Elsevier

Surface Science

Volume 709, July 2021, 121837
Surface Science

Conformational behavior of naphtho-merocyanine dimers on Au(111)

https://doi.org/10.1016/j.susc.2021.121837Get rights and content

Highlights

  • Study of naphtho-merocyanine/spiropyran on Au(111).

  • STM analysis under UHV conditions identifies merocyanine dimers on substrate.

  • Computational chemistry analysis including DFT and molecular mechanics.

  • Identification of merocyanine conformers using computational chemistry and adsorption energies and geometries.

  • First step in understanding island/thin film growth of naphtho-merocyanine.

Abstract

Thin film growth of molecules on substrates are governed by the convolution of inter-molecular forces and template-adsorbate interactions. Often the initial dimer formation plays a crucial role in the outcome of the molecular structure on the surface. Here, the behavior of naphtho-merocyanine on an Au(111) substrate was investigated using computational chemistry methods and Scanning Tunneling Microscopy. The experiments show a strong preference of dimer formation of the merocyanine molecules. Topographical measurements are used to identify two distinguished configurations, an elongated/oval dimer and a compact/rectangular dimer. With the addition of computational chemistry calculations including Density Functional Theory (DFT) and Molecular Mechanics calculations using the AMBER 3 force field, these two configurations of two specific merocyanine conformers, namely CTC and CTT, could be identified. Intermolecular binding energy calculations could be performed. This is a first step in understanding the possible island/thin film growth of naphtho-merocyanine and subsequently possible pathways toward the switching behavior of merocyanine and spiropyran on this substrate.

Introduction

Molecular switches on substrates are an important sub-class for functionality of molecular electronics. Two or more isomers with different conductance can be used as an electronic switch in molecule-based devices [1], [2], [3], [4], [5], [6]. The switching properties and thresholds are influenced by the interaction of these molecules with the underlying substrates [7], [8], [9]. For instance, metallic substrates can introduce additional excitation routes of molecular switching due to their reservoirs of electrons and holes [10], [11]. However, other studies have shown that adsorption on metallic substrates could also lead to a loss of photoisomerization [12], [13], [14]. Using different types of substrates, the electronic properties of semi-conductors and other non-metals can suppress switching mechanisms due to electronic surface features which means the switching process can be better controlled by external stimuli [15]. Overall, the combination of a suitable substrate and a molecular switch with appropriate end groups can lead to a sufficiently high photoisomerization potential [16]. As a stimulus, light for instance has various advantages: it can be used to stimulate the switch locally with high precision, it can be tuned to specific wavelengths and energies, and it does not introduce any contamination to the molecular structure. Switches activated by heat usually are stimulated over a wider area of the sample. Studies have shown that environmental factors, such as Ultra-High Vacuum vs. ambient conditions, can also create differences in the isomerization processes and yield [17].

A promising candidate chosen for the present study of molecular switches is a naphtho-spiropyran/merocyanine molecule (see Fig. 1). In general, the class of spiropyran/merocyanine molecules is especially interesting for a variety of applications since their isomers have very different properties. Spiropyran (SP) molecules consist of an indoline and a benzopyran unit connected together via a spiro junction. Cleavage of the central C–O bond leads to the creation of a merocyanine (MC). This class of molecules can undergo reversible isomerization caused by a variety of stimuli depending on the environment of the molecule. The switching between these isomers has been investigated extensively in solution, and only recently on selected metal substrates [11], [18], [19], [20]. The class of SP/MC-based molecules is especially interesting since the isomers have vastly different properties concerning the size, dipole moment, color, and emission properties. Both isomers are structurally very distinguishable: spiropyran is a three-dimensional molecule where the spiro junction leads to aromatic rings being at 90 angles. However, the merocyanine with the central C–O bond broken is a much more planar molecule. These properties play an important role in their adsorption behavior when anchored to a substrate. The charge separation within the molecule leads to pronounced dipole moments in both, the spiropyran and the merocyanine isomers. Density Functional Theory (DFT) calculations as well as electro-optical absorption measurements have shown that the dipole moment of the merocyanine molecules is much bigger than that of spiropyran, increasing from about (1015)×1030 C⋅m to (5060)×1030 C⋅m, or by a factor of 4 to 5 [21], [22]. The optical properties of spiropyran-based molecules in solutions are well known for over half a century [23], [24]. The spiropyran isomer is colorless, meaning optically transparent in the visible region. However, the merocyanine isomer absorbs strongly in the yellow region of the spectrum (550 nm–600 nm) and therefore appears blue. Additionally to these absorption behavior differences, the isomers also differ in their emission properties. Spiropyran does not show a strong emission, however, merocyanine has a strong red emission region around 650 nm. The photochromic behavior of this class of molecules is very interesting for a wide variety of applications such as reusable sensors, high-resolution imaging in biological samples, and detection and imaging of mechanical stress [25].

