Coordination-Controlled One-Dimensional Molecular Chains in

Article pubs.acs.org/IC

Coordination-Controlled One-Dimensional Molecular Chains in Hexapodal Adenine−Silver Ultrathin Films R. Kamal Saravanan,† Prithwidip Saha,† Viruthakasi Venkatesh,‡ Thiruvancheril G. Gopakumar,* and Sandeep Verma* Department of Chemistry and Center for Nanoscience and Soft Nanotechnology, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: Growth of a silver coordination polymer of a C3-symmetric hexaadenine ligand is studied on highly oriented pyrolytic graphite (HOPG), using high-resolution atomic force microscopy (AFM). This unusual ligand offers 6-fold multidentate coordination sites, and consequently, a multidimensional growth of coordination polymer is expected. Notably, each discrete hexapodal unit is bridged by two silver ions along one of the crystallographic directions, resulting in high interaction energy along this direction. When the polymer was deposited on an HOPG surface from a dilute solution, we observed abundant one-dimensional (1D) coordination polymer chains, with a minimum width of approximately 4.5 nm. The single-crystal structure using X-ray analysis is compared with the surface patterns to reconcile and understand the structure of the 1D polymer on an HOPG surface. The energy levels of Ag-L1 within the proposed model were calculated, on the basis of the X-ray crystal structure, and compared to the ligand states to gain information about the electronic structure of ligand upon Ag coordination. On the basis of the wave functions of a few molecular orbitals (MOs) near the Fermi energy, it is surmised that unfilled MOs may play a crucial role in the transport properties of the Ag-L1 adlayer.

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INTRODUCTION A multitude of coordination polymers of varying dimensionalities are known in the literature, of which one-dimensional coordination polymers (1D CPs) are considered to be structurally the least interesting in comparison to twodimensional (2D) and three-dimensional (3D) motifs. A recent surge in their potential magnetic, electrical, mechanical, and optical properties has spurred interest in their synthesis and study of relevant conformations and morphologies.1−7 Thus, immense efforts have been invested in preparing CPs on surfaces, under ultrahigh vacuum, either through coevaporation of metal atoms and ligands containing carbonitrile or carboxylate functional groups8−15 or via coordination of ligands with adatoms (generation at elevated temperatures) on adsorbing surfaces.16−18 Efforts are also underway for surface deposition of CPs under ambient conditions.1,2,19 Although different types of CPs such as flexible 1D molecular chains,13,16−18 2D networks,9−12 clusters,17 and thick fibers1,2 are known, approaches in creating micrometer-length, nonflexible (rigid) molecular CP chains are scarce.1 As illustrated by their interesting electronic structure,1,2,8,14,15 a reduction in the energy barrier for injection of holes/electrons, due to electron transfer between metal and ligand molecules in long and nonflexible CP chains on surfaces, presents significant potential for electronic and optical communication applications. © 2017 American Chemical Society

Nitrogenous nucleobases, with suitably arranged metal binding sites, offer a variety of coordination possibilities for constructing novel metal−nucleobase frameworks. Adenine is a nitrogen-rich purine heterocycle with five potential coordination sites for metal ion interaction.20−27 Our earlier efforts have particularly focused on silver−adenine complexes and formation of metallamacrocycles and coordination polymers in the solid state as well as on surfaces.20,21 As Ag and Au ions have been extensively used for creating stable CPs with higher stability and varied geometries,28−31 we decided to create a C3symmetric hexaadenine ligand (L1) from C8-thiolated adenine to enhance the number of potential interaction sites to achieve hierarchically complex geometries (Figure 1 and sections S2 and S3 in the Supporting Information). Thiol substitution, absent in natural nucleobases, in L1 is expected to confer additional sites for interaction with silver ions.32 The ensuing one-dimensional coordination polymer with Ag(I) ions was deposited on the basal plane of HOPG, followed by AFM analysis, and a subsequent X-ray crystal structure analysis revealed the role of a multicentered L1-Ag coordination in governing the formation of 1D CP. Strikingly, their growth is Received: December 19, 2016 Published: March 14, 2017 3976

DOI: 10.1021/acs.inorgchem.6b03090 Inorg. Chem. 2017, 56, 3976−3982

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Synthetic procedures, characterization data, and crystal structure details of L1 and Ag-L1 are given in the Supporting Information.

