Quantum coherent energy transport in the Fenna-Matthews-Olson complex at low temperature.

In the primary step of natural light harvesting, the solar photon energy is captured in a photoexcited electron-hole pair, or an exciton, in chlorophyll. Its conversion to chemical potential occurs in the special pair reaction center, which is reached by downhill ultrafast excited-state energy transport through a network of chromophores. Being inherently quantum, transport could in principle occur via a matter wave, with vast implications for efficiency. How long a matter wave remains coherent is determined by the intensity by which the exciton is disturbed by the noisy biological environment. The stronger this is, the stronger the electronic coupling between chromophores must be to overcome the fluctuations and phase shifts. The current consensus is that under physiological conditions, quantum coherence vanishes on the 10-fs time scale, rendering it irrelevant for the observed picosecond transfer. Yet, at low-enough temperature, quantum coherence should in principle be present. Here, we reveal the onset of longer-lived electronic coherence at extremely low temperatures of ∼20 K. Using two-dimensional electronic spectroscopy, we determine the exciton coherence times in the Fenna-Matthew-Olson complex over an extensive temperature range. At 20 K, coherence persists out to 200 fs (close to the antenna) and marginally up to 500 fs at the reaction center. It decays markedly faster with modest increases in temperature to become irrelevant above 150 K. At low temperature, the fragile electronic coherence can be separated from the robust vibrational coherence, using a rigorous theoretical analysis. We believe that by this generic principle, light harvesting becomes robust against otherwise fragile quantum effects.

DNA-Modulated and Mechanoresponsive Excitonic Couplings Reveal Chiroptical Correlation of Conformation, Tension, and Dynamics of DNA Self-Assembly.

Study of the conformational and mechanical behaviors of biomolecular assemblies is vital to the rational design and realization of artificial molecular architectures with biologically relevant functionality. Here, we revealed DNA-modulated and mechanoresponsive excitonic couplings between organic chromophores and verified strong correlations between the excitonic chiroptical responses and the conformational and mechanical states of DNA self-assemblies irrespective of fluorescence background interference. Besides, the excitonic chiroptical effect allowed sensitive monitoring of DNA self-assembled nanostructures due to small molecule bindings or DNA strand displacement reactions. Moreover, we developed a new chiroptical reporter, a DNA-templated dimer of an achiral cyanine5 and an intrinsically chiral BODIPY, that exhibited unique multiple-split spectral line shape of exciton-coupled circular dichroism, largely separated response wavelengths, and enhanced anisotropy dissymmetry factor (g-factor). These results shed light on a promising chiroptical spectroscopic tool for studying biomolecular recognition and binding, conformation dynamics, and soft mechanics in general.

Frenkel and Charge-Transfer Excitonic Couplings Strengthened by Thiophene-Type Solvent Enables Binary Organic Solar Cells with 19.8% Efficiency.

Overcoming the trade-off between short-circuited current (Jsc) and open-circuited voltage (Voc) is important to achieving high-efficiency organic solar cells (OSCs). Previous works modulated energy gap between Frenkel local exciton (LE) and charge-transfer (CT) exciton, which is served as driving force of exciton splitting. Differently, our current work focuses on modulation of LE-CT excitonic coupling (tLE-CT) via a simple but effective strategy that the 2-chlorothiophene (2Cl-Th) solvent is utilized in treatment of OSC active-layer films. The results of our experimental measurements and theoretical simulations demonstrated that 2Cl-Th solvent initiates the tighter intermolecular interactions with non-fullerene acceptor in comparison with that of traditional chlorobenzene solvent, thus suppressing the acceptor's over-aggregation and retarding the acceptor crystallization with reduced trap. Importantly, the resulted shorter distances between donor and acceptor molecules in the 2Cl-Th treated blend efficiently strengthen tLE-CT, which not only promotes the exciton splitting but also reduces non-radiative recombination. The champion efficiencies of 19.8% (small-area) with a superior operational reliability (T80: 586 hours) and 17.0% (large-area) were yielded in 2Cl-Th treated cells. This work provided a new insight into modulating the exciton dynamics to overcome the trade-off between Jsc and Voc, which can productively promote the development of OSC field.

A BF2 Chelate Exhibiting Excimer-like Fluorescence with an Unusually Large Stokes Shift in the Crystalline Phase.

