Photoactive Zn(II)Porphyrin–multi-walled carbon nanotubes nanohybrids through covalent β-linkages

https://doi.org/10.1016/j.matchemphys.2013.09.002Get rights and content

Highlights

  • β-linked Zn(II)Porphyrin–MWCNT nanohybrids were prepared through direct or amide bond.

  • Efficient and mild functionalizations were achieved using diazonium chemistry.

  • Good nanohybrid dispersibility was obtained in low boiling point solvent.

  • Nanohybrids showed strong photoinduced electronic transfer.

  • The emission quenching was higher for the π-expanded system.

Abstract

Donor–acceptor nanohybrids by a covalent linkage between the β-position of a Zn(II)Porphyrin and multi-walled carbon nanotubes are reported for the first time, in a closer analogy to the natural light harvesting systems, which are based on β-substituted porphyrinoid structures, the chlorophylls. An unique and direct connection was established through the immobilization of the Zn(II)(β-NH2-tetraphenylporphyrin), using diazonium chemistry, in order to afford i) a short and conjugated linkage between the two aromatic systems and ii) an amide bond resulting from a three-step functionalization synthesis. Electronic and steady-state fluorescence spectroscopies confirmed high photoinduced electron communication through the β-linkage when compared to analogous meso-phenyl linkers, stating its positive effect. The procedure involving the amide linkage allowed higher chromophore loadings; however, the direct conjugated bond showed improved photoinduced activity and a different emission pattern that can be associated with intense communication within the expanded π-system MWCNT–metalloporphyrin.

Graphical abstract

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Preparation and photo-induced activity of two donor–acceptor nanohybrids is reported based on different linkages through β-position of porphyrin core to MWCNT, direct conjugation and amide bond.

Introduction

Light-harvesting structures and processes, acting during natural photosynthesis, have been a fundamental source of inspiration on the fabrication of solar energy conversion systems. This relevant issue in the actual eco-energetic context is addressing intense research [1], [2], [3], and a large number of porphyrinoid chromophores have been applied as biomimetic models of chlorophylls, aiming to understand the natural processes and to prepare efficient devices [4], [5].

The (metallo)porphyrins studied as light harvesters-electron/energy donors evidence the effect of the porphyrin chemical structure, the central metal and the spacer used to bind the acceptor material [6]. In the context of porphyrin sensitized solar cells (PSSC), high conversion efficiencies were achieved using zinc as central metal [7] and conjugated linkers [6]; in addition, the use of expanded porphyrins with a broader absorption window also showed good potentiality [8]. The position of the linker on the porphyrin ring, namely on β-pyrrollic or on meso-positions, (Fig. 1) was also tested in PSSC; when a Zn(II) β-substituted derivative was compared with the equivalent Zn(II) meso-phenyl porphyrins, the β-substituted ones had considerably better performance in the same device conditions [9]. A number of asymmetric meso-porphyrins, i.e. with different meso-substituents and with the linker held through one specific position, in order to direct the connection, have also been applied as photovoltaics. In this context, the derivatives with a double/triple bond directly connected to the porphyrin ring, also showed to be more efficient than derivatives connected through meso-phenyl moieties [10]. Nevertheless, these asymmetric porphyrins are associated with complex and low efficient synthetic procedures [11]. Recently, we have reported a versatile synthesis of Zn(II) β-NH2Porphyrins [12], which allow to combine an easy synthesis of the porphyrin structure with the possibility to establish an unique and direct linkage to the porphyrin ring not using a meso-phenyl group [13].

Carbon nanomaterials, with an extended π-system, raised vast interest on the unique features of these materials for photo-electronic and photo-catalytic applications [14], namely the highly delocalized electronic structure, mechanical robustness, structural homogeneity, high surface areas, together with the possibility of appropriate chemical derivatization [15]. Several hybrids of carbon nanomaterials as acceptors with metalloporphyrins as donors have been reported, leading to improved ratios of charge separation to charge recombination rates [16]. These derivatives showed to be useful materials for a variety of light induced applications, such as photovoltaics [17], photoelectronic [18], photocatalysis [19], fluorescent imaging [20] or materials showing optical limiting properties [21]. In particular, the high aspect ratio of carbon nanotubes (CNT) allow efficient charge/electron transportation along the 1D-tubular structure and thereby show reduced probability of back transfer [22].