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RESULTS AND DISCUSSION Given the unique topology of L1 and documented interactions between adenine and HOPG surfaces,33−35 we investigated the growth of ultrathin films (coverage ∼0.5 monolayer (ML)) of L1 on a freshly cleaved HOPG surface, through AFM topography measurements (Figure 2). Micrographs at different resolutions displayed monolayer islands (height distribution ∼0.45 nm) of L1 on HOPG terraces. Terrace edges are marked by yellow dashed lines in Figure 2a. Two types of molecular islands that appeared as bright patches on the surface were identified on the basis of the nature of their growth: the first type presented well-defined edges (indicated with dashed blue lines), whereas the second type possessed indefinite peripheries (indicated with dashed green lines). We assigned these islands as compact and loosely packed, respectively, with crystalline nature given the observation of molecular level ordering in high-resolution images (indicated using arrows in Figure 2b,c). The molecular rows are spaced at 5.8 ± 0.4 and 7.1 ± 0.3 nm in compact and loose-packed islands, respectively. In addition, the islands displayed only three preferred growth directions, each rotated by approximately 60°, indicating its registry with the graphite lattice. The abundance of compact islands was relatively less in comparison to loosely packed islands, where the former has rigid edges and the latter exhibited a dynamic nature. Compare the island boundary in the dashed square in Figure 2a with Figure 2b, which are subsequent images. Next, a drop-casted ultrathin film (coverage ∼0.35 ML) of a mixture of L1 and AgClO4 in a 1/16 ratio (v/v) was prepared, and AFM phase and topography images were acquired at different resolutions (Figure 3a,b). The surface was populated with extensive long and thin needle-like features, denoted as 1D chains throughout the text. To understand the nature of their growth, we performed a statistical analysis of relative orientation. Three orientations peaking at around 0, 60, and 120° with a broadening (±7.5°) were observed and are assigned to rotational domains. The broadening is presumably due to the presence of mirror domains of three rotational domains. For example, a pair of mirror domains is indicated using green and white dashed lines in Figure 3a. All possible rotational and mirror domains are indicated using green and

Figure 1. (a) C3-symmetric hexaadenine ligand (L1). N1, N3, N6, N7, and N9 are labels of different nitrogen atoms (see text for details). (b) Ag-coordinated L1. Ag atoms with different coordination numbers are indexed Ag1, Ag2/Ag2′, and Ag3. The N9-propyl group on adenine, hydrogen atoms, and counterions are omitted for clarity.

only limited by multiatomic step edges and other chains leading to few micrometer-length CPs.

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EXPERIMENTAL SECTION

Adenine, thiourea, and bromine were purchased from Spectrochem Pvt Ltd., n-propyl bromide, K2CO3, hexakis(bromomethyl)benzene, and AgClO4 were obtained from Alfa Aesar, India, and perchloric acid (about 70% GR) was purchased from Merck. Dimethylformamide was purchased from S. D. Fine-Chem Ltd., Mumbai, India, and acetonitrile from Merck. Methanol and dichloromethane were acquired from Emparta. Highly oriented pyrolytic graphite (HOPG) was purchased from μ-masch. 1H and 13C NMR spectra were recorded on JEOL DELTA2 500 spectrometer operating at 500 MHz and JEOL JNMECS400 spectrometer operating at 400 MHz, respectively. The spectra were recorded in DMSO-d6 as solvent, and chemical shifts were referenced with respect to tetramethylsilane. High-resolution (ESI+ mode) mass spectra were obtained on a Waters HAB 213 machine. Powder X-ray diffraction (PXRD) of complex Ag-L1 was performed with a PANalytical X’Pert PRO diffractometer with Cu Kα radiation (1.5405 Å). Melting points of both ligand L1 and Ag-L1 were determined on DBK-programmable melting point apparatus. Caution! metal perchlorate salts are potentially hazardous, explosive under certain conditions and should be handled with care. All microscopic structural analysis on the surface was carried out using an Agilent 5500 atomic force microscope (AFM). Aluminum-coated silicon cantilevers from Budget Sensors were used as AFM probes. During scanning (tapping mode), the resonance frequency of the cantilever was set around 270 kHz and the corresponding force constant was around 35 N/m. Images were processed using WSxM software from Nanotec.

