1973 Volume 46 Issue 10 Pages 3056-3061
Absorption and fluorescence spectra of 1- and 2-naphthoic acid (1- and 2-NA) have been studied in a hydrocarbon rigid-glass matrix at 77 K and in the crystal. Each of these acids is associated to form a hydrogen-bonded dimer. On increasing the concentration of 1- or 2-NA, the dimer is further associated to give a higher-order complex that may be assumed to be a tetramer. The fluorescence spectra of the monomer and dimer are in mirror-image relation to their respective absorption spectra in both 1- and 2-NA. When the tetramer of 1-NA is photo-excited, a broad structureless fluorescence emission appears, with the maximum at ca. 7000 cm−1 to the red from the 0–0 band of the dimer fluorescence. The structureless fluorescence spectrum, which has no mirror-image relation to the absorption spectrum of the tetramer, is supposed to originate from an “excimer-like tetramer” of 1-NA. Photo-excitation of the 2-NA tetramer results in a structured fluorescence emission which is considered to come from the simple excited state of the tetramer. The fluorescence lifetimes, quantum yields, and fluorescence-polarization spectra were measured for various molecular species of 1- and 2-NA. The natural fluorescence lifetime of the excimer-like tetramer of 1-NA is evaluated to be 320 nsec, which is 51 times as long as that of the dimer, and the fluorescence from the excimer-like tetramer is polarized mainly along the short molecular axis of the naphthalene nucleus. It is concluded from these observations that the excimer-like tetramer has a symmetrical configuration, with the planes of the constituent dimer molecules parallel to each other, and its fluorescence process is essentially symmetry forbidden.
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