Supramolecular Electronic Interactions in PorphyrinâSWCNT Hybrids

Letter pubs.acs.org/OrgLett

Supramolecular Electronic Interactions in Porphyrin−SWCNT Hybrids through Amidinium−Carboxylate Connectivity Laura Rodríguez-Pérez,†,§ Sonia Vela,†,§ Carmen Atienza,† and Nazario Martín*,†,‡ †

Departamento Química Orgánica, Facultad C. C. Químicas, Universidad Complutense de Madrid, Av Complutense s/n, 28040 Madrid, Spain ‡ IMDEA-Nanociencia, C/Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain S Supporting Information *

ABSTRACT: New supramolecular (metal)porphyrin/SWCNT hybrids have been synthesized through a combination of hydrogen bond and electrostatic interactions. Our experimental findings reveal through different techniques (XPS, TGA, UV−vis, Raman, and TEM) an efficient n-doping of the SWCNT from the electron donor (metal)porphyrin through the efficient and strong amidinium−carboxylate connectivity.

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ince its discovery by Iijima and Bethune in 1993,1 singlewall carbon nanotubes (SWCNTs) have received significant attention due to their singular structural features and excellent mechanical, thermal, and electronic properties which make them materials of choice for applications in the development of advanced carbon-based materials for molecular electronics.2 However, a main drawback of CNTs is their poor solubility, which severely limits their use as materials for practical devices. Hence, different approaches have been investigated in order to improve the solubility of SWCNT, namely covalent, supramolecular, and mechanically interlocked bonds,3 and simultaneously be able to obtain electroactive donor−acceptor (D−A) systems, which are suitable for application in artificial solar cells and other non-natural optoelectronic devices.4 On the one hand, covalent D−A dyads based on SWCNT have been reported in which the donor units are covalently linked to the sidewalls and cap of SWCNTs.5 In such cases, the electronic structure of the SWCNTs is irreversibly modified, and in turn, the overall π-system reveals a notable perturbation. On the other hand, non-covalent functionalization of SWCNTs has mostly been based on relatively weak π−π supramolecular interactions. In the resulting D−A dyads, SWCNT are coated/ wrapped, and thus, intertube interactions are minimized, whereas the intrinsic electronic properties are preserved.6 As a representative example of electron-donor moiety, porphyrins represent a class of fascinating molecules that exhibit remarkable spectroscopic, redox, and photophysical properties with long lifetimes up to milliseconds that make them promising candidates for applications in optoelectronics.7 Hence, a great variety of metalloporphyrins, porphyrin− © 2017 American Chemical Society

oligomers, and porphyrin−polymers have been used to form donor−acceptor dyads with SWCNTs through both covalent and supramolecular chemical approaches.8 With this in mind, here we present a new approach, which combines two important aspects with respect to the previously reported examples, namely, (i) the robustness of the covalent functionalization through a simple and straightforward Tour reaction on the SWCNT and (ii) the simpler synthetic availability and wider versatility of the supramolecular chemistry. For this purpose, we have decorated the surface of the SWCNT with carboxyl groups by introducing benzoic acid units. These systems can be considered as versatile scaffolds for connecting to other electroactive systems by means of straigthforward non-covalent connectivities. To assemble the donor moieties, we have synthesized different porphyrins (2a− c) endowed with amidinium functional groups as a complementary group in order to be anchored to the SWCNT’s surface through efficient H-bonding and electrostatic interactions (Scheme 1). To the best of our knowledge, this is the first example of noncovalently associated SWCNTporphyrin ensembles (1a·2a−c), where the electronic communication occurs through the strong electrostatic and H-bond supramolecular forces existing between the strong and robust amidinium−carboxylate groups. The syntheses of the benzoic acid modified SWCNT (1a) and porphyrinamidines (2a−c) were carried out by following a previously described procedure.9 To obtain the supramolecular ensembles (1a·2a−c), the SWCNT-Ph−COOH (1a) was Received: July 21, 2017 Published: September 6, 2017 4810

