Adsorption, Ordering, and Metalation of Porphyrins on MgO Nanocube

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Adsorption, Ordering and Metalation of Porphyrins on MgO Nanocube Surfaces: The Directional Role of Carboxylic Anchoring Groups Johannes Schneider, Fabian Kollhoff, Torben Schindler, Stephan Bichlmaier, Johannes Bernardi, Tobias Unruh, Jörg Libuda, Thomas Berger, and Oliver Diwald J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08956 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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The Journal of Physical Chemistry

Adsorption, Ordering and Metalation of Porphyrins on MgO Nanocube Surfaces: the Directional Role of Carboxylic Anchoring Groups

Johannes Schneider1#, Fabian Kollhoff2#, Torben Schindler3, Stephan Bichlmaier1, Johannes Bernardi4, Tobias Unruh3*, Jörg Libuda2,5*, Thomas Berger1, Oliver Diwald1*

Address: 1

Chemistry and Physics of Materials, Paris Lodron University of Salzburg, Hellbrunnerstraße

34/III, A-5020 Salzburg, Austria 2

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg,

Egerlandstraße 3, D-91058 Erlangen, Germany 3

Lehrstuhl für Kristallografie und Strukturphysik, Friedrich-Alexander-Universität Erlangen-

Nürnberg, Staudtstraße 3, D-91058 Erlangen, Germany 4

University Service Center for Transmission Electron Microscopy, Vienna University of

Technology, Wiedner Hauptstrasse 8-10, A-1040 Vienna, Austria 5

Erlangen Catalysis Resource Center and Interdisciplinary Center for Interface-Controlled

Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, D-91058 Erlangen, Germany

Corresponding authors: [email protected], 0043 662 8044-5444 [email protected], [email protected] # These authors contributed equally to this work 1 ACS Paragon Plus Environment


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Abstract The understanding of porphyrin adsorption on oxide nanoparticles including knowledge about coverages and adsorbate geometries is a prerequisite for the improvement and optimization of hybrid materials. The combination of molecular spectroscopies with small angle X-ray scattering provides molecular insights into porphyrin adsorption on MgO nanocube dispersions in organic solvents. In particular we addressed the influence of terminal carboxyl groups on the adsorption of free base porphyrins, on their chemical binding, on the metalation reaction as well as on the coverage and orientation of adsorbate molecules. We compared the free base forms 5,10,15,20tetraphenyl-21,23H-porphyrin (2H TPP) with the carboxyl-functionalized 5,10,15,20-tetrakis(4carboxyphenyl)-21,23H-porphyrin (2H TCPP) and show that without carboxylic anchoring groups the free base form metalates on the nanocube surface and adopts a flat-lying adsorbate geometry. The saturation limit for flat-lying adsorption on nanocubes with an average edge length of 6 nm corresponds to 90 ± 14 molecules per particle. This limit is surpassed when 2H TCPP molecules attach via their terminal carboxyl groups to the surface. The resulting upright adsorption geometry suppresses self-metalation, on the one hand, and allows for much higher porphyrin coverages, on the other (at porphyrin concentrations in the stock solution of 210-2 molL-1). UV Vis Diffuse Reflectance results are perfectly consistent with conclusions from SAXS data analysis. The experiments reveal concentration dependent 2H TCPP coverages in the range between 0.4 to 1.9 molecules nm-2 which correspond to the formation of a shell of upright standing porphyrin molecules around the MgO nanocubes. In contrast, after adsorption and metalation of non-functionalized 2H TPP the resulting porphyrin shells are in the range of a tenth of a nanometer and thus too thin to be captured by SAXS measurements. Related insights advance our opportunities to prepare well-defined nanohybrids containing highly organized porphyrin films.

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Introduction The efficient functionalization of metal oxide nanostructures with organic and metal-organic dyes is a prerequisite to realize hybrid materials for photosensitization1,2, for (electro)chemical sensors3,4 or for molecular switches.5 Porphyrins have received increased attention as promising photosensitizers because of the variety of modifications through which their light absorption, chemical and electrochemical properties can be altered at the molecular level. For example, the optical and electronic properties of porphyrins can be tuned by attaching different functional groups to the porphyrin ring.6,7 Moreover, linker groups for binding to the surface can be attached to different regions of the molecules and - last but not least - the central metal atom can be varied.6,8,9 Carboxylic groups are often used to covalently anchor porphyrins to oxides.2,8,10 For planar 2D surfaces there is an advanced understanding of the impact of the these linking groups on the geometry of adsorbed porphyrins, their assembly and dynamics.11–17 Functional nanocrystalline materials such as photoelectrodes are much more complex material with respect to composition, structure, and microstructure. Being porous particle networks, they provide surfaces and interfaces in all three dimensions and proximity effects may come into play during adsorption.18 Although the chemical understanding of porphyrin adsorption and the knowledge on coverages and adsorbate geometries on particle systems are an important prerequisite to enhance the performance of these hybrid materials, there exists only little qualitative and essentially no quantitative information on porphyrin binding and associated surface chemistry. The rational development of functional hybrids connects to another key aspect in the utilization of porphyrins, namely to their organization in supra-molecular nanostructures.19 Diverse shapes of assembled porphyrin particles can be adjusted just by the solvent composition.20 The surfactant-assisted hierarchical self-assembly involving oil water interfaces can be used to generate a variety of nanostructures, including hollow nanospheres, solid nanospheres, 3 ACS Paragon Plus Environment


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nanotubes, nanorods, and nanofibers.21,22 While these approaches are performed in homogeneous liquid phase, the controlled assembly of porphyrins on high surface area materials such as nanoparticle networks18 or monolithic porous solids23 opens an important opportunity to prepare well-defined nanohybrids with 2D crystal phases or crystalline porphyrin nanofilms.24,25 The way that porphyrins adsorb to the metal oxide surface does also determine its subsequent surface chemistry.26 In recent studies we identified porphyrin metalation reactions on highly dispersed MgO particles of cubic shape leading to the complexation of Mg cations at the oxide surface and, thus, to the metalation of the porphyrin. Importantly, it turned out that the surface cations of lowest coordination (such as 3- and 4-fold coordinated Mg2+ ions in MgO nanocube corners and edges) show a high reactivity in the porphyrin metalation reaction, whereas surface cations of higher coordination (i.e. 5-fold coordinated Mg2+ ions in the (100) plane) do not. This observation holds for the reaction of cubic shaped MgO particles of different sizes with 2H TPP molecules at both the oxide/ vacuum27 and the oxide/ toluene interface.28

Figure 1: MgO nanocube powders (TEM images and digital micrograph) and different porphyrin types in organic solution. The difference in color arises from changes in the metalation state of the porphyrins.


