The High-Pressure Response of 9,9'-Spirobifluorene Studied by Raman Spectroscopy

The pressure response of crystalline 9,9'-spirobifluorene up to 8 GPa was studied by means of Raman spectroscopy using a diamond anvil cell as a pressure chamber. With increasing pressure, the observed Raman peaks shifted to higher frequencies, reflecting the bond hardening upon volume reduction, which was much more pronounced for the initially weaker intermolecular interactions than for the stronger intramolecular covalent bonds. The significant changes in the Raman spectrum and the pressure evolution of the frequencies at ~1.3 GPa for both the intermolecular and the intramolecular Raman peaks signaled a pressure-induced structural and molecular conformation transition with a little hysteretic behavior (~0.5 GPa) upon pressure release. For P > 4 GPa, the reversible decrease of the pressure coefficients of the majority of the intermolecular and some intramolecular peak frequencies indicated another structural modification of the studied molecular crystal. A value of ~9 GPa for the bulk modulus of the system at zero pressure was estimated from the logarithmic pressure coefficients of the frequencies of the intermolecular modes in the low-pressure phase. These coefficients were reduced by ~6 times at 4.2 GPa, indicating that the considerable stiffening of the material in the high-pressure phase emanated from the selective strengthening of the intermolecular interactions.