Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems

Two-dimensional photon-echo experiments indicate that excitation energy transfer between chromophores near the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides occurs coherently with decoherence times of hundreds of femtoseconds, comparable to the energy transfer time scale in these systems. The original explanation of this observation suggested that correlated fluctuations in chromophore excitation energies, driven by large scale protein motions could result in long lived coherent energy transfer dynamics. However, no significant site energy correlation has been found in recent molecular dynamics simulations of several model light harvesting systems. Instead, there is evidence of correlated fluctuations in site energy-electronic coupling and electronic coupling-electronic coupling. The roles of these different types of correlations in excitation energy transfer dynamics are not yet thoroughly understood, though the effects of site energy correlations have been well studied. In this paper, we introduce several general models that can realistically describe the effects of various types of correlated fluctuations in chromophore properties and systematically study the behavior of these models using general methods for treating dissipative quantum dynamics in complex multi-chromophore systems. The effects of correlation between site energy and inter-site electronic couplings are explored in a two state model of excitation energy transfer between the accessory bacteriochlorophyll and bacteriopheophytin in a reaction center system and we find that these types of correlated fluctuations can enhance or suppress coherence and transfer rate simultaneously. In contrast, models for correlated fluctuations in chromophore excitation energies show enhanced coherent dynamics but necessarily show decrease in excitation energy transfer rate accompanying such coherence enhancement. Finally, for a three state model of the Fenna-Matthews-Olsen light harvesting complex, we explore the influence of including correlations in inter-chromophore couplings between different chromophore dimers that share a common chromophore. We find that the relative sign of the different correlations can have profound influence on decoherence time and energy transfer rate and can provide sensitive control of relaxation in these complex quantum dynamical open systems.     

Bioinspired molecular design of light-harvesting multiporphyrin arrays

Recent progress in fundamental studies on multiporphyrin arrays has provided structural parameters for the molecular design of artificial light-harvesting antennae which mimic the wheel-like antenna complexes of photosynthetic purple bacteria. Covalent and noncovalent approaches have been employed for the construction of artificial light-harvesting multiporphyrin arrays. Such arrays are categorized into ring-shaped, windmill-shaped, star-shaped, and dendritic architectures. In particular, dendritic multiporphyrin arrays have been proven to be promising candidates for both providing a large absorption cross-section and enabling the vectorial transfer of energy over a long distance to a designated point. Such molecular and supramolecular systems are also expected to be potent components for molecular electronics and photonic devices.     

Perspectives in Supramolecular Chemistry—From Molecular Recognition towards Molecular Information Processing and Self-Organization

The selective binding of a substrate by a molecular receptor to form a supramolecular species involves molecular recognition which rests on the molecular information stored in the interacting species. The functions of supermolecules cover recognition, as well as catalysis and transport. In combination with polymolecular organization, they open ways towards molecular and supramolecular devices for information processing and signal generation. The development of such devices requires the design of molecular components performing a given function (e.g., photoactive, electroactive, ionoactive, thermoactive, or chemoactive) and suitable for assembly into an organized array. Light-conversion devices and charge-separation centers have been realized with photoactive cryptates formed by receptors containing photosensitive groups. Eleclroactive and ionoactive devices are required for carrying information via electronic and ionic signals. Redox-active polyolefinic chains, like the “caroviologens”, represent molecular wires for electron transfer through membranes. Push-pull polyolefins possess marked nonlinear optical properties. Tubular mesophases, formed by organized stacking of suitable macro-cyclic components, as well as “chundle”-type structures, based on bundles of chains grafted onto a macrocyclic support, represent approaches to ion channels. Lipophilic macrocyclic units form Langmuir-Blodgett films that may display molecular recognition at the air-water interface. Supramolecular chemistry has relied on more or less preorganized molecular receptors for effecting molecular recognition, catalysis, and transport processes. A step beyond preorganization consists in the design of systems undergoing self-organization, that is, systems capable of spontaneously generating a well-defined supramolecular architecture by self-assembling from their components under a given set of conditions. Several approaches to self-assembling systems have been pursued: the formation of helical metal complexes, the double-stranded helicates, which result from the spontaneous organization of two linear polybipyridine ligands into a double helix by binding of specific metal ions; the generation of mesophases and liquid crystalline polymers of supramolecular nature from complementary components, amounting to macroscopic expression of molecular recognition; the molecular-recognition-directed formation of ordered solid-state structures. Endowing photo-, electro-, and ionoactive components with recognition elements opens perspectives towards the design of programmed molecular and supramolecular systems capable of self-assembly into organized and functional supramolecular devices. Such systems may be able to perform highly selective operations of recognition, reaction, transfer, and structure generation for signal and information processing at the molecular and supramolecular levels.     