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We subjected different porphyrin derivatives, with carboxyl groups on each of the four phenyl rings (2H TCPP and Mg TCPP) and without (2H TPP and Mg TPP) and performed a comparative adsorption study (Figure 1). Surface attachment and anchoring of these molecules can occur either via a metalation reaction involving the central region of the molecule or via the carboxyl groups at its periphery. Specifically, 2H TCPP contains four carboxylic acid groups at the para position of the phenyl rings. These functional groups can act as linkers and covalently bind to metal oxide surfaces. Bonding may take place via one or more carboxylic acid groups which allows for different molecule orientations ranging from flat lying to upright standing on the particle surface. In the present study we combine a multitude of methods to explore the relation between anchoring, structure formation and metalation: using UV/Vis spectroscopy we quantitatively investigated 2H TPP and 2H TCPP adsorption on MgO nanocubes and determined adsorbate coverages as a function of the porphyrin concentration in solution. At the same time analysis of the porphyrin specific Q-band region revealed information about whether metalation does occur or not. Independent from optical spectroscopy we obtained consistent information from Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements, where we specifically tracked the complexation of surface Mg2+ ions by 2H TPP via the loss of the IR active ν(NH) band. The covalent bonding of 2H TCPP via carboxylic linker groups is monitored via the depletion of ν(C=O) bands and the emergence of a νas(OCO) band. Small angle X-ray scattering (SAXS) measurements provided complementary and consistent information on porphyrin coverage and the assembly of the film on the surface of the MgO nanocubes. Experimental Section Material synthesis and activation



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MgO nanocube powders were prepared by chemical vapor synthesis (CVS).29 The CVS reactor consists of two concentrical quartz glass tubes placed inside a cylindrical furnace. The inner tube hosts ceramic ships containing Mg pieces (99.98 %, Alfa Aesar). Heating to 913 K guarantees a metal vapor pressure of 1 mm Hg column (1.33 mbar). An argon stream (Ar 5.0) transports the metal vapor from the evaporation zone to the end of the inner glass tube. At this position the argon/ metal vapor mixture gets in contact with molecular oxygen from the outer glass tube. The exothermic oxidation reaction gives rise to a bright flame in the reactor and MgO nanoparticles form after homogeneous nucleation in the gas phase. Stable process conditions are guaranteed by the spatial separation of the Mg evaporation and oxidation zone. Furthermore, continuous pumping keeps the residence time of resulting nuclei within the flame short and prevents substantial coarsening and coalescence. After production, the MgO nanocubes were transferred into quartz glass cells, which allow thermal activation of the powder in high vacuum (p < 10-5 mbar) as well as in defined gas atmospheres. Cleaning of the as-obtained MgO powders and removal of organic contaminants is carried out by heating to 1123 K at a rate of 10 K min-1 in high vacuum (p < 10-5 mbar) and subsequent exposure to molecular oxygen at this temperature. Then, at p < 5 · 10-6 mbar the sample temperature was raised to 1173 K and kept for 1 h at this temperature until full dehydroxylation of the sample surface was achieved.30 The activated MgO nanocubes are characterized by a narrow particle size distribution below 10 nm. Adsorption experiments For porphyrin adsorption we followed a protocol that is outlined in detail elsewhere.28 A defined mass of 5,10,15,20-Tetrakis(4-carboxyphenyl)-21,23H-porphyrin, 2H TCPP, (Porphyrin Systems, 97.0 %) was dissolved in methanol (Sigma-Aldrich, ≥ 99.8 % no further purification). After 24 h of stirring approximately 400 mg of MgO nanocube powder was dispersed in 25 mL of the porphyrin solution and stirred for another period of 24 h. At higher concentrations, cTCPP > 110-3 6 ACS Paragon Plus Environment

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mol L-1, some remnant 2H TCPP crystals were found to continuously dissolve upon progressing adsorption. To prevent photochemical reactions the dye solution was kept in the dark during the adsorption process by wrapping the flask in aluminum foil. In a next step the suspension was centrifuged (8 min, 4000 min-1) and the MgO particles separated from the liquid phase. Then the powder was washed twice in fresh methanol to remove loosely bound porphyrin from the MgO nanocube surface. The porphyrin concentrations of the supernatant solutions were determined photometrically. The number of adsorbed porphyrin molecules was estimated from the porphyrin concentration difference between the pristine and the supernatant solutions. For powder studies the solid fraction was dried first using a membrane pump, then transferred into a quartz glass spectroscopic cell and, to remove remaining methanol adsorbates, pumped with a turbomolecular pump down to pressures below p < 510-6 mbar. Material characterization: structure, morphology and size distribution Before and after porphyrin adsorption X-ray diffraction (XRD) on the MgO powders was measured on a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ = 154 pm). Small amounts of the powders were cast on a carbon grid for investigation with a TECNAI F20 transmission electron microscope equipped with a field emission gun and an S-twin objective lens to check whether particle size and morphology are retained in the course of the powder processing step in methanol or toluene. Spectroscopy A Perkin Elmer Lambda 750 spectrometer was used to record UV/Vis transmission spectra of porphyrin solutions. UV/Vis diffuse reflectance (DR-UV/Vis) spectra of MgO powders where acquired using an integrating sphere. After porphyrin adsorption out of liquid methanol the samples were transferred to an optical high vacuum cell which allows for measurements either at pressures of p < 510-6 mbar or in defined gas atmospheres. All UV/Vis DR spectra were 7 ACS Paragon Plus Environment


