate fashion—complementary solution

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phase moieties (i.e., entities containing functional groups that will assemble/react with each other) onto solid substrates.7 For the most part, non-covalent interactions are utilized to fabricate functional thin films, due to the ease of assembling partners via supramolecular chemistry. Furthermore, non-covalent assembly can allow for fine-tuning affinity, error-correction, and imparting stimuli responsiveness.8, 9 An area of thin film fabrication that has received recent attention is the hierarchical assembly of ordered chromophores since such films have applications in light-harvesting, photovoltaics, optoelectronics, and chemical/biological sensors.10,

11, 12 13

While elegant LBL assembly of

chromophores has been achieved using covalent growth approaches, these LBL deposition schemes often require organic solvents, and metal catalysts (that may need to be removed for biological applications).14,

15

Further, these covalent systems are not readily amenable to

modulation by external stimuli. Thus, there is a need to develop complementary LBL deposition strategies that utilize non-covalent interactions and function in water. Our group16, 17 and others18, 19, 20, 21, 22

have been interested in constructing multi-chromophoric nanostructures using the self-

assembly capacity of β-cyclodextrins (β-CDs). In terms of fabricating LBL thin films, β-CDs are prime candidates since these macrocycles can be easily tethered to various functional molecules, and are good host molecules in water for various small molecule guests.23 For instance, the adamantane moiety is a well-studied guest that forms inclusion complexes with β-CDs displaying a robust association constant (104-105 M-1 in water).24 Water is a particularly attractive assembly solvent for LBL technology as it is an environmentally friendly, costeffective, and sustainable solvent. Although a variety of moieties including nanoparticles,25, 26 quantum dots,27 polymers,28 29and proteins30 have been conjugated with β-CDs and incorporated into functional mono-or multi-layer films via β-CD based host-guest chemistry, there has been

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no exploration of using such β-CD based host-guest interactions in conjunction with LBL fabrication to construct organic multi-chromophore array derived thin films with defined chromophore placement and orientation.31 Herein we report an aqueous LBL deposition strategy that exploits repetitive β-CDadamantane host-guest interactions to prepare multiple layers (up to 19) of perylene diimide (PDI) chromophores built upon a scaffolding multivalent porphyrin monolayer. The resultant multi-chromophore assembly exhibits an absorption band-width covering a significant portion of the visible spectrum. Furthermore, for a thin film that contains the porphyrin scaffolding layer and one PDI monolayer, we demonstrate that the LBL technique is reversible, since the PDI containing monolayer can be stripped-off by simply washing with DMF thereby regenerating the covalently fastened porphyrin scaffolding. The porphyrin monolayer can then be re-used for another cycle of PDI assembly.

Results and Discussion The first two components for the LBL thin film (Figure 1) are an azide terminated alkyl siloxane monolayer on glass substrate and tetraphenyl porphyrin 1 that is attached, via the metaphenyl positions, to four adamantane arms and four alkyne arms. This judicious positioning of functional groups allows for the porphyrin molecule to access multiple attachments to the azide monolayer via the “click” reaction which then (a) places the porphyrin plane roughly parallel to the glass substrate32, 33 (since the porphyrin and meso phenyl rings are nearly orthogonal) and (b) positions the adamantane arms facing upwards from the glass substrate (which is critical for further vectorial LBL growth). In addition to these structural considerations, the porphyrin

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chromophore was specifically chosen since it is an attractive component in photonic, optoelectronic, and light-harvesting systems.34 The multilayer LBL growth process was envisioned to be conducted using PDI monomers, Specifically, PDIs 2 and 3 were designed to project β-CD hosts and adamantane guests, respectively, via attachment at each imide position. The PDI unit was chosen as the multilayer growth component since these chromophores have large extinction coefficients in the visible spectrum, possess high fluorescence quantum yields, and are photo-chemically and thermally stable. In fact, PDIs have found a number of promising applications including in electrophotography and photovoltaics.35 In addition, PDIs 2 and 3 absorb in the 500-600 nm region and thus strongly absorb in a region that largely lies between the porphyrin Soret and Qabsorption bands, hence nicely complementing the porphyrin chromophore. 36

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Figure 1. Illustration of the layer by layer deposition method facilitated by host-guest interactions. (i) The azide terminated alkyl siloxane monolayer on glass substrate was reacted with porphyrin 1 resulting in a porphyrin monolayer. (ii) The porphyrin containing substrate was immersed in an aqueous solution of PDI 2 resulting in the first PDI layer. (iii) After rinsing with water, the substrate was dipped into an aqueous solution of PDI 3 generating the second PDI layer. Repetition of the deposition steps ii and iii in an alternate manner leads to a multilayer PDI thin film.

