Pancake Bonding in Organic Radicals

Abstract

Interactions between two closed-shell molecules are usually non-covalent such as van der Waals (vdW) interactions. A covalent-like multi-electron/multi-center (me/mc) intermolecular interaction, often termed as “pancake bond”, was recently found between some open-shell radical and multiradicals. Pancake bonding features highly orientated configurations and low-lying triplet/multiplet states. Pancake bonding was observed among a number of newly synthesized radical-based materials exhibiting exciting magnetic and electronic properties. Unlike closed-shell molecules with only vdW interactions, open-shell molecules with pancake bonding pose many challenges to theoretical chemists including the consideration of contribution from the multireference states, the special through-space dispersion interactions and the much shorter than vdW intermolecular distances. In this dissertation, we systematically studied a series of pancake-bonded systems using density functional theory (DFT) with the purpose to better understand their properties and to make predictions about yet to be synthesized pancake-bonded systems.

Phenalenyl and its derivatives are stable radicals with strong pancake bonding. Using the unrestricted M05-2X DFT, we investigated a series of homogeneous phenalenyls including two newly synthesized derivatives: triphenyl-phenalenyl and trimethyl-phenalenyl. We found that these phenalenyls could aggregate through π-dimerizations or σ-dimerizations based on their substitution groups and the relative interaction energy. We further explained an experimentally observable interchange between π-dimers and σ-dimers of trimethyl-phenalenyl by calculating the potential energy surface along the interchange. In addition to the study on homo-phenalenyls, we investigated on hetero-phenalenyls and suggested that large energy difference between singly occupied molecular orbitals (SOMOs) between two different phenalenyls could be utilized to generate very stable pancake-bonded hetero-phenalenyl π-dimers.

Next, we studied the possibly multiple pancake-bonded systems on a series of triangulenes which can be viewed as graphene flakes. Some triangulenes are multiradicals where phenalenyl is single radical. The DFT study we conducted on triangulenes suggested that multiple degenerate SOMOs among these multiradicals can serve as the basis of pancake-bonded π-dimers with a formal pancake bond order up to five. Surprisingly, we found a linear relationship between the pancake bond strength and the pancake bond orders.

We validated over fifty DFTs regarding their performance on pancake bonds. Dispersion corrections were included in the validation process. Comparing to the high-level MR-AQCC results on four different pancaked bonded systems, we found no universal DFT applicable for all pancake-bonded systems. However, we identified DFTs that worked well for each individual system.