Magnesium porphine supermolecules and two-dimensional

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New Concepts at the Interface: Novel Viewpoints and Interpretations, Theory and Computations

Magnesium porphine supermolecules and two-dimensional nanoaggregates formed using Langmuir-Schaefer technique Larissa Alexandrovna Maiorova, Nagao Kobayashi, Sergei Vitalievich Zyablov, Victor A. Bykov, Sergei Nesterov, Aleksei V. Kozlov, Charles H. Devillers, Alexandr Vladimirovich Zavyalov, Viktor Veniaminovich Alexandriysky, Mario Orena, and Oskar I. Koifman Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00905 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Magnesium porphine supermolecules and twodimensional nanoaggregates formed using Langmuir-Schaefer technique Larissa A. Maiorova†*, Nagao Kobayashi‡, Sergey V. Zyablov†, Victor A. Bykov‫׀‬, Sergei I. Nesterov‫׀׀‬, Aleksei V. Kozlov†,+, Charles H. Devillers†, Alexandr V. ZavyalovV†, Viktor V. Alexandriysky†, †

§

& Oskar I. Koifman†,++

Research Institute of Macroheterocyclic Compounds, Ivanovo State University of Chemistry

and Technology, 153000 Ivanovo, Russia ‡

‫׀‬

Faculty of Textile Science and Technology, Shinshu University, Tokida, Ueda 386-8567, Japan NT-MDT, Moscow, 124482 Zelenograd, Russia

‫׀׀‬

F.V. Lukin State Research Institute of Physical Problems, Moscow, 124460 Zelenograd, Russia

+

Institute of Problems of Chemical Physics of Russian Academy of Sciences (IPCP RAS),

Academician Semenov avenue 1, Chernogolovka, Moscow region, 142432 Russian Federation (Present address) † ICMUB UMR 6302, CNRS, Univ. Bourgogne Franche-Comté, F-21000 Dijon, France §

Department Di.S.V.A. - Polytechnic University of Marche, 60131 Ancona, Italy

++

G.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, 153045 Ivanovo,

Russia (Second address)


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*E-mail: [email protected] KEYWORDS: Air-water interface, 2D M-nanoaggregates, supermolecules, magnesium porphine, photophysical properties.

ABSTRACT: Porphyrins are functional elements of important biomolecules, whose assemblies play a central role in fundamental processes such as electron transfer, oxygen transport, enzymatic catalysis and light-harvesting. Here we report an approach to formation of porphyrin supermolecules, particular type of nanoparticles with unusually strong noncovalent intermolecular interactions. Key differences of the supermolecules from noncovalent nanostructures described earlier are (1) supermolecules consist of molecules of the same type without side groups promoting the self-assembly and without any spacers; no surfactant or catalyst to assist the process is needed (2) they exhibit unusual photophysical properties and remain stable even in organic solvents. Their formation occurs under specially selected conditions at the air-water interface at room temperature. Following this route, we have formed supermolecules of magnesium porphine, a functional element of chlorophyll. The properties of these supermolecules are markedly different from those of the constituent molecules: for example, in contrast to the pink color of the monomer solution, solutions of supermolecules are transparent for visible light and absorb in the ultraviolet and near-infrared regions. We also present AFM visualization of the porphyrin two-dimensional nanoaggregates forming at airwater interface which were predicted in our previous works. The present approach offers a guideline for the discovery of new supermolecules, including complex biological ones, and the formation of supermolecular materials with novel properties.


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1. INTRODUCTION The design of porphyrin nanostructures, including self-assembled architectures, has been mainly focused on the artificial reproduction of organization and functions of natural assemblies. It was shown that the properties of the nanostructures, in particular the energy transfer rate between adjacent molecules, are strongly affected by the structure of the ensemble. This makes them promising for the use of these materials to convert solar energy into chemical or electrical energy in, e.g., photocatalytic applications, sensors and organic photovoltaics.1–9 To date, diverse porphyrin architectures have been prepared in three-dimensional (3D) media.10-14 Low-dimensional molecular nanostructure arrays are obtained on solid surfaces by using two-dimensional (2D) templates, exploiting surface-assisted covalent bonding15-19 or various intermolecular noncovalent interactions.18, 20-25 Self-assembled nanostructures arise from molecules which bear side groups promoting the process, such as through weak van der Waals, π−π, electrostatic, hydrophobic and hydrogen bonding interactions.26,27 Self-assembly, accomplished by the formation of coordinative bonds between the electron-accepting and the electron-donating atoms in spin-coated films, has also been reported.28 Nanostructures and supramolecular polymers were constructed via diverse self-assembly strategies29-35 including self-assembly protocol, wherein different kinds of surfactants are employed for a controllable assembly. 35-40 Furthermore, while self-assembled architectures were obtained at liquid-solid and liquid-liquid interfaces41,42, synthesis of supermolecular porphyrin – spacer – porphyrin heterostructures was also reported.43 Further, J- and H-aggregation of some porphyrins were detected at the water surface by using various spectroscopic44-47 and potential-induced Jaggregation at liquid-liquid interface was reported.48


