Assignment of the Q-Bands of the Chlorophylls: Coherence Loss via Qx − Qy Mixing

Abstract

We provide a new and definitive spectral assignment for the absorption, emission, high-resolution fluorescence excitation, linear dichroism, and/or magnetic circular dichroism spectra of 32 chlorophyllides in various environments. This encompases all data used to justify previous assignments and provides a simple interpretation of unexplained complex decoherence phenomena associated with Q_x → Q_y relaxation. Whilst most chlorophylls conform to the Gouterman model and display two independent transitions Q_x (S₂) and Q_y (S₁), strong vibronic coupling inseparably mixes these states in chlorophyll-a. This spreads x-polarized absorption intensity over the entireQ-band system to influence all exciton-transport, relaxation and coherence properties of chlorophyll-based photosystems. The fraction of the total absorption intensity attributed to Q_x ranges between 7% and 33%, depending on chlorophyllide and coordination and is between 10% and 25% for chlorophyll-a. CAM-B3LYP density-functional-theory calculations of the band origins, relative intensities, vibrational Huang-Rhys factors and vibronic coupling strengths fully support this new assignment.