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recorded in the presence of 100 mbar O2 to quench luminescence, which would give rise to false and negative absorption contributions in the spectrum. The IR spectra presented here were performed in DRIFTS mode using an open sample cup placed in a Praying Mantis DR accessory (Harrick). The IR spectrometer is a Bruker 80/v spectrometer. Spectra were recorded with a resolution of 2 cm-1 and 151 scans. The detector used to acquire the spectra was a liquid nitrogen cooled HgCdTe midrange detector in combination with a SiC globar. All windows in the beam path as well as the beamsplitter are manufactured from KBr. The sample chamber was evacuated or purged with dry air for at least 30 min before the measurements to allow pumping of atmospheric water and CO2. For the background we acquired a spectrum on non-functionalized MgO nanocube powder. TEM measurements The powders were analyzed by dipping a holey carbon grid into the powder and using a TECNAI F20 field emission TEM in order to investigate the structural features of materials sticking to the grid. Electron micrographs were recorded using a Gatan Orius CCD camera. SAXS measurements SAXS images were collected using the VAXSTER instrument which is equipped with a GaMetalJet D2 70 kV X-ray source (EXCILLUM, Kista, Sweden). The beam is focused by a 150 mm Montel optics (INCOATEC, Geesthacht, Germany) which extracts the Ga-Kα1,2 radiation with a wavelength of 1.34 Å and focuses it to the detector at about 3.5 m distance to the source. The beam size was set to 0.3 x 0.3 mm² by 2 double slit systems with low scattering blades. The beam path was evacuated except for 50 mm around the sample position, which was held at ambient conditions. A water-cooled, vacuum-tight Pilatus3 300 K device was used as detector, which was located at a sample-to-detector distance of 357 mm, resulting in a Q-range of 0.2 – 4 nm-1. Mica foils of 10 µm thickness were used as beam windows in the sample cell. The Q-scale 8 ACS Paragon Plus Environment

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was calibrated using a silver behenate standard sample with a d-spacing of 5.838 Å (Rose Chemicals Ltd.). For calibration of the absolute scale a glassy carbon sample kindly provided by the 15ID-D USAXS beamline at the Advanced Photon Source, Argonne, IL, was used.31 Each sample was measured for 600 s at room temperature and the transmission of the sample was determined from integrated intensity measurements (measuring time: 1 s) of the attenuated beam with and without sample in the beam and with the beamstop removed. The scattered intensity of isolated particles can be written in the small angle approximation by = ∆ ∙ ∙ ∙ , The contrast ∆ = − is defined by the difference of the scattering length densities (SLDs) of the particles ( ) and the solvent ( ), designates the particle number, the particle size distribution, the volume and , the form factor of the particles. For data analysis the program SASfit was used and a cubic core-shell model was implemented. For cubic nanoparticles the form factor is given by

, =

!"# % $

with = 2 % , where 2 is the edge-length of the cubes. For cubic core-shell structures the scattered intensity of a particle can be written as: IQ, R, ΔR, Δρ, , Δρ = -F/Q, R + ΔR1 ∙ Δρ ∙ VR + ΔR − FQ, R ∙ Δρ − Δρ, ∙ VR3

With Δρ,/ = ρ6789/:;9 310-3 molL-1), however, the supernatant solutions retain their originally purple color. The uptake curve shows a saturation limit which corresponds to an approximate number of 90 adsorbed porphyrin molecules per nanocube (Figure 3 a).28 UV/Vis measurements of the supernatant solutions reveal that the remaining porphyrin molecules in solution remain as a free base species. In strong contrast to 2H TPP adsorption, 2H TCPP is adsorbed quantitatively up to cTCPP = 510-2 molL-1, i.e. the maximum concentration we investigated (Figure 3 b). This is evident from the complete decoloration of the supernatant solutions for all concentrations studied. The maximum number of adsorbed 2H TCPP molecules can be as high as 1500 molecules per MgO nanocube (Figure 3 b) and shows a linear dependence on the concentration of initially dissolved porphyrin molecules. The residual concentration of porphyrin molecules in solution was found to be negligible, i.e. less than 1 % of the adsorbed molecules can be removed after four washing cycles at room temperature.



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Figure 3: Proposed adsorption geometries (i-iii) of non-metalated or metalated 2H TPP and 2H TCPP molecules on the MgO nanocube surface and uptake-curves for 2H TPP (a) and 2H TCPP (b) on MgO nanocubes. For 2H TPP adsorption the coverage saturates at approximately 90 molecules per nanocube. -2

-1

2H TCPP adsorbs quantitatively in the concentration range up to c = 510 mol L . This concentration corresponds roughly to 1500 molecules per nanocube.

It will be discussed in detail below, that the saturation limit of 90 2H TPP molecules per nanocube results from the space requirement of 2 nm2 per flat lying porphyrin molecule. For MgO nanocubes with an average length of 6 nm, 110 porphyrin molecules correspond to a monolayer of flat lying molecules (see (i) in Figure 3). Apart from the adsorbed and metalated porphyrin, further porphyrin molecules remain as 2H TPP in solution. In sharp contrast we did not find such saturation limit for 2H TCPP up to porphyrin concentrations as high as c = 510-2 molL-1. The corresponding number of 1500 adsorbed porphyrin molecules per nanocube requires a very different adsorption geometry and selfassembly (Figure 3 ii and iii). Possible geometries are discussed in detail below. 2) DRIFTS measurements In comparison to the DRIFTS spectra of pure 2H TPP (a) and Mg TPP (d) as reference, Figure 4 b and c show the spectra measured for MgO nanocube powders after contact with 2H TPP porphyrin solutions in toluene with concentrations of 310-3 and 210-2 molL-1, respectively. Both spectra of MgO nanocubes treated with 2H TPP solution are very similar. Consistent with the spectrum of Mg TPP (d), there are no features detected related to the ν(NH) vibration of 2H TPP. This observation indicates porphyrin metalation which is further corroborated by the changes in the characteristic bands related to skeletal porphin vibrations of Mg TPP. A more detailed assignment of corresponding bands based on literature and previous DFT calculations38–42 can be found in Table 1.


