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Non-radiative relaxation of photoexcited chlorophylls: theoretical and experimental study.

Bricker WP, Shenai PM, Ghosh A, Liu Z, Enriquez MG, Lambrev PH, Tan HS, Lo CS, Tretiak S, Fernandez-Alberti S, Zhao Y - Sci Rep (2015)

Bottom Line: Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis.Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales.This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

View Article: PubMed Central - PubMed

Affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University, Saint Louis, Missouri 63130, USA.

ABSTRACT
Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

No MeSH data available.


Related in: MedlinePlus

Excited state energy histogram of the ground state structures of (a) ChlA and (b) ChlB.The average of 500 conformations shows relative oscillator strengths of the Qy, Qx, B1, B2, and B3 bands, and total oscillator strength (solid black), compared to experimental spectra in ethanol (dashed black). Within the simulations, each Chl molecule was initially excited at the B1 band using a Gaussian pulse (red). In the experimental study, both of the Chl molecules were excited at 442 nm.
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f4: Excited state energy histogram of the ground state structures of (a) ChlA and (b) ChlB.The average of 500 conformations shows relative oscillator strengths of the Qy, Qx, B1, B2, and B3 bands, and total oscillator strength (solid black), compared to experimental spectra in ethanol (dashed black). Within the simulations, each Chl molecule was initially excited at the B1 band using a Gaussian pulse (red). In the experimental study, both of the Chl molecules were excited at 442 nm.

Mentions: To model internal conversion processes in ChlA and ChlB we employ the non-adiabatic excited-state molecular dynamics (NA-ESMD) simulations (full details are given in Computational Methods), which require sampling of the ground state potential energy surfaces (PESs). Generation of such an ensemble consisting of 500 input configurations is achieved via ground state molecular dynamics simulations (details are given in Ground State Molecular Dynamics) at 300 K. In order to guide the set-up of subsequent excited state dynamics, we calculated the absorption spectrum for each molecule after averaging over the profiles of excited state energies and the corresponding oscillator strengths for the 500 configurations. Figure 4 shows a comparison of the calculated absorption spectra of ChlA and ChlB to their corresponding experimentally measured steady-state spectra in ethanol. Although the calculated excited state energies and oscillator strengths cannot be compared precisely with experimental results owing to the well known shortcomings of the semi-empirical methods, several key features of the experimental spectra are still found to be well reproduced in the calculated spectra. We note that although semiempirical methods are poor predictors of absolute excitation energies, most experimental features are retained when comparing relative excitation energies between ChlA and ChlB as well as between excited states in each molecule. First, the Qy and Qx bands compare well with the experimental spectra in terms of energetic ordering, with ChlB states having the higher energies, followed by ChlA. Second, the relative oscillator strengths of the Qy bands are in agreement to those in experiment, where ChlB has lower oscillator strength than ChlA. ChlB also has the most distinguishable Qx band of the two chlorophyll molecules, but Qx band energies and oscillator strengths in experimental spectra are more difficult to ascertain than the Soret and Qy bands. Finally, the energy gap between the well distinguished Soret and Qy band peaks in the experiment spectrum is 1.01 eV in ChlA and 0.76 eV in ChlB. The calculated spectrum, on the other hand, yields an energy gap of 1.24 eV in ChlA and 1.12 eV in ChlB, exhibiting a trend that is in reasonable agreement with the experimental counterpart. It is important to note that the calculated Qx bands in both ChlA and ChlB are closer in excitation energy to their respective B bands than the Qy bands, a trend which is not supported by experiment. Thus, this study may reproduce trends in overall B → Qy internal conversion more accurately than in B → Qx and Qx → Qy internal conversion. Notably, such reproduction in the trends of experimental gaps between excited states bears significance for the NA-ESMD results and particularly for calculated non-radiative relaxation timescales.


Non-radiative relaxation of photoexcited chlorophylls: theoretical and experimental study.

Bricker WP, Shenai PM, Ghosh A, Liu Z, Enriquez MG, Lambrev PH, Tan HS, Lo CS, Tretiak S, Fernandez-Alberti S, Zhao Y - Sci Rep (2015)

Excited state energy histogram of the ground state structures of (a) ChlA and (b) ChlB.The average of 500 conformations shows relative oscillator strengths of the Qy, Qx, B1, B2, and B3 bands, and total oscillator strength (solid black), compared to experimental spectra in ethanol (dashed black). Within the simulations, each Chl molecule was initially excited at the B1 band using a Gaussian pulse (red). In the experimental study, both of the Chl molecules were excited at 442 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4561917&req=5

f4: Excited state energy histogram of the ground state structures of (a) ChlA and (b) ChlB.The average of 500 conformations shows relative oscillator strengths of the Qy, Qx, B1, B2, and B3 bands, and total oscillator strength (solid black), compared to experimental spectra in ethanol (dashed black). Within the simulations, each Chl molecule was initially excited at the B1 band using a Gaussian pulse (red). In the experimental study, both of the Chl molecules were excited at 442 nm.
Mentions: To model internal conversion processes in ChlA and ChlB we employ the non-adiabatic excited-state molecular dynamics (NA-ESMD) simulations (full details are given in Computational Methods), which require sampling of the ground state potential energy surfaces (PESs). Generation of such an ensemble consisting of 500 input configurations is achieved via ground state molecular dynamics simulations (details are given in Ground State Molecular Dynamics) at 300 K. In order to guide the set-up of subsequent excited state dynamics, we calculated the absorption spectrum for each molecule after averaging over the profiles of excited state energies and the corresponding oscillator strengths for the 500 configurations. Figure 4 shows a comparison of the calculated absorption spectra of ChlA and ChlB to their corresponding experimentally measured steady-state spectra in ethanol. Although the calculated excited state energies and oscillator strengths cannot be compared precisely with experimental results owing to the well known shortcomings of the semi-empirical methods, several key features of the experimental spectra are still found to be well reproduced in the calculated spectra. We note that although semiempirical methods are poor predictors of absolute excitation energies, most experimental features are retained when comparing relative excitation energies between ChlA and ChlB as well as between excited states in each molecule. First, the Qy and Qx bands compare well with the experimental spectra in terms of energetic ordering, with ChlB states having the higher energies, followed by ChlA. Second, the relative oscillator strengths of the Qy bands are in agreement to those in experiment, where ChlB has lower oscillator strength than ChlA. ChlB also has the most distinguishable Qx band of the two chlorophyll molecules, but Qx band energies and oscillator strengths in experimental spectra are more difficult to ascertain than the Soret and Qy bands. Finally, the energy gap between the well distinguished Soret and Qy band peaks in the experiment spectrum is 1.01 eV in ChlA and 0.76 eV in ChlB. The calculated spectrum, on the other hand, yields an energy gap of 1.24 eV in ChlA and 1.12 eV in ChlB, exhibiting a trend that is in reasonable agreement with the experimental counterpart. It is important to note that the calculated Qx bands in both ChlA and ChlB are closer in excitation energy to their respective B bands than the Qy bands, a trend which is not supported by experiment. Thus, this study may reproduce trends in overall B → Qy internal conversion more accurately than in B → Qx and Qx → Qy internal conversion. Notably, such reproduction in the trends of experimental gaps between excited states bears significance for the NA-ESMD results and particularly for calculated non-radiative relaxation timescales.

Bottom Line: Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis.Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales.This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

View Article: PubMed Central - PubMed

Affiliation: Department of Energy, Environmental and Chemical Engineering, Washington University, Saint Louis, Missouri 63130, USA.

ABSTRACT
Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

No MeSH data available.


Related in: MedlinePlus