Optically nonlinear energy transfer in light-harvesting dendrimers

Dendrimeric polymers are the subject of intense research activity geared towards their implementation in nanodevice applications such as energy harvesting systems, organic light-emitting diodes, photosensitizers, low-threshold lasers, and quantum logic elements, etc. A recent development in this area has been the construction of dendrimers specifically designed to exhibit novel forms of optical nonlinearity, exploiting the unique properties of these materials at high levels of photon flux. Starting from a thorough treatment of the underlying theory based on the principles of molecular quantum electrodynamics, it is possible to identify and characterize several optically nonlinear mechanisms for directed energy transfer and energy pooling in multichromophore dendrimers. Such mechanisms fall into two classes: first, those where two-photon absorption by individual donors is followed by transfer of the net energy to an acceptor; second, those where the excitation of two electronically distinct but neighboring donor groups is followed by a collective migration of their energy to a suitable acceptor. Each transfer process is subject to minor dissipative losses. In this paper we describe in detail the balance of factors and the constraints that determines the favored mechanism, which include the excitation statistics, structure of the energy levels, laser coherence factors, chromophore selection rules and architecture, possibilities for the formation of delocalized excitons, spectral overlap, and the overall distribution of donors and acceptors. Furthermore, it transpires that quantum interference between different mechanisms can play an important role. Thus, as the relative importance of each mechanism determines the relevant nanophotonic characteristics, the results reported here afford the means for optimizing highly efficient light-harvesting dendrimer devices.     

Electrochemical and photochemical control of host-guest complexation at surfaces

The application of electrochemistry or photochemistry to modulate supramolecular interactions between host-guest systems in solution is a burgeoning field. In particular, the speed and reversibility associated with electrochemically or photochemically actuated supramolecular interactions has allowed the creation and modulation of novel solution-based devices. In recent years, great advances have been made in transferring these systems from the solution to the solid state, to facilitate the development of molecular-electronics components that can operate in unison under the influence of an externally applied stimulus. These studies pave the way for the creation of responsive surfaces with advanced materials and nanotechnology applications.     

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light‐harvesting and charge‐separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF₂‐chelated dipyrromethene (BODIPY), “porphyrin’s little sister”, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron‐donor and ‐acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto‐ and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy‐ and electron‐transfer reactions and to determine their rates and efficiencies.     

Advancement of Electrochemical Thermoelectric Conversion with Molecular Technology

Thermocells are a thermoelectric conversion technology that utilizes the shift in an electrochemical equilibrium arising from a temperature difference. This technology has a long history; however, its low conversion efficiency impedes its practical usage. Recently, an increasing number of reports have shown drastic improvements in thermoelectric conversion efficiency, and thermocells could arguably represent an alternative to solid thermoelectric devices. In this Minireview, we regard thermocells as molecular systems consisting of successive molecular processes responding to a temperature change to achieve energy generation. Various molecular technologies have been applied to thermocells in recent years, and could stimulate diverse research fields, including supramolecular chemistry, physical chemistry, electrochemistry, and solid-state ionics. These research approaches will also provide novel methods for achieving a sustainable society in the future.     

Elucidating excited state electronic structure and intercomponent interactions in multicomponent and supramolecular systems

Rational design of supramolecular systems for application in photonic devices requires a clear understanding of both the mechanism of energy and electron transfer processes and how these processes can be manipulated. Central to achieving these goals is a detailed picture of their electronic structure and of the interaction between the constituent components. We review several approaches that have been taken towards gaining such understanding, with particular focus on the physical techniques employed. In the discussion, case studies are introduced to illustrate the key issues under consideration.     

二茂铁-酞菁-富勒烯超分子三元体系的构筑及光物理和电化学性质

采用富勒吡咯烷衍生物中的吡啶或咪唑基与二茂铁修饰的金属酞菁轴向配位构筑了二茂铁-酞菁-富勒烯超分子三元体系, 通过紫外-可见光谱滴定法测定了其配位稳定性(K_assoc约为8.58×10⁴ L/mol). 稳态和时间分辨荧光光谱研究结果表明, 在该超分子三元体系中发生了快速的光诱导电子转移(k_CS约为10⁹ s^-1), 并具有较高的电荷分离态量子产率(Ф_CS=0.88). 循环伏安法数据表明, 其电荷分离驱动力ΔG_CS为负值(-0.60 eV), 说明酞菁和富勒烯之间容易形成电荷分离态.     

