Vibrational probes of anharmonic interactions in the molecular ferroelectric 2-methylbenzimidazole

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We report experimental and computational characterization of the anharmonic interactions between the constituents of the molecular ferroelectric material 2-methylbenzimi-dazole (MBI). Using experimental Raman spectroscopy measurements across a wide range of temperatures we assess the anharmonic properties of this material’s vibrations we expect to play a major role in proton tautomerism (PT) mechanism that explain ferroelectric switching in MBI. Through an application of the standard theory of anharmonic contributions to the interatomic potential energy of MBI we develop and apply a method to decipher how cubic and quartic anharmonic coupling to low frequency lattice vibrations affect the frequency of an intramolecular ring vibration of the molecular constituents of this material capable of modulating carbon-nitrogen double bonds that alternate in the PT mechanism. In addition, we use the theory of vibrational solitons to show the appearance of satellites peaks red-shifted from the N-H stretching vibration of MBI stem from self-trapped vibron states. We assess the number of these trapped states and the modes along which trapping occurs to show MBI possesses distinct phases in which excitations of the N-H stretching vibration couple to different low frequency lattice vibrational modes. We propose increased N-H stretching vibrational trapping at low temperature helps explain reductions in the temperature-dependent dielectric constant of MBI measured previously. These results highlight the important role vibrational spectroscopy can play in understanding how intermolecular interactions impact the structural dynamics of materials central to sustainable information storage and nonlinear optical technologies