The interface orientation of perylene red and phthalocyanine molecules vapor deposited on aluminum

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Abstract
Reflection absorption infrared (RAIR) spectroscopy has been used to study thin films of photo‐sensitizing dyes deposited by vacuum sublimation onto aluminum. Strong monotonic variations in relative band absorbances were observed for perylene red films of varying thicknesses (from 1400 to 30 Å) sublimed onto air‐oxidized vapor‐deposited aluminum substrates. Normalizing to constant film thickness, the bands with dipole transition moments perpendicular to the major symmetry plane of the molecule strongly attenuate as the interface is approached. Those with moments parallel to the major symmetry axis double in intensity while those in the third direction remain constant. The RAIR selection rule that only dipole moment changes normal to the surface can be seen implies, then, that at the interface the perylene red molecules are preferentially oriented and standing one‐third on edge and two‐thirds on end but none flat‐on. This behavior is in contrast to that observed from similarly depositedfilms of metal‐free phthalocyanine which appear to remain disordered as film thickness decreases. Comparison with recent three‐dimensional x‐ray‐produced crystal structures for perylene red and the role of the substrate are discussed.

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