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Table 1: IR signals and assignments for 2H TPP and Mg T PP on MgO nanocubes

ωexp / cm–1 ωDFT / cm–1

ωexp / cm–1

ωexp / cm–1

2HTPP

MgTPP

2HTPP on MgO NC

Description

3311

3424

−

−

νas(NH)

1441

1429

1441

1439

Phenyl

1212/1222

1210

1201

1199

1176/1187

1183

1175

1178

967/982/

996/980/

995/ 1009

991/ 1009

1002

986

Pyrrole + Porphyrin backbone structure, splitting reduced by metal Pyrrole + Porphyrin backbone structure, splitting reduced by metal Porphyrin ring vibrations, shifted and reduced by metal

While DRIFTS is not an inherently quantitative method, trends can be observed at a semiquantitative level. Despite the fact that between the concentrations of 310-3 and 210-2 molL-1 there is a concentration difference by a factor of 7, the features in the spectra of Figure 3 b and d show comparable signal intensities. This observation provides additional evidence for the self-limiting adsorption of porphyrin on the nanocubes close to a porphyrin concentration of 310-3 molL-1.



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Figure 4: DRIFTS spectra of 2H TPP (a), Mg T PP (d) and MgO nanocube powders after contact with 310 3

-1

-2

-

-1

molL (b) and 210 molL 2H TPP (c) in toluene solution.

Figure 5 shows from top to bottom the spectra of bulk 2H TCPP in KBr (a) and on MgO nanocubes treated with 110-3 (b), 310-3 (c) and 210-2 (d) molL-1 2H TCPP solution. Three different ranges are of interest: 1.) As for 2H TPP we can find the ν(NH) vibration around 3316 cm-1.38 2.) In the range between 1800 and 1600 cm-1 we find vibrations related to the C=O stretching vibrations of free carboxylic acid groups which carry information about the binding mechanism.43 3.) As for the 2H TPP related spectra the porphyrin specific fingerprint region is in the range 1600 and 950 cm-1. The respective spectral range contains information on the chemical structure within the porphyrin molecules.38–42 While we did not observe a ν(NH) band after 2H TPP adsorption on MgO nanocubes, for 2H TCPP a ν(NH) band at 3316 cm-1 was observed for both the reference spectrum as well as for the spectra acquired for the 2H TCPP contacted MgO nanocube powders.


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With bands at 966, 981, 993 and 1020 cm-1, pairs of bands 1176 and 1187 cm-1 as well as 1212 and 1223 cm-1 the fingerprint range is very similar to that of 2H TPP. Because of the similarities of the backbone structure of 2H TPP and 2H TCPP, on the one hand, and the very good agreement in the band positions, on the other, we follow a band assignment that is analogous to that previously performed for 2H TPP.38,42,44 In terms of band positions and intensities the spectra of porphyrin in contact with MgO nanocubes are comparable irrespective of the type of the porphyrin used and very similar to the bulk 2H TCPP spectrum. (The distinct phenyl band above 1600 cm-1 shifts slightly to higher wavenumbers.) From this we can rule out that metalation or any other type of chemical transformation takes place in the backbone structure, which is very different to the behavior found for 2H TPP.38–42 Table 2: IR band assignments for 2H TPP and 2H TCPP adsorbed on MgO nanocube powders

ωexp / cm–1

ωexp / cm–1

2HTCPP

2HTCPP on MgO NC

3316

3320

νas(NH)

1726/1689

1690

COOH

-

1588

νasym(OCO)

1312

-

Phenyl + COOH

1215/1223

1213/1221

Pyrrole + Porphyrin backbone structure

1176/1187

1178

Pyrrole + Porphyrin backbone structure

1020

1022

Phenyl

994/985/ 967/

994/981 967

Porphyrin ring vibrations

Description

In the C=O and OCO-region, we identify pronounced changes between the reference 2H TCPP spectrum (a) and the spectra of the MgO nanocube powders (b, c and d). In comparison to the reference spectrum we observe for the C=O stretching vibration at 1686 and 1716 cm-1a strong loss in intensity. For the reference spectrum we attribute the splitting to molecule-molecule interactions within the porphyrin crystallites.45 For 110-3 molL-1 there are only small signals observable, while for 310-3 molL-1 and 210-2 molL-1 we can detect features at this position. A 17 ACS Paragon Plus Environment


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combination vibration that includes free acid groups at 1312 cm-1 43,46–48 is not observed in the nanocube powder spectra. The nanocube powder spectra show an additional signal at 1588 cm-1 for higher concentrations. After powder contact with the 110-3 molL-1 sample there exists only a shoulder at 1600 cm-1. As it overlaps with the signal at 1600 cm-1, a quantitative evaluation is not feasible. The ν(NH) band is similar in both relative intensity and band position for all spectra.

-3

-1

-3

Figure 5: DRIFTS spectra of bulk 2H TCPP (a) and MgO NC after contact with 110 molL (b), 310 -1

-2

-1

molL (c) and 210 molL (d) 2H TCPP solution respectively.

The large changes in the C=O and C-O signals indicate porphyrin binding involving the carboxylic acid groups. UHV experiments on a similar system

49

suggest a bidentate binding

mode via the carboxylic group. This binding mode also agrees with the new, broad bands around 1410 and 1588 cm-1 found in all spectra. Typically signals in this range are assigned to bidentate carboxylates.47,49–51


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The additional band at 1759 cm-1 scales with the amount of adsorbed porphyrin and its intensity levels off at concentrations above 310-3 molL-1. This signal was not found for the porphyrin with only one carboxy group (not shown). We, therefore, assume that the signal is associated with an interaction between acid groups not bound to the surface. Indeed, the band is in the range of anhydride ν(C=O) signals,49,52 pointing to an intermolecular condensation reaction between the porphyrin molecules. Considering the possible configurations illustrated in Figure 3 we can extend the discussion of the binding geometry of 2H TCPP. A flat lying geometry with four carboxy groups attached to the oxide surface (Figure 3ii) is unlikely for steric reasons. An alternative scenario pointed out in literature47,50 involves only two of the acid groups attached to the oxide surface as bidentate carboxylates with the other two carboxylic acid groups pointing away from the surface. In such a case, an adsorption induced reduction of the intensity of C=O related bands by ~ 50% would be expected. Indeed, for carboxy-functionalized porphyrins adsorbed on TiO2,