Photoinduced Electron Transfer in Organized Assemblies—Case Studies

In this review, photoinduced electron transfer processes in specifically designed assembled architectures have been discussed in the light of recent results reported from our laboratories. A convenient and useful way to study these systems is described to understand the rules that drive a light-induced charge-separated states and its subsequent decay to the ground state, also with the aim of offering a tutorial for young researchers. Assembled systems of covalent or supramolecular nature have been presented, and some functional multicomponent systems for the conversion of light energy into chemical energy have been discussed.     

Supramolecular electron transfer by anion binding

Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation. This difference in rates ensures that the linkage between electron donor and acceptor moieties is maintained over the course of most forward and back electron-transfer processes. A particular example of this principle is illustrated by electron-transfer ensembles based on tetrathiafulvalene calix[4]pyrroles (TTF-C4Ps). In these ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after anion binding. Competition with non-redox active substrates is also observed. Anion binding to the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes formed with fullerenes also results in acceleration of the photoinduced electron-transfer process but deceleration of the back electron transfer; again, this is ascribed to favourable structural and electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes between sulphonated tetraphenylporphyrin anions ([MTPPS](4-): M = H(2) and Zn) and a lithium ion encapsulated C(60) (Li(+)@C(60)); the resulting ensemble produces long-lived charge-separated states upon photoexcitation of the porphyrins.     

基于分子催化剂光驱动水氧化器件的研究进展

光解水制氢是人们解决未来能源危机的一种重要构想,构建高效的光电化学池是实现这一构想的重要途径,而光驱动水氧化的顺利进行是实现这一过程的关键.本文总结了近年来基于分子催化剂及染料敏化的光驱动水氧化分子器件的研究和进展,介绍了这些分子器件的组装和性能研究等,并根据这些研究的优点和不足提出一些浅见.     

Redox-Guest-Induced Multimode Photoluminescence Switch for Sequential Logic Gates in a Photoactive Coordination Cage

Molecular or supramolecular level photoluminescence (PL) modulation combining chemical and photonic input/output signals together in an integrated system can provide potential high-density data memorizing and process functions intended for miniaturized devices and machines. Herein, a PL-responsive supramolecular coordination cage has been demonstrated for complex interactions with redox-active guests. PL signals of the cage can be switched and modulated by adding or retracting Fc derivatives or converting TTF into different oxidation states through chemical or photochemical pathways. As a result, reversible or stepwise PL responses are displayed by these host–guest systems because of the occurrence of photoinduced electron-transfer (PET) or fluorescence resonance energy transfer (FREnT) processes, providing unique nanodevice models bearing off/on logic gates or memristor-like sequential memory and Boolean operation functions.     

Structural Design and Application of Azo-based Supramolecular Polymer Systems

This article presents a brief overview of recent advances in azo-containing supramolecular systems. In literature, it has been shown that azo supramolecular polymers and their composite materials exhibit fast and intelligent responses to various external stimuli,such as temperature, p H change, redox reagents, ligands, coupling reagents, etc. In applications, these systems are widely used for molecular motors, shape memory, liquid crystal, solar thermal energy storage, signal transmission, intelligent encryption, and other purposes. Furthermore, these systems can function as key components for device upgrade processing. However, the design and rules of azo supramolecular polymers are still not supported by an exact theory. Information about the relationship between the spatial structure and behavior is lacking, and new supramolecular materials cannot be designed by adding functional moieties to known azo polymers.Based on the current research status, this review mainly summarizes the structural design principles as well as structures and applications of known azo supramolecules; meanwhile, it highlights the emerging development fields, recent advances, and prospects in fabricating self-assembling intelligent supramolecular systems with azo supramolecular polymers as responsive units. The goal of this review is to bring new inspiration to researchers who want to optimize the chemical structure, steric conformation, electrostatic environment, and specific molecular functionalization.     

