Aromatic Molecular Dimer on Single-Walled Carbon Nanotubes
Authors: Baek Jinseok , Umeyama Tomokazu , Imahori Hiroshi 
Published: February 2020

In this study, we focus on the sidewall of a single-walled carbon nanotube (SWNT) as a unique platform to probe ‘the smallest molecular assembly’ (dimers), which can be fully characterized microscopically, spectroscopically, and theoretically [1]. Theoretical calculations predicted that reactions of SWNT with aryl radicals lead to the single bond formation between the attached aryl radicals and the SWNT sidewall [2], although no experimental evidence is available regarding the binding mode. Then, carbon atoms near the binding site are activated, resulting in the preferential addition of of the next aryl radical at the nearby carbon atoms. 

   On the basis of this hypothesis, we successfully synthesized the dimeric structure of a model π-aromatic compound, pyrene, on SWNT and the monomeric structure for comparison. Scheme 1 presents the synthetic strategy for the selective preparation of paired and unpaired pyrenes on the sidewall of SWNT. The paired pyrenes were attached to purified SWNTs (p-SWNT) using a one-step method, that is, the direct addition of 4-(1-pyrenyl)phenyl (PP) radicals onto the SWNT sidewall (Py-1-SWNT). In addition, unpaired pyrenes on SWNT sidewalls were also synthesized selectively using a two-step method, that is, a Suzuki coupling reaction between a pre-prepared iodophenyl-functionalized SWNT (PhI-SWNT) and a pyrene boronic ester (Py-2-SWNT). The dimeric and monomeric natures of the structures located on the SWNT sidewalls in Py-1-SWNT and Py-2-SWNT, respectively, were substantiated using UV–vis absorption measurements. The Py-2-SWNT spectrum exhibited only a pyrene π-π* band at 350 nm and a broad structureless absorption band extending to the near-infrared region corresponding to the SWNT. On the other hand, the Py-1-SWNT spectrum revealed an additional distinct band at 450 nm with an intensity comparable to that of a pyrene π-π* band, which is due to the existence of a dimeric interaction between the pyrene rings in Py-1-SWNT.

   More importantly, we have successfully visualized for the first time the linking of small polycyclic planar molecules onto the outside of nanocarbon scaffolds at a single-molecule level using the HR-TEM technique (Figure 1a,b). The stability of the pyrene stacking and unpaired pyrene was corroborated by analyzing sequential HR-TEM images; the dimeric pyrenes did not separate or vanish even though the SWNT scaffold vibrated due to the electron-beam energy used (60 kV). In addition, on the basis of the structures observed in the HR-TEM, we performed density functional thoeory (DFT) (Figure 1c-f) and time-dependent density functional theory (TD-DFT) calculations to obtain the dimer structure, relative energy attached to the SWNT sidewall, and excitation energy, which are well consistent with experimental results.

   Furthermore, pump-probe transient absorption (TA) measurements allowed us to elucidate the interaction of the pyrene dimer in the excited state. Py-2-SWNT exhibited nearly identical TA spectrum compared to that of PhI-SWNT, that is, the sole occurrence of energy transfer from the pyrene excited state to the SWNT, followed by the decay of the SWNT excited state to the ground state. In contrast, the TA spectrum of Py-1-SWNT showed the occurrence of electron transfer from the excited state of the pyrene dimer to the SWNT, which was complimented with spectroelectrochemical measurements. Notably, this difference in the TA spectra of Py-1-SWNT and Py-2-SWNT results from the π-π interaction between the pyrenes, which also affects the oxidation potential of the pyrene dimer to become lower than that of the pyrene monomer, making it possible to form the charge-separated state consisting of pyrene dimer radical cation and SWNT radical anion.

References

[1] T. Umeyama, J. Baek, Y. Sato, K. Suenaga, F. Abou-Chahine, N. V. Tkachenko, H. Lemmetyinen and H. Imahori, Nat. Commun.
6, 7732 (2015).

[2] J.-X. Zhao and Y. H. Ding, J. Phys. Chem. C,
112, 13141–13149 (2008).

Figure 1