Chemical Modifications Suppress Anharmonic Effects in the Lattice Dynamics of Organic Semiconductors

Abstract

The lattice dynamics of organic semiconductors has a significant role in determining their electronic and mechanical properties. A common technique to control these macroscopic properties is to chemically modify the molecular structure. These modifications are known to change the molecular packing, but their effect on the lattice dynamics is relatively unexplored. Therefore, we investigate how chemical modifications to a core [1]benzothieno-[3,2-b]benzothiophene (BTBT) semiconducting crystal affect the evolution of the crystal structural dynamics with temperature. Our study combines temperature-dependent polarization-orientation (PO) low-frequency Raman measurements with first-principles calculations and single-crystal X-ray diffraction measurements. We show that chemical modifications can indeed suppress specific expressions of vibrational anharmonicity in the lattice dynamics. Specifically, we detect in BTBT a gradual change in the PO Raman response with temperature, indicating a unique anharmonic expression. This anharmonic expression is suppressed in all examined chemically modified crystals (ditBu-BTBT and diC8-BTBT, diPh-BTBT, and DNTT). In addition, we observe solid−solid phase transitions in the alkyl-modified BTBTs. Our findings indicate that π-conjugated chemical modifications are the most effective in suppressing these anharmonic effects.

Phonon–Phonon Interactions in the Polarization Dependence of Raman Scattering

“…However, some of us recently showed that the polarization–orientation (PO) Raman response, i.e., the polarization dependence of the Raman signal, evolves with temperature for some organic crystals. , These kinds of PO patterns and their temperature dependence cannot be captured by any superposition of irrep-specific matrices, and a generalization to a fourth-rank formalism is necessary. We argue this is the result of specific phonon–phonon interactions entering the Raman scattering process, an effect which is highly temperature and material-dependent.…”

Phonon-induced localization of excitons in molecular crystals from first principles

“…Anharmonic low-frequency phonons are common in molecular materials [27] and can couple to singlets when these approach the Wannier-Mott limit, in a manner which is in turn determined by the size [10] and packing [56] of the molecular building blocks. Our work lays foundations for a deep understanding and controlled enhancement of exciton transport in molecular crystals, for example by suppressing anharmonicity through chemical modifications [57].…”

Mechanistic View on the Order–Disorder Phase Transition in Amphidynamic Crystals

“…Below the phase transition temperature, we see red-shifting and broadening of the spectra as temperature increases. These observations are common in temperature-dependent Raman measurements. ,− Their physical origin is mainly thermal expansion which causes a weakening of the intermolecular interaction (red-shifting) and an increase in phonon population, which decreases the vibrational lifetime (broadening) …”