Laterally π-Extended Polyhelicenes

Abstract

Helically coiled, semiconducting graphenic nanostructures show exceptional promise for nanoelectronics, yet their synthesis has remained challenging due to their inherently strained backbone and the difficulties associated with structural characterization. In this work, we demonstrate the synthesis and characterization of laterally π-extended polyhelicenes (EPHs), achieved through regioselective cyclodehydrogenation. Spectroscopic and microscopic analyses, including mass spectrometry, solid-state NMR, scanning-probe microscopy, and transmission electron microscopy, confirm the well-defined helical, layered architecture of the EPHs. Ultrafast terahertz spectroscopy reveals pronounced intrahelix photoconductivity, demonstrating their potential as carbon-based nanoscale conductors. The scalable synthetic approach described in this work unlocks the application potential of carbon-based helical nanostructures, paving the way for nanoinductors in nanoscale solenoids, spin-selective electronics, and future high-frequency nanoelectronic devices.

1

Synthesis of E[12]H 2 and EPH 4 by the regioselective cyclodehydrogenation reaction and their nonoptimized 3D models (when n = 5 for EPH 4), showing the helically coiled structures and Connolly surfaces. R groups are omitted for clarity. The colored dots indicate the positions of C–C bond formation during regioselective cyclodehydrogenation.

2

(A) Synthetic route to E[12]H 2. The colored dots represent the positions of chemical bond formation from 1 to 2. (B) Side-view and top-view of the single-crystal structure of E[12]H 2b. All hydrogen atoms and the alkyl chains are omitted for clarity. (C) UV–vis absorption spectra of E[12]H 2b, EPH 4, and 3, as well as emission spectra of E[12]H 2b and EPH 4 in CH₂Cl₂.

3

(A) Synthetic route to EPH 4. The colored dots represent the positions of chemical bond formation from 3 to 4. (B) MALDI-TOF mass spectra of EPH 4. A list of observed and calculated mass values of the most abundant isotopes of the oligomers (as marked with * for the pentamer) is displayed on the right. (C) The experimental and simulated isotopic distribution of pentamer [M]⁺ along with its chemical structure. (D) Solid-state ¹³C CP-MAS NMR spectrum of EPH 4.

4

(A) STM topography of an isolated EPH 4 after HV-ESD on Au(111) followed by annealing at 100 °C (I = 1 pA, U = −400 mV). Scale bar: 5 nm. (B) Zoomed STM (I = 1 pA, U = −200 mV) and (C) CO-AFM images (A = 40 pm, f = 26 kHz) of EPH 4. Scale bar: 1 nm. (D) Height and frequency shift profiles extracted from panels (B, C), respectively. (E) HR-TEM image of EPH 4 in a double-walled BNNT. Scale bar: 1 nm. (F) A TEM simulation image of heptameric EPH 4. (G) An atomic-number-correlated molecular model corresponding to (F). (H) Intensity profile extracted from panel (E) (cyan line).

5

(A) Time-resolved complex terahertz photoconductivity of E[12]H 2b, 9b, and EPH 4 (film and dispersion in toluene) normalized to the absorbed photon density, following optical excitations by 400 nm laser pulses. (B) Frequency-resolved terahertz conductivity measured at ∼1.2 ps after photoexcitation for EPH 4. The solid lines are fitted to the Drude–Smith model.