The effect of ethynyl substitution and cyclopenta fusion on the ultraviolet absorption spectra of polycyclic aromatic hydrocarbons

We have examined the UV spectra of six newly-synthesized ethynyl-PAH (1-eth ynylnaphthalene, 5-ethynylacenaphthylene, 1-ethynylanthracene, 9-ethynylphe nanthrene, 3-ethynylfluoranthene, and 1-ethynylpyrene) and five correspondi ng CP-PAH (acenaphthylene, aceanthrylene, acephenanthrylene, cyclopenta[cd] fluoranthene, and cyclopenta[cd]pyrene) and have found the following system atic behavior: ethynyl-substitution results in average bathochromic shifts of 5 nm for the lower-wavelength beta- and beta'-bands and of 18 nm for the higher-wavelength p-band, as well as an amplification of the p-band with r espect to the P-band; the addition of a peripheral cyclopenta ring results in average bathochromic shifts of 7 nm in the beta-band, 16 nm in the beta' -band, and 44 nm in the p-band, compared to the wavelengths in the correspo nding unsubstituted PAH. The addition of a cyclopenta ring also decreases t he resolution of vibrational fine structure in the cases where the fine str ucture is particularly intense in the spectrum of the unsubstituted PAH. Th e effects of ethynyl substitution on the UV spectra of PAH are sufficiently consistent that new ethynyl-PAH should be identifiable based on their UV s pectra alone. (C) 2000 Elsevier Science B.V. All rights reserved.