Electronic Structure Effects During Cooperative Phase Transitions in Organic Semiconductor Single Crystals

ABSTRACT Cooperative phase transitions in organic crystals are attracting growing interest due to their reversible, diffusionless nature, their ability to generate macroscopic dynamic phenomena, and their strong influence on the physicochemical properties of molecular materials relevant to organic electronics. Yet, the microscopic mechanisms governing these lattice‐ and disorder‐driven transformations remain only partially understood. Because phase transitions amplify electron–phonon interactions by pushing low‐frequency modes into the anharmonic regime and reorganizing molecular packing within a few kelvins, they offer a unique opportunity to directly probe lattice–electronic coupling in organic semiconductors. Here, we investigate single crystals of 2,7‐di‐tert‐butyl‐[1]benzothieno[3,2‐b]benzothiophene (ditBu‐BTBT), a high‐performance organic semiconductor undergoing a diffusionless polymorphic transition near 70°C, to reveal how its electronic landscape, specifically the distribution of electronic states and optical bandgap, evolves across a cooperative phase transition. Variable‐temperature soft x‐ray reflectivity at the carbon K‐edge and UV–vis absorption spectroscopy show the order–disorder transition, evidenced by energy shifts of specific resonances attributed to increased dynamics of the lateral tert‐butyl groups, which become intramolecularly disordered at high temperature. Concurrently, the optical bandgap increases, reflecting reduced intermolecular interactions and band dispersion. These results directly demonstrate how cooperative polymorphic transitions modulate the electronic structure of organic semiconductors.