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A Technique to Functionalize and Self-assemble Macroscopic Nanoparticle-ligand Monolayer Films onto Template-free Substrates
Published on: May 9, 2014
In situ spectroscopic characterization of rectifying molecular monolayers self-assembled on gold
Alberto Girlando1, Cristina Sissa, Francesca Terenziani
1Dip. Chimica GIAF, Università di Parma and INSTM UdR Parma, Parco delle Scienze 17 A, 43100 Parma, Italy. alberto.girlando@unipr.it
Summary
Self-assembled monolayers (SAMs) of a donor-pi-bridge-acceptor chromophore exhibit a neutral ground state, unlike zwitterionic states in other forms. This difference is linked to molecular packing in SAMs, impacting their function as molecular rectifiers.
Area of Science:
- Molecular electronics
- Surface science
- Spectroscopy
Background:
- Donor-(pi-bridge)-acceptor chromophores are key components in molecular electronics.
- Previous studies indicate a zwitterionic (D+-pi-A-) ground state for these chromophores in solution, solid state, and Langmuir-Blodgett (LB) films.
- Understanding the electronic structure of chromophores in self-assembled monolayers (SAMs) is crucial for device applications.
Purpose of the Study:
- To investigate the electronic ground state of a specific donor-(pi-bridge)-acceptor chromophore (CH3CO-S-CnH2n-Q3CNQ) within SAMs on gold substrates.
- To compare the spectral properties of chromophores in SAMs with those in solution, solid state, and LB films.
- To elucidate the relationship between molecular packing, electronic structure, and rectification properties in SAMs.
Main Methods:
- Synthesis of Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ) with n=8, 10.
- Formation of SAMs on gold-coated substrates.
- Characterization using visible, Raman, and infrared spectroscopy.
- Comparison with spectra from solution, solid state, and LB films.
- Electrical characterization of SAMs to determine electron flow direction.
Main Results:
- Spectral analysis revealed a neutral, quinoid ground state for the chromophore in SAMs, contrasting with the zwitterionic state observed in solution, solid state, and LB films.
- The distinct ground state in SAMs is attributed to the near-vertical molecular packing of chromophores on the gold substrate.
- LB films showed chromophores oriented at approximately 45 degrees, consistent with a zwitterionic ground state.
Conclusions:
- The molecular packing within SAMs significantly alters the electronic ground state of the donor-(pi-bridge)-acceptor chromophore.
- The vertical orientation in SAMs promotes a neutral quinoid state, distinct from the zwitterionic state in less ordered or differently oriented films.
- These findings provide insights into the mechanism of molecular rectification by Q3CNQ molecules in SAMs, correlating spectroscopic and electrical properties.