Figure 2. (a) Constant height AFM topography of ultrathin film of L1 on an HOPG (0001) surface. Different types of islands are marked with green and blue polygons. High resolution AFM topographs of (b) loose-packed (dashed square in (a)) and (c) compact islands. 3977


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had their structure altered on scanning, suggesting the high stability and polymeric nature of these chains. To understand the molecular level structure of L1 and to correlate observations gathered in thin film studies, single crystals of ligand L1 were subjected to single-crystal X-ray analysis. Refinement of crystallographic data suggested that L1 crystallized into a triclinic unit cell with space group P1̅. Careful analysis of the crystal data revealed that the asymmetric unit consisted of three modified adenine units along with a halfshared benzene ring and a noncoordinated DMF solvent molecule (section S2 in the Supporting Information). The packing of L1 was primarily stabilized by intermolecular hydrogen-bonding interactions between N6−H···N3 and N6−H···N1, with bond lengths ranging between 2.20 and 2.47 Å. Further stabilization was offered by π−π stacking interactions between adenine rings with a centroid-to-centroid distance of 3.67 Å (section S2 in the Supporting Information). We extracted a lattice plane enclosing N6−H···N3 and N6− H···N1 hydrogen bonding and π−π stacking between adenine rings from bulk X-ray data (plane diagonal to the bulk ba plane, Figures S3 and S9 in the Supporting Information), to compare the observed packing of L1 on the surface. This is the plane with maximum possible nonbonding interactions in bulk packing. Additionally, the plane offers a maximum adsorption area on HOPG due to six adenine units, allowing us to conclude that the close-packed pattern of L1 could be ascribed to the above plane. The spacing between experimentally observed molecular rows (arrows in Figure 2c) is ∼4 times that of intermolecular packing distance (1.5 nm) in the proposed lattice plane. This is presumably due to the formation of a superlattice owing to the variable adsorption geometry of molecules in adjacent rows. On the basis of favorable interactions described for adenine and the HOPG surface,33−35 further relaxation of adenine orientation (from perpendicular to parallel) may be assumed within the plane. Crystal refinement data of Ag-L1 (16/1 Ag(I) to ligand ratio) suggested that it belonged to the triclinic system, in the space group P1̅. The crystal lattice properties of Ag-L1 revealed an unusual adenine coordination mode with the help of two ring nitrogen atoms (N3, N7) and a sulfur atom, which was also found coordinated to silver ions (Figure 1b). Three crystallographically unique silver ions were identified (labeled Ag1, Ag2/Ag2′, and Ag3; cf. Figure 1b), each exhibiting pentacoordinated distorted-square-pyramidal geometry. Ag1 was coordinated to two N7 nitrogen atoms from two different adenine rings (2.24 and 2.25 Å), one perchlorate counterion (2.65 Å), one bridging perchlorate counterion (2.67 Å), and one water molecule (2.62 Å); Ag2 coordinates via the N7 nitrogen atom (2.28 Å), the C8-sulfur atom (2.51 Å), two perchlorate counterions (2.62 and 2.65 Å), and a water molecule (2.64 Å), while Ag3 is coordinated to the N3 nitrogen atom (2.29 Å), one perchlorate counterion (2.72 Å), one bridging perchlorate counterion (2.76 Å), and two water molecules (2.31 and 2.37 Å) (section S3 in the Supporting Information). High-resolution positive ion electrospray ionization mass spectrometry (HR-ESI-MS) analysis confirmed the presence of an [L + Ag]+ peak in complex Ag-L1 (Figure S4 in the Supporting Information). Powder X-ray diffraction analysis confirmed the crystallinity and bulk purity of the complex AgL1 (Figure S5 in the Supporting Information). We compared crystallographic planes shown in Figure 4, where a and c are lattice vectors (same as in bulk packing) and the corresponding lattice directions are indicated using arrows.

Figure 3. (a) AFM phase image of ultrathin film of Ag-L1 on an HOPG surface. (b) AFM topographic image of Ag-L1 (dashed square in (a)). Yellow dashed lines are graphite multiatomic terrace edges, while green and white arrow pairs indicate growth directions of rotational and mirror domains of 1D chains. White and green arrows are rotated by ±7.5°. Blue circles indicate amorphous clusters at terrace edges. White arrowheads indicate 1D chains crossing a monoatomic terrace of HOPG. (c) Statistical analysis of widths of different 1D chains. Arrow heads indicate equispaced peaks corresponding to 1D chains with different widths.