47

a decrease of the

respective band intensities has been observed. In contrast, spectra of carboxy-functionalized porphyrins adsorbed on ZnO do not feature C=O bands.46 Importantly, on MgO nanocube samples C=O related bands at 1686 and 1716 cm-1 are completely absent at low coverages (90 molecules per nanocube) (Figure 5b) We therefore propose that some of the free linker groups are deprotonated or interact with neighboring porphyrin molecules via e.g. hydrogen bonding. Such intermolecular interactions seem to be especially relevant at low coverages (< 90 molecules per nanocube), where we do not observe any C=O related bands (Figure 5b). On the other hand, bands at 1686 and 1716 cm-1 point to the presence of free acid groups at higher coverages (≥ 90 molecules per nanocube Figure 5c and d). Intermolecular interactions give rise to the appearance of new bands. The band at 1761 cm-1, whose intensity levels off at concentrations above 310-3 molL-1, may be indicative of intermolecular interactions at low coverages. Finally, the band at 1440 cm-1 is exclusively observed for the sample after contact with the 210-2 molL-1 solution, i.e. at high coverages. As in this case the amount of adsorbed 19 ACS Paragon Plus Environment


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porphyrin corresponds to more than a monolayer we attribute this signal to the formation of 2H TCPP multilayers on the surface of MgO nanocubes. 3) SAXS measurements The small angle scattering curves of the MgO nanocubes in toluene and methanol are displayed in Figure 6.

Figure 6: Small angle scattering curves of MgO nanocubes dispersed in toluene and methanol respectively (black lines) and those after addition of porphyrins at two different concentrations ((b) 310 -1

-2

molL , red and (c) 210

-3

-1

molL , green). The blue arrow indicates the region, where the scattering

curves clearly vary between the MgO nanocubes in porphyrin-free dispersion (a) and dispersions containing porphyrin (b and c). The scattering curves for the samples in methanol (MeOH) are shifted by a factor of 3 for clarity.

The scattering curves of the different MgO nanocube samples in toluene differ only to a minor extent, which is caused by slight and unavoidable variations of the exact sample composition. Within this experimental error the addition of 2H TPP to the MgO dispersion in toluene does not affect the shape of the curves. Fitting the structure model described above (polydisperse cubes 20 ACS Paragon Plus Environment

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with stabilizer shell) to the data supports this finding. In this case the scattering curves can also be fitted without assuming a shell at all. For the methanolic dispersion loaded with 2H TCPP, however, we find significant deviations of the scattering curves in the mid-Q regime (blue arrow in Figure 6). The scattering curves with porphyrin loaded samples are shifted to smaller Qvalues, which points to an increase of the average size of the MgO-porphyrin particles as it would be expected for MgO nanocubes functionalized by a monolayer of porphyrin molecules. This assumption is clearly supported by the fitting results. Similar to MgO in toluene dispersions the porphyrin free nanocubes in methanolic dispersion can be fitted without a shell. In contrast to 2H TPP in toluene dispersions, an additional shell needs to be included to achieve a satisfactory fit for the scattering curves of 2H TCPP loaded samples. Due to the low electron density contrast between the porphyrin shell and the MgO nanocubes, only the volume of the porphyrin shell can be determined. From these values the porphyrin coverage of the nanocubes could be estimated. The coverage corresponds to about 0.4 molecules/ nm² for a 2H TCPP concentration of 310-3 molL-1 and 1.9 molecules/nm² for a 2H TCPP concentration of 210-2 molL-1. These values are in good agreement with the quantitative absorption analysis by UV-Vis spectroscopy yielding values of 0.4 and 2.6 molecules/ nm² at 2H TCPP concentrations of 310-3 and 210-2 molL-1 (Figure 3). From the SAXS data no indication of 2H TPP adsorption at the MgO-toluene-interface could be found. However, a very thin layer of flat lying molecules which would be as thin as about 0.1 nm could most probably not be resolved by these measurements. There is general agreement that the functional properties of metal oxide/ porphyrin hybrids are subject to the hierarchy of different interaction types, to surface bonding and to the geometry of the molecular layer.53,54 In devices the complex microscopic architecture, however, makes it very demanding to establish firm correlations between the specific oxide/porphyrin geometries and the macroscopic materials properties. One possible approximation to this problem is the use of novel nanoscale characterization techniques.53,55 On the other hand, appropriate model systems for adsorption and charge transfer studies are needed. Gas-phase engineered and aggregate21 ACS Paragon Plus Environment


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free MgO nanocube powders feature a high definition with respect to surface composition and structural properties.27,28 The present study shows consistently that the metalation of 2H TPP on MgO nanocubes is associated with the formation of a monolayer of flat-lying molecules.28 In the case of 2H TCPP, which features four carboxyl moieties as potential linker groups to the MgO surface, the situation is more complex. Based on the consistent picture concluded from our multi-technique approach (spectroscopy and scattering) an adsorbate organization type where 2H TCPP molecules adsorb to the surface via covalent bonds in a flat lying geometry until monolayer coverage is reached (Figure 3 ii) is very unlikely due to steric reasons.43 On the other hand, a weak bonding of flat lying molecules is in contrast with our observation that adsorbed molecules remain at the MgO surface even after repeated washing steps with pure solvent. Our observations are in line with an adsorption model where the 2H TCPP molecule binds via one or two carboxyl groups. The remaining carboxyl groups would be available for multilayer formation. In that case the density of molecules inside the monolayer would be significantly higher and multilayer formation via interacting carboxyl groups (H-bonds, anhydride formation) would be expected. With regard to the consecutive surface chemistry we found that 2H TPP molecules metalate at the MgO surface, whereas the 2H TCPP molecules do not (Figure 2). Possibly, surface anchoring via external carboxylic acid groups and steric limitations do suppress the porphyrin molecule center’s approach to the nanocube surface and - consequently - the metalation step.27,28 These insights can be generalized and are easily transferable to other particulate metal oxide systems with an appreciable abundance of flat surface planes. Porphyrin molecules with strong-interacting groups such as carboxyl groups inhibit/ or may even prevent consecutive surface reactions that involve the central part of the macrocycle. In addition to adsorption, changes in position of the carboxyl group with respect to the benzene ring will allow for adjustment of the porphyrin’s electronic properties and ultimately its chemical reactivity. This is particularly important for hybrid materials where a hierarchy of possible interactions determine the actual adsorption chemistry and the nature of the oxide-organic interface. 22 ACS Paragon Plus Environment

