# Thermal-mediated modulation of binary supramolecular self-assembly from phase separation to co-crystallization at the liquid–solid surface

## TL;DR

This paper shows how heating can change how two molecules assemble together on a surface, leading to stable structures instead of separating.

## Contribution

The study demonstrates thermal control over supramolecular co-crystallization, avoiding phase separation in binary systems.

## Key findings

- Thermal activation from 25 °C to 60 °C enables co-crystallization instead of phase separation in a host–guest system.
- STM data shows co-crystal structures transition from chicken-wire to flower types with increasing temperature.
- Molecular interactions and adsorption energy determine the stability and transformation of co-crystals.

## Abstract

Significant research in materials chemistry has focused on the design and fabrication of organic materials and their self-assembled architectures for a wide range of applications, such as organic transistors, photovoltaic cells, and surface functionalization, to name just a few. For binary supramolecular systems, however, the increased complexity that involves hetero-molecular interactions often leads to challenges, for instance, undesired phase segregation. Using scanning tunnelling microscopy (STM), we show that thermal activation (from 25 °C to 60 °C) can drive a transition from phase separation to thermodynamically stable co-crystallization for a host–guest system comprising trimesic acid and a tetrathiafulvalene derivative. Our STM data revealed that the co-crystals varied from the chicken-wire type to a flower type as a function of annealing temperature (from 60 °C up to 80 °C). Their molecular interactions and adsorption energy and thus the corresponding stability constitute the energy landscape, which is derived from force-field simulations. This transformation could be governed by the modulation of molecule–substrate interactions, intermolecular bonding, and hetero-molecular attractions, offering a thermally tuneable route toward supramolecular co-assemblies.

This study elucidates the thermal-mediated modulation to form vertically stacked bilayers in a binary supramolecular system, which otherwise would form phase-separated domains on a graphite surface.