white arrows, respectively. These results indicate that 1D chains are crystalline and their initial growth is templated by the surface structure. A few amorphous aggregations were also observed at the terrace edges of HOPG (Figure 3a) (marked with blue circles). Statistical analysis of the width of the 1D chain presents two distinct features (Figure 3c): equispaced peaks (∼4.5 nm) and a relatively high abundance of peaks between 10 and 25 nm. Equispaced peaks (marked with arrow heads in Figure 3c) indicate that the assembly of 1D chains lead to the formation of thicker chains, while the high relative abundance of thin chains in comparison to thicker chains suggests that 1D chains are energetically stable on the surface. These chains tend to grow over surface kinks and monatomic steps, and their length seems to be restricted only by other chains and multiatomic terrace edges.36 Such striking growth is indicative of a strong bonding interaction along the length of the 1D chain. This suggests that 1D chains are coordination polymers of L1 and Ag(I). It should be noted that the onset of the statistics, the width of the thinnest 1D chain, is broader than the spacing between the peaks, which is most likely due to broadening induced by the AFM tip, tip radius, and interaction of the 1D chain and tip. To prove the stability and the effect of coverage, we also studied the growth of a mixture of L1 and AgClO4 (1/16 v/v ratio) at low coverage (∼0.15 ML; data included in section S5 in the Supporting Information). Isolated 1D chains were observed on free terraces, whose growth is in registry with graphite, as is evident from the 3-fold growth directions. However, 1D chains appear broader, ascribed to the lateral dynamic nature arising from their interaction with the AFM tip while scanning. Interestingly, none of the 1D chains broke or 3978


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Figure 5. (a) AFM constant height phase image of 2D domains of AgL1 on HOPG. (b) High-resolution AFM topography image of region indicated by the black square in (a). The lower part shows a meshaveraged data (see text for details). Arrows spanning between raw and mesh averaged data indicate adjacent molecular rows; different colors depict different brightnesses between adjacent rows.

Thus, we conclude that the rows observed in AFM correspond to the lattice along the a axis. To further verify the influence of metal to L1 ratio on the formation of 1D chains, we have investigated an ultrathin film of Ag-L1 from a 10/1 metal to L1 (v/v) solution. Growth of 1D islands extending over monatomic terraces was observed. The high-resolution images revealed types of molecular level patterns similar to those observed above (section S6 in the Supporting Information). We have calculated (B3LYP/LanL2DZ) the energy levels of Ag-L1 within the proposed model (cf. Figure 4), on the basis of the X-ray crystal structure, and compared it to the ligand states (Figure 6), to gain information about the electronic structure of

Figure 4. Plane containing Ag-coordinated polymer as extracted from the bulk crystal structure. Lattice vectors of the plane are marked with arrows, and a and c are the corresponding unit vectors (in accordance with the bulk lattice vectors). Along the a axis two Ag ions (Ag2 and Ag2′) are coordinating with L1, which is assigned as the 1D coordination polymer growth direction. Only bridging perchlorate counteranions are given. The N9-propyl group, counteranions, water, and all hydrogen atoms are omitted for clarity.

Interestingly the crystal lattice shows that along the a axis two Ag ions (Ag2 and Ag2′) are coordinated to two neighboring L1s. Further, the Ag···Ag distance in this lattice (3.15 Å) shows an argentophilic interaction between Ag2 and Ag2′.37 L1 is coordinated to its neighbors through a perchlorate ion along the c axis. The coordination of L1 and Ag along the a axis points to high interaction energy (high coordination) along this axis in comparison to the other. We assign this lattice to the observed 1D chains on HOPG. It is believed that growth on the surface occurs more quickly along energetically favorable directions, thus justifying growth along one dimension. However, the abundance of 1D chains is dominant at low coverage, but as coverage increases growth occurs along other directions and a 2D film is formed. We surmise that 2D growth in the proposed adlayer is mediated by bridging 1D polymeric chains via perchlorate counteranion interactions with Ag1 and Ag3 ions (along the c axis, Figure 4). 2D domains were also observed (Figure 5a), and the highresolution images of these domains revealed molecular level ordering, as observed by the presence of linelike contrast marked using arrows (Figure 5b). We assign these lines to adjacent molecular rows, where the upper half of Figure 5b is a measured data, while the lower half has been mesh-averaged.38 Curiously, adjacent rows periodically vary in contrast, with every fourth row appearing as identical, affording a perpendicular distance between every fourth row as 6.2 ± 0.3 nm, whereas the distance between adjacent rows is 2.1 ± 0.2 nm. We assume that this variation in the contrast of adjacent rows is due to variable adsorption geometry of adjacent molecular rows with respect to the HOPG lattice. This is generally observed for incommensurate adlayers and is known as a Moire pattern.39 We compared these molecular rows to the plane depicted in Figure 4 to find that the distance between molecules along the c axis is 1.95 nm. This is well in agreement with the observed row spacing between the adjacent rows.