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The detailed synthesis of porphyrin 1, PDIs 2 and 3, and LBL deposition protocols are provided in the General Experimental. Briefly, the synthesis of porphyrin 1 is shown in Scheme 1. Here it was critical to develop a method to differentially functionalize the two meta positions within each meso-phenyl ring of porphyrin 1. This was done by first selectively functionalizing one of the hydroxyl groups of 3, 5-dihydroxybenzaldehyde with an alkyne unit, using 1.25 equivalents of propargyl bromide under basic conditions, to afford benzaldehyde 4. Next, the adamantane arm was attached to the newly formed alkyne unit on 4 via the copper catalyzed Huisgen click reaction with 1-azidoadamantane to yield 5. Benzaldehyde 5 was subsequently reacted with propargyl bromide to afford porphyrin-forming precursor 6 with the appropriate differently functionalized arms. Last, zinc-containing porphyrin 1 was synthesized by refluxing 6 with freshly distilled pyrrole in propionic acid, followed by insertion of the metal using zinc acetate.

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Scheme 1. Synthetic procedure for the preparation of zinc porphyrin 1. PDI 2 was prepared following a previously reported procedure,37 whilst PDI 3 was synthesized in three steps as outlined in Scheme 2. First, commercially available perylenetetracarboxylic dianhydride was reacted with N,N-diethylethylenediamine to afford condensation product 838 that is flanked with two amine arms. These two arms were further functionalized with alkyne units through reaction of 8 with propargyl bromide which afforded dicationic PDI 9. Compound 9 was used in the click reaction step along with 1-azidoadamantane to yield PDI 3.

Scheme 2. Schematic depicting the synthesis of PDI 3.

The azido terminated alkyl siloxane monolayer on oxide silica glass substrate was prepared as reported by Bard and colleagues.39 The LBL fabrication process starts by reacting porphyrin 1 with the azide modified glass substrate in the presence of a Cu(I) catalyst via the Huisgen click reaction,40 yielding a porphyrin monolayer that is terminated with adamantane moieties (Figure 1i). Next, a layer of PDI 2 is deposited on the porphyrin functionalized glass substrate by

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immersing the substrate into an aqueous solution of PDI 2 (Figure 1ii). After washing with DI water to remove any unbound species, the substrate was submerged into an aqueous solution (with 1% formic acid) of PDI 3 to generate a second PDI layer (Figure 1iii). Each deposition step was performed by immersion of the substrate into the respective PDI solutions for 10 min.41 These two non-covalent deposition processes were repeated in an alternating manner to obtain multiple PDI layers. Preliminary evidence for the incorporation of porphyrin 1 on the glass substrate came from UV-vis spectroscopy. While porphyrin 1 has a λmax (Soret Band) at 426 nm in THF solution, the surface bound chromophore reveals a λmax that is red shifted by 9 nm to 435 nm (see SI, Figure S3). Such a red shift is typically found in ordered porphyrin thin films wherein the porphyrin moieties are in a closely packed arrangement.10 The surface density (Γ) was calculated using the following equation. Γ= [(Aλ/2)ελ-1NA] x10-3 Here Aλ is the absorbance of the porphyrin in the film at wavelength λ, ελ is the extinction coefficient in solution (M-1 cm-1) at λ, and NA is Avogadro’s number.42 On the basis of the extinction coefficient of porphyrin 1 in THF solution (ελ = 260699 M-1 cm-1, where λ = 426 nm, SI Figure S4), the surface density of 1 was estimated to be 3.76 x 1013 molecules/cm2. This value corresponds to an average molecular area of 2.6 nm2. Previous literature reports from Langmuir– Blodgett films show that when tetra-aryl porphyrin macrocycles are placed perpendicular to the surface, the average molecular area of the tetra-aryl porphyrin is about 0.9 nm2. On the other hand, when the plane of the porphyrin lies parallel to the surface, the average molecular area is 2.25 nm2.43 Our results thus suggest that porphyrin 1 is arranged roughly parallel to the surface,

