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A ReaxFF-based thermomechanical analysis of N-carbophenes: phase-change, thermal expansion, and high temperature synthesis pathway

Chad E. Junkermeier, Kat Lavarez, R. Martin Adra, Valeria Aparicio Diaz, Heather Osterstock, Pal Casinto, M. Verano, Ricardo Paupitz, Adri C. T. van Duin

Abstract

N-carbophenes are a class of two-dimensional covalent organic frameworks with potential for solid-state gas storage and as 2D topological materials. Previous studies have demonstrated that variations in their bonding, topology, and functionalization enable the tuning of their chemical, electrical, and mechanical properties. Yet, the thermal stability and high-temperature behavior of pristine and functionalized N-carbophenes remain unexplored. Using ReaxFF-based reactive molecular dynamics (RMD) simulations with extensive statistical validation, we performed temperature-ramp MD simulations of pristine and functionalized N-carbophenes. We demonstrate that N-carbophenes remain stable up to temperatures above 1000 K. The phase-change onset temperatures decrease as the N-phenylene chain length increases in pristine N-carbophenes, attributed to increasing antiaromaticity in the central phenylene segments, thereby contributing to the foundational understanding of aromatic versus antiaromatic bonding in 2D carbon networks, a topic of considerable interest in theoretical chemistry. Pristine N-carbophenes exhibit negative area thermal expansion (NATE), whereas functional groups modulate this, leading to either negative or positive expansion. Functional groups remain stably bonded well above the transition temperature. We also show that a temperature-induced phase transition from graphenylene (2-carbophene) to γ-graphyne is possible. Our results provide upper bounds on N-carbophene stability, clarify the relationships between structure and thermal properties, and identify a new transformation pathway. These results will have applications in tunable band gaps, porous architectures, or chemically accessible sites.

A ReaxFF-based thermomechanical analysis of N-carbophenes: phase-change, thermal expansion, and high temperature synthesis pathway

Abstract

N-carbophenes are a class of two-dimensional covalent organic frameworks with potential for solid-state gas storage and as 2D topological materials. Previous studies have demonstrated that variations in their bonding, topology, and functionalization enable the tuning of their chemical, electrical, and mechanical properties. Yet, the thermal stability and high-temperature behavior of pristine and functionalized N-carbophenes remain unexplored. Using ReaxFF-based reactive molecular dynamics (RMD) simulations with extensive statistical validation, we performed temperature-ramp MD simulations of pristine and functionalized N-carbophenes. We demonstrate that N-carbophenes remain stable up to temperatures above 1000 K. The phase-change onset temperatures decrease as the N-phenylene chain length increases in pristine N-carbophenes, attributed to increasing antiaromaticity in the central phenylene segments, thereby contributing to the foundational understanding of aromatic versus antiaromatic bonding in 2D carbon networks, a topic of considerable interest in theoretical chemistry. Pristine N-carbophenes exhibit negative area thermal expansion (NATE), whereas functional groups modulate this, leading to either negative or positive expansion. Functional groups remain stably bonded well above the transition temperature. We also show that a temperature-induced phase transition from graphenylene (2-carbophene) to γ-graphyne is possible. Our results provide upper bounds on N-carbophene stability, clarify the relationships between structure and thermal properties, and identify a new transformation pathway. These results will have applications in tunable band gaps, porous architectures, or chemically accessible sites.
Paper Structure (13 sections, 6 figures, 1 table)

This paper contains 13 sections, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methodology
  3. Results and Discussion
  4. Comparison with experiment
  5. Phase-change onset
  6. Two-Way ANOVA and Tukey's HSD
  7. Negative Area Thermal Expansion
  8. Functional group stability
  9. graphenylene to gamma-graphyne
  10. Conclusion
  11. Acknowledgements
  12. CReDiT authorship contribution statement
  13. Declaration of generative AI and AI-assisted technologies in the manuscript preparation process

Figures (6)

  • Figure 1: (a) A 3-Carbophene model that showcases different supercell sizes: size 2, 4, and 6. Nomenclature of bond types in (b) graphenylene, (c) 3-carbophene, (d) 4-carbophene, and (e) 5-carbophene PyMOL.
  • Figure 2: Comparison of the bond lengths in Angstroms and valence bond angles in degrees of 3-phenylene as computed by ReaxFF and experimentally determined through XRD (parenthesized values).
  • Figure 3: (a) - (d) Examples of the first bond breaking in each type of material in this study patchwork. Each frame presents the type of bond that broke and the temperature at which it broke. Gold lines outline supercell boundaries. Initial bond breaking temperatures of each type of (e) pristine N-carbophene (g) functionalized 3-carbophene. Treemaps of the ratios of the type of the first broken bond according to (f) type of pristine N-carbophene and (h) functionalized 3-carbophene.
  • Figure 4: Graphs showing the area thermal expansion versus temperature for each simulation are discussed. The jagged multicolor graph within each facet consists of overlapping plots of area thermal expansion, with each simulation plotted in a different color. The smooth blue line represents the LOWESS smoothing of the data in each facet. (a) Facet plot of the pristine N-carbophenes. (b) Facet plot of the functionalized 3-carbophenes.
  • Figure 5: Box-and-whiskers plots of mean desorption temperature of the first atom or molecule to desorb from functionalized 3-carbophenes. Each box-and-whiskers plot shows the molecule that first desorbed and the temperature at which it desorbed.
  • ...and 1 more figures