The Effect of Substituent Location on the Ultraviolet Absorption Spectra of Ethynyl-Substituted Polycyclic Aromatic Hydrocarbons

Abstract

We have examined the ultraviolet absorption spectra of 11 ethynyl-substituted polycyclic aromatic hydrocarbons—1-ethynylnaphthalene, 2-ethynylnaphthalene, 1-ethynylacenaphthylene, 5-ethynyl-acenaphthylene, 1-ethynylanthracene, 2-ethynylanthracene, 2-ethynyl-phenanthrene, 3-ethynylphenanthrene, 9-ethynylphenanthrene, 1-ethynylpyrene, and 4-ethynylpyrene—in order to determine the effect of substituent location on spectral features. We have found that, as predicted by theory, substitution on the long axis of the molecule produces a bathochromic shift of the lower-wavelength β-band; substitution on the short axis of the molecule produces a bathochromic shift in the higher-wavelength p-band. The features of the spectra of the unsubstituted compound are preserved, with the following exceptions: substitution on the long axis also produces a split in the β-band peak; and the p-band always increases with respect to the β-band, regardless of substituent location. The effects of substituent location are sufficiently regular that the position of the ethynyl group with respect to the axial orientation of the molecule can be distinguished by the UV spectrum alone.

Key Words:

• UV spectra
• polycyclic aromatic hydrocarbons
• ethynyl-PAH
• substitution effects