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which is consistent with the notion that the alkyne arms have reacted covalently via the click reaction and the four adamantane moieties are projected upwards from the surface. Shown in Figure 2a is the UV-vis spectrum of the scaffolding porphyrin 1 monolayer along with the host-guest derived PDI multilayer (19 PDI monolayers are shown via alternative deposition of PDIs 2 and 3). Each monolayer composed of PDIs 2 or 3 show dual absorbance bands at 506 nm (A01) and 536 nm (A00). Plots of the PDI absorption intensity at 506 nm (Figure 2b) or 536 nm (SI, Figure S5) versus the monolayer number indicate a linear increase of the absorption intensity with the number of layers, suggesting that our LBL deposition process is well-controlled. Indeed, the absorption shape of each PDI monolayer stays constant (with the exception of increasing intensity) further suggesting a uniform deposition process with no substantial changes in the aggregate state of the PDIs between the different layers. The ratio of the absorbance intensity of the two PDI peaks (i.e., A00/A01 which corresponds to the monomer/aggregate ratio) can reveal information about the degree of PDI based πaggregation, with a monomer PDI having a ratio close to 1.65 whilst the aggregate displays a ratio near 0.7.44,

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In aqueous solution, the A00/A01 ratio for PDI 2 and 3 (under the same

concentrations, 5 uM, used for the LBL assembly process) was found to be 0.67 and 0.65, respectively (SI, Figure S6). These values clearly indicate that both PDIs have significant selfaggregation in solution. Interestingly, the A00/A01 ratio calculated for each PDI bilayer (i.e., after deposition of PDI 2 and 3) on the solid substrate (SI, Figure S7) remains constant at ca. 0.79. The fact that the ratio remains constant suggests a uniform LBL assembly process. Further, we speculate that the slightly higher ratio (i.e., lower aggregation) on the surface compared to the individual PDIs in solution may imply a more ordered π-assembly on the surface.

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UV-vis spectroscopy of the initial PDI monolayer (i.e., after first PDI 2 deposition) was used to gather a rough estimate of the surface density of this PDI layer. The density was estimated to be 1.38 x 1014 molecules/cm2, (when ελ =14237M-1 cm-1, λ = 506 nm, see SI, Figure S8). This value is 3.7 times larger than the surface density of the porphyrin 1 monolayera finding that is consistent with our model as each porphyrin has four adamantane guests and thus can bind 4 PDI units.

Figure 2. (a) UV-vis absorption spectra of 1-20 monolayers composed of a porphyrin scaffolding layer followed by nineteen PDI monolayers, on glass substrate. (b) Absorbance values at 506 nm for each PDI monolayer (layer number 2-20). Important control experiments were also performed by UV-vis to confirm the host-guest driven assembly process. First, when the covalent reaction to fasten the porphyrin 1 scaffolding layer was conducted in the absence of Cu(I) catalyst, no absorption bands corresponding to the porphyrin unit was observed (SI, Figure S9a). Second, when only PDI 2 was used as the growth layer (i.e., by extending the deposition time of PDI 2 on the porphyrin 1 functionalized substrate, no additional increase in absorbance was observed even after a 24 hr incubation (SI, Figure S9b); excluding the possibility of simple physisorption. Third, the adamantane functionalized PDI 3

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does not get deposited on the substrate (containing porphyrin 1 and one PDI 2 layer) when excess β-CD was present in the solution containing PDI 3 (SI, Figure S10). This result shows that when the host-guest interaction is blocked, PDI 3 cannot bind to the surface of the PDI 2 monolayer, and clearly suggests that the host-guest interaction is a major driving force in the multilayer fabrication process. In addition to the key role that the host-guest interaction plays, intra-layer PDI–based aromatic stacking is also important in the LBL assembly. For instance, when a three layer thin film (containing the porphyrin 1 scaffolding layer along with one layer each of PDI 2 and PDI 3) was attempted to be dis-assembled by the addition of excess competitive agent, 2-adamantylamine hydrochloride (that can compete for host-guest interactions but should not affect aromatic stacking interactions), only a 25% decrease in the PDI absorption (SI, Figure S11) was observed. This result suggests that the close packing of the PDI molecules via aromatic stacking is an important contributor to the integrity of the thin film as the competitive host-guest disruptor is precluded from significantly dis-assembling the multilayers.

