Stereo-isomeric selection upon adsorption on ionic thin film : a structural an optical study

Curcuminoid derivatives are known to be versatile dye molecules with an electrophile character. They are used as charge transport materials in organic photovoltaics and optoelectronics such as organic field effect transistors and organic light emitting diodes, in bioimaging and in nonlinear optics as well. Part of those properties rely on the chemical topology of the molecule that possesses a pi-conjugated donor-acceptor-donor architecture.

However, the way the condensed phase influences the intrinsic properties of the molecule is rarely studied. In this work, we have investigated the relationship between the structural and the optical properties of a supramolecular phase of curcuminoid derivatives on ionic substrates of KCl. Among three possible stereoisomers of the molecule, we have shown that the molecule-substrate interactions preferentially select one of them during the adsorption process. Our study was carried out by combination of:

1-structural measurements down to the molecular scale by scanning tunneling microscopy at low temperature, hence on a conducting substrate consisting of 2 monolayers of KCl on Au(111) (2ML KCl/Au(111)),

2-absorption spectroscopy in solvent

and 3-differential reflectance spectroscopy measurements on a bulk KCl(001) single cristal. To compare the relevance of both absorption spectroscopy methods (solvent vs. bulk KCl), it was mandatory to make sure that the condensed phase on bulk KCl(001) was similar to the one observed on 2ML KCl(Au(111). Thereto, additional noncontact-atomic force microscopy structural measurements have been performed at room temperature. The as-observed phase is similar to the one observed on 2ML KCl/Au(111). The structural study points towards an epitaxy relationship between the organic layer and the KCl substrate involving an interaction between the CN groups of the molecule and the K+ species, as well as an intramolecular interaction between BF2 groups and phenyl groups of the curcuminoids. The proposed structural model was confirmed by DFT and electrostatic interactions calculations. The comparison between optical spectra in solvent (single molecule) and in the condensed phase on surface points towards a redshift of the first electronic transition and of its vibronic replica which is found to be consistent with the stereoisomeric selection of the molecules induced by the adsorption process.

This work was performed within the framework of the inter-labs CINaM IM2NP NanoMatMol team and the molecules have been synthesized by the IMMF department at the CINaM.

Ref. : Stereoisomeric selection upon adsorption: A structural and optical study of curcuminoid derivatives on ultrathin films of KCl on Au(111) and on bulk KCl(001)

Thomas Leoni, Laurent Nony, Elena Zaborova, Sylvain Clair, Frédéric Fagès, Franck Para, Alain Ranguis, Conrad Becker, and Christian Loppacher

Phys. Rev. B 104, 205415 – Published 9 November 2021

For more information :

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.205415