Photosynthetic bacteria, algae, and plants convert solar energy into biochemical fuel via light-harvesting complexes comprised of proteins and pigments. Pigments absorb photons by exciting electron-hole pairs that move readily to other pigments. How biology utilizes such quantum processes at room temperature to achieve efficiency of energy transfer is of interest to physicists. This article reviews computational and spectroscopic advances in understanding the energy conversion and energy transport of such a magnificent nanorectenna.

Delocalized excitons in natural light-harvesting complexes

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.

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

An efficient artificial light harvesting (LH) system was developed via sequential two-step energy accumulation. A periodic mesoporous organosilica with bridging biphenyl groups in the framework (Bp–PMO) was used as an LH antenna. The center of a linear-shaped Re(I) pentanuclear complex was connected to a Ru(II) trisdiimine complex through a covalent bond, these served as the first and second energy acceptors, respectively, (Ru–Re5). Hybridization was achieved with the non-ionic surfactant C12H25(OCH2CH2)4OH in an acetonitrile solution, and in the hybrid (Ru–Re5–Bp–PMO), the Ru–Re5 molecules were adsorbed in an orderly fashion in the mesopores of the Bp–PMO. Photons absorbed by 437 ± 43 of the Bp units were first accumulated in the five Re units in Ru–Re5 and then transferred to only one Ru unit, which emitted the light strongly.

Efficient light harvesting via sequential two-step energy accumulation using a Ru–Re5 multinuclear complex incorporated into periodic mesoporous organosilica

The control of molecular-level quantum effects in artificial photosynthetic membranes is a powerful tuning knob for optimizing long-range energy transport, according to a theoretical study.

Design Principles for Long-Range Energy Transfer at Room Temperature

Under physiological conditions, ballistic long-range transfer of electronic excitations in molecular aggregates is generally expected to be suppressed by noise and dissipative processes. Hence, quantum phenomena are not considered to be relevant for the design of efficient and controllable energy transfer over significant length and time scales. Contrary to this conventional wisdom, here we show that the robust quantum properties of small configurations of repeating clusters of molecules can be used to tune energy transfer mechanism that take place on much larger scales. With the support of an exactly solvable model, we demonstrate that coherent exciton delocalization and dark states within unit cells can be used to harness dissipative phenomena of varying nature (thermalization, fluorescence, non-radiative decay and weak inter-site correlations) to support classical propagation over macroscopic distances. In particular, we argue that coherent delocalization of electronic excitations over just a few pigments can drastically alter the relevant dissipation pathways which influence the energy transfer mechanism, and thus serve as a molecular control tool for large-scale properties of molecular materials. Building on these principles, we use extensive numerical simulations to demonstrate that they can explain currently not understood measurements of micron-scale exciton diffusion in nano-fabricated arrays of bacterial photosynthetic complexes. Based on these results we provide quantum design guidelines at the molecular scale to optimize both energy transfer speed and range over macroscopic distances in artificial light-harvesting architectures.

Design principles for long-range energy transfer at room temperature

Excitation energy transport in a biomimetic molecular nanoarray constructed from LH2 antenna complexes is investigated by a master equation approach including the effect of coherent hopping. Calculated stationary and transient fluorescence signals upon incidence of a diffraction-limited light pulse are compared with measurements. Energy transport was established from the influence of active energy-guiding layers on the observed fluorescence emission. Energy migration occurs as a result of efficient coupling between many hundreds of LH2 complexes. We obtain an analytical expression of stationary fluorescence distribution solving the master equations of the system. The time-dependent fluorescence intensity is derived using the same formalism. The numerical results show a reasonable consistency with the experimental result in the engineered nanoarray of LH2 complexes. In addition, our results show that quantum coherence mechanism is necessary to explain the long distance energy transport. These results demon...

/pdf/theoretical-examination-of-long-range-energy-propagation-in-lk1n0ki5w9.pdf

Theoretical examination of long-range energy propagation in nano-engineered light-harvesting antenna arrays

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Theoretical examination of long-range energy propagation in nano-engineered light-harvesting antenna arrays