Figure 6. Energy level diagram of L1 and building blocks of the lattice along the a axis (Ag-L1a) and c axis (Ag-L1c) and Ag. Configurations of L1, Ag-L1a, and Ag-L1c are depicted in the lower panel (full size images of L1, Ag-L1a, and Ag-L1c configurations are provided in the Supporting Information).

ligands upon Ag coordination. Two configurations of Ag-L1 were used, which are the building blocks of the lattice along the a axis (Ag-L1a) and c axis (Ag-L1c). The configurations as obtained from the X-ray crystal structure are included in the lower panel of Figure 6. For both configurations, ClO4− and H2O (relative geometry and positions as obtained from the Xray crystal structure) are enclosed around Ag to ensure the 3979


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of ligand and the Ag states in Ag-L1c. The unfilled MOs of AgL1c are relatively lower compared to those of Ag-L1a. We are expecting therefore that the unfilled MOs may play a crucial role in the transport properties of the adlayer of Ag-L1.

exact coordination sphere as in the experiment. Each Ag ion is connected to one ClO4− to maintain the charge neutrality. Thus, all Ag ions are in the +1 oxidation state and are diamagnetic. For the a axis (Ag-L1a) parts of the adjacent ligands are added to satisfy the coordination sphere. Refer to configurations provided in Figure 6 and section S7 in the Supporting Information. Ag-L1a corresponds to the 1D chain direction. Two striking observations are the outputs of the calculations, as follows. The effective highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap of Ag-L1 is reduced to 3.552 eV (Ag-L1a) and 3.716 eV (Ag-L1c) in comparison to that of L1 (4.470 eV). The density of states (DOS) near the Fermi energy (both filled and unfilled states) is increased for Ag-L1 in comparison to L1. This reduction in the gap as well as the increased DOS near the Fermi energy foresees that the adlayer of Ag-L1 will be useful for transport of charge carriers. Since Ag-L1a is directly linked (Ag-L1c is linked via perchlorate along the c axis) along the a axis as well as has a lower HOMO−LUMO gap, high transport of charge carriers may be proposed along this lattice within the adlayer. The wave functions of few MOs near the Fermi energy are plotted in Figure 7. While the filled MOs near the Fermi energy are localized on the ligand, the unfilled frontier MOs of Ag-L1a originate from the 5s orbitals of Ag and show no hybridization with the ligand states. However, the higher unfilled states show a continuum and are expected to facilitate transport of electrons. Similar to the case of Ag-L1a, there is no mixing

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CONCLUSION In summary, we have synthesized a ligand containing a C3symmetric hexapodal framework with six modified adenine rings appended at six arms of the central benzene ring which on interaction with Ag(I) ion results in a 1D coordination polymer, where each monomeric hexapodal unit is bridged by two silver ions also characterized independently by X-ray crystallography. AFM investigations of L1 growth as well as of silver-coordinated L1 (Ag-L1) on an HOPG surface reveal formation of ultrathin films of Ag-L1 on the HOPG surface, with abundant one-dimensional coordination polymer patterns. We propose that multivalent Ag coordination directly to ligand framework, which is an electron-rich system, will be helpful in tuning electron/hole injection bands in adenine-based CPs, as is also evident from the theoretical calculations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03090. AFM sample preparation, synthesis and crystal structure analysis of L1, synthesis and crystal structure analysis of silver complex of L1 (Ag-L1), comparison of adlayer pattern of L1 on surface with bulk packing, AFM images of Ag-L1 at 0.15 and 0.37 ML coverage, AFM images of Ag-L1 with a metal salt to L1 volume ratio of 10/1, and configuration of L1, Ag-L1a and Ag-L1c used in the calculations (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.G.G.: [email protected]. *E-mail for S.V.: [email protected]. ORCID

Sandeep Verma: 0000-0002-2478-8109 Present Address ‡

Department of Chemistry, University of Warwick, Warwick, U.K. Author Contributions †

R.K.S. and P.S. contributed equally to this paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the single-crystal CCD X-ray facility at IITKanpur, CSIR, for an SPM fellowship (R.K.S.), the DST for an INSPIRE fellowship (P.S.), and the UGC for a fellowship (V.V.). T.G.G. thanks the DST for financial support, and S.V. thanks the DST for a JC Bose National Fellowship.

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Figure 7. Calculated MOs of L1 and Ag-L1 along the a axis (Ag-L1a) and c axis (Ag-L1c). The geometry of Ag-L1 is obtained from the Xray structure of Ag-L1. Only MOs close to the Fermi energy are depicted.

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