Optical Absorption Spectrum of Metal-Free Phthalocyanine Single Crystals

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Abstract
The optical absorption spectrum of the molecular crystal metal-free phthalocyanine has been investigated in the region 25 000 to 3500 Å. The spectrum of the phthalocyanine molecule retains much of its identity in the crystal, as evidenced by the similarity in the regions of main absorption. This is a consequence of the weak intermolecular binding forces. The crystal absorptions in the region 8000 to 5000 Å, and below 4000 Å can be attributed to excitations from the highest filled molecular orbital to the vibrationally broadened and shifted excited states of the molecule which occur in the crystal field. Vibrational structure was observed which was attributed to a symmetrical breathing vibration of the peripheral benzene rings and, quite possibly, a similar vibration of the entire phthalocyanine molecule.

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