(Invited) Diameter-Dependent Stacking of Squaraine Dye Molecules inside Chirality-Sorted Single-Wall Carbon Nanotubes

Filling of single-wall carbon nanotubes (SWCNTs) with organic dye molecules is a promising strategy to add new functionalities, for instance by aligning dipolar dyes in a one-dimensional (1D) head-to-tail coupled chain to obtain a giant enhancement of their nonlinear optical response,[1] or by photo-sensitizing the tubes in new wavelength ranges by excitation energy transfer (EET) from the dye to the SWCNTs, useful for photo-conversion applications.[2] The 1D stacking of molecules inside SWCNTs modulates both the properties of the molecules and of the SWCNTs. We found that the EET of squaraine-dye-filled SWCNTs reveals a pronounced SWCNT-diameter-dependence of the dye absorption, indicating a diameter-dependent stacking. By performing IR fluorescence excitation spectroscopy on a range of chirality-enriched samples of dye-filled SWCNTs with different chirality distributions, each shifted dye absorption wavelength observed through the EET can be unambiguously assigned to a specific encapsulating SWCNT chirality.[3] This reveals a systematic trend with diameter that can be associated with the J-aggregate- and H-aggregate-like interactions expected in close-packed single- and double-file stackings, as corroborated by a simple geometric model, and enables tuning of the optical properties of the 1D molecular arrays through the selection of proper SWCNT diameters. [1] S. Cambré, J. Campo, C. Beirnaert, C. Verlackt, P. Cool, W. Wenseleers, Nature Nanotechnol. 10 (3) 248-252 (2015). [2] S. van Bezouw, D.H. Arias, R. Ihly, S. Cambré, A.J. Ferguson, J. Campo, J.C. Johnson, J. Defillet, W. Wenseleers, J.L. Blackburn, ACS Nano 12 (7) 6881-6894 (2018). [3] S. Forel, H. Li, S. van Bezouw, J. Campo, L. Wieland, W. Wenseleers, B.S. Flavel, S. Cambré, Nanoscale 14 , 8385-8397 (2022).