Conductance switching in diarylethenes bridging carbon nanotubes

Area of Science:

• Molecular electronics
• Organic semiconductor physics
• Nanotechnology

Background:

• Diarylethene derivatives are photoswitchable molecules with potential in molecular electronics.
• Carbon nanotubes (CNTs) offer unique electrical properties for nanoscale devices.
• Understanding the interplay between molecular structure, electronic states, and CNT contacts is crucial for photoswitchable CNT-based systems.

Purpose of the Study:

• To theoretically analyze the photoswitching mechanism of diarylethene molecules bridging CNT contacts.
• To investigate the influence of molecular electronic states (HOMO, LUMO) on photochromism and charge transport.
• To explore the impact of CNT contact geometry (armchair vs. zigzag) and molecular interactions on device performance.

Main Methods:

• Theoretical analysis of molecular electronic structures and energy levels.
• Investigation of excited state lifetimes and photoexcitation pathways.
• Modeling of charge transport through molecular junctions with CNT electrodes.
• Examination of the effects of molecular isomerism, substituent groups, and multi-molecule coherence.

Main Results:

• Short lifetimes of the lowest unoccupied molecular orbital (LUMO) prevent efficient photoexcitation for photochromic ring opening.
• Photoexcitation leads to oxidation, potentially reducing quantum yield for ring closing or enabling oxidative ring closing.
• Optimal charge transport requires the highest occupied molecular orbital (HOMO) energy levels to align with the CNT Fermi level.
• Conductance in zigzag CNTs is dominated by surface states, leading to low switching ratios, while armchair CNTs show isomer-dependent resistance due to HOMO shifts.
• Coherent interactions between multiple bridging molecules result in nonlinear conductance changes, with a significant conductance increase observed for three molecules.

Conclusions:

• The photoswitching of diarylethene-CNT junctions is limited by molecular excited state lifetimes and photo-induced oxidation.
• Molecular energy level alignment, particularly the HOMO level relative to the CNT Fermi level, dictates transport properties.
• CNT contact type significantly influences device sensitivity to molecular changes, with armchair contacts being more responsive.
• Coherent intermolecular interactions can dramatically enhance conductance, offering a pathway for high-performance molecular switches.