(135c) Understanding Polymorphic Transition Mechanisms of n-Type Semiconductors

Cooperativity has long been used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structure transition involves simultaneous, concerted displacement of molecules in a crystalline material, in stark contrast to the typical nucleation and growth mechanism occurring in a molecule-by-molecule fashion that often disrupt the material structural integrity. Cooperative transitions have acquired much attention in the research community for its low transition barrier, ultrafast kinetics, and structural reversibility. On the other hand, cooperative transition is rarely observed in molecular crystals and its molecular origin is not well understood. In 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type semiconductor our research has shown prolific diversity in polymorphism and transition behavior resulting in 5 obtainable polymorphs. For each of these crystal structures, we report dramatic changes in electronic and optical performance as an organic semiconductor. Along with the diversity of structure, both a cooperative and nucleation and growth transition has been reported simultaneously in the same system through 2 different thermally activated phase transitions. In situ microscopy, single crystal and grazing incidence X-ray diffraction, and Raman spectroscopy suggest a reorientation the alkyl side chains results in a cooperative transition behavior. While in stark contrast, we find the nucleation and growth mechanism occurs based on through a combination of side chain melting and increased core interactions resulting from the biradical nature of 2DQTT-o-B and is confirmed through in situ electron paramagnetic resonance. This is the first time, to our knowledge, that biradical interactions result in a structural change. Through studying these fundamental mechanisms, we may establish design rules to rationally obtain polymorphic behavior for novel electronic applications.