Dielectric permittivity of small-molecule matrices for organic optoelectronics: The key contribution of solid state molecular dynamics

The dielectric response of amorphous organic solids at the timescale of atomic motion has broad implications for their photophysics, governing spectroscopic properties and excited-state relaxation, among other phenomena. We present an original approach to the calculation of the vibrational contribution to the frequency-dependent dielectric permittivity. Our methodology, combining molecular dynamics simulations, density functional theory calculations, and linear response theory, is applied to a set of amorphous carbazole-based small-molecule matrices, widely used as host materials in organic light-emitting diodes. Our analysis reveals the crucial role of hindered molecular rotations (librations) in the solid-state matrices of dipolar compounds. These modes manifest in the very-low frequency ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo><</a:mo> <a:mn>100</a:mn> <a:mspace width="0.28em"/> <a:msup> <a:mrow> <a:mi>cm</a:mi> </a:mrow> <a:mrow> <a:mo>−</a:mo> <a:mn>1</a:mn> </a:mrow> </a:msup> </a:mrow> </a:math> ) region of the dielectric function, accounting for up to one-third of the total vibrational contribution to the static dielectric constant in a dipolar compound like 3,3'-di(carbazol-9-yl)-5-cyano-1,1'-biphenyl. This low-frequency contribution is found to be crucial for determining the variations in the static dielectric constant across various materials.