Limitations arising in the study of the fluorescence quenching of rhodamine 6G by iodides using cw-laser thermal lens spectrometry

Fluorescence quenching of rhodamine 6G by iodides has been carried out in ethanol and aqueous micellar solutions using cw-laser thermal lens spectrometry. Comparison of the photothermal results with those obtained from conventional fluorescence has shown some limitations of the photothermal method. Depending on the medium and the quencher concentration, the quenching level calculated from the photothermal data is higher than that derived from fluorescence. In some cases, the thermal lens signal is even higher than that expected for a nonfluorescent compound. A detailed study of the thermal lens behaviour has shown a converging effect on the probe beam arising simultaneously with the normal diverging effect of the thermal lens. The time scale of this abnormal signal is longer than that generated by thermal relaxation of the excited states and could originate from an endothermic (photo)chemical reaction subsequent to the quenching reaction.