On the origin of high transient anisotropies: an exemplification in a Cd-porphyrin

Transient anisotropy is a widely used spectroscopic method to access the polarization dynamics of a molecular sample. In this contribution, we present results on 5,10,15,20-tetraphenyl-porphyrinato cadmium (II) in tetrahydrofuran which exhibits values exceeding the typical range between 0.4 and -0.2 in dependence of the probe wavelength. These findings are explained by varying contributions from excited state absorption and ground state bleaching∕stimulated emission. Model calculations show that time zero values and time decays are complex values that often do not correlate with the underlying physical processes. As a consequence, the interpretation of anisotropy experiments necessitates extreme care.