Nonlinear Optical Contrast in Azobenzene-Based Self-Assembled Monolayers

Conjugated organic photochromes such as azobenzene derivatives can show remarkable nonlinear optical (NLO) properties and rapid responses, two essential requirements for the realization of optoelectronic switching devices. These applications also require the control of the molecular organization over the micrometric scale, which in principle can be achieved by arranging chromophore units in self-assembled monolayers (SAMs). To rationalize the interplay between the NLO responses of isolated molecules and those of photoresponsive materials, we implement here a computational approach combining molecular dynamics simulations and DFT calculations for predicting the contrast NLO responses of azobenzene-based SAMs with different surface densities. We show that collective switching of the chromophores is indeed possible, even though trans -> cis photoisomerization yields decrease when increasing the chromophore concentration. The magnitude of the second-order NLO response of trans SAMs is dominated by the component normal to the surface, which is considerably larger than the parallel one and significantly increases with the packing density. Photoswitching has the neat effect of halving the first hyperpolarizability, allowing for large NLO contrasts exploitable for storing and reading information in selected portions of the surface.