In order to investigate the average molecular orientation of the porphyrin and PDI chromophores with respect to the glass substrate we conducted polarized UV-vis spectroscopy. In particular, the porphyrin 1 monolayer and a multilayer composed of seven PDI depositions were probed with incident beam angles of 45° and perpendicular to the substrate. At each incident angle the light beam was polarized horizontally and vertically. The average tilt angle of the chromophore plane assembled on the film was calculated according to the following equation.15, 40, 46,

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Where the D (λ) is the dichroic ratio between the absorption of horizontally (Ah), and vertically (Av) polarized light, α is the angle of the incident light beam relative to the substrate surface (i.e., 45°); γ is the angle of the chromophore plane with respect to the substrate surface. When the incident light is at 45° the porphyrin monolayer film demonstrates polarization dependence (Figure 3a).47 From the above equation, the tilt angle γ was estimated to be 25° for the porphyrin 1 layer. The non-zero degree tilt angle could be ascribed to the surface roughness of the glass substrate (as it is not perfectly flat) as well as the possible flexibility/disorder of the alkyl siloxane layer.46 In contrast to the relatively low tilt angle for porphyrin 1, for the seven multilayer PDI thin films the average tilt angle was found to be 50°, suggesting that the PDI layers are grown in a relatively upright manner with respect to the substrate surface. These results are consistent with our proposed LBL assembly scheme, wherein the adamantane arms of the porphyrin units are projecting upright from the surface and the PDI units are attached end-on via sequential host-guest interactions. Furthermore, when the light incidence is perpendicular to the substrate, no appreciable anisotropy was observed for the mono- and multi-layer thin films (see SI, Figure S12), suggesting that the LBL solution deposition method employed does not induce any preferential direction in the plane of the substrate.

Figure 3. Polarized UV-vis spectra of the porphyrin 1 monolayer (a) and seven PDI multilayers

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(b) on glass substrate when the light beam is polarized horizontally (black) and vertically (red). The incident light was set at a 45° angle. Water contact angle measurements were employed to characterize the surface compositions of each layer of the thin film. It was expected that the surface hydrophilic properties of the multilayer film would change upon alternatively depositing PDIs 2 and 3 since the β-CD and adamantane projected surfaces have differing hydrophobicities. Figure 4 shows the water contact angle for all 20 layers. The zeroth layer (i.e., the 11-azidoundecylsiloxane monolayer on glass substrate) has an angle of 84°, and is consistent with literature precedence.14 The porphyrin 1 monolayer exhibited a water contact angle value of 75°. Interestingly, each type of PDI layer demonstrated characteristic water contact angles, with the PDI 2 (even numbered layers in Figure 4) terminated surface consistently producing a lower average water contact angle of 50°. In contrast, the PDI 3 layers (odd numbered layers in Figure 4), were more hydrophobic with an average contact angle of 65°. These water contact angle measurements provide further evidence for the well-defined LBL self-assembly.

Figure 4. Water contact angle measurements. The 0th layer is the glass slide functionalized with the 11-azidoundecylsiloxane, the 1st layer is the porphyrin 1 containing monolayer, the even numbered layers correspond to PDI 2 deposition and the odd numbered layers correspond to PDI

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3 deposition. Note: each water contact angle data point is an average of three measurements and the maximum standard deviation was 4 degrees. A salient feature of supramolecular self-assembly is that the constructed systems can be modulated via external stimuli. Since the β-CD-adamantane interaction is largely hydrophobic in nature,24 the addition of an organic solvent should attenuate the interactions resulting in disassembly of the monolayers. Indeed, when only one PDI monolayer is deposited, this PDI layer could be readily removed by washing with a DMF solution, leaving a re-usable porphyrin 1 layer on the surface (Figure 5a). Additional layers of assembly were, however, more difficult to remove completely (Figure 5b). We postulate that the reason for this maybe that the selfassembled layers are rather robust since in addition to host-guest interactions that act to hold the different layers together, within each layer the PDIs are involved in significant π-stacking.

Figure 5. UV-vis absorption profiles for various stages of the LBL assembly. (a) Black: porphyrin 1 attached to the glass substrate. Red: upon deposition of the 1st layer of PDI 2. Blue: upon washing the assembled layer with DMF. Pink: Re-deposition of PDI 2 after the DMF wash. (b) Black: porphyrin 1 attached to the glass substrate. Red: upon deposition of the 1st layer of

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PDI 2. Blue: upon deposition of the 1st and second PDI layers (i.e., PDI 2 + PDI 3). Pink: upon washing the assembled PDI bilayer with DMF. Green: after the DMF wash and re-deposition of PDI 2. Orange: after the DMF wash and re-deposition with PDIs 2 and 3.