The Importance of Charge Transfer between the Retinal Chromophore and the Protein Environment in Bacteriorhodopsin: A Theoretical Analysis on Reduced and Oxidized Chromophores

Abstract— The importance of charge transfer(CT) between the retinal chromophore and the protein environment in the ground state of bacteriorhodopsin(BR) has been verified by using ab initio and semiempirical molecular orbital methods. We hypothesize that the chromophore is stabilized in BR by highest occupied molecular orbital-lowest unoccupied molecular orbital(HOMO-LUMO) interaction with the protein environment. If sufficient charge is transferred between two sites due to the strong HOMO-LUMO interaction, the chromophore might be treated as a one-electron reduced species(when it behaves as an electron acceptor), or as a one-electron oxidized one (when it acts as an electron donor).In both optimized geometries, the -conjugated systems exhibit a drastic decrease in bond alternation. To estimate the rotational barrier for thermal isomerization between the al-trans and the 13,15-dicis form, the potential energy curve around these two bonds was computed. The first -_* transition energy was also calculated for an inspection of the opsin shift. The barrier height and the transition energy became much lower as a result of the chromophore reduction. The site selectivity in photo- and thermal isomerization and the opsin shift in BR can be well explained by considering CT from the protein environment to the chromophore.     

Novel triptycene-derived hosts: synthesis and their applications in supramolecular chemistry

The development of new classes of macrocyclic hosts has always been one of the most important topics in supramolecular chemistry. During the past several years, based on the triptycene with unique three-dimensional rigid structure, several different kinds of novel triptycene-derived hosts including triptycene-derived cylindrical macrotricyclic polyether, triptycene-derived tris(crown ether)s, triptycene-derived molecular tweezers, triptycene-derived calixarenes, triptycene-derived heterocalixarenes, triptycene-derived tetralactam macrocycles and molecular cage have been designed and synthesized. Then, by exploring the applications of some of the triptycene derived hosts in molecular recognition and molecular assemblies, a series of new supramolecular systems with specific structures and properties have been developed. This feature article highlights our recent advances in the synthesis of triptycene-derived hosts and their applications in supramolecular chemistry.     

Visualization of Stereoselective Supramolecular Polymers by Chirality‐Controlled Energy Transfer

Chirality‐driven self‐sorting is envisaged to efficiently control functional properties in supramolecular materials. However, the challenge arises because of a lack of analytical methods to directly monitor the enantioselectivity of the resulting supramolecular assemblies. Presented herein are two fluorescent core‐substituted naphthalene‐diimide‐based donor and acceptor molecules with minimal structural mismatch and they comprise strong self‐recognizing chiral motifs to determine the self‐sorting process. As a consequence, stereoselective supramolecular polymerization with an unprecedented chirality control over energy transfer has been achieved. This chirality‐controlled energy transfer has been further exploited as an efficient probe to visualize microscopically the chirality driven self‐sorting.     

Design of molecular photonic wires based on multistep electronic excitation transfer.

Light-harvesting complexes, one of nature's supreme examples of nanoscale engineering, have inspired researchers to construct molecular optical devices, such as photonic wires, which are optimised for efficient transfer of excited-state energy over large distances. The control parameters for the design and the advantages of single-molecule fluorescence spectroscopy for the study of such complex systems are discussed with respect to energy-transfer mechanisms, chromophore selection and arrangement as well as static and dynamic heterogeneity.     

Concatenation of reversible electronic energy transfer and photoinduced electron transfer to control a molecular piston

Reversible electronic energy transfer and photoinduced electron transfer conspire in the light-driven dethreading of a molecular piston, showing the potential of combining these processes in supramolecular systems.     

Interrogation of cobaloxime-based supramolecular photocatalyst architectures

This perspective provides a discussion of recent work focused on elucidating the fundamental interactions of artificial photosynthesis in newly developed supramolecular photocatalysts composed of linked chromophore and catalyst modules. Supramolecular photocatalyst architectures are of particular interest because of their potential to overcome many of the limitations of molecular or multimolecular systems and amenability to conventional and emerging physical characterization techniques. As such, changes to the oxidation state and/or physical structure of either chromophore or catalyst modules in response to light excitation is readily monitored with high spatial and temporal resolution. To illustrate this approach, the design evolution of photocatalysts based on Ru(II)poly(pyridyl) chromophores linked to cobaloxime-based H₂ catalysts is discussed. In this work, new synthesis, transient optical spectroscopy, and X-ray scattering were combined to develop next generation photocatalysts capable of ultrafast charge transfer and identification of a key intermediate for hydrogen photocatalysis. Recent and upcoming advances in light source capabilities are ideally suited to monitor light-generated transient structures and well-poised to dramatically impact the drive toward technologically relevant systems for artificial photosynthesis.