Second Harmonic Response of Azobenzene Self-Assembled Monolayers: the Effect of Push/Pull Substitution

A computational methodology combining classical molecular dynamic simulations and density functional theory calculations is employed to characterize the second harmonic generation (SHG) response of self-assembled monolayers (SAMs) functionalized with azobenzene photochromic units. Chemical substitution of the azobenzene core is explored to investigate how the nature of the substituent influences the morphology and nonlinear optical (NLO) properties of the SAMs. The interfacial NLO responses are computed, accounting for the effect of structural fluctuations and steric interactions of individual azobenzene units, as well as electrostatic intermolecular interactions. It turns out that the para functionalization of the terminal phenyl of the azobenzene core with an electron-acceptor group is promising for applications, since it enhances the NLO contrast perpendicular to the surface with respect to the nonsubstituted system, while maintaining well-separated SHG signal ranges for the two E and Z isomeric forms, making them easily distinguishable for polarization-resolved SHG measurements.