Abstract
Significant asymmetry found between the high-resolution Qy emission and absorption spectra of chlorophyll-a is herein explained, providing basic information needed to understand photosynthetic exciton transport and photochemical reactions. The Qy spectral asymmetry in chlorophyll has previously been masked by interference in absorption from the nearby Qx transition, but this effect has recently been removed using extensive quantum spectral simulations or else by analytical inversion of absorption and magnetic circular dichroism data, allowing high-resolution absorption information to be accurately determined from fluorescence-excitation spectra. To compliment this, here, we measure and thoroughly analyze the high-resolution differential fluorescence line narrowing spectra of chlorophyll-a in trimethylamine and in 1-propanol. The results show that vibrational frequencies often change little between absorption and emission, yet large changes in line intensities are found, this effect also being strongly solvent dependent. Among other effects, the analysis in terms of four basic patterns of Duschinsky-rotation matrix elements, obtained using CAM-B3LYP calculations, predicts that a chlorophyll-a molecule excited into a specific vibrational level, may, without phase loss or energy relaxation, reemit the light over a spectral bandwidth exceeding 1,000 cm−1 (0.13 eV) to influence exciton-transport dynamics.
Highlights
Chlorophyll-a (Chl-a) is the most common chlorophyllide utilized in natural photosynthesis (van Grondelle et al, 1994; Blankenship et al, 2004; Grimm et al, 2006; Laisk et al, 2009)
We demonstrate that use of gas-phase calculations allows for the key qualitative features that control absorption-emission asymmetry in Chl-a to be identified
The experimental data fitting is done adopting a HuangRhys model (Huang et al, 1950). This assumes that just two electronic states are involved, that the Born-Oppenheimer (Born and Oppenheimer, 1927) and Franck-Condon (Condon, 1928) approximations hold, that the potentials-energy surfaces are harmonic, that the ground- and excited-state vibrational frequencies are identical, and that the Duschinsky rotation matrix (Duschinsky, 1937) is the unit matrix, yielding vibrational frequencies νi, Huang-Rhys factors Si, and associated reorganization energies hνiSi for each state
Summary
Chlorophyll-a (Chl-a) is the most common chlorophyllide utilized in natural photosynthesis (van Grondelle et al, 1994; Blankenship et al, 2004; Grimm et al, 2006; Laisk et al, 2009). Only relatively recently has the differential fluorescence line-narrowing techniques ( FLN) (Rätsep and Freiberg, 2003, 2007; Rätsep et al, 2009a, 2019a) been developed to deliver similar results These techniques provide critical vibrational data required for studies of, e.g., coherent energy transport in photosystems (Renger et al, 1996; Huo and Coker, 2010; Rivera et al, 2013; Kreisbeck et al, 2014; Müh et al, 2014; Romero et al, 2014; Malý et al, 2016; Duan et al, 2017; Ren et al, 2018; Cao et al, 2020; Tomasi and Kassal, 2020)
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