DOI: 10.1021/acs.orglett.7b02239 Org. Lett. 2017, 19, 4810−4813

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Organic Letters

Csp3, CO, C−O, and a shake up peak π−π*)10 and (ii) the atomic percentage of O 1s increases from 4% in pristine SWCNT (1) to 12% in 1a (Table S1) due to the covalent functionalization with p-phenylcarboxyl groups on the SWCNT surface (Table S1). Note here that no detectable signal of nitrogen (N 1s) was observed in pristine SWCNT (1) and 1a (for more details, see Figures S1 and S2). Furthermore, the survey analysis of the supramolecular ensembles (1a·2a−c) presented the same deconvolution pattern as 1a for the C 1s and O 1s peaks together with the N 1s signal at ∼400 eV (Figure 1). It should be taken into account that for 1a·2b the analysis of the Zn 2p region showed the presence of two sharp peaks at 1022.2 and 1045.2 eV corresponding to Zn 2p3/2 and Zn 2p1/2. In the case of 1a·2c, two sharp lines were observed in the Ni region at 872.0 and 855.5 eV, which are attributed to Ni 2p1/2 and Ni 2p3/2, respectively (Figure 1). In XPS, we could analyze in more detail the N 1s signal observed in the supramolecular ensembles (1a·2a−c), which indicates the porphyrin nature anchored to the SWCNTs (Figure 1 and Figure S3) and the presence of the amidinium group. In this regard, the N 1s signal was shifted from 400 eV (1a·2a) to 398.8 eV (1a·2b-c) due to the increase of the electron donor character in the metal−porphyrin unit. Indeed, we can assign for 1a·2a a double-component peak shape as expected with almost equal abundance due to the difference in electronic structure between the protonated and unprotonated nitrogen atoms in the porphyrin core and the contribution of the amidinium group at 400.2 and 398.9 eV (Figure S3). However, the N 1s signal obtained for 1a·2b,c presented one main deconvolution band (relative abundance 84.6% for 1a·2b and 88.9% for 1a·2c, Table S2) together with a small contribution probably due to the amidinium group. This fact is due to their metal complexation process, which results in four equivalent nitrogen atoms as it has previously been reported for these systems. Furthermore, the weak shake up satellite structure of the main N 1s line at 403.1 eV (1a·2a), 403.6 eV (1a·2b), and 403.8 eV (1a·2c) were attributed to electronic transitions into π* antibonding orbitals. To estimate the amount of porphyrinamidine (2a−c) anchored to the SWCNT (1) surface, thermogravimetic analyses (TGA) were accomplished (Figure S4). Initially, TGA for 1a showed a weigh loss of 17% attributed to the covalent functionalization present on the SWCNT that could be averaged to one organic functional group per 49 carbon atoms. Likewise, supramolecular ensembles (1a·2a−c) showed the same tendency under the same analytical conditions with a 25% of total weigh loss corresponding to the organic supramolecular assembly. Further studies on the supramolecular interaction between the SWCNT−carboxylate acid and porphyrin−amidinium were investigated by means of absorption and fluorescence spectroscopies. For UV−vis titrations with 2a or 2b, a suspension of benzoic acid enriched SWCNT (1a) was used; for more details, see the Supporting Information. Figure 2 shows the electronic absorption spectral changes of 2a (1.6 × 10−5 M) upon addition of 1a in o-DCB at room temperature. Depletion of the Soret band of the porphyrin is accompanied by the presence of an isosbestic point at 439 nm and the emergence of a new transition at 457 nm, which is assigned to the supramolecular ensemble 1a·2a. Furthermore, the increasing amount of 1a also produces a bathochromic shift in the Q bands affording a broad band at 677 nm (Figure 2A). In order to confirm the supramolecular interaction between amidinium−carboxylate

Scheme 1. Molecular Structures with Schematic Supramolecular Structure of SWCNT (1a) and Porphyrin Derivatives (2a−c)

dispersed in 1,2-dichlorobenzene (o-DCB), and an excess of the corresponding 2a−c was added. The supramolecular ensembles (1a·2a-c) were filtered through a membrane filter (OMNIPORE, 0.1 μm pore size), and the resulting solid was washed with o-DCB to remove the excess of unbounded porphyrinamidine. These supramolecular ensembles (1a·2a−c) were studied through different techniques such as X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and UV−vis, Raman, and transmission electron microscopy (TEM). The first evidence of the presence of supramolecular ensembles (1a·2a-c) was obtained using the XPS spectroscopy. XPS has proven to be a powerful tool to identify the surface groups; their oxidation state and the relative abundance in the SWCNT functionalized, either covalent functionalization as SWCNTPh−COOH (1a) or supramolecular ensembles (1a·2a-c). First, the XPS was recorded for pristine SWCNT (1) and 1a, giving rise to the spectroscopic signature of the carbon (C 1s) and oxygen (O 1s) signals at 284.6 and 533.3 eV, respectively (Figure 1 and Figures S1 and S2). Herein, we can highlight two important aspects due to the covalent functionalization: (i) the deconvolution of the C 1s energy level signals for 1a was accomplished with five different Gaussian−Lorentzian curves corresponding to the different carbon oxidized states (Csp2,

Figure 1. XPS survey spectra of the supramolecular ensembles 1a·2a (blue), 1a·2b (purple), and 1a·2c (violet). 4811


