Quantum dynamics of ultrafast energy transfer processes in light harvesting systems
How to harvest sunlight efficiently for residential and industrial consumption is one of the most challenging problems faced by the modern energy-hungry world. However, underlying mechanisms of the nearly perfect energy transfer efficiency in light harvesting systems are not well understood despi...
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Format: | Theses and Dissertations |
Language: | English |
Published: |
2017
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Online Access: | http://hdl.handle.net/10356/69483 |
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Institution: | Nanyang Technological University |
Language: | English |
Summary: | How to harvest sunlight efficiently for residential and industrial consumption is one of the most
challenging problems faced by the modern energy-hungry world. However, underlying mechanisms
of the nearly perfect energy transfer efficiency in light harvesting systems are not well
understood despite many experimental and theoretical efforts devoted to unravelling these intricate
processes. By formulating an accurate theoretical framework aided by the highly parallel
Graphic Processing Unit (GPU) architecture, this thesis presents a comprehensive mechanistic
study of the energy transfer processes in light harvesting systems.
We first develop two theoretical methods, i.e., a time-dependent variational approach utilizing
a superposition of the Davydov D1 and the Davydov D2 ans¨atze, and an extended discretized
Hierarchy Equation of Motion (HEOM) method that can accurately and efficiently
characterize dynamics of the Holstein polaron in the presence of both diagonal and off-diagonal
exciton-phonon coupling. Polaron dynamics of the Holstein model in various parameter regimes
are systematically scrutinized by a combination of the two methods. Our formalism making
use of the Davydov D1 trial state is also applied to explore the coherent energy transport in
engineered nano-arrays of pigments taken from natural photosynthetic systems.
Next, the HEOM method is applied to study dissipative dynamics at a conical intersection,
which plays an important role in the ultrafast energy relaxation processes in polyatomic
systems. We analyze the ultrafast nonadiabatic dynamics for a two-state two-mode conical
intersection in pyrazine which is bilinearly coupled to a bath of an infinite number of harmonic
oscillators by calculating the diabatic and adiabatic populations of electronic states as well as
wave packets of individual vibrational modes. While the population of the upper adiabatic electronic
state is effectively quenched by the system-bath coupling, that of the diabatic electronic
state exhibits long-lived quantum beating driven by the motion of the tuning mode. It is found
that even weak coupling to the bath may effectively suppress the erratic oscillatory features
of the nonadiabatic wave packet motion of an isolated two-mode conical intersection, and the
main effect of the bath is to direct the wave packet to the lower adiabatic energy surface.
Furthermore, a first principles recipe has been developed for describing femtosecond doublepump
single molecule signals of molecular aggregates by incorporating the vibrational modes
with significant exciton-phonon coupling into a system Hamiltonian and treating the corresponding
dynamics in terms of the Davydov D1 ansatz, while the effect of remaining intra and
inter-molecular modes treated as a bath are accounted for through the lineshape function formalism.We then apply our theory to calculate the single molecule signal of LHCII, and it is
found that signal oscillations are of vibronic origin, and the oscillation periods are dependent
on the exciton-phonon coupling strength.
Finally, a theoretical framework is developed to calculate 2D spectra of molecular aggregates
by integrating the Davydov trial states into the non-linear response theory. We first apply
this theory to calculate a model J aggregate in the presence of exciton-phonon coupling and
bath-induced relaxation and dephasing. It is found that the 2D spectra at weak diagonal (offdiagonal)
exciton-phonon coupling exhibits similar patterns as those of a two-level system,
while multi-peak structures accompanied with the population relaxation from high to low energy
regions for large population time dominate for the large diagonal (off-diagonal) coupling.
The application of our theory to the calculation of 2D spectra of LHCII reveals coherent energy
transport between different excitonic states within Chlb and Chla bands.
To summarize, through this work we have developed theoretical frameworks to systematically
study energy transfer processes starting from basic model systems to realistic light
harvesting systems. Specifically, we have investigated nonadiabatic wave packet motion of a
two-state two-mode conical intersection in pyrazine, oscillatory features of the single molecule
signal of LHCII, and the coherent energy transport between different excitonic states within
Chlb and Chla bands of LHCII. It is our hope that this work may help provide guiding principles
for the relationship between the structure of the light harvesting systems and optimal
energy transfer pathways. |
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