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Time-Resolved Observation of Chiral-Index-Selective Wrapping on Single-Walled Carbon Nanotube with Non-Aromatic Polysilane
In the present paper, we ascertain two novel findings
on chiral-index-selective
binding/separating of single-walled carbon nanotubes (SWNTs) with
a nonaromatic polymer, polyĀ(dialkylsilane) (PSi). PSi is a typical
Ļ-conjugated polymer, composed of alkyl side chains attached
to the silicon (Si)-catenated main chain. First, PSiās with
linear alkyl side chains showed significant diameter-selective wrapping
for SWNTs with ca. 0.9 nm in diameter, resulting in the selective
separation of (7,6) and (9,4) SWNTs. Its driving force was demonstrated
to be cooperative CHāĻ interactions among the alkyl side
chains of PSiās and the curved graphene of SWNTs. Second, the
dynamic wrapping behavior of PSiās onto SWNTs was elucidated
with time-resolved UV spectroscopy. Highly anisotropic UV absorption
of PSi along the Si main chain was utilized as a āchromophoric
indicatorā to monitor the global/local conformations, which
enabled us to track kinetic structural changes of PSiās on
SWNTs. Consequently, we concluded that upon wrapping, flexible/helical
PSi with an average dihedral angle (Ļ) of 145Ā° and Kuhnās
segment length (Ī»<sup>ā1</sup>) of 2.6 nm interconverted
to the more stiffer/planar conformation with 170Ā° and Ī»<sup>ā1</sup> of 7.4 nm. Furthermore, through kinetic analyses
of the time-course UV spectra, we discovered the fact that PSiās
involve three distinct structural changes during wrapping. That is,
(i) the very fast adsorption of several segments within dead time
of mixing (<30 ms), following (ii) the gradual adsorption of loosely
wrapped segments with the half-maximum values (Ļ<sub>1</sub>) of 31.4 ms, and (iii) the slow rearrangement of the entire chains
with Ļ<sub>2</sub> of 123.1 ms, coupling with elongation of
the segment lengths. The present results may be useful for rational
design of polymers toward chiral-index-selective binding/separating
of desired (<i>n</i>,<i>m</i>) SWNTs