Abstract
In hydrogen-bonded systems, nuclear quantum effects such as zero-point motion and tunneling can significantly affect their material properties through underlying physical and chemical processes. Presently, direct observation of the influence of nuclear quantum effects on the strength of hydrogen bonds with resulting structural and electronic implications remains elusive, leaving opportunities for deeper understanding to harness their fascinating properties. We studied hydrogen-bonded one-dimensional quinonediimine molecular networks which may adopt two isomeric electronic configurations via proton transfer. Herein, we demonstrate that concerted proton transfer promotes a delocalization of π-electrons along the molecular chain, which enhances the cohesive energy between molecular units, increasing the mechanical stability of the chain and giving rise to distinctive electronic in-gap states localized at the ends. These findings demonstrate the identification of a class of isomeric hydrogen-bonded molecular systems where nuclear quantum effects play a dominant role in establishing their chemical and physical properties. This identification is a step toward the control of mechanical and electronic properties of low-dimensional molecular materials via concerted proton tunneling.
Highlights
These findings demonstrate the identification of a new class of isomeric hydrogen bonded molecular systems where nuclear quantum effects play a dominant role in establishing their chemical and physical properties
We show that the concerted proton motion in a H-bonded 1D molecular system enhances its mechanical stability but directly modifies its electronic structure, forming new electronic in-gap states localized at the ends of the chain
The molecular DABQDI building blocks exist in solution as two tautomers in equilibrium, whose mutual alternation can be realized via a fast intramolecular double proton transfer that generates a structure of higher symmetry which directly alters the π-conjugation of the whole DABQDI molecule.[17]
Summary
The precursor molecule 2,5-diamino-1,4-benzoquinonediimines (DABQDI) was synthesized via the procedure described in the literature[19]. To form the canted chains, molecules were evaporated in the preparation chamber (crucible temperature 90-120°C) onto a sample thermalized to 20-60°C before being immediately transferred to the microscope head. DFT Calculations Density functional theory (DFT) calculations were performed using the FHI-AIMS code[26] within exchange-correlation functional B3LYP27,28 to describe the electronic properties of the gas-phase DABQDI molecule and of its protonated form adsorbed on the Au(111) substrate using a 6x6 unit cell. PIMD calculations All the simulations were performed with 3 quinone molecules in local orbital DFT with local basis set Fireball code[35]; the surface was simulated using the interface forcefield[38]. To obtain the free energy profile we performed umbrella sampling with the bias applied to the reaction coordinate of the path integral centroid configuration at 20 K and of two contracted replicas at 10 K.
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