Intramolecular dipole coupling and depolarization in self-assembled monolayers

Quantum mechanical and classical atomistic computational methods are used to simulate the chain-length dependence of depolarization effects in S(CH2)(n-1)CH3 and S(CH2)(n-1)COOH self-assembled monolayers on gold (111) surface. These calculations show that due to weak cooperative effects, the electrostatic properties of alkanethiol monolayers are well described by the gas phase dipole moments of the molecules. However, depolarization in monolayers with the molecules carrying head- and tail-group dipoles, such as COOH-terminated monolayers, strongly depends on the degree of intramolecular dipole coupling. Thus the electrostatic properties of self-assembled monolayers can be engineered by changing the length of the aliphatic spacer between the polar groups. The transition from strong to weak coupling regime was found to be accompanied by the change in the sign of the asymptotic value of electrostatic potential above the surface of the monolayers and hence in the sign of the metal work function change. Therefore, the use of weakly polarizable spacers between the polar groups inside the molecules forming the SAM is beneficial for accessing a wider range of work-function changes.