Polymorph Transition Behaviors. Crystals of 2DQTT-o-B (Figure ) were either grown via drop casting from a 1:1 para-dichlorobenzene and decane mixture or slow evaporation from a 1:1 dichloromethane and ethyl acetate mixture to form both small (50-300µm) and large (1-3mm) crystals, respectively (Methods Section). Single crystal X-ray diffraction showed crystals of 2DQTT-o-B packed in a C 2/c unit cell with 1-D π-stacks which was designated polymorph I . Based on the BFDH morphology (Figure ), we observe the π-π stacking occurs along the long axis of the crystal (Figure ). These π-stacks then pack into layers separated by the alkyl side chains (Figure ), which results in the thin needle like crystals observed in polarized optical microscopy (POM). We set to investigate two thermally induced structural transitions in single crystals of 2DQTT-o-B using polarized optical microscopy, where we heated and cooled crystals at a constant 5 °C min -1 . We observed two reversible phase transitions designated the I-II transition at 164 °C and II-III at 223 °C. The phase transitions were accompanied by modulation of the brightness and color, reflecting changes in the refractive index of the material (Figure ,e, Movies 1-2,3-4). We observed distinct transition behavior suggesting a cooperative transition mechanism for the I-II transition as opposed to a nucleation and growth mechanism during the II-III transition. Below we discuss evidence from in situ POM in detail and this inference is further supported by in situ GIXD, Raman spectroscopy and EPR spectroscopy discussed later.