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Figure 2. (A) Absorption spectral changes of 2a in o-DCB (1.6 × 10−5 M) upon addition of 1a. (B) UV−vis spectra of the reversible process of the supramolecular ensemble 1a·2a formed after addition of MeOH.

moieties, two additional experiments were recorded: (i) the addition of methanol to the supramolecular ensemble formed should produce a break of the hydrogen bond recovering the initial features of the free porphyrin;11 (ii) UV−vis titrations of 2a or 2b with pristine SWCNT (1), as control experiment, were carried out in order to evaluate the π−π interactions between porphyrin and SWCNT. On one hand, a reversible process was observed after the addition of methanol to the supramolecular ensemble 1a·2a formed. A decrease of the new transition bands at 457 and 677 nm and the appearance of the features bands of the free porphyrin were observed (Figure 2B). On the other hand, the UV−vis titration of 2a with pristine SWCNT (1) as control experiment showed an increase of the absorption over the whole spectrum due to increase of SWCNT. However, negligible changes were observed on the features bands of the porphyrin (Figure S5). These facts confirm that the supramolecular ensembles occur through the amidinium− carboxylate interaction instead of π−π interactions between porphyrin and SWCNT. For the supramolecular ensembles (1a·2b,c), the evolution of the spectra during the titrations against 1a is similar to that previously observed for 1a·2a. The spectral changes showed, namely, a decrease of the absorbance at λ= 428 nm (1a·2b) and 421 nm (1a·2c) during the first addition of 1a and the appearance of two isosbestic points with the concomitant increase in the absorbance between 450 and 700 nm (Figure S6). The Raman scattering spectra shown in Figure 3 were recorded (λexc= 785 nm) for pristine SWCNT (1), 1a, and the supramolecular ensembles (1a·2a−c) considering three important signatures: the radial breathing mode (RBM) bands, the D mode, and the MT-tangential mode or G band. The Raman-active RBM bands, located at 260 cm−1 and related with the SWCNT diameter and chiralities, confirmed the presence of only semiconducting tubes and their pretty sharp shape indicated a quite small size distribution in all the supramolecular ensembles (Figure 3).12 The D-band with maximum around 1293 cm−1 is active by disorder in the sp2 carbon network and has been considered as sensitive prove to SWCNT functionalization. Hence, upon covalent modification the D band increases up to 0.43 due to the increase of the SWCNT sp3 character remaining constant for the subsequent supramolecular ensembles (1a·2a−c). More interesting is the behavior of the G band located at 1590 cm−1 that can be related with several SWCNT properties as the charge transfer arising from doping a SWCNT. In this regard, shifts toward lower frequencies compared with pristine SWCNT reveal an ntype SWCNT doping after supramolecular functionalization

Figure 3. Raman spectra of 1 (black), 1a (dash black line), 1a·2a (red), 1a·2b (green), and 1a·2c (blue) excited 785 nm. The inset shows the magnification of the G band.

being around 4 cm−1 for 1a·2a, 18 cm−1 for 1a·2b, and 9 cm−1 for 1a·2c (Figure 3, inset). Finally, transmission electron microscopy (TEM) were performed for the supramolecular ensembles (1a·2b,c). Pioneering studies by Nakamura and co-workers have offered extensive evidence of the observation of a variety of small organic molecules under TEM in the vicinity of carbon nanotubes.13 In our cases, images of the pristine SWCNT (1) functionalized SWCNT (1a) and supramolecular ensembles (1a·2b) obtained under a JEOL-JEM 2100F microscope are shown in Figure S6. In TEM images for the supramolecular ensembles (1a·2b), it is possible to appreciate high dispersion of individual SWCNTs and densely covered walls compared to 1 (Figure S7). In summary, we have reported the synthesis and characterization of a SWCNT functionalized through Tour reaction with benzoic acid moieties to anchor supramolecularly different porphyrins endowed with an amidininum group. Our investigations revealed a strong electronic interaction between carboxylate−amidinium pairs by a variety of techniques and a high degree of chemical functionalization.

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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.orglett.7b02239. Experimental methods and relevant spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nazario Martín: 0000-0002-5355-1477 Author Contributions §

L.R.-P. and S.V. contributed equally.

Notes

The authors declare no competing financial interest. 4812


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ACKNOWLEDGMENTS Financial support by the European Research Council (ERC320441-Chirallcarbon), the Ministerio de Economiá y Competitividad (MINECO) of Spain (project CTQ2014-52045-R) and the Comunidad Autónoma de Madrid (PHOTOCARBON project S2013/MIT-2841) is acknowledged. C.A. is thankful for the Ramón y Cajal grant, and S.V. thanks the Spanish Ministry of Education for an FPU grant.

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Supramolecular Electronic Interactions in PorphyrinâSWCNT Hybrids

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