Synthesis and characterization of a highly strained donor–acceptor nanohoop

Abstract

A highly-strained, nitrogen-doped cycloparaphenylene (CPP), aza[6]CPP, was synthesized and then converted to a donor-acceptor nanohoop, N-methylaza[6]CPP, via alkylation of the nitrogen center. The energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for both molecules were then probed by cyclic voltammetry (CV), which revealed that the donor-acceptor nanohoop had a significantly lower LUMO energy relative to [6]CPP and aza[6]CPP. Density functional theory (DFT) revealed that the donor-acceptor nanohoop underwent a redistribution of the frontier molecular orbital (FMO) density such that a significant portion of the LUMO density resided upon the electron-deficient nitrogen-containing ring. This localization of LUMO density caused a large lowering in the LUMO energy of nearly a full electron volt, while the HOMO energy was less affected due to a large centralization of the FMO on the electron-rich phenylene backbone. This ultimately resulted in a net lowering of the HOMO-LUMO energy gap which was observed both experimentally and computationally. In addition, N-methylaza[6]CPP has a significantly lower energy LUMO than N-methylaza[8]CPP, illustrating that the FMO levels of donor-acceptor nanohoops can be tuned by adjusting the hoop size.

N‐doped cycloparaphenylenes: Tuning electronic properties for applications in thermally activated delayed fluorescence

“…Whereas for the aza‐[ n ]CPP series of compounds, the HOMO‐LUMO gap increases monotonically from 2.87 to 3.37 eV when going from the aza‐[6]CPP to the aza‐[10]CPP system, as it also happens for the unsubstituted [ n ]CPPs, the gap does not significantly alters (up to 0.1‐0.2 eV) in the other two cases, reflecting the strong localization of both frontier molecular orbitals in spatially separated fragments of the molecule. Thus, the conclusions drawn before about the impact of inserting pyridinium and methylpyridinium units are expected to also approximately hold, independently of the nanoring size, as it was also recently inferred experimentally after comparing aza‐[6]CPP with aza‐[8]CPP and N ‐methylaza‐[6]CPP + with N ‐methylaza‐[8]CPP + . The same trend regarding the evolution of the HOMO‐LUMO gap is also found for the N‐doped linear paraphenylenes, although to a lesser extent due to edge effects: the gap decreases monotonically from 3.98 to 3.81 eV for the aza‐[6]LPP and aza‐[10]LPP cases, and changes up to 0.1‐0.3 eV in the other cases, again reflecting some localization of both frontier molecular orbitals in spatially separated fragments of the charged molecules.…”

Nanohoop Rotaxanes from Active Metal Template Syntheses and Their Potential in Sensing Applications

“…Additionally, as a consequence of breaking molecular orbital symmetry, it should be noted that both 3 and 4 were fluorescent, with 3 emitting at λ max of 509 nm ( Φ =0.14) and 4 at λ max of 476 nm ( Φ =0.62). Of note, the all para ‐substituted nanohoop with a single embedded pyridine is non‐emissive; a detailed investigation of the result of symmetry breaking on nanohoop fluorescence is disclosed in a separate study…”

Expanding the Chemical Space of Biocompatible Fluorophores: Nanohoops in Cells

“…[30] Additionally, due to Laporte forbidden HOMOàLUMO transitions, all nanohoops share a common absorption maxima (labs = 340 nm) with high absorption coefficients (e) and large effective Stokes shifts ranging from 100-200 nm depending on size. [31][32][33][34] Taken together, the nanohoop scaffold offers the possibility of multiplexed imaging using a single excitation source. Moreover, the non-planarity of the benzene rings in the nanohoop also leads to better solubility when compared to planar aromatic systems.…”