Based on this assignment, we plotted the intensity ratio of the 1442 cm -1 to the 1387 cm -1 peaks, representing the ratio of aromatic to quinoidal forms, as we increased temperature (Figure , top). We observe a significant increase in the biradical form starting at the II-III transition. Moreover, the intensity ratio of trans to gauche C-C stretching also changed during the transition (Figure , bottom). As polymorph III formed, this ratio decreases drastically, suggesting significant changes in the alkyl chain dynamics. In previous works, the intensity ratio of these two conformers provide an indication of disorder in the alkyl side chains . As more gauche isomers form in the system, the alkyl chains become bent and twisted within the occupied volume. A complete melting of the alkyl side chains is represented by a ratio of 1, as the chains become free to bend in any direction. In the 2DQTT-o-B system, the intensity ratio of trans to gauche decreases from 8 to 1 across the II-to-III transition. These results strongly suggest that the formation of biradical species and alkyl chain melting are the molecular origins of the II-III phase transition. To confirm the formation of biradicals, we also simulated and compared the absorption spectroscopy as well as measured the spin states in the crystals through in situ EPR spectroscopy to directly observe the formation of these biradical states. UV-Vis was simulated for both the quinoidal and aromatic forms via setting the BLA value for each of the systems (see methods). The simulated UV-Vis spectra for the quinoidal molecule showed good agreement with the experimental spectra (Figure ) though we expect the redshift and vibronic fine features resulting from crystalline packing not to be captured in the simulation. For the aromatic molecule, the simulation showed a significantly red shifted and broadened peak compared to the quinoidal form, similar to the redshift observed in the experimental spectra (Figure ). Based on Raman, we expect a mixture of quinoidal and aromatic forms, which may account for a smaller extent of red shift observed than predicted by simulation (Figure ). Nonetheless, the good qualitative match between simulated and experimental spectra suggests that the biradical aromatic form provides a good model for describing the molecular system in polymorph III. Finally, as expected from our hypothesis, in situ EPR showed an increasing presence of spins in our system as we increased temperature (Figure ). EPR directly measures the increasing unpaired spins associated with the biradical state (Figure ) and indeed, we see a dramatic increase in the concentration of spins within the material (40x increase) upon reaching the II-III transition (Figure ). This confirms that the II-III transition not only exhibits a large structural change, but also that a key part of this transition involves accommodating the abruptly increased concentration of biradical species through a reconstructive structural transition (Figure ). ). Much like stacks of dominos tilting all at once, molecules along the b-axis of the crystal constituting the 1-D π stacks stand up during the I-II transition. In this case, the tipping of the molecule is caused by a conformation change in the alkyl side chains, which results in the molecule standing upright during heating and allowing the molecule to slip back into a tilted position during cooling. We suggest engineering of these alkyl side chains may be crucial in facilitating the I-II cooperative behavior. While recently rotation of bulky side chains have been shown to be critical for expressing cooperative behavior in molecular crystals , cooperative polymorph transition has rarely been studied in molecules with alkyl side chain motifs -a dominant side chain design in organic crystals . Until now, their role in facilitating cooperative transitions has not been well understood.