Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy

Area of Science:

• Molecular electronics
• Charge transport phenomena
• Organic materials science

Background:

• Charge delocalization influences charge transport in conjugated molecules, vital for organic electronics.
• Dimerization reactions model intermolecular charge delocalization, but studies on radical anions are limited.
• Radical anion dimerization with neutral molecules is proposed to involve electron delocalization.

Purpose of the Study:

• To investigate the dimerization of radical anions of 4-n-hexyl-4'-cyanobiphenyl (6CB).
• To demonstrate and quantify electron delocalization in dimer radical anions.
• To explore the role of molecular structure and interactions in this process.

Main Methods:

• Time-resolved infrared (TRIR) spectroscopy coupled with pulse radiolysis.
• Monitoring nitrile ν(C≡N) vibrational band shifts to quantify charge distribution.
• Electronic absorption spectroscopy and electronic structure calculations for corroboration.

Main Results:

• Radical anions of 6CB undergo dimerization with neutral 6CB molecules.
• An electron is equally delocalized over the two molecules in the dimer radical anion.
• The dimerization constant (Kdim) was determined to be 3 × 10^4 M^-1.
• Hexyl chain presence and π-π interactions are essential for dimerization.

Conclusions:

• The study provides clear evidence of spatial electron delocalization over two molecular fragments in 6CB dimer radical anions.
• Nitrile IR shifts serve as a sensitive probe for electron localization in anions, independent of solvent polarity.
• Findings advance the understanding of charge transport mechanisms in organic electronic materials.