Orientation of the Electronic Qy Transition Dipole Moment in Chlorophyll a

The orientation of the Qy 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 Qy 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 Qy tdm and different probed vtdms, which determine cones of possible orientations of the Qy tdm. Calculating the intersection of three cones from linearly independent vtdm vectors, we determine the orientation of the Qy tdm. We find that the Qy tdm for hexacoordinated Chl a is oriented along the y-axis of the macrocycle, exhibiting only a small angular deviation within −1° to 4°.