Using python-based image analysis, we were able to select individual crystals and track the progression of the phase transition by calculating an average intensity for each frame. The I-II transition proceeded remarkably fast upon heating (Figure , Movie S1,2). Across many crystals, the heating transition typically occurred within one frame, less than 0.1 second, providing a lower bound for the propagation speed of at least 2000 µm s -1 , implying a cooperative-type mechanism. While difficult to capture under normal circumstances due to the speed, this rapid transition is accompanied by a well-defined phase front propagating through the crystal, distinct from typical diffuse phase boundaries under nucleation and growth mechanisms. Initiation of such phase front typically occurred at the crystal tips, or at cracks within the crystal. Even when the transition initiated at both tips simultaneously, the resulting crystal was a single domain after transition, suggesting a close orientational relationship between the two phases. This is consistent with our earlier work by Chung and Diao et. al. which showed defects facilitated phase boundary initiation in similar cooperative transitions of ditBu-BTBT followed by cooperative propagation.