Hydrogen-bonded supramolecular assembly of didodecylmelamine and thymine at the air/water interface studied by heterodyne-detected vibrational sum frequency generation

Langmuir monolayers of didodecylmelamine (2C12mela) on thymine aqueous solutions were studied by in situ heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. The HD-VSFG spectrum of the monolayer on a 10 mM thymine aqueous solution exhibited a positive band at 1665 cm-1, a negative band at 1710 cm-1, and an extremely broad, positive band at the low wavenumber side (2600-2850 cm-1) of the CH, NH, and OH stretching region. The three bands were assigned to the 4-carbonyl, 2-carbonyl, and strongly hydrogen-bonded NH stretching modes of thymine. The signs of the bands indicate that thymine molecules were oriented with the oxygen atoms of the 4-carbonyl pointing downward, the oxygen atoms of the 2-carbonyl pointing upward, and the hydrogen atoms of the NH groups of thymine pointing upward along the interface normal, respectively. The spectral data, supported by finite-cluster spectral simulations, favor a polymer-like supramolecular structure over trimeric models. In this model, thymine molecules are bound to neighboring 2C12mela molecules in an ∼1:1 arrangement, with their methyl groups pointing downward along the interface normal and their 4-carbonyl groups sharing a common lateral orientation along the supramolecular chain.