Three-Dimensional Orientation of the Q_y Electronic Transition Dipole Moment within the Chlorophyll a Molecule Determined by Femtosecond Polarization Resolved VIS Pump−IR Probe Spectroscopy

Abstract

Chlorophyll a (Chl a) is the most abundant pigment on earth. In all plants, algae, and cyanobacteria, it plays a pivotal role as an antenna and reaction center pigment in the primary steps of photosynthesis. In the past, a true three-dimensional (3D) experimental determination of the Qy electronic transition dipole moment orientation could not be obtained. With combined femtosecond polarization resolved VIS pump-IR probe experiments and theoretical calculations of the infrared transition dipole moments (tdm's) in the electronic ground state, we determined the 3D orientation of the Qy electronic tdm of Chl a within the molecular structure. Polarization resolved experiments provided angles of the Qy electronic tdm with three different infrared tdm's, whose orientations within the molecular structure were taken from our theoretical calculations. The orientation of the Qy tdm results from the intersection of all three angles and was found to have an angle of (78 +/- 3)degrees with the x-axis, (12 3)degrees with the y-axis, and (86 +/- 2)degrees with the z-axis.

Determining the Three‐Dimensional Electronic Transition Dipole Moment Orientation: Influence of an Isomeric Mixture

“…The determination of the Q y electronic tdm orientation was performed [8] using three molecular vibrations of the chlorin ring segment, that is, the chlorin ring n(C=C) r absorbing at 1288 cm…”

“…[8,9] Briefly, the method consists of determining experimentally the angles between the electronic tdm and vibrational tdm vectors, where the latter have been calculated using DFT. For a single vibrational tdm the angle produces a cone of possible orientations of the electronic tdm.…”

“…Combining cones for minimally three linearly independent vibrational tdm vectors yields at most a single unique 3D electronic tdm vector solution. [8,9] Thus far, the results of this method were reported for the Q y transition of Chlorophyll a [8] and for the S 0 !S 1 transition of Coumarin 314 (C314) [9] (see Figure 1 for the molecular structures). The importance of determining the Q y electronic tdm orientation is self-evident, given its enormous importance for all kinds of photosynthetic biological apparatus.…”

“…The vibrational tdm vectors were obtained [8] for these modes in the gas phase lowest energy conformation by DFT calculations performed with Gaussian03, using the B3LYP exchange-correlation functional and 6-31G* or 6-31G** basis sets. .…”

Structure, Dynamics, and Function in the Major Light-Harvesting Complex of Photosystem II

“…The excited state transition dipole moments of the individual chlorophyll are the Q y , or S 0 -S 1 , transitions. [2,30,31] This is illustrated for three coupled chlorophylls within the structural model of monomeric LHCII in Fig. 2a and b.…”