On Anomalous Fluorescence of Symmetrical Polymethine Dyes

Abstract

The group-theoretical analysis of polymethine dyes (PMD) showed that relaxation processes between the states S3 and S1 are forbidden, either by radiation or by internal conversion. From the state S3, only transition to the ground state of the molecule is possible. Since the experimental data state that the quantum yield of S3 ⟶ S0 fluorescence does not exceed 1%, it is indicated that the internal conversion rate can be 2 orders of magnitude higher than the radiative relaxation rate of the molecule. Concerning the reasons for the appearance of fluorescence from the higher excited states of molecules, it can be asserted that the necessary condition for the appearance of S3 ⟶ S0 fluorescence is the absence of S0 ⟶ S1(v)-absorption in the region of the S0 ⟶ S3 transition. The sufficient condition is the corresponding symmetry of the excited states, which imposes a prohibition on the S3 ⟶ S1 relaxation process.