Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet–triplet annihilation to upconvert excitation, have been extensively studied. ...Comparatively little work has sought to examine the properties of the intermediate state in both of these processesthe bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin |X1⟩ ≈ 1TT and quintet spin |Q⟩ ≈ 5TT. It is shown that triplet pair states can be separated by a triplet–triplet energy-transfer mechanism to give a separated, yet entangled triplet pair 1T···T. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton–exciton annihilation to form the correlated triplet pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the separated correlated triplet pair (geminate recombination).
Ultrafast energy transfer is used to transmit electronic excitation among the many molecules in photosynthetic antenna complexes. Recent experiments and theories have highlighted the role of coherent ...transfer in femtosecond studies of these proteins, suggesting the need for accurate dynamical models to capture the subtle characteristics of energy transfer mechanisms. Here we discuss how to think about coherence in light harvesting and electronic energy transfer. We review the various fundamental concepts of coherence, spanning from classical phenomena to the quantum superposition, and define coherence in electronic energy transfer. We describe the current status of experimental studies on light-harvesting complexes. Insights into the microscopic process are presented to highlight how and why this is a challenging problem to elucidate. We present an overview of the applicable dynamical theories to model energy transfer in the intermediate coupling regime.
The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the ...excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment−pigment and pigment−protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment molecules. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.
Polaritons and excitons Scholes, Gregory D.
Proceedings of the Royal Society. A, Mathematical, physical, and engineering sciences,
10/2020, Letnik:
476, Številka:
2242
Journal Article
Recenzirano
Odprti dostop
The primary questions motivating this report are: Are there ways to increase coherence and delocalization of excitation among many molecules at moderate electronic coupling strength? Coherent ...delocalization of excitation in disordered molecular systems is studied using numerical calculations. The results are relevant to molecular excitons, polaritons, and make connections to classical phase oscillator synchronization. In particular, it is hypothesized that it is not only the magnitude of electronic coupling relative to the standard deviation of energetic disorder that decides the limits of coherence, but that the structure of the Hamiltonian—connections between sites (or molecules) made by electronic coupling—is a significant design parameter. Inspired by synchronization phenomena in analogous systems of phase oscillators, some properties of graphs that define the structure of different Hamiltonian matrices are explored. The report focuses on eigenvalues and ensemble density matrices of various structured, random matrices. Some reasons for the special delocalization properties and robustness of polaritons in the single-excitation subspace (the star graph) are discussed. The key result of this report is that, for some classes of Hamiltonian matrix structure, coherent delocalization is not easily defeated by energy disorder, even when the electronic coupling is small compared to disorder.
The intermediate coupling regime for electronic energy transfer is of particular interest because excitation moves in space, as in a classical hopping mechanism, but quantum phase information is ...conserved. We conducted an ultrafast polarization experiment specifically designed to observe quantum coherent dynamics in this regime. Conjugated polymer samples with different chain conformations were examined as model multichromophoric systems. The data, recorded at room temperature, reveal coherent intrachain (but not interchain) electronic energy transfer. Our results suggest that quantum transport effects occur at room temperature when chemical donor-acceptor bonds help to correlate dephasing perturbations.
The sophistication with which we can now prepare and characterize inorganic nanoparticles has inspired new areas of research into the fundamental properties and applications of these fascinating ...nanoscale systems. In this article some of the recent ideas concerning control of their optical properties are examined and explained, focusing on semiconductor nanocrystals. It is known that the optical properties of nanocrystals can be size‐tunable, but it is less obvious how shape matters. To explain how size as well as shape matters, the electronic structure of nanocrystals is sketched in relatively simple terms, leading to an introduction to deeper concepts at the heart of spectroscopy such as the exciton fine structure. The exciton fine structure states, although obscured by inhomogeneous line broadening, dictate selection rules for optical excitation. These viewpoints are compared and contrasted to well‐established principles in molecular spectroscopy that provide inspiration as well as perspective. The control of optical poperties is founded on our ability to prepare good quality colloidal particles. Recent advances in nanocrystal shape control are described. The current status of heterostructures is examined, with an emphasis on charge separation in CdSe–CdTe nanorods.
Inorganic nanoparticles exhibit unique optical properties. It is well known that they can be tuned by size, but what other factors influence the electronic states, optical properties, and excited state dynamics? What is the exciton fine structure? How does shape make a difference? How are the properties of two components transformed in a heterostructure?.
Here we investigate the photophysics and photochemistry of Ni(II) aryl halide complexes common to cross-coupling and Ni/photoredox reactions. Computational and ultrafast spectroscopic studies reveal ...that these complexes feature long-lived 3MLCT excited states, implicating Ni as an underexplored alternative to precious metal photocatalysts. Moreover, we show that 3MLCT Ni(II) engages in bimolecular electron transfer with ground-state Ni(II), which enables access to Ni(III) in the absence of external oxidants or photoredox catalysts. As such, it is possible to facilitate Ni-catalyzed C–O bond formation solely by visible light irradiation, thus representing an alternative strategy for catalyst activation in Ni cross-coupling reactions.
Recent research suggests that electronic energy transfer in complex biological and chemical systems can involve quantum coherence, even at ambient temperature conditions. It is particularly notable ...that this phenomenon has been found in some photosynthetic proteins. The role of these proteins in photosynthesis is introduced. The meaning of quantum-coherent energy transfer is explained, and it is compared to Förster energy transfer. Broad, interdisciplinary questions for future work are noted. For example, how can chemists use quantum coherence in synthetic systems (perhaps in organic photovoltaics)? Why did certain photosynthetic organisms evolve to use quantum coherence in light harvesting? Are these electronic excitations entangled?
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