Most of the previous work on (metallo)porphyrin-CNT nanohybrids have considered single-walled carbon nanotubes (SWCNT) [23]. When compared to multi-walled carbon nanotubes (MWCNT), they show better solution processability and easier covalent functionalization, as a result of the higher curvature of the graphene wall [24]. However, the extensive covalent functionalization of SWCNT leads to a significant disruption of the conjugation and decrease of their charge mobility, thus the electronic applications of SWCNT have been largely developed using non-covalent functionalizations [7], [25], [26], although, covalent linkages would be preferable since afford more efficient energy/electron communication [27], [28] and more robust structures. In the covalent functionalization of MWCNT [29], the modification of the nanotube outer-shell keeps inner shells protected, which account for the preservation of their initial conductivity properties [24]. Moreover, the covalent functionalization should avoid the commonly used, strong oxidative treatments [30], since these led to open caps and extensive disruption of the CNT graphitic surface and the functional groups are mainly introduced on the edges and are not homogenously spread on the sidewalls. Therefore, less aggressive functionalization methods are preferable, as the in situ generation of diazonium salts, which also allow to prepare C–C direct bonds with the carbon materials [31].

The approaches previously reported to prepare covalent hybrids (metallo)porphyrin-carbon nanotube considered asymmetric porphyrins carrying meso-phenyl groups and in one of the positions a hydroxyphenyl [29], or an aminophenyl functionality [27], namely, the meso-aminophenyltriphenyporphyrin was covalently immobilized, using diazonium chemistry, to afford both hybrids SWCNT-meso-TPP [28] and MWCNT–meso-TPP [21]. The same porphyrin was also immobilized on oxidized MWCNT through amide bonds MWCNT–CONH–meso-TPP [20]. These approaches have exclusively considered binding through the meso-phenyl groups of the non-metallated porphyrin. Consequently, there is still room for optimization and further understanding of structure–activity effects, namely, it is important to exploit the linkage through the β-position of the porphyrin core. Herein, novel Zn(II)Porphyrin–MWCNT hybrids were prepared by reaction of MWCNT with a zinc β-aminoporphyrin derivative prepared through an acid free procedure [12]. Two different linkages were established using diazonium chemistry, a direct conjugation between the two aromatic systems, MWCNT and metalloporphyrin, or an amide linker. The two hybrid materials were evaluated in terms of anchoring efficiency, solution processability and photo-induced electron/energy communication.

Section snippets

Instruments and measurements

X-ray photo-emission spectroscopy (XPS) was performed in Centro de Materiais da Universidade do Porto (CEMUP, Porto, Portugal) in a VG Scientific ESCALAB 200A spectrometer using non-monochromatic Al Kα radiation (1486.6 eV). The measurements were carried out using pellets of the materials. Binding energies were calibrated relative to the C1s peak at 284.6 eV. The XPS spectra were deconvoluted using the software XPSPEAK 4.1 with no preliminary smoothing. Symmetric Gaussian–Lorentzian product

Results and discussion

The anchoring methods used for the grafting of Zn(II)β-amino-meso-tetraphenylporphyrin (Znβ-NH2TPP) onto the surface of multi-walled carbon nanotubes (MWCNT) are shown in Fig. 1. Nanohybrid I was prepared through in situ formation of a ZnTPP β-diazonium salt in the presence of MWCNT, to afford a direct conjugation between porphyrin moiety and MWCNT (Fig. 1a).

Attending to the steric hindrance associated with the direct reaction of the big chromophore with the carbon nanotube sidewalls, the

Conclusions

Novel donor–acceptor nanohybrids of a Zn(II)Porphyrin covalently linked through the β-position to MWCNT were prepared, using direct conjugated or amide bond linkages. The functionalizations were based on the in situ generation of diazonium salts avoiding opening and shortening of the nanotubes and strong disruptions on the CNT sidewalls; in addition, improved dispersibility was obtained in a low boiling point solvent, such as THF in the presence of ammonium acetate, accounting for easier

Acknowledgments

This work was funded by Fundação para a Ciência e a Tecnologia (FCT) and FEDER through grant nr. PEst-C/EQB/LA0006/2011. Monika E. Lipińska also thanks FCT for a PhD grant SFRH/BD/66297/2009.

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