In an effort to determine the thickness of the multilayer thin films, atomic force microscopy (AFM) was used. A razor was first employed to create a scratch on the glass substrate containing the multilayers and the thickness of the film was determined by scanning across the scratch with the AFM tip using the tapping mode. The thickness of a four layer thin film (i.e.; the porphyrin 1/alkyl siloxane layer + PDI 2 + PDI 3 + PDI 2) was found to be ca. 10 nm (SI, Figure S13) whilst an eight layer film exhibited a thickness of ~ 20 nm (SI, Figure S14). From these results we estimate that the thickness per layer is ca. 2.5 nm. The thickness was also verified by spectroscopic ellipsometry.48 The ellipsometry data as a function of the number of layers (SI, Figure S15) also gave a calculated thickness of 2.5 nm per layer. Importantly, the molecular lengths of free porphyrin 1-alkyl siloxane, PDI 2, and PDI 3 (SI, Figure S16) range between 2.5 to 3.4 nm. Thus it is reasonable to postulate that the non-covalent LBL growth of the thin film exploits monolayers of PDIs 2 or 3, respectively.

Conclusions In conclusion, we have designed a LBL fabrication technique that, utilizes water as the immersion solvent, to construct a multilayer thin film that absorbs across a large portion of the visible spectrum as it is functionalized with nineteen PDI monolayers placed on top of a scaffolding porphyrin monolayer. The resultant film is well-ordered with the porphyrin scaffold oriented close to parallel to the substrate whilst the PDI units are aligned closer to the surface

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normal. The multilayer thin films are self-assembled via host-guest interactions based on the βcyclodextrin/adamantane molecular recognition couple in addition to aromatic stacking interactions of the PDI units. Importantly, washing with DMF leads to clean dis-assembly of a thin film containing only one PDI monolayer and indicates the possibility of reusability at this monolayer level. This work is expected to be of broad interest for researchers developing wellorganized chromophoric materials that have applications in the next generation of electronic and optical devices/materials that are prepared using a sustainable and cost-effective solvent.

Experimental Section 1. General Experimental All chemicals were purchased from Sigma-Aldrich or TCI America and solvents were obtained from Fisher Scientific. Glass slides were purchased from Delta Technologies. NMR spectra were recorded on a Varian 400 MHz spectrometer in (CD3)2CO, CDCl3, (CD3)2SO or CF3COOD solvents. High resolution mass spectrometry was performed at the Bioanalytical Mass Spectrometry Facility at Georgia Institute of Technology or the Mass Spectrometry Facility at Indiana University. UV-vis spectra were undertaken using a Varian Cary Eclipse spectrophotometer. Water contact angle measurements were performed on a Rame-Hart model 100 goniometer with a 5 µL drop of DI water. The reported water contact angle measurements are an average of three data points.

2. Synthesis of zinc porphyrin 1. (a) Synthesis of 3-hydroxy-5(prop-2-yn-1-yloxy) benzaldehyde (4)

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Propargyl bromide (2.39 ml, 25 mmol, 80% v/v solution in toluene) was slowly added to a suspension of 3,5-dihydroxybenzaldehyde (2.76 g, 20 mmol) and K2CO3 (2.76 g, 20 mmol) in 100 ml of acetone. The reaction mixture was then refluxed under argon for 6 hr. After cooling to room temperature, water (100 ml) and dichloromethane (100 ml) were sequentially added to the reaction mixture. Subsequently the organic layer was isolated and the aqueous layer was extracted a second time with another aliquot of dichloromethane (100 ml). The organic layers were combined and dried with MgSO4 and the solvents were removed under reduced pressure. The crude residue was purified on a silica column, using hexane/ethyl acetate (5/1) as the eluent, to afford yellow solid 4 (1.2 g, 35%). 1H NMR (400 MHz, (CD3)2CO, 298 K): δ = 9.91 (s, 1H), 9.01 (s, 1H), 7.05 (s, 1H), 7.03(s, 1H), 6.78(s, 1H), 4.85 (s, 2H), 3.13 (s, 1H);

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C NMR (100

MHz, (CD3)2CO, 298 K): δ = 192.5, 160.3, 159.9, 139.8, 110.3, 109.1, 107.4, 79.3, 77.4, 56.5; HRMS (EI): calcd m/z: 176.0473 [C10H8O3] +: found: 176.0468.

