Template Induced Highly Ordered Aggregates of Merocyanines for the Tuning of Optical Properties

Merocyanines are widely investigated molecules with large dipole moments, adjustable optical bandgaps, and high absorption coefficients. They are of interest for the use in optoelectronic devices, in particular, for the application in organic solar cells [1]. By controlling the molecular packing, either via template mediated film growth on crystalline surfaces or by modification of the molecular structure such as a variation of the peripheral substituents, the optical properties can be tuned by the formation of J- or H-aggregates [2].<br/><br/>Here, we present a study on the adsorption and molecular ordering of the prototype merocyanine 2-[5-(5-dibutylamino-thiophen-2-yl-methylene)-4-<i>tert</i>-butyl-5<i>H</i>-thiazol-2-ylidene]-malononitrile (HB238) and its derivatives on single crystalline Ag(100) and KCl(100) surfaces. Measurements were performed on ultrathin films (range of about one monolayer) by a variety of surface sensitive techniques such as high resolution low energy electron diffraction (SPA-LEED), scanning tunneling microscopy (STM), and photoelectron spectroscopy (XPS, UPS). The results are complemented by density-functional theory (DFT) calculations.<br/>Remarkably, the thermodynamically preferred phase of HB238 on the Ag(100) surface at room temperature is a commensurate superstructure which is considerably different from known bulk structures [3]. We observe domains of several hundreds of Angstroms in diameter composed of homochiral aggregates of four molecules which are arranged in a nearly circular manner. Hence, properties of J-aggregates are expected for this phase. This structure is stabilized mainly by hydrogen bonds between donor and acceptor groups of neighboring molecules. Interestingly, the molecular packing between the above described homochiral aggregates can be affected by a change of the sterically demanding peripheral substituents. We will demonstrate this by comparing the structure formation of HB238 to that of derivatives on the Ag(100) surface.<br/><br/>Finally, the question arises if similar structures can be prepared on different template surfaces, for example those of insulating layers that are needed to investigate the resulting optical properties. To answer this question it is important to understand how the structure formation on the Ag(100) surface depends on the interfacial bonding. For this purpose, we discuss the role of the functional groups on the interfacial bonding and compare the structure formation of HB238 on Ag(100) to its structure formation on a thin insulating layer of KCl(100) epitaxially grown on Ag(100). On the basis of these results we discuss the interplay of intermolecular and interfacial bonds essential for the film formation and hence most important for the tuning of the optical properties.<br/><br/>MCs were kindly provided by the group of Prof. K. Meerholz (Cologne).<br/>Supported by the DFG through the research training group 2591.<br/><br/>[1] A. Arjona-Esteban et al., JACS 137 (2015) 13524. [2] A. Liess et al., Adv. Funct. Mater. 29 (2019) 1805058. [3] N. Gildemeister et al., J. Mater. Chem. C 9 (2021) 10851-10864.