Modulation Mechanisms of Charge Transport in Nitrogen-Containing π-Conjugated Molecules by Acidity, Light, and Solvent Polarity

Abstract

This dissertation presents a systematic investigation of single-molecule molecular electronics, focusing on controllable strategies and physical mechanisms for modulating the conductance of molecular junctions through external stimuli and chemical environments. Using the scanning tunneling microscope break-junction (STM-BJ) technique as the core experimental platform, and integrating density functional theory (DFT) with quantum transport calculations, the work elucidates how acid–base interactions, optical excitation, and solvent polarity influence charge transport in nitrogen-containing π conjugated molecules represented by diazapentalene and triazole derivatives. By establishing a closed-loop chain of evidence from experimental characterization to theoretical interpretation, the study offers methodological foundations and mechanistic insights for the design of single-molecule switches, and chemical/light-responsive devices.

Methodologically, the dissertation first reviews the principles and data processing workflow of STM-BJ, including quantum tunneling, conductance traces and histogram analysis, instrument noise mitigation and stability control, and configuration selection based on repetitive opening/closing cycles. A standardized experimental protocol is then established for constructing and characterizing single-molecule junctions. In parallel, transport calculations within the DFT and non-equilibrium Green’s function frameworks are employed to analyze molecular level alignment, electrode-molecule coupling strength, and configuration sensitivity, enabling consistent cross-validation between experiment and theory.

The research proceeds along three thematic lines. First, acid-mediated conductance modulation in diazapentalene junctions is investigated, revealing the impact of protonation/deprotonation on level alignment and electrode coupling, and the resulting voltage-dependent conductance control. Second, the cooperative modulation of diazapentalene conductance by light and solvent polarity is examined, assessing how light-induced states and the dielectric environment affect transport channels. Third, triazole derivative junctions are studied under optical and pH perturbations, comparing structure-property relationships across anchoring and substitution patterns and identifying their potential as dual-responsive (chemical/optical) molecular components. Furthermore, the dissertation explores STM-BJ signatures of light-induced organic long-persistence behavior, providing initial evidence and an experimental paradigm for single-molecule units exhibiting memory or hysteresis effects.

The principal contributions include: establishing an STM-BJ measurement and analysis workflow tailored to multidimensional coupling of external fields and chemical environments; clarifying modulation pathways and plausible mechanisms by which PH, light, and solvent polarity regulate conductance in representative nitrogen-containing π systems; and, through reciprocal validation between experiment and DFT based transport theory. The findings support verifiable strategies for versatile control of molecular conductance, and contributes to the continued development of newly molecular circuit design.

Date of Award	9 Feb 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Haixing LI (Supervisor)