Exciton Trapping at Shape-Persistent Molecular Nanotubes
Abstract
We report a series of shape-persistent molecular nanotubes with top rim connectivity traversing from an all-meta- (m4) to an all-para-phenylene (p4) bridged species, including all possible members in between them. Single-crystal X-ray diffraction (SCXRD) and microcrystal electron diffraction (MicroED) data show a large torsional angle for meta-phenylenes relative to para-phenylene rings. Density functional theory (DFT) calculations reproduce the experimental torsional angles and also establish a correlation indicating a gradual increase in strain energy from m4 (∼31 kcal mol-1) to p4 (∼90 kcal mol-1). Structural transitions from m4 to p4 lead to additional correlations such as a shift in the lowest absorption wavelength from 330 to 394 nm, a sizeable red shift in the maximum emission wavelength from 444 to 546 nm, and a decrease in fluorescence quantum yield from 0.76 to 0.20, respectively. Time-dependent (TD)-DFT analysis of the relaxed excited state (S1') geometry shows a progression of exciton delocalization as para-phenylenes are introduced into m4 en route to p4, while the overall molecular size remains constant. This effect is directly related to increased π-conjugation within the nanotube's top-segment and demonstrates how exciton trapping can take place without changing the nanotube's physical size, e.g., diameter and length.
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