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First experiments point to the crystallization of porphyrins at high 2H TCPP loadings as a result of adsorption (Supporting Information, Figure S1). What remains an open issue is the strength of the covalent bond between carboxyl groups and the MgO surface. This is intimately related with the nucleation and growth kinetics of assembled porphyrin layers. Porphyrin nucleation is favored when surface bond formation is fast with respect to π-π stacking of adsorbed porphyrin molecules. In that case, surface bonding must also exhibit some reversibility so that surface assembly can take place56 and porphyrin molecules can organize within a highly ordered structure at the MgO nanocube surface. In our laboratories a more detailed investigation is underway to address related questions. This is motivated by the fact that the controlled and adsorption induced organized accommodation of porphyrins on high surface area materials opens an important opportunity region towards well-defined nanohybrids.24,25 Conclusions In the present study we used UV/Vis spectroscopy, DRIFTS and small angle X-ray scattering to investigate the adsorption and consecutive surface chemistry of 2H TPP and 2H TCPP molecules on MgO nanocubes. For 2H TCPP we observed porphyrin adsorption that is linear with respect to the porphyrin concentration in the stock solution and that no saturation effects occur up to coverages of 1500 ± 225 molecules per nanocube. The 2H TCPP does not metalate on the MgO nanocubes at room temperature. In sharp contrast 2H TPP undergoes rapid metalation at the MgO nanocubes at room temperature and its adsorption shows a clear saturation at 90 ± 14 molecules per MgO nanocube. Differences in the adsorption geometry and the consecutive surface chemistry are attributed to the difference in adsorbate binding and associated space requirements. While 2H TPP is bound by complexation and remains at the surface as a flat lying molecule, 2H TCPP covalently binds to the surface through its carboxyl groups. The associated upright-standing adsorption geometry requires less space per molecule and allows for substantially larger coverages of porphyrin molecules. From uptake curves, scattering results and the adsorption induced crystallization behavior of the porphyrins we 23 ACS Paragon Plus Environment


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conclude that multilayer formation is mediated by interacting carboxyl groups between porphyrins in adjacent porphyrin layers. Acknowledgements This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Research Unit FOR 1878 “funCOS – Functional Molecular Structures on Complex Oxide Surfaces”. Supporting Information X-ray diffractograms of MgO nanocube powders before and after porphyrin adsorption are provided within the supporting information. In addition we added photoluminescence spectra of the samples shown in figure 2.

TOC


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References (1) Hasobe, T. Porphyrin-Based Supramolecular Nanoarchitectures for Solar Energy Conversion. J. Phys. Chem. Lett. 2013, 4, 1771–1780. (2) Higashino, T.; Imahori, H. Porphyrins as Excellent Dyes for Dye-Sensitized Solar Cells: Recent Developments and Insights. Dalton Trans. 2015, 44, 448–463. (3) Sivalingam, Y.; Martinelli, E.; Catini, A.; Magna, G.; Pomarico, G.; Basoli, F.; Paolesse, R.; Di Natale, C. Gas-Sensitive Photoconductivity of Porphyrin-Functionalized ZnO Nanorods. J. Phys. Chem. C 2012, 116, 9151–9157. (4) D’Urso, A.; Di Mauro, A.; Cunsolo, A.; Purrello, R.; Fragalà, M. E. Solvophobic versus Electrostatic Interactions Drive Spontaneous Adsorption of Porphyrins onto Inorganic Surfaces: A Full Noncovalent Approach. J. Phys. Chem. C 2013, 117, 17659–17665. (5) Liljeroth, P. Molecular Electronics: Flipping a Single Proton Switch. Nat. Nanotechnol. 2012, 7, 5–6. (6) Gouterman, M. Study of the Effects of Substitution on the Absorption Spectra of Porphin. J. Chem. Phys. 1959, 30, 1139. (7) Lu, X.; Shao, Y.; Wei, S.; Zhao, Z.; Li, K.; Guo, C.; Wang, W.; Zhang, M.; Guo, W. Effect of the Functionalized p-Bridge on Porphyrin Sensitizers for Dye-Sensitized Solar Cells: an in-depth Analysis of Electronic Structure, Spectrum, Excitation, and Intramolecular Electron Transfer. J. Mater. Chem. C. 2015, 3, 10129–10139. (8) Brennan, B. J.; Llansola Portolés, M. J.; Liddell, P. A.; Moore, T. A.; Moore, A. L.; Gust, D. Comparison of Silatrane, Phosphonic Acid, and Carboxylic Acid Functional Groups for Attachment of Porphyrin Sensitizers to TiO2 in Photoelectrochemical Cells. Phys. Chem. Chem. Phys. 2013, 15, 16605–16614. (9) Franke, M.; Marchini, F.; Jux, N.; Steinrück, H.-P.; Lytken, O.; Williams, F. J. Zinc Porphyrin Metal-Center Exchange at the Solid–Liquid Interface. Chem. - Eur. J. 2016, 22, 8520–8524.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