The target mono-BF2 complex is weakly emissive in fluid solution because radiationless decay of the excited-singlet state is promoted through an intramolecular N⋅⋅⋅H-N hydrogen bond. The lack of mirror symmetry for this compound is attributed to vibronic effects, as reported previously for the bis-BF2 complex (BOPHY). Red-shifted fluorescence is observed from single crystals, the emission quantum yield approaching 30 % with a fluorescence lifetime of 2 ns. The large Stokes shift of 5,700 cm-1 helps minimize self-absorption. Crystallography indicates that the internal fold and twist angles are increased substantially in the crystal, but the hydrogen bond is weakened relative to solution. The crystal structure is compiled from pairs of head-to-tail molecules having a shift of ca. 4.1 Šand closest approach of ca. 3.5 Å. These molecular pairs are arranged in columns, which, in turn, assemble into sheets. The proximity favors excitonic coupling between individual molecules, with the coupling strength obtained by analysis of the absorption spectrum reaching ca. 1,000 cm-1 . Both the ideal dipole approximation and the extended dipole methodology seriously overestimate the coupling strength, but the atomic transition charge density procedure leads to good agreement with experiment. Emission is attributed to the closely coupled molecular pair functioning in an excimer-like manner with the exciton trapped in a local minimum. Increasing temperature causes a slight blue shift and loss of fluorescence.

Molecular Dynamic Studies of Dye-Dye and Dye-DNA Interactions Governing Excitonic Coupling in Squaraine Aggregates Templated by DNA Holliday Junctions.

Dye molecules, arranged in an aggregate, can display excitonic delocalization. The use of DNA scaffolding to control aggregate configurations and delocalization is of research interest. Here, we applied Molecular Dynamics (MD) to gain an insight on how dye-DNA interactions affect excitonic coupling between two squaraine (SQ) dyes covalently attached to a DNA Holliday junction (HJ). We studied two types of dimer configurations, i.e., adjacent and transverse, which differed in points of dye covalent attachments to DNA. Three structurally different SQ dyes with similar hydrophobicity were chosen to investigate the sensitivity of excitonic coupling to dye placement. Each dimer configuration was initialized in parallel and antiparallel arrangements in the DNA HJ. The MD results, validated by experimental measurements, suggested that the adjacent dimer promotes stronger excitonic coupling and less dye-DNA interaction than the transverse dimer. Additionally, we found that SQ dyes with specific functional groups (i.e., substituents) facilitate a closer degree of aggregate packing via hydrophobic effects, leading to a stronger excitonic coupling. This work advances a fundamental understanding of the impacts of dye-DNA interactions on aggregate orientation and excitonic coupling.

Azobenzene-Substituted Triptycenes: Understanding the Exciton Coupling of Molecular Switches in Close Proximity.

Herein, we report a series of azobenzene-substituted triptycenes. In their design, these switching units were placed in close proximity, but electronically separated by a sp3 center. The azobenzene switches were prepared by Baeyer-Mills coupling as key step. The isomerization behavior was investigated by 1 H NMR spectroscopy, UV/Vis spectroscopy, and HPLC. It was shown that all azobenzene moieties are efficiently switchable. Despite the geometric decoupling of the chromophores, computational studies revealed excitonic coupling effects between the individual azobenzene units depending on the connectivity pattern due to the different transition dipole moments of the π→π* excitations. Transition probabilities for those excitations are slightly altered, which is also revealed in their absorption spectra. These insights provide new design parameters for combining multiple photoswitches in one molecule, which have high potential as energy or information storage systems, or, among others, in molecular machines and supramolecular chemistry.

Complete Set of Diketopyrrolopyrrole Centrosymmetrical Cofacial Stacked Pairs.

Stacked centrosymmetrical dimers and simultaneously H-bonded and stacked hexamers of thiophene-substituted diketopyrrolopyrrole (ThDPP) were studied using DFT as models for crystals with slipped-stacked molecules in 1D columns. Eight stacked dimer arrangements were found, six of which are driven by the minimisation of electron repulsion and realised by placing the partially negatively charged atoms of the diketopyrrolopyrrole rings below the centre of an adjacent thiophene ring. Four of these stacks are related to N,N'-dialkylated derivatives of ThDPP found in the literature, while a further one is related to an N,N'-diacylated derivative. An analogous set of eight stacks was discovered computationally for phenyl-substituted DPP (PhDPP), four of which are known among H-bonded DPP pigments, and one more among N,N'-dialkylated PhDPP derivatives. The results shed more light on the mechanisms that drive the formation of stacks between nonaromatic (DPP) and aromatic (Th, Ph) rings. The excitation energies of the lowest four singlet states computed by TD DFT enabled excitonic coupling and energy separation between Frenkel-resonance-type and charge-transfer states to be established, depending on the equilibrium stack geometry.

Photocrosslinking Probes Proximity of Thymine Modifiers Tethering Excitonically Coupled Dye Aggregates to DNA Holliday Junction.

