Exciton Trapping at Shape‐Persistent Molecular Nanotubes

Abstract

Abstract We report a series of shape‐persistent molecular nanotubes with top rim connectivity traversing from an all‐ meta ‐ ( m 4 ) to an all‐ para ‐phenylene ( p 4 ) 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 m 4 (∼31 kcal mol −1 ) to p 4 (∼90 kcal mol −1 ). Structural transitions from m 4 to p 4 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 (S 1’ ) geometry shows a progression of exciton delocalization as para ‐phenylenes are introduced into m 4 en route to p 4 , 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.