Perylene Crystal Spectra under Pressure

The absorption and fluorescence shifts of perylene at high pressures are compared for the two crystalline forms. The red shift $\mathrm{-\hspace{0.17em}\Delta \nu }$ at 20 kbar is greater for β‐perylene than for α‐perylene. Also, it is observed that ${\mathrm{-\hspace{0.17em}\Delta \nu }}^f{\mathrm{\hspace{0.17em}<\hspace{0.17em}-\hspace{0.17em}\Delta \nu }}^a$ for α‐perylene and ${\mathrm{-\hspace{0.17em}\Delta \nu }}^f{\mathrm{\hspace{0.17em}\sim \hspace{0.17em}-\hspace{0.17em}\Delta \nu }}^a$ for β‐perylene. The smaller shift of the excimer fluorescence compared to the pressure shift in absorption of α‐perylene is indicative of large specific interactions in the excited state. The condition ${\mathrm{-\hspace{0.17em}\Delta \nu }}^f{\mathrm{\hspace{0.17em}<\hspace{0.17em}-\hspace{0.17em}\Delta \nu }}^a$ has been previously observed in charge‐transfer complexes so that charge‐transfer forces could be responsible for the stabilization of the excimeric state although the present data cannot exclude other explanations for the specific interactions. Intensity changes in the crystal absorption band are not easily related to vibronic coupling mechanisms. Fluorescence intensities are reduced for both crystals under compression.