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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 potential energy surface (PES) events during 1 ps NA-ESMD simulations of ChlA (black) and ChlB (red).Times of effective (a) B → Qx and (b) Qx → Qy hops, energy gaps (ΔE) between (c) B and Qx, and (d) Qx and Qy excited states during effective hops, frequency of nonadiabatic coupling magnitude during effective (e) B → Qx and (f) Qx → Qy hops, and times of effective hops with ΔE > 0.1 eV (solid) and ΔE ≤ 0.1 eV (dashed) for (g) B → Qx and (h) Qx → Qy. In (a,b,g,h), t = 0 refers to the moment the donor excited state (B – (a) and (g), Qx – (b) and (h)) is initially populated on each trajectory.
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f7: Excited state potential energy surface (PES) events during 1 ps NA-ESMD simulations of ChlA (black) and ChlB (red).Times of effective (a) B → Qx and (b) Qx → Qy hops, energy gaps (ΔE) between (c) B and Qx, and (d) Qx and Qy excited states during effective hops, frequency of nonadiabatic coupling magnitude during effective (e) B → Qx and (f) Qx → Qy hops, and times of effective hops with ΔE > 0.1 eV (solid) and ΔE ≤ 0.1 eV (dashed) for (g) B → Qx and (h) Qx → Qy. In (a,b,g,h), t = 0 refers to the moment the donor excited state (B – (a) and (g), Qx – (b) and (h)) is initially populated on each trajectory.

Mentions: In Fig. 7, various properties of the B → Qx and Qx → Qy pathways are plotted, including the distributions of the time (fs) and energy gap, ΔE (eV), of effective excited state hops, and a distribution of the nonadiabatic coupling magnitudes at these times. From the cumulative plot in Fig. 7(a), it becomes evident that the effective hops from B → Qx occur on a shorter time-scale in ChlB than those in ChlA. In ChlB, most effective hops have occurred within 200 fs of the initial B excitation, while ChlA continue to have effective hops until 500 fs. Also, when comparing Fig. 7(a,b), all B → Qx effective hops occur on a shorter time-scale than Qx → Qy effective hops. This observation is consistent with the calculated absorption spectra, which show a much greater overlap of B and Qx bands in ChlB, and is also consistent with the proposed mechanism of sequential internal conversion from B → Qx → Qy. Figure 7(c) further shows that during the effective hops in B → Qx transfer in ChlB, the exciton appears to pass close to the seam of the conical intersection. In contrast, B → Qx transfer evolves further away from the conical intersection seam in ChlA. This B → Qx relaxation pathway through the conical intersection seam can explain the shorter lifetime for ChlB with respect to ChlA. Figure 7(e,f) show the distribution of nonadiabatic (NA) coupling magnitudes for B/Qx and Qx/Qy excited states during effective hops. The B/Qx NA couplings in Fig. 7(e) are related to the fact that ChlB effective hops occur close to the seam of the conical intersection, thus stronger NA couplings are more common in ChlB.


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 potential energy surface (PES) events during 1 ps NA-ESMD simulations of ChlA (black) and ChlB (red).Times of effective (a) B → Qx and (b) Qx → Qy hops, energy gaps (ΔE) between (c) B and Qx, and (d) Qx and Qy excited states during effective hops, frequency of nonadiabatic coupling magnitude during effective (e) B → Qx and (f) Qx → Qy hops, and times of effective hops with ΔE > 0.1 eV (solid) and ΔE ≤ 0.1 eV (dashed) for (g) B → Qx and (h) Qx → Qy. In (a,b,g,h), t = 0 refers to the moment the donor excited state (B – (a) and (g), Qx – (b) and (h)) is initially populated on each trajectory.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f7: Excited state potential energy surface (PES) events during 1 ps NA-ESMD simulations of ChlA (black) and ChlB (red).Times of effective (a) B → Qx and (b) Qx → Qy hops, energy gaps (ΔE) between (c) B and Qx, and (d) Qx and Qy excited states during effective hops, frequency of nonadiabatic coupling magnitude during effective (e) B → Qx and (f) Qx → Qy hops, and times of effective hops with ΔE > 0.1 eV (solid) and ΔE ≤ 0.1 eV (dashed) for (g) B → Qx and (h) Qx → Qy. In (a,b,g,h), t = 0 refers to the moment the donor excited state (B – (a) and (g), Qx – (b) and (h)) is initially populated on each trajectory.
Mentions: In Fig. 7, various properties of the B → Qx and Qx → Qy pathways are plotted, including the distributions of the time (fs) and energy gap, ΔE (eV), of effective excited state hops, and a distribution of the nonadiabatic coupling magnitudes at these times. From the cumulative plot in Fig. 7(a), it becomes evident that the effective hops from B → Qx occur on a shorter time-scale in ChlB than those in ChlA. In ChlB, most effective hops have occurred within 200 fs of the initial B excitation, while ChlA continue to have effective hops until 500 fs. Also, when comparing Fig. 7(a,b), all B → Qx effective hops occur on a shorter time-scale than Qx → Qy effective hops. This observation is consistent with the calculated absorption spectra, which show a much greater overlap of B and Qx bands in ChlB, and is also consistent with the proposed mechanism of sequential internal conversion from B → Qx → Qy. Figure 7(c) further shows that during the effective hops in B → Qx transfer in ChlB, the exciton appears to pass close to the seam of the conical intersection. In contrast, B → Qx transfer evolves further away from the conical intersection seam in ChlA. This B → Qx relaxation pathway through the conical intersection seam can explain the shorter lifetime for ChlB with respect to ChlA. Figure 7(e,f) show the distribution of nonadiabatic (NA) coupling magnitudes for B/Qx and Qx/Qy excited states during effective hops. The B/Qx NA couplings in Fig. 7(e) are related to the fact that ChlB effective hops occur close to the seam of the conical intersection, thus stronger NA couplings are more common in ChlB.

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