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In our previous studies, we demonstrated that supramolecular design at the air-water interface with controlled self-assembly of organic compounds in 2D and 3D nanostructures becomes possible if one has quantitative information on the structure of the floating layer. Studies of floating layers of azaporphyrins and their analogs showed that the size, morphology and stability of nanoaggregates are defined not only by the molecular structure of the macroheterocycle (the presence of side substituents, their nature, length and position, the size of the macrocycle), but also by the layer formation conditions.49,50 Depending on the initial conditions during film formation, the same compound may exhibit both edge-on and face-on orientations at the water surface. A novel concept of structuring of organic compounds at air-water interface and a model of a Langmuir layer, as the layer consisting of two-dimensional major nanoaggregates with diameters in the range of 5-20 nm (so-called M-nanoaggregates) were presented.51 A method for determining its structure and properties at the quantitative level was also developed,51 and applied to phthalocyanines and porphyrins.52–54 Accordingly, the boundaries of existence of the monolayer state and some characteristics of the monolayer were determined, such as the size of nanoaggregates formed in the layer, the number of molecules in the nanoaggregates, the interaggregate spacing, the water content in a layer, and layer compressibility. The model enabled the explanation of a lot of experimental data inconsistent with prior models where the layer was considered as consisting of randomly positioned individual molecules. However, the agreement of the theoretical description of the nanostructured layer with direct experimental data has not yet been attained. In addition, the first two- and four-stacked 3D nanoaggregates of substituted phthalocyanine55 and porphyrazine (which were assigned as supermolecules56–58) were detected in Langmuir-Schaefer (LS) films by using small-angle X-ray scattering. Further, we hypothesized that purposeful formation of stable porphyrin supermolecules at the air-water


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interface is possible, even from molecules where substituents are missing, due to specific conditions for intermolecular interactions within M-nanoaggregates having specially selected structures. Thus, unsubstituted magnesium(II) porphine (MgPor), the parent of all porphyrins and the functional element of chlorophyll, has been used to examine this hypothesis. Here, we report two main results. (1) The agreement of the theoretical description of the monolayer consisting of two-dimensional nanoaggregates of MgPor with the experimental data. (2) An approach to formation of the particular type of nanoparticles – M-supermolecules at the air-water interface which display the following significant features. They consist of molecules of the same type, without any spacers, while no surfactant is used to assist the process. Moreover, they are obtained starting from molecular building blocks in which side groups promoting the noncovalent self-assembly are missing. The formed supermolecules of MgPor are very stable and exhibit optical properties radically different from those of the parent molecules. In order to investigate stability, photophysical properties and the evolution of the supermolecular entities, experiments on floating layers have been combined with a study of LS-films arising from the layers, as well as of solutions of the films. The study of the compression of the layers at the airwater interface is supplemented by AFM microscopy, as well as UV-Vis, near IR, fluorescence, MALDI-TOF and NMR spectroscopy of LS-films and their solutions.

2. EXPERIMENTAL SECTION

2.1. Formation of the floating at air-water interface layers, thin films on solid support and solution of the film. To produce a floating layer, the solution of the magnesium(II) porphine in dichloromethane (DCM, С = 2,28 10-4 M) was deposited onto the surface of twice distilled water in the Langmuir trough (NIMA) at (20±1) C. Then 15 min after the deposition


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the layer was compressed at speed varied from 19.2 to 4.4 Å2 per molecule per minute depending on the initial degree of water surface coverage by the porphyrin, cedge, which was varied from 8 to 35%; the accuracy of determination of the area per molecule A is 2%. The surface pressure was measured with the accuracy of 0.02 mN m-1. Films on solid supports, including the monolayer film, were prepared using the horizontal lift (Langmuir-Schaefer) method by transferring floating layers onto a quartz or silicon plate under conditions represented by point 1 and point 2 in Figure 1. Solutions of the film with 6 polylayers each consisting of 3 edge-on monolayer decks of MgPor in dimethyl sulfoxide (DMSO) and in DMSO-d6 were prepared and used in photophysical and spectrometric characterization. Estimated concentration of the solutions was 1·10-5 M for DMSO and 5·10-5 M for DMSO-d6.