(10) Killian, M. S.; Gnichwitz, J.-F.; Hirsch, A.; Schmuki, P.; Kunze, J. ToF-SIMS and XPS Studies of the Adsorption Characteristics of a Zn-Porphyrin on TiO2. Langmuir 2010, 26, 3531– 3538. (11) Marbach, H.; Steinrück, H.-P. Studying the Dynamic Behaviour of Porphyrins as Prototype Functional Molecules by Scanning Tunnelling Microscopy Close to Room Temperature. Chem. Commun. 2014, 50, 9034–9048. (12) Röckert, M.; Ditze, S.; Stark, M.; Xiao, J.; Steinrück, H.-P.; Marbach, H.; Lytken, O. Abrupt Coverage-Induced Enhancement of the Self-Metalation of Tetraphenylporphyrin with Cu(111). J. Phys. Chem. C 2014, 118, 1661–1667. (13) Marbach, H. Surface-Mediated in Situ Metalation of Porphyrins at the Solid-Vacuum Interface. Acc. Chem. Res. 2015, 48, 2649–2658. (14) Auwärter, W.; Écija, D.; Klappenberger, F.; Barth, J. V. Porphyrins at Interfaces. Nat. Chem. 2014, 7, 105–120. (15) Gottfried, J. M. Surface Chemistry of Porphyrins and Phthalocyanines. Surf. Sci. Rep. 2015, 70, 259–379. (16) Zhang, L.; Patil, A. J.; Le Li; Schierhorn, A.; Mann, S.; Gösele, U.; Knez, M. Chemical Infiltration during Atomic Layer Deposition: Metalation of Porphyrins as Model Substrates. Angew. Chem., Int. Ed. 2009, 48, 4982–4985. (17) Zhao, Y.; Kornienko, N.; Liu, Z.; Zhu, C.; Asahina, S.; Kuo, T.-R.; Bao, W.; Xie, C.; Hexemer, A.; Terasaki, Osamu, et al. Mesoscopic Constructs of Ordered and Oriented Metal– Organic Frameworks on Plasmonic Silver Nanocrystals. J. Am. Chem. Soc. 2015, 137, 2199– 2202. (18) Baumann, S. O.; Elser, M. J.; Auer, M.; Bernardi, J.; Husing, N.; Diwald, O. Solid-Solid Interface Formation in TiO2 Nanoparticle Networks. Langmuir : the ACS journal of surfaces and colloids 2011, 27, 1946–1953.


Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Leishman, C. W.; McHale, J. L. Morphologically Determined Excitonic Properties of Porphyrin Aggregates in Alcohols with Variable Acidity. J. Phys. Chem. C 2016. (20) Ma, J.; Zhang, W.; Li, Z.; Lin, Q.; Xu, J.; Han, Y. Competition of Major Forces Dominating the Structures of Porphyrin Assembly. Cryst. Growth Des. 2016, 16, 1942–1947. (21) Qiu, Y.; Chen, P.; Liu, M. Evolution of Various Porphyrin Nanostructures via an Oil/Aqueous Medium: Controlled Self-Assembly, Further Organization, and Supramolecular Chirality. J. Am. Chem. Soc. 2010, 132, 9644–9652. (22) Wang, L.; Chenb, Y.; Jiang, J. Controlling the Growth of Porphyrin Based Nanostructures for Tuning Third-Order NLO Properties. Nanoscale 2014, No. 6, 1871–1878. (23) Feinle, A.; Elsaesser, M. S.; Husing, N. Sol-Gel Synthesis of Monolithic Materials with Hierarchical Porosity. Chem. Soc. Rev. 2016, 45, 3377–3399. (24) Motoyama, S.; Makiura, R.; Sakata, O.; Kitagawa, H. Highly Crystalline Nanofilm by Layering of Porphyrin Metal−Organic Framework Sheets. J. Am. Chem. Soc. 2011, 133, 5640– 5643. (25) Hill, J. P.; Wakayama, Y.; Ariga, K. How Molecules Accommodate a 2D Crystal Lattice Mismatch: an Unusual ‘Mixed’ Conformation of Tetraphenylporphyrin. Phys. Chem. Chem. Phys. 2006, 8, 5034–5037. (26) Munir, S.; Shah, S. M.; Hussain, H.; Siddiq, M. Adsorption of Porphyrin and Carminic Acid on TiO2 Nanoparticles: A Photo-Active Nano-Hybrid Material for Hybrid Bulk Heterojunction Solar Cells. J. Photochem. Photobiol., B 2015, 153, 397–404. (27) Schneider, J.; Franke, M.; Gurrath, M.; Röckert, M.; Berger, T.; Bernardi, J.; Meyer, B.; Steinrück, H.-P.; Lytken, O.; Diwald, O. Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems. Chem. - Eur. J. 2016, 22, 1744–1749.



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(28) Schneider, J.; Kollhoff, F.; Bernardi, J.; Kaftan, A.; Libuda, J.; Berger, T.; Laurin, M.; Diwald, O. Porphyrin Metalation at the MgO Nanocube/Toluene Interface. ACS Appl. Mater. Interfaces 2015, 7, 22962–22969. (29) McKenna, K. P.; Koller, D.; Sternig, A.; Siedl, N.; Govind, N.; Sushko, P. V.; Diwald, O. Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders. ACS Nano 2011, 5, 3003–3009. (30) Diwald, O.; Sterrer, M.; Knözinger, E. Site selective Hydroxylation of the MgO Surface. Phys. Chem. Chem. Phys. 2002, 4, 2811–2817. (31) Zhang, F.; Ilavsky, J.; Long, G. G.; Quintana, J. P. G.; Allen, A. J.; Jemian, P. R. Glassy Carbon as an Absolute Intensity Calibration Standard for Small-Angle Scattering. Metall. Mater. Trans. A 2010, 41, 1151–1158. (32) Siedl, N.; Koller, D.; Sternig, A. K.; Thomele, D.; Diwald, O. Photoluminescence Quenching in Compressed MgO Nanoparticle Systems. Phys. Chem. Chem. Phys. 2014, 16, 8339–8345. (33) Schwaiger, R.; Schneider, J.; Bourret, G. R.; Diwald, O. Hydration of Magnesia Cubes: a Helium Ion Microscopy Study. Beilstein J. Nanotechnol. 2016, 7, 302–309. (34) Baumann, S. O.; Schneider, J.; Sternig, A.; Thomele, D.; Stankic, S.; Berger, T.; Grönbeck, H.; Diwald, O. Size Effects in MgO Cube Dissolution. Langmuir 2015, 31, 2770–2776. (35) Starting with methanolic porphyrin solution we performed control experiments to investigate their crystallization behavior. During continuous methanol evaporation and in the absence of dispersed MgO nanocubes we acquired diffractograms and tracked the process of porphyrin solidification on the time scale of hours. In the absence of MgO nanocubes the solid residue was found to be amorphous with no XRD evidence for crystallinity. Interestingly, isopropanol addition induced the precipitation of a crystalline material. Identification of the underlying crystal phase and further mechanistic details about the MgO adsorption induced porphyrin crystallization process are currently under way.


