Self-Assembled Monolayers of Double-Chain Disulfides of Adenine on Au: An IR-UV Sum-Frequency Generation Spectroscopic Study

We have synthesized double-chain disulfides of adenine with different chain lengths (n = 2, 4, 5, 9, and 10) and studied their self-assembled monolayers (SAM) on gold surface by IR-UV doubly resonant sum frequency generation spectroscopy with the help of DFT calculation. A versatile way to investigate the orientation angle of functional groups of SAMs and their surface coverage has been demonstrated. It was revealed that the IR dipoles of the band at around 1630 cm−1, which were almost parallel to the long molecular axis of the adenine ring, were less tilted with respect to the substrate surface in the SAMs with longer chains (n = 9 and 10) in comparison to those with shorter chains (n = 2, 4, and 5).