Asymmetry in the Qy Fluorescence and Absorption Spectra of Chlorophyll a Pertaining to Exciton Dynamics

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.

Density functionals with asymptotic-potential corrections are required for the simulation of spectroscopic properties of materials

“…This means that excited-state equilibrium geometries and spectroscopic reorganisation energies become better described, as well as vibrational line intensities arising from both Franck–Condon (Huang–Rhys) and Herzberg–Teller mechanisms. 27,28,91…”

“… a Vertical excitation energies at the CAM-B3LYP/6-31G*/D3(BJ) ground-state geometry. 91 b Gas-phase, 92 with the Q x band centre (an absorption minimum) estimated using established methods: 26 Q y = 1.94 eV, Q x = 2.14 eV, B x = 3.08 eV, B y = 3.38 eV, N x , N y > 3.2 eV. …”

“…This means that excitedstate equilibrium geometries, spectroscopic reorganisation energies become better described, as well as vibrational line intensities arising from both Franck-Condon (Huang-Rhys) and Herzberg-Teller mechanisms. 27,28,91 b. Overview of how corrections to the long-range potential can significantly improve calculated properties of simple materials.…”

“…excitation energies at the CAM-B3LYP/6-31G*/D3(BJ) ground-state geometry91 . b: gas-phase92 , with the Q x band centre (an absorption minimum) estimated using established methods26 : Q y = 1.94 eV, Q x = 2.14 eV, B x = 3.08 eV, By = 3.38 eV, N x ,N y > 3.2 eV.Chemical ScienceARTICLEThis journal is © The Royal Society of Chemistry 20xxChemical Science, 2013, 00, 1-3 | 5…”

Towards a quantitative description of excitonic couplings in photosynthetic pigment–protein complexes: quantum chemistry driven multiscale approaches

“…These results on one hand justify our linear scaling procedure and on the other hand they demonstrate a great robustness of our methods with respect to the details of the quantum chemical method used. Note that, besides the redistribution of oscillator strength discussed in Section 2.7, there are additional subtleties as a Dushinsky rotation of normal modes, 101 Jahn-Teller corrections 102 to the Condon approximation that have an influence on the exact value of the (effective) Huang-Rhys factor. However, these additional effects are small compared to the 3.5 fold correction found above (eqn ( 44)).…”