2.2. Materials and molecular modelling. Magnesium(II) porphine monomer (Figure S1) was synthesized (according to Lindsey’s procedure59) by Charles H. Devillers. Dichloromethane and dimethyl sulfoxide were purchased from Sigma-Aldrich and DMSO-d6 from ABCR GmbH & CO. KG, respectively. The energy-minimized structure of compound under study (Figure S1) was calculated using HyperChem 7.01 software exploiting MM+ calculation procedure.

2.3. Method for quantitative analysis of compression isotherms. The structure of the floating layers was analyzed within the framework of an original model and method for quantitative analysis of compression isotherms of nanostructured M-monolayers.51,52 Details can be found in Supporting Information (SI-1). Characteristics of monolayers were calculated as described in Supporting Information (SI-2) and in previous works.52–54 The data were analyzed by using the Origin 7.5 software.


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2.4. Thin film and film solutions characterization. AFM (atomic force microscopy) images of magnesium porphyrin film on silicon were obtained in ambient conditions by AFM (NTEGRA Prima microscope, NT-MDT, Russia) working in a tapping mode. A rectangular silicon probe (NSG01, NT-MDT, Russia) with 150 kHz resonant frequency and 3 N/m force constant was used. The AFM images were obtained with a scanning range of 1 µm x 1 µm and usual scanning frequencies in the range of 0.7–1.0 Hz per line. All the images were flattened by using the analysis software provided with the AFM instruments. Electronic absorption spectra were recorded with an UV/VIS Lambda 20 spectrophotometer with the wavelength accuracy of ±0.1 nm, wavelength reproducibility of ±0.05 nm and photometric accuracy ±0.003. Photoluminescence (PL) spectra were recorded with a Solar CM2203 spectrofluorimeter and near infrared (NIR) spectra with Shimadzu UV-3600 UV-VISNIR-Spectrometer. Mass spectrometric analyses were performed in the reflectron/linear, positive mode at +20 kV accelerating potential with a time of flight mass spectrometer (Shimadzu AXIMA Confidence; Kratos Analytical Ltd., Manchester, UK). Mass spectral data sets were acquired using MALDIMS Application Shimadzu Biotech Launchpad v.2.9 software (© Kratos Analytical Ltd.) in the range of m/z 200–2,000 with 200-400 laser shots per spectrum. Samples were prepared by mixing a matrix and solutions of MgPor (in different forms) in DCM. Fresh matrix solution of 10 mg/mL DHBA (2,5-dihydroxybenzoic acid) in an aqueous solution of 50% (v/v) acetonitrile, 0.1% (v/v) trifluoroacetic acid (TFA) using ultrahigh purity water was prepared daily. The 1H NMR (proton nuclear magnetic resonance) spectra of MgPor monomer and film solutions in dimethyl sulfoxide-d6 were recorded on a Bruker Avance III 500 spectrometer


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(Bruker Biospin AG, Germany, Rheinstetten, 500.17 MHz for 1H) at 294 K. A 5 mm 1H/31P/DBB z-GRD Triple Resonance Broad Band Probe (TBI) was used.

3. RESULTS AND DISCUSSION 3.1. Formation and characterization of nanostructured layers of magnesium porphyrin at air-water interface. Two-dimensional M-nanoaggregates of MgPor have been formed at the air-water interface at the initial degree of coverage of the water surface by MgPor molecules, cedge, varying within the 8%–35% range. A dichloromethane solution of MgPor has been used to produce the floating layer. A quantitative analysis of the compression isotherms has shown that stable monolayers of different structure with edge-on arrangements of MgPor molecules in nanoaggregates can be formed (Figure S2 and Table S1). Dependences of the key parameters of the monolayers (such as the nanoaggregate size, number of constituting molecules etc.) on cedge have been studied. A theoretical description of MgPor monolayer was performed and a quantitative model for the monolayer was built (Equations S1-S13). Using the data obtained, the conditions appropriate for forming films on a solid support have been selected. The films have been formed by transferring the layers having the maximum number of molecules in M-nanoaggregates and almost the highest density of the latter. These conditions are met by layers formed at cedge = 35%, which are stable in the 0.18

Magnesium porphine supermolecules and two-dimensional

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