Azobenzene versus 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) at Au(111): Characterizing the role of spacer groups

TL;DR

This study combines DFT calculations and experimental measurements to analyze how spacer groups affect the adsorption and vibrational properties of azobenzene derivatives on gold surfaces, challenging previous assumptions about their role in molecular switching.

Contribution

It provides a detailed comparison of TBA and azobenzene adsorption on Au(111), highlighting the limitations of DFT-D and questioning the assumed decoupling role of tert-butyl groups.

Findings

DFT-D overbinds molecules but predicts accurate geometries.

Structural and vibrational properties of the N=N group are insensitive to spacer groups.

Spacer groups may not decouple the azobenzene moiety as previously thought.

Abstract

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface chemical bond. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N=N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.