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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

Species-associated absorption difference spectra of the Sn state and S1 state of Chl resulting from fitting a two-component sequential kinetic model Sn → S1 to the experimental transient absorption measured in a 2.5 ps window, for (a) ChlA in ethanol and (b) ChlB in ethanol.The Sn → S1 lifetime is shown in parentheses. The S1 state is quasi-stationary (fixed 5 ns lifetime).
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f3: Species-associated absorption difference spectra of the Sn state and S1 state of Chl resulting from fitting a two-component sequential kinetic model Sn → S1 to the experimental transient absorption measured in a 2.5 ps window, for (a) ChlA in ethanol and (b) ChlB in ethanol.The Sn → S1 lifetime is shown in parentheses. The S1 state is quasi-stationary (fixed 5 ns lifetime).

Mentions: To obtain a quantitative measure of the time constant of internal conversion, the TA data were subjected to global multiexponential analysis39, using the program Glotaran40, whereby the TA kinetics at all wavelengths was described by a single-exponential rise convoluted with a Gaussian (55 fs FWHM) instrument response function. More precisely, a two-component sequential kinetic scheme was used - Sn → S1 - with one component representing higher Sn excited states and a quasi-stationary component (with a fixed lifetime of 5 ns), representing the S1 state. The lifetime of the Sn states and the pump-probe spectra of the Sn and S1 states (species-associated absorption difference spectra, SADS) were obtained by fitting the model to the experimental TA data. The model also accounted for chirp (group velocity dispersion) and coherent artifact. The best fit was obtained with lifetimes of 143 fs for ChlA and 162 fs for ChlB (standard errors of fit ca. 5%). The SADS are shown in Fig. 3(a,b) for ChlA and ChlB, respectively. In both cases, the initial SADS has a smaller amplitude, blue-shifted maximum (664 nm and 647 nm for ChlA and ChlB, respectively), and no appreciable excited-state absorption at 600–640 nm, confirming that the spectrum represents higher excited states, whereas the second SADS represents the emitting S1 state. Owing to these spectral differences, global analysis of the TA could extract the time constant of Sn → S1 internal conversion with much greater accuracy than what would be possible by analyzing time traces at single probe wavelengths. From this analysis it can be concluded that internal conversion time constant in ChlB is very close to, or within the error estimates, slightly larger than that in ChlA.


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)

Species-associated absorption difference spectra of the Sn state and S1 state of Chl resulting from fitting a two-component sequential kinetic model Sn → S1 to the experimental transient absorption measured in a 2.5 ps window, for (a) ChlA in ethanol and (b) ChlB in ethanol.The Sn → S1 lifetime is shown in parentheses. The S1 state is quasi-stationary (fixed 5 ns lifetime).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Species-associated absorption difference spectra of the Sn state and S1 state of Chl resulting from fitting a two-component sequential kinetic model Sn → S1 to the experimental transient absorption measured in a 2.5 ps window, for (a) ChlA in ethanol and (b) ChlB in ethanol.The Sn → S1 lifetime is shown in parentheses. The S1 state is quasi-stationary (fixed 5 ns lifetime).
Mentions: To obtain a quantitative measure of the time constant of internal conversion, the TA data were subjected to global multiexponential analysis39, using the program Glotaran40, whereby the TA kinetics at all wavelengths was described by a single-exponential rise convoluted with a Gaussian (55 fs FWHM) instrument response function. More precisely, a two-component sequential kinetic scheme was used - Sn → S1 - with one component representing higher Sn excited states and a quasi-stationary component (with a fixed lifetime of 5 ns), representing the S1 state. The lifetime of the Sn states and the pump-probe spectra of the Sn and S1 states (species-associated absorption difference spectra, SADS) were obtained by fitting the model to the experimental TA data. The model also accounted for chirp (group velocity dispersion) and coherent artifact. The best fit was obtained with lifetimes of 143 fs for ChlA and 162 fs for ChlB (standard errors of fit ca. 5%). The SADS are shown in Fig. 3(a,b) for ChlA and ChlB, respectively. In both cases, the initial SADS has a smaller amplitude, blue-shifted maximum (664 nm and 647 nm for ChlA and ChlB, respectively), and no appreciable excited-state absorption at 600–640 nm, confirming that the spectrum represents higher excited states, whereas the second SADS represents the emitting S1 state. Owing to these spectral differences, global analysis of the TA could extract the time constant of Sn → S1 internal conversion with much greater accuracy than what would be possible by analyzing time traces at single probe wavelengths. From this analysis it can be concluded that internal conversion time constant in ChlB is very close to, or within the error estimates, slightly larger than that in ChlA.

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