Orientation of the Electronic Qy Transition Dipole Moment in Chlorophyll a

The orientation of the Q_y transition dipole moment (tdm) in Chlorophyll a (Chl a) is a fundamental parameter for optimizing energy transfer in light harvesting systems. Here, we apply polarization resolved transient VIS-pump/IR-probe and 2D-IR spectroscopy in combination with density functional theory (DFT) calculations to determining the orientation of the Q_y tdm for hexacoordinated Chl a. Analysis of the 2D-IR spectra provide a comprehensive study of the C═O vibrations, allowing us to refine the modeling of the vibrational tdm (vtdm) vectors. Polarization resolved transient IR spectra provide relative angles between the Q_y tdm and different probed vtdms, which determine cones of possible orientations of the Q_y tdm. Calculating the intersection of three cones from linearly independent vtdm vectors, we determine the orientation of the Q_y tdm. We find that the Q_y tdm for hexacoordinated Chl a is oriented along the y-axis of the macrocycle, exhibiting only a small angular deviation within −1° to 4°.