The effect of substituent location on the ultraviolet absorption spectra of ethynyl-substituted polycyclic aromatic hydrocarbons

We have examined the ultraviolet absorption spectra of 11 ethynyl-substitut ed polycyclic aromatic hydrocarbons-1-ethynylnaphthalene, 2-ethynylnaphthal ene, 1-ethynylacenaphthylene, 5-ethynylacenaphthylene, 1-ethynylanthracene, 2-ethynylanthracene, 2-ethynylphenanthrene, 3-ethynylphenanthrene, 9-ethyn ylphenanthrene, 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 pro duces a bathochromic shift of the lower-wavelength beta -band; substitution on the short axis of the molecule produces a bathochromic shift in the hig her-wavelength p-band. The features of the spectra of the unsubstituted com pound are preserved, with the following exceptions: substitution on the lon g axis also produces a split in the beta -band peak; and the p-band always increases with respect to the beta -band, regardless of substituent locatio n. The effects of substituent location are sufficiently regular that the po sition of the ethynyl group with respect to the axial orientation of the mo lecule can be distinguished by the UV spectrum alone.