Photophysics of ANS. II: Charge transfer character of near-UV absorption and consequences for ANS spectroscopy

We continue our investigation of the photophysics of 1,8-anilinonaphthalenesulfonate in protein and solvent systems. In this report, we concentrate on the nature of the excited states as observed in UV spectra. We develop a fairly general formalism for handling the coupled transitions we observe in the partial systems aniline and naphthalene. We assign one of the near-UV transitions, which is more clearly discernible in congeners of 1,8-ANS, but still present in 1,8-ANS as we postulate it, to a charge-transfer band. The other transition is from aniline itself. The expected energies of these now coupled bands in anilinonaphthalene and ANS are calculated, and the transition dipole moment for these transitions is derived.