Grazing incidence X-ray Diffraction. GIXD was performed at beamline 8-ID-E of the Advanced Photon Source at Argonne National Laboratory . The data were collected at 10.91 keV on a 2D Pilatus 1M detector. Films of polymorph I were obtained by solution coating from a solution of 2DQTT-o-B dissolved in chloroform and chlorobenzene at 6 mg mL -1 onto SiO 2 treated with trichloro(phenyl)silane (PTS). Solution coating was performed at 85 °C at a blade speed of 0.3 mm s -1 and a blade gap of 100 μm. These films were annealed under nitrogen atmosphere for 30 minutes at 100 °C to convert the films to polymorph I. In situ thermal annealing measurements were conducted in a He environment with the sample on a commercial thermal stage (Linkam HFSX350-GI), with the temperature ramped at 10 °C min - 1 and the exposure taken after equilibration at the target temperature for approximately 5 min. GIXD videos (Movies S7,13) were obtained via taking continuous exposures at a constant heating of 5 °C min -1 . GIXSGUI software was used to correct for detector nonuniformity, beam polarization and to reshape the 2D data into the representation q z vs q r 65 . The incident angle was set at 0.14° right above the critical angle for total reflectance of the organic thin film. Unit cells were extracted based on the discussion in the previous paper by Davies and Diao et. al., 2021. Raman Spectroscopy. A Raman confocal imaging microscope with a 532 nm laser (Laser Quantum Ventus 532 with max power 50 mW) and 50× long working distance objective lens (HORIBA LabRAM HR 3D) equipped with HORIBA Synapse back-illuminated deep-depletion CCD camera was used to collect spectra. Using a 300 g mm -1 grating, we used a scan exposure time of between 20-60 s. An optical density filter of OD = 0.2 was used (OD = log(power transmission factor)), and no beam damage was observed only at the highest temperatures after prolonged periods of laser exposure (>5min). To eliminate this effect, each spectrum was recorded in a new position on the crystal to prevent overexposure of any particular area. For variable-temperature experiments, the samples were collected using a Linkam THMS600 heating stage with a closed chamber. The heating and cooling rate was kept at 10 °C min -1 . Each temperature was equilibrated until the temperature reading stabilized and the Raman laser was refocused to account for substrate thermal expansion (approximately 5 minutes).