Length-Dependent Conduction of Polyynes: Searching for the Limit of the Tunneling Regime
Bibliographic record
Abstract
Rigid, conjugated molecules are excellent candidates as molecular wires since they can achieve full extension between electrodes while maintaining conjugation. Molecular design can be used to minimize the accessible pi surface and interactions between the bridging wire and the electrode. Polyynes are archetypal molecular wires that feature a rigid molecular framework with a cross-section of a single carbon atom. Understanding the behavior of polyynes in molecular junctions is essential for testing models of length versus electron transport. We report the construction of molecular junctions using polyynes with a well-defined length up to ca. 5 nm in devices characterized by scanning tunneling microscopy break junctions. The polyynes, Py**[ n ] ( n = 4, 6, 8, 10, 12, 16), are end-capped with pyridyl groups, and we demonstrate good agreement between the length of the molecular junction and the calculated molecular length, with an average discrepancy of just 0.1 nm. This highlights the power of STM-BJ experiments to accurately determine the molecular length. The range of molecular lengths, from 1.8 to 4.8 nm, mark this as the most accurate determination of β in polyynes to date (β = 2.2 ± 0.1 nm –1 ). We have applied a model based on the single and triple bond lengths to interpret β-values, which predicts β = 1.9 nm –1, consistent with the experimental value. This model also confirms that electronic coupling in polyynes is unaffected by the rotation about the single bonds. At all molecular lengths, we observe conductance in the tunneling regime due to the long effective conjugation length of polyynes.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".