Collective Excitonic Structure Governs Anomalously Weak Thermal Optical Dephasing in Conjugated Polymers

Abstract

Conjugated polymer aggregates exhibit optical decoherence in the presence of strong vibronic coupling and substantial diversity in chemical structure, solid-state organization, and excitonic character. Here, we use coherence-detected and population-detected two-dimensional electronic spectroscopies to examine the temperature dependence of the homogeneous optical linewidth across a series of semiconducting polymers. Despite substantial differences in molecular architecture and solid-state organization, all polymers studied exhibit remarkably weak thermal linewidth scaling over the measured temperature range. Comparison between complementary detection modalities further shows that, while this weak temperature dependence is obust, the absolute homogeneous linewidth depends on the measured observable. This behavior reflects the different ways in which coherence- and population-detected measurements project population relaxation and pure dephasing onto the spectroscopic response. These results establish the weak thermal scaling of optical decoherence across a diverse series of conjugated polymers and show that its experimental manifestation must be interpreted in the context of the detection observable.