On Anomalous Fluorescence of Symmetrical Polymethine Dyes

The group-theoretical analysis of polymethine dyes (PMD) showed that relaxation processes between the states S 3 and S 1 are forbidden, either by radiation or by internal conversion. From the state S 3 , only transition to the ground state of the molecule is possible. Since the experimental data state that the quantum yield of S 3 ⟶ S 0 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 S 3 ⟶ S 0 fluorescence is the absence of S 0 ⟶ S 1 (v)-absorption in the region of the S 0 ⟶ S 3 transition. The sufficient condition is the corresponding symmetry of the excited states, which imposes a prohibition on the S 3 ⟶ S 1 relaxation process.