Abstract
The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.
Highlights
That a focus on resilience mechanisms is still not specific enough for two reasons: 1) recovery and resistance are reductionist concepts, while persistence is a holistic one, and 2) we need to take into account different time horizons and, decision contexts
The strength and scope of resilience mechanisms depend on the context, for example whether we focus on individuals, populations, communities, ecosystems or landscapes
Resilience is an important concept that refers to the ability of ecosystems to self-organize in a way that they can ‘absorb changes ... and still persist’ (Holling 1973)
Summary
It focusses on the ‘persistence of relationships’ (Holling 1973) and the functioning and self-organization of entire systems This holistic focus is in contrast to more reductionist interpretations, which prevail in ecology and focus on specific state variables and decompose resilience into the ability to resist, or recover from, disturbances and thereby persist (Oliver et al 2015). That a focus on resilience mechanisms is still not specific enough for two reasons: 1) recovery and resistance are reductionist concepts, while persistence is a holistic one, and 2) we need to take into account different time horizons and, decision contexts. It certainly has been known before that management actions to bolster resilience will depend on the time scale under consideration, we believe that the real problem lies deeper and can be traced back to different interpretations and schools of thought
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