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(36) Edwards, L.; Dolphin, D.; Gouterman, M.; Adler, A. Porphyrins XVII. Vapor Absorption Spectra and Redox Reactions: Tetraphenylporphins and Porphin. J. Mol. Spectrosc. 1971, 38, 16–32. (37) Rubio, M.; Roos, B. O.; Serrano-Andrés, L.; Merchán, M. Theoretical Study of the Electronic Spectrum of Magnesium-Porphyrin. J. Chem. Phys. 1999, 110, 7202. (38) Saini, G. S. S.; Sharma, S.; Kaur, S.; Tripathi, S. K.; Mahajan, C. G. Infrared Spectroscopic Studies of Free-base Tetraphenylporphine and its Dication. Spectrochim. Acta, Part A 2005, 61, 3070–3076. (39) Rush, T. S.; Kozlowski, P. M.; Piffat, C. A.; Kumble, R.; Zgierski, M. Z.; Spiro, T. G. Computational Modeling of Metalloporphyrin Structure and Vibrational Spectra: Porphyrin Ruffling in NiTPP. J. Phys. Chem. B 2000, 104, 5020–5034. (40) Li, X. Y.; Zgierski, M. Z. Porphine Force Field: In-plane Normal Modes of Free-base Porphine; Comparison with Metalloporphines and Structural Implications. J. Phys. Chem. 1991, 95, 4268–4287. (41) Ambre, R.; Chen, K.-B.; Yao, C.-F.; Luo, L.; Diau, E. W.-G.; Hung, C.-H. Effects of Porphyrinic meso -Substituents on the Photovoltaic Performance of Dye-Sensitized Solar Cells: Number and Position of p-Carboxyphenyl and Thienyl Groups on Zinc Porphyrins. J. Phys. Chem. C 2012, 116, 11907–11916. (42) Bedel-Cloutour, C. H.; Barois-Gacherieu, C.; Marchand, A. Infrared Studies of Symmetrically and Asymmetrically Substituted Tetrapyrrolic Macrocycles: Free Bases and Indium Derivatives. Spectrochim. Acta, Part A 1988, 44, 567–573. (43) Roales, J.; Pedrosa, J. M.; Cano, M.; Guillén, M. G.; Lopes-Costa, T.; Castillero, P.; Barranco, A.; González-Elipe, A. R. Anchoring Effect on (Tetra)Carboxyphenyl Porphyrin/TiO2 Composite Films for VOC Optical Detection. RSC Adv. 2014, 4, 1974–1981. (44) Alben J.; Choi S.; Adler A.; Chaughey W. Infrared Spectroscopy of Porphyrins. Ann. N. Y. Acad. Sci. 1973, 206, 278–295. 29 ACS Paragon Plus Environment


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(45) Reva, I. D.; Stepanian, S. G. An Infrared Study on Matrix-Isolated Benzoic Acid. J. Mol. Struct. 1995, 349, 337–340. (46) Tu, W.; Lei, J.; Wang, P.; Ju, H. Photoelectrochemistry of Free-base-porphyrinfunctionalized Zinc Oxide Nanoparticles and Their Applications in Biosensing. Chem. - Eur. J. 2011, 17, 9440–9447. (47) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position. J. Am. Chem. Soc. 2007, 129, 4655–4665. (48) Werner, K.; Mohr, S.; Schwarz, M.; Xu, T.; Amende, M.; Dopper, T.; Gorling, A.; Libuda, J. Functionalized Porphyrins on an Atomically Defined Oxide Surface: Anchoring and CoverageDependent Reorientation of MCTPP on Co3O4(111). J. Phys. Chem. Lett. 2016, 7, 555–560. (49) Xu, T.; Mohr, S.; Amende, M.; Laurin, M.; Döpper, T.; Görling, A.; Libuda, J. Benzoic Acid and Phthalic Acid on Atomically Well-Defined MgO(100) Thin Films: Adsorption, Interface Reaction, and Thin Film Growth. J. Phys. Chem. C 2015, 119, 26968–26979. (50) Tacconi, N. R. de; Chanmanee, W.; Rajeshwar, K.; Rochford, J.; Galoppini, E. Photoelectrochemical Behavior of Polychelate Porphyrin Chromophores and Titanium Dioxide Nanotube Arrays for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2009, 113, 2996–3006. (51) Rochford, J.; Galoppini, E. Zinc(II) Tetraarylporphyrins Anchored to TiO2, ZnO, and ZrO2 Nanoparticle Films Through Rigid-rod Linkers. Langmuir 2008, 24, 5366–5374. (52) Mohr, S.; Xu, T.; Dopper, T.; Laurin, M.; Gorling, A.; Libuda, J. Molecular Orientation and Structural Transformations in Phthalic Anhydride Thin Films on MgO(100)/Ag(100). Langmuir 2015, 31, 7806–7814. (53) Ye, S.; Kathiravan, A.; Hayashi, H.; Tong, Y.; Infahsaeng, Y.; Chabera, P.; Pascher, T.; Yartsev, A. P.; Isoda, S.; Imahori, Hiroshi, et al. Role of Adsorption Structures of Zn-Porphyrin on TiO2 in Dye-Sensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy. J. Phys. Chem. C 2013, 117, 6066–6080. 30 ACS Paragon Plus Environment

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(54) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and Orientation of Molecular Adsorption as a bottom-up Methodology for Nanomaterials. Langmuir 2013, 29, 2108–2119. (55) Rogero, C.; Pickup, D. F.; Colchero, J.; Azaceta, E.; Tena-Zaera, R.; Palacios-Lidon, E. Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO Films. ACS Appl. Mater. Interfaces 2016, 8, 16783–16790. (56) McCreery, R. L.; Bergren, A. J. Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication. Adv. Mater. 2009, 21, 4303–4322.


Adsorption, Ordering, and Metalation of Porphyrins on MgO Nanocube

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