Precise determination of the orientation of the transition dipole moment in a Bodipy derivative by analysis of the magnetophotoselection effect

Magnetophotoselection effects, observed for the lowest triplet state of a boron-dipyrromethene (Bodipy) derivative by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy, have been exploited to determine with a good precision of about 5° the 3D orientation of the singlet-singlet transition dipole moment (TDM) in the molecular frame. We have shown that the TR-EPR spectrum lineshape, obtained using a polarized laser pulse, is different from that obtained by using depolarized light. We first developed a new model for the simulation of the TR-EPR spectra obtained with polarized light excitation; the model explicitly takes into account the orientation of the TDM in the molecular frame. We then applied the method for the determination of the TDM orientation, directly from glassy frozen isotropic solutions of Bodipy. The experimentally determined direction has been compared with that obtained from quantum mechanical calculations.