Locally functionalized single-walled carbon nanotubes (lf-SWNTs) have local defects on the sp² carbon networks of the tube wall through a limited amount of chemical modification, by which they emit red-shifted and quantum yield-enhanced photoluminescence (PL) in comparison to that of pristine SWNTs.[1-6] The newly generated PL (E₁₁*) of lf-SWNTs shows spectral shifts depending on the chemical structures of the modified molecules and dynamic wavelength shifts have been observed by protonation of amino groups[1] and saccharide recognition[6] at the doped sites. Such external stimuli responsiveness would be applicable to develop advanced sensing and imaging materials and optical devices using the near infrared (NIR) PL functions.

Based on findings in the saccharide recognition study[6], we extend molecular designs of our molecular recognition approach for development of stimuli-responsive wavelength-shift systems using lf-SWNTs. Crown ethers, which bind cations such as metal ions with selectivity based on ring sizes and donor atoms, are useful molecular motifs for metal sensing and supramolecular self-assemblies. We newly synthesized azacrown ether-modified lf-SWNTs (crown-SWNTs) and crown-SWNTs showed a new PL peak at 1138 nm (E₁₁*), which was red-shifted and intensified in comparison to E₁₁ PL at 989 nm. When a AgNO₃ solution was added to the crown-SWNTs solution, the E₁₁* PL showed a larger peak shift to a longer wavelength region compared to the E₁₁ PL. According to the cation recognition mechanism of the azacrown ether, it is considered that the observed wavelength shift is induced by electronic character changes from electron donating to withdrawing of the modified groups due to the Ag⁺ inclusion in the ring. The molecular recognition approach, therefore, is promising to create new types of NIR PL nanomaterials whose PL properties can be modulated by the molecular actions.

References: [1] Schatz, G. C. and Wang, Y. et al., J. Am. Chem. Soc. 2016, 138, 6878-6885. [2] Htoon, H. and Doorn, S. K. et al., Nat. Photon. 2017, 11, 577-582. [3] Shiraki, T. and Nakashima, N. et al., Chem. Commun. 2017, in press, DOI:10.1039/C7CC06663E. [4] Shiraki, T. and Nakashima, N. et al., Nanoscale. 2017, in press, DOI:10.1039/C7NR05480G. [5] Shiraki, T. and Nakashima, N. et al., Sci. Rep. 2016, 6, 28393. [6] Shiraki, T. and Nakashima, N. et al., Chem. Commun. 2016, 52, 12972-12975.