In order to achieve large scale molecular domains which can be used as molecular switch, film growth of these molecules has to be established. Many factors play crucial roles during the adsorption interactions, one of them the initial dimer formation between two molecules [26]. Energetically favorable configurations of dimers in anti-parallel orientation (a rotation of 180 around the axis along the surface normal) can lead to repulsive interactions between dimers and therefore to self-organized growth of molecular wires [27]. On the other hand, dimers oriented in chevron orientation can be the building blocks of wide-scale island/film growth [28]. In all events, the configuration of the initial dimers can be used as a precursor to larger scale adsorption studies necessary for molecular switches.

The present study investigates naphtho-spiropyran/merocyanine adsorption on Au(111) for the first time. Previous studies have looked at other, related molecular switches on gold or on semimetals [13], [29], [30], however, the specific adsorbate/substrate combination used here has not been investigated. The aforementioned studies have researched various reaction paths for the switching between the spiropyran/merocyanine isomers, the present case investigates the behavior of annealing/heat. Lastly, the combination of microscopic imagining combined with computational chemistry allows for the first time to explore geometries, identify isomers of merocyanine, and calculate adsorption and binding energies for the Naphtho-SP/MC on Au(111) system.

Section snippets

Experiment

The molecule for this study was a commercially purchased naphtho-spiropyran molecule (TCI: 1,3,3-Trimethylspiro[indoline-2,3-[3H]naphth[2,1-b]pyran] – C23H21NO) at 98.0% purity. A Lewis structure of this naphtho-spiropyran and naphtho-merocyanine molecule can be seen in Fig. 1.

All experiments were carried out under UHV conditions at pressure of about 5×1010 mbar. The Au(111) sample was cleaned through repeated Ne+ sputtering cycles and subsequent annealing of the sample to 750 K. Afterwards

Results and discussion

Fig. 2 shows a typical STM image of the surface structure taken at a bias voltage of V=+1.0 V and tunneling current of I=20 pA. The size of the image is 200 Å × 200 Å. After annealing the sample to a temperature of 288 K, the molecules organize themselves into structures with two pronounced protrusions. The apparent lateral size of these features (>20 Å) clearly indicates that they can not be a single molecule where two of the aromatic rings would be cause for the protrusions, but that they are

Conclusion

The adsorption behavior of individual naphtho-merocyanine molecules on an Au(111) substrate was investigated using Scanning Tunneling Microscopy (STM) and computational chemistry methods. The experiments show a strong preference of dimer formation of the merocyanine molecule. It should be noted that for the system of naphtho-SP/MC on Au(111), the reaction path between the isomers is different than in solution as in the current system heat drives the reaction towards MC isomers, whereas in

CRediT authorship contribution statement

Andreas Riemann: Conceptualization, Writing - original draft, Investigation, Visualization, Funding acquisition. Lucas Browning: Investigation. Hunter Goff: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported financially by NSF (Grant 1807460). A.R. likes to thank Prof. Katharina Franke for technical support and acknowledge the use of the facilities in her research group at the Freie Universität Berlin.

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