(b) Synthesis of adamantane tethered benzaldehyde (5) A solution of 4 (1 g, 5.7 mmol) and 1-azidoadamantane (1.64 g 10 mmol) in THF (50 ml) was first prepared in 250ml round bottle flask. The flask was sealed and bubbled with argon for 0.5 hr. Then a solution of CuSO4 5H2O (14 mg dissolved in 1 ml water), and a solution of sodium ascorbate (47 mg dissolved in 1 ml water) were injected to the reaction sequentially. The reaction was charged with argon and left to stir at 40 ºC for 24 hr. After cooling to room temperature, water (50 ml) and dichloromethane (50 ml) were added to the reaction. Subsequently the organic layer was isolated and the aqueous layer was extracted a second time with another aliquot of dichloromethane (50 ml). The organic layers were combined and dried over MgSO4 and the solvent was removed under reduced pressure. The crude residue was purified on a silica column,

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using hexane/ethyl acetate (3/1) as the eluent, to afford 5 as a white solid (1.6 g, 82 %). 1H NMR (400 MHz, (CD3)2CO, 298K): δ = 9.91(s, 1H), 8.97 (s, 1H), 8.14 (s, 1H), 7.10 (s, 1H), 7.00 (s, 1H), 6.83 (t, J = 2 Hz, 1H), 5.21(s, 2H), 2.27-2.24 (m, 9H), 1.82 (s, 6H);

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C NMR (100 MHz,

(CD3)2CO, 298 K): δ = 192.7, 161.4, 160.1, 143.1, 139.9, 121.3, 109.7, 109.1, 107.7, 62.9, 60.2, 43.6, 36.6, 29.3; HRMS (ESI): calcd m/z: 354.1812 [C20H23N3O3H]+: found: 354.1802.

(c) Synthesis of benzaldehyde flanked with adamantane and alkyne arms (6) Propargyl bromide (239 ul 2.5 mmol, 80% v/v solution in toluene) was slowly added to a suspension of 5 (706 mg, 2 mmol) and K2CO3 (276 mg, 2 mmol) in 40 ml of acetone. The reaction mixture was then refluxed under argon for 8 hr. After cooling to room temperature, water (100 ml) and dichloromethane (100 ml) were added to the reaction. Subsequently the organic layer was isolated and the aqueous layer was extracted a second time with another aliquot of dichloromethane (100 ml). The organic layers were combined and dried over MgSO4 and the solvent was removed under reduced pressure. The crude residue was purified on a silica column, using hexane/ethyl acetate (2/1) as the eluent, to afford 6 as a white solid (711 mg, 91%). 1H NMR (400MHz, CDCl3, 298K): δ=9.85(s, 1H), 7.68(s, 1H), 7.11(s, 1H), 7.05(s, 1H), 6.84(s, 1H), 5.16(s, 2H), 4.67(s, 2H), 2.51(s, 1H,), 2.19(s, 9H), 1.74(s, 6H);13C NMR (100MHz, CDCl3, 298K): δ=191.7, 160.1, 159.1, 142.1, 138.4, 119.6, 108.8, 108.6, 108.3, 77.9, 76.3, 62.5, 59.9, 56.2, 42.9, 35.9, 29.4; HRMS (ESI): calcd m/z: 392.1969 [C23H25N3O3H]+: found: 392.1956.

(d) Synthesis of tetra-adamantane porphyrin (7)

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To a solution of 6 (586 mg, 1.5 mmol) in propionic acid (60 ml) was heated till reflux, freshly distilled pyrrole (107 ul, 1.5 mmol) was added. The reaction mixture was maintained under reflux for 3 hr. Subsequently the propionic acid solvent was evaporated under reduced pressure and triethylamine (1 ml) was then added to the resultant residue. After completely evaporating the solvents, the crude residue was purified on a silica column using dichloromethane as the eluent to yield purple solid 7 (394 mg, 15%).1H NMR (400 MHz, CDCl3, 298K): δ=8.92 (s, 8H), 7.75 (s, 4H), 7.49 (s, 8H), 7.08 (s, 4H,), 5.35 (s, 8H), 4.85 (s, 8H), 2.59 (s, 4H), 2.17-2.25(br s, 36H), 1.68-1.79 (br s, 24H), -2.88 (s, 2H);

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C NMR (75MHz, CDCl3, 298K): δ=157.8, 157.1,

144.2, 142.9, 131.3, 119.7, 119.5, 115.6, 115.2, 102.1, 78.6, 76.1, 62.8, 59.9, 56.4, 43.2, 36.1, 29.6; HRMS (MALDI-TOF): calcd m/z: 1756.8489 [C108H106N16O8H] +: found: 1756.8364.

