Simultaneous dual superradiance from the planar and twisted conformations of a laser dye

This paper reports the observation of simultaneous superradiant bands, one around 415 nm, and another around 435 nm, from Coumarin 450 in certain solvents, under nitrogen laser pumping. The short wavelength band is attributed to the Normal Conformation (NF) and the long wavelength band to the TICT conformation. It appears that these two conformation exist in a dynamic equilibrium in the excited state and for the stabilisation of the latter, specific solvent-solute complexation is necessary. This exciplex association is strongly temperature dependent. At high temperature (about 65°C), the complexation is easily broken so that ASE at shorter wavelength (due to NF) is stronger than (due to TICT) that at longer wavelength. The reverse is the situation at low temperature (about 5°C).