A DNA Holliday junction (HJ) has been used as a versatile scaffold to create a variety of covalently templated molecular dye aggregates exhibiting strong excitonic coupling. In these dye-DNA constructs, one way to attach dyes to DNA is to tether them via single long linkers to thymine modifiers incorporated in the core of the HJ. Here, using photoinduced [2 + 2] cycloaddition (photocrosslinking) between thymines, we investigated the relative positions of squaraine-labeled thymine modifiers in the core of the HJ, and whether the proximity of thymine modifiers correlated with the excitonic coupling strength in squaraine dimers. Photocrosslinking between squaraine-labeled thymine modifiers was carried out in two distinct types of configurations: adjacent dimer and transverse dimer. The outcomes of the reactions in terms of relative photocrosslinking yields were evaluated by denaturing polyacrylamide electrophoresis. We found that for photocrosslinking to occur at a high yield, a synergetic combination of three parameters was necessary: adjacent dimer configuration, strong attractive dye-dye interactions that led to excitonic coupling, and an A-T neighboring base pair. The insight into the proximity of dye-labeled thymines in adjacent and transverse configurations correlated with the strength of excitonic coupling in the corresponding dimers. To demonstrate a utility of photocrosslinking, we created a squaraine tetramer templated by a doubly crosslinked HJ with increased thermal stability. These findings provide guidance for the design of HJ-templated dye aggregates exhibiting strong excitonic coupling for exciton-based applications such as organic optoelectronics and quantum computing.

Thermally Activated and Aggregation-Regulated Excitonic Coupling Enable Emissive High-Lying Triplet Excitons.

Room-temperature phosphorescence (RTP) originating from higher-lying triplet excitons remains a rather rarely documented occurrence for purely organic molecular systems. Here, we report two naphthalene-based RTP luminophores whose phosphorescence emission is enabled by radiative decay of high-lying triplet excitons. In contrast, upon cooling the dominant phosphorescence originates from the lowest-lying triplet excited state, which is manifested by a red-shifted emission. Photophysical and theoretical studies reveal that the unusual RTP results from thermally activated excitonic coupling between different conformations of the compounds. Aggregation-regulated excitonic coupling is observed when increasing the doping concentration of the emitters in poly(methylmethacrylate) (PMMA). Further, the RTP quantum efficiency improves more than 80-fold in 1,3-bis(N-carbazolyl)benzene (mCP) compared to that in PMMA. This design principle offers important insight into triplet excited state dynamics and has been exploited in afterglow-indicating temperature sensing.

Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid-BF2 Dimers.

We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid-BF2 dyes and the detailed investigation of their solvent-dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly-coupled H-aggregates. The extent of folding and, in turn, of ground-state aggregation is strongly dependent on the nature of the flexible linker. Steady-state and time-resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in-depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two-dimensional 1 H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore.

A Complex Comprising a Cyanine Dye Rotaxane and a Porphyrin Nanoring as a Model Light-Harvesting System.

A nanoring-rotaxane supramolecular assembly with a Cy7 cyanine dye (hexamethylindotricarbocyanine) threaded along the axis of the nanoring was synthesized as a model for the energy transfer between the light-harvesting complex LH1 and the reaction center in purple bacteria photosynthesis. The complex displays efficient energy transfer from the central cyanine dye to the surrounding zinc porphyrin nanoring. We present a theoretical model that reproduces the absorption spectrum of the nanoring and quantifies the excitonic coupling between the nanoring and the central dye, thereby explaining the efficient energy transfer and demonstrating similarity with structurally related natural light-harvesting systems.

Phonon-Driven Oscillatory Plasmonic Excitonic Nanomaterials.

We demonstrate that coherent acoustic phonons derived from plasmonic nanoparticles can modulate electronic interactions with proximal excitonic molecular species. A series of gold bipyramids with systematically varied aspect ratios and corresponding localized surface plasmon resonance energies, functionalized with a J-aggregated thiacarbocyanine dye molecule, produces two hybridized states that exhibit clear anticrossing behavior with a Rabi splitting energy of 120 meV. In metal nanoparticles, photoexcitation generates coherent acoustic phonons that cause oscillations in the plasmon resonance energy. In the coupled system, these photogenerated oscillations alter the metal nanoparticle's energetic contribution to the hybridized system and, as a result, change the coupling between the plasmon and exciton. We demonstrate that such modulations in the hybridization are consistent across a wide range of bipyramid ensembles. We also use finite-difference time domain calculations to develop a simple model describing this behavior. Such oscillatory plasmonic-excitonic nanomaterials offer a route to manipulate and dynamically tune the interactions of plasmonic/excitonic systems and unlock a range of potential applications.

First example of a modular porphyrinoid assembly capable of stabilizing different metal ions in a single molecular scaffold.

We report the synthesis and characterization of porphyrin-corrole-porphyrin (Por-Cor-Por) hybrids directly linked at the meso-meso positions for the first time. The stability and solubility of the trimer are carefully balanced by adding electron-withdrawing substituents to the corrole ring and sterically bulky groups on the porphyrins. The new hybrids are capable of stabilizing more than one metal ion in a single molecular scaffold. The versatility of the triad has been demonstrated by successfully stabilizing