Anomalous fluorescence behaviour of organic dyes

Subject to certain excitation conditions, organic-dye molecules in solution can fluoresce at higher energies than those of the S₁ → S₀ transition. This paper investigates the anomalous fluorescence behaviour of differently substituted polymethine dyes when excited by ruby-laser radiation and the ruby harmonic respectively. All dyes, including those which are not aromatically substituted, show a short-wave fluorescence. The results of investigation suggest that in the excited state the conjugation over the total molecule is lost and that the anomalous fluorescence reflects the transition between two levels of this de-coupled system.