Assigning a structural motif using spontaneous molecular dipole orientation in thin films

Area of Science:

• Solid-state physics and physical chemistry.
• Molecular solid thin films.
• Ab initio quantum chemistry.

Background:

• Spontaneous orientation of molecular dipoles in solid thin films generates bulk electric fields, known as spontelectric fields.
• These fields have been observed in various molecular solids formed via gas-phase condensation.
• cis-methyl formate (cis-MF) exhibits these spontelectric fields, making it a candidate for structural investigation.

Purpose of the Study:

• To investigate the structural basis of spontelectric fields in cis-methyl formate (cis-MF) crystalline thin films.
• To correlate the observed spontelectric behavior with the molecular structure and aggregation state of cis-MF.
• To predict the unit cell structure of crystalline cis-MF using spontelectric field measurements as a key indicator.

Main Methods:

• Experimental observation of spontelectric fields in cis-MF thin films.
• High-quality ab initio calculations on cis-MF monomers and dimers.
• Correlation of experimental spontelectric field data with theoretical structural predictions.

Main Results:

• Spontelectric fields were observed in cis-MF thin films.
• Ab initio studies indicated that cis-MF forms non-polar dimers in the crystalline phase.
• The formation of non-polar dimers was linked to the collapse of the spontelectric field at deposition temperatures above 90 K.

Conclusions:

• The crystalline phase of solid cis-MF likely possesses a unit cell structure dominated by non-polar dimers.
• Dimer formation above 90 K is proposed as the mechanism responsible for the observed decrease in spontelectric fields.
• This study demonstrates the utility of spontelectric behavior as a sensitive probe for predicting molecular structures in solid thin films.