Orientation of transition moments in excited-state absorption of polymethine and squarylium dyes

Summary form only given. Over the past several years our efforts have been directed toward a systematic investigation of nonlinear properties of a new series of polymethine dyes (PDs) and squarylium dyes (SDs) in liquid solutions and polymeric media. These studies led to the development of several new dyes showing strong excited-state absorption (ESA) in the visible region with the ratios of excited-to ground-state absorption cross-sections up to 200 at 532 nm. One of these dye´s (labeled PD2338) chemical structure is shown along with its singlet energy-level diagram. The study of the excited state transition moments will lead to a deeper understanding of the nature of ESA in organic molecules and will aid in the quantum-chemical calculations for the prediction of their nonlinear properties. To measure the orientation of the transition moments we use a pump-probe geometry in which the probe polarization is fixed and the pump polarization can be rotated using a halfwave plate. In a pump-probe experiment an intense pump beam induces a nonlinear response in the medium that is then sensed by the weak probe beam. This experimental procedure may be used to determine population decay kinetics and rotational relaxation times. These decay times can be measured by conducting three measurements for the pump polarization-parallel, perpendicular, and at the magic angle (57.3 degrees) to the probe. The results for PD2338, using the second harmonic of a picosecond (25 ps FWHM) Nd:YAG laser are shown.