(e) Synthesis of Zinc tetra-adamantane porphyrin 1 To a solution of 7 (30 mg, 0.017 mmol) in chloroform (15 ml), a solution of zinc acetate (0.1 g, 5.6 mmol) in methanol (1.5 ml) was added. The mixture was stirred at room temperature for 3 hr and then was washed with water and dried over anhydrous MgSO4. The solvents were removed via reduced pressure to yield purple solid 1 (29mg, 93%). 1H NMR (400 MHz, (CD3)2SO, 298 K): δ =8.85 (d, 8H, J = 4Hz), 8.35(s, 4H), 7.43 (br m, 8H), 7.12(s, 4H), 5.31(s, 8H), 4.98(s, 8H), 3.72(s, 4H), 2.17(s, 36H), 1.76(s, 24H);

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C NMR (125 MHz, CDCl3, 298K):

δ= 157.2, 156.6, 149.9, 145.7, 142.0, 131.7, 120.0, 119.1, 115.4, 101.4, 78.8, 76.0, 61.6, 59.7, 56.3, 42.8, 36.9, 29.4; HRMS (MALDI-TOF): calcd m/z: 1817.7593 [C108H104N16O8ZnH] +: found: 1816.7625

3. Synthesis of PDI 3.

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(a) Synthesis of Precursor PDI 8 In a round bottom flask, a mixture of 0.350 g (0.89 mmol) of 3, 4, 9, 10perylenetetracarboxylic dianhydride and 0.500 ml (3.56 mmol) of N, N-diethyl ethylene diamine were suspended in 5 ml of DMF. The mixture was heated at 80 ºC overnight under argon. Upon cooling to room temperature, 50 ml of acetone was added and the precipitate was allowed to form overnight. The precipitate was then filtered using a medium porosity fritted glass filter and washed with acetone three times. The product was dried under vacuum and then suspended in 3% (w/w) KOH solution for 3 hr to remove unreacted perylenetetracarboxylic dianhydride. Finally, the precipitate was then again filtered through a medium porosity fritted glass filter and washed with distilled water. The solid recovered was dried under vacuum at 65 ºC overnight to yield 0.446 g (0.76 mmol, 85%) of 8. 1H NMR (400 MHz, CF3COOD, 298 K): δ = 8.77-8.73 (m, 8H), 4.71 (br s, 4H), 3.63-3.43 (m, 12H), 1.39 (br s, 12H); 13C NMR (100 MHz, CF3COOD, 298K) δ = 168.4, 138.8, 135.6, 131.7, 128.7, 126.8, 123.7, 54.4, 51.2, 38.6, 9.6; HRMS (ESI): calcd m/z: 589.2815 [C36H36N4O4H]+: found: 589.2806.

(b) Synthesis of PDI 9 0.30 g of 8 (0.51 mmol) was placed in a round bottom flask and to this flask was added 5 ml of propargyl bromide (44.89 mmol). The mixture was refluxed for 72 hr at 65 ºC under argon gas. The product was suspended in 100 ml of acetone and the solid mass was collected using a medium porosity fritted glass filter, then dried under vacuum at 65 °C overnight. The solid mass was then dissolved in distilled water and filtered using a medium porosity fritted glass filter. The filtrate was collected and dried by rotary evaporation to yield 0.34 g (0.41 mmol, 80 %) of PDI9. 1H NMR (400 MHz, CF3COOD, 298 K): δ = 9.02-8.55 (br s, 8H), 4.82 (br s, 4H), 4.30 (s,

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4H), 4.0-3.51 (m, 12H), 2.87 (s, 2H), 1.79-1.36 (br s, 12H);

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C NMR (100 MHz, CF3COOD,

298 K) δ 167.7, 138.4, 135.5, 131.6, 128.5, 126.8, 124.0, 83.9, 70.6, 57.6, 55.9, 51.4, 36.5, 8.8; HRMS (ESI): calcd m/z: 333.1598 [C42H42N4O4H]2+: found: 333.1552.

(c) Synthesis of PDI 3 A mixture of PDI 9 (50 mg, 0.06 mmol) and 1-azido adamantane (42 mg, 0.24 mmol) were suspended in a 1:1 mixture of water and tert-butyl alcohol (4 ml). The mixture was degassed with argon for 15 minutes. To this solution was introduced a freshly prepared (0.1 ml) aqueous solution of sodium ascorbate (0.24 mmol), followed by the addition of copper (II) sulfate pentahydrate (15 mg, 0.06 mmol). The reaction was stirred under argon at 50 °C overnight, after which acetone was added and the precipitate was filtered through a medium porosity fritted glass filter and washed with a 1:1 mixture of water and acetone. The solid residue was dried under vacuum at 40 ºC overnight to yield 49.8 mg (0.04 mmol, 70%) of PDI 3. 1H NMR (400 MHz, CF3COOD, 298 K): δ = 9.20 (s, 2H), 8.86-8.84 (m, 8H), 5.24 (br s, 4H) 4.94 (br s, 4H), 4.0-3.58 (m, 12H), 2.60-1.36 (m, 42H);

13

C NMR (100 MHz, CF3COOD, 298 K) δ 167.8, 138.9, 135.6,

132.0, 131.8, 131.4, 128.8, 126.8, 123.8, 70.5, 57.9, 56.0, 52.1, 44.3, 36.8, 36.2, 31.9, 8.7; HRMS (ESI): calcd m/z: 510.2869 [C61H72N10O4] 2+: found: 510.2885.

4. Multilayer thin film fabrication. (a) Azide terminated self-assembled monolayer formation on glass substrate (adapted from Ref39). Prior to use, glass slides (7x50x0.7mm) were sonicated in dilute aqueous solution of Alconox for 10 min, rinsed with water, acetone, dichloromethane and lastly water. Then the glass slides

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were immersed in concentrated sulfuric acid for 30 min to generate the hydroxide surface. The slides were then washed with DI water and dried under a stream of air. Next, the cleaned slides were immersed in a solution of 1% 11-bromoundecyltrichlorosilane (v/v) in anhydrous dichloromethane for 1 hr. This was followed by washing with dry dichloromethane, ethanol and DI water and drying under a stream of air. To substitute the bromide functional group with a terminal azido group, the glass slides were submerged in a saturated solution of NaN3 in DMF under 90 ºC for 48 hr, then rinsed with DI water, and dried under a stream of air.

(b) Fabrication of a covalent monolayer composed of porphyrin 1. The azide functionalized glass slides were immersed in a solution of porphyrin 1 (30 uM) in 18 ml THF in a vial with septum. Then the vial was bubbled with argon for 0.5 hr. Next a solution of CuSO4 5H2O (8 mg dissolved in 0.5 ml water), and a solution of sodium ascorbate (32mg dissolved in 0.5 ml water) were injected to the vial sequentially. The reaction was charged with argon and left to stir at 40 ºC for 24 hr. After that, the slides were washed and sonicated in acetone and DI water, and dried under a stream of air.

(c) Layer by layer deposition of PDIs 2 and 3 to form a multilayer assembly. The porphyrin 1 modified glass substrate was placed into a solution of PDI 2 (5 uM) in H2O for 10 min, followed by washing with copious amounts of DI water, and drying under a stream of air. Then the slide was submerged in a solution of PDI 3 (5 uM) in 1% HCOOH/H2O for 10 min, followed by washing with copious amount of DI water, and drying under a stream of air. The multilayer film was constructed by repeating both these deposition steps in an alternating manner.

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Fax: (+1) 504-865-5596. Tel: (+1)504-862-3580. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was conducted using funds from the National Science Foundation (Grant CHE1112091 to JJ). Notes The authors declare no competing financial interest ACKNOWLEDGMENT We would like to thank Qi Zhao for his assistance with NMR experiments. We also gratefully acknowledge Baraka S Lwoya and Chunlei Yue for their assistance with AFM. Supporting Information Supplementary data concerning conventional and polarized UV-vis spectroscopy as well as AFM and spectroscopic ellipsometry are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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Octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin Studied by Ultraviolet−Visible and Infrared Spectroscopies. Langmuir 1997, 13 (16), 4422-4427. 48. Palomaki, P. K. B.; Krawicz, A.; Dinolfo, P. H. Thickness, Surface Morphology, and Optical Properties of Porphyrin Multilayer Thin Films Assembled on Si(100) Using Copper(I)Catalyzed Azide−Alkyne Cycloaddition. Langmuir 2011, 27 (8), 4613-4622.

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