The 40 year riddle of chlorophyll Q bands resolved: Vibronic coupling and dephasing
Chlorophylls are amongst the most important molecules on Earth. They serve to absorb solar radiation, transferthe excitation to reaction centres where specialised chlorophylls initiate the photochemistry that provide the energy for most life processes.
Most critical are the properties of the Q bands Qy and Qx. The assignment of Qx has been continuously argued, particularly for chl-a, this chlorophyll being the most important in photosynthesis. Using a combination of computational chemistry, theoretical chemistry and MCD spectroscopy, we have been able to completely resolve this assignment riddle for a very wide range of chlorophillides. The previous assignments for Qx were frequently fundamentally incorrect!
For chl-a in particular, the breakdown of the Born-Oppenheimer approximation leads to strong vibronic coupling between a vibrational sideband of Qy and a quasi-resonant Qx. This leads to two 'Qx features'. The new analysis has great significance in both excitation transfer calculations and in the viability of quantum coherence processes in photosynthesis. The dephasing induced by vibronic coupling will fundamentally limit coherence to less than 100 fs.