To elucidate the driving force behind this change in tilt, we turned to in situ Raman spectroscopy to provide insight into the molecular changes occurring in the I-II transition (Figure ). Raman has shown to be a powerful tool for tracking polymorphic transitions and understanding the molecular changes that occur as a result of these transitions . Under Raman spectroscopy crystals exhibit both low frequency peaks related to the intermolecular phonon vibrations, sensitive to the crystal structure (< 500 cm -1 ), as well as intramolecular vibrations related to the conformation and chemical environment of the molecule (900-1800 cm -1 ). In the phonon region, we observe two key peaks which are associated with lattice vibrational modes, indicated with black arrows in Figure . We observe a ratio change between these two peaks, as the lowest peak increases in intensity during the I-II transition (Figure ). This is a small effect, indicating modest changes to the lattice packing consistent with our observation of changing molecular tilt from GIXD. Phonon intensity modulation, as opposed to peak shifts or splitting has been shown to be typical in well studied martensitic type cooperative transitions . On the other hand, in the intramolecular vibration region (900-1800 cm -1 ), we observed significant changes to the intensity of the peak at 1409 cm -1 relative to the most intense peak at 1387 cm -1 indicated by the red and blue arrows in Figure , respectively and illustrated in Figure . Based on a combination of DFT calculations (Figure ) and literature, we assigned the peak at 1387 cm -1 to C=C stretching along the backbone, corresponding to the effective conjugation coordinate (ECC) mode for the quinoidal form which we denote as ν(C=C) ECC,Q (Figure , Movie S8,9); the peak at 1522 cm -1 was assigned to other C=C stretching modes in the backbone (Figure , Movie s10); the peak at 1772 cm -1 was assigned to C=O stretching (Figure , Movie S11). While the peak at 1409 cm -1 may correspond to core stretching (Figure , Movie S12), this was assigned to CH 2 deformation in the alkyl chains denoted as δ(CH 2 ) based on wellestablished literature on alkyl chain vibrations. Along with these peaks, we also tracked the peaks at 1025 and 1063 cm -1 which were assigned to C-C stretching of the trans ν(C-C) T and gauche ν(C-C) G isomers in the alkyl chains. The alkyl chain peak assignments (δ(CH 2 ), ν(C-C) T , ν(C-C) G ) were confirmed based on Raman spectroscopy measured on a similar molecule with shorter side chains (Figure ). A complete explanation for these assignments can be found in the supplementary information.