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) oil 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 I R 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 W) in comparison to those with shorter chains (n = 2, 4, and 5).