Photo-induced electronic excitations drive polymerization of carbon monoxide: A first-principles study

TL;DR

The paper addresses how laser-induced electronic excitations influence the polymerization of carbon monoxide (CO) under reduced pressure. Using first-principles simulations—FPMD, TDDFT/TDA, and LR-TDDFT—the authors show that electronic excitation strengthens C-C bonding and lowers the pressure threshold for the molecular-to-polymeric transition, with absorption spectra shifting under compression. Key findings include smoother, excitation-enabled transitions and the persistence of polymeric structures upon decompression, along with a compression-induced reduction in the optical gap that enables potential excitation by visible light. These results suggest a feasible photo-assisted route to synthesize polymeric CO-based materials under milder conditions and provide guidance for exploring laser-driven polymerization in related energetic materials.

Abstract

Under pressure, carbon monoxide (CO) transforms into a polymer that can be recovered to ambient conditions. While this transformation can occur without additional stimuli, experimental observations have shown that laser irradiation can induce a similar transformation at reduced pressure. The resulting polymeric phase, which is metastable under ambient conditions, releases energy through decomposition into more stable configurations. Using time-dependent density functional theory and Born-Oppenheimer molecular dynamics simulations, we investigate the mechanism by which electronic excitation facilitates CO polymerization. Our calculations reveal that electronic excitation enhances carbon-carbon bonding, enabling polymerization at pressures significantly lower than those required by conventional compression methods. These findings suggest that a photo-assisted approach could be employed to synthesize novel, potentially energetic materials under less demanding pressure conditions.

Figures (5)

• Figure 1: Atomic structures of 64 CO molecules at initial density of 1.12 g/cm$^3$ and after isothermal compression to a density of 2.4 g/cm$^3$ at 300 K. The structure shown on the left is at the initial density and without any electronic excitation, which is in the pure molecular phase. On the right is the structure at the initial density but with electronic excitation, which contains bonds between carbon atoms. In the middle is a typical polymeric structure obtained after isothermal compression up to 2.4 g/cm$^3$, for both cases with and without electronic excitation. The transition pressures (P$_\text{trans}$) are also indicated for both cases. Gray atoms represent carbon and red atoms denote oxygen.
• Figure 2: Results from FPMD simulations of isothermal compression of CO molecules at 300 K. The simulations were performed under four conditions: a standard Born-Oppenheimer FPMD simulation where electrons occupy the ground state corresponding to ionic positions (GSMD), and three levels of electronic excitation as described in the text. (a) Evolution of pressure as a function of density. (b) Change in the number of chains/molecules versus density. Circular markers on the curves in (a) and (b) indicate the approximate onset of the molecular-to-polymeric transition. (c) Number of 4-member rings versus density. (d) Number of 5-member rings versus density. Each plot includes four curves corresponding to the simulation conditions: blue for GSMD, orange for level 1, green for level 2, and red for level 3 electronic excitation. Panels (c) and (d) also illustrate typical 4-member and 5-member rings formed during compression. Gray atoms represent carbon and red atoms denote oxygen.
• Figure 3: Radial distribution functions of C-O, C-C, and O-O atom pairs at various densities during the isothermal compression of CO molecules at 300 K. Each plot includes four colored curves representing the different simulation conditions considered in this work: blue for GSMD, orange for level 1 electronic excitation, green for level 2 electronic excitation, and red for level 3 electronic excitation.
• Figure 4: Ground- and excited-state energies of two CO molecules as a function of the distance between the two carbon atoms, obtained from TDDFT/TDA calculations. A schematic configuration of the two CO molecules is also shown: the two carbon atoms face each other, while the two oxygen atoms lie outside, facing away from each other. Gray atoms represent carbon, and red atoms represent oxygen. The TDDFT/TDA calculations did not converge for separations less than 1.45 Å between the two carbon atoms.
• Figure 5: Optical absorption coefficients for the molecular phase of CO at two different densities: (a) 1.12 g/cm$^3$ and (b) 1.70 g/cm$^3$, calculated using linear response Time-Dependent Density Functional Theory (TDDFT). Vertical lines indicate the electronic band gap in each plot.