Forbidden character in electronic spectra of mono-halonaphthalenes

The singlet-singlet absorption and emission spectra of α-fluoro and the absorption spectra of α-chloro-, α-iodo-, and β-bromo-naphthalenes have been photographed and analyzed. The absorption spectra of all the four molecules show the presence of two discrete systems of bands, and the emission spectrum of α-fluoronaphthalene corresponds to the longer-wavelength system of absorption. With the α-chloro- and α-iodo-naphthalenes, the longer-wavelength system shows the appearance of pure electronic transition with weak bands, while the corresponding transitions in case of α-fluoro- and β-bromo-naphthalenes appear with the most intense bands of the system, as expected theoretically. The appearance of strong vibronic and weak electronic bands in the longer-wavelength system of the former two molecules has been interpreted as the interaction of totally symmetric vibrations, a rare example in the literature.