Can we correlate the nature of electronic disorder to structural dynamics in polymer semicondcutors?

First-generation polymer semiconductors typically have been of a flexible-chain nature, leading typically to highly heterogeneous solid-state microstructures, comprised of molecular disordered (“amorphous”) and ordered regions (crystallites, aggregates). This heterogeneity can have direct impact on the material’s electronic disorder and is believed to lead to pronounced static electronic disorder. More recent polymers are comprised of a relatively rigid back bone, requiring highly complex side chain substitutions to render them processable. Because of the high back-bone rigidity, the individual chains of these macromolecular semiconductors do not entangle, supporting a “liquid-crystalline”-like behaviour with pronounced structural dynamics. Here, we discuss on the example of PBTTT, which is known to be thermotropic, that the liquid-crystalline behavior strongly influences the optoelectronic landscape of such “hairy-rod” polymers. For instance, in two-dimensional coherent excitation spectroscopy we observe well-defined (0-0) and (0-1) features on the diagonal, and (0-0/0-1) and (0-1/0-0) cross peaks. Asymmetries of cross peaks between two spectral features are indicative of cross-correlations beyond the sharing of a common ground state. Importantly, time evolution analysis of the two-dimensional spectra allows us to identify a periodic change of the (0-1) intensity that is identically to a low-wavenumber vibrational mode found in resonance Raman spectroscopy and can be attributed to in-plane and out-of-plane torsional motions of the backbone; i.e. backbone dynamics can be directly correlated with specific photophysical processes. This implies that structurally more disordered, next-generation hairy-rod semiconductors are overall more homogeneous leading to dynamic electronic disorder, with molecular motions being large enough to cause transfer integral fluctuations on a timescale comparable to or faster than the timescale associated with the coupling energy.