Modeling the Monolayer Formation of Merocyanine HB238 on the Ag(100) Surface

We recently published a study on the adsorption of the merocyanine HB238 (2-[5-(5-dibutyl-amino-thiophene-2-yl-methylene)-4-tert-butyl-5H-thiazol-2-ylidene]-malononitrile) on the Ag(100) surface using scanning tunneling microscopy (STM), spot profile analyzing low electron diffraction (SPA-LEED), x-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) techniques [Kny et al., Nanoscale, 2023, 15, 10319]. We observed that HB238 self-organizes as a chiral tetramer upon adsorption. The adsorbate structure, denoted as α-phase, is completely different from the known bulk structures of HB238. However, the previous study was mainly experimental; in the present study, we focus on the results of quantum chemical calculations to corroborate the previous experimental findings and interpret the measured spectra in order to deepen the understanding of the adsorption process. For this purpose, we first screened the full conformer space of the HB238 molecule, particularly with respect to its flexible alkyl side chains, which obtain several degrees of freedom due to possible rotations along their single bonds. The most stable structures were then used to determine the optimal orientations of HB238 on the Ag(100) surface and to compare to the experimental α-phase structure. We generated various initial configurations of the HB238 monolayer on a three-layer Ag slab model. For the most stable adsorption model, STM images, projected density of states (PDOS), x-ray absorption spectra (XAS), and adsorption height profiles were calculated and compared with the previous experimental results. Our observations reveal a significant alteration in the HB238 conformation upon adsorption, with monolayers forming through stabilization mainly by electrostatic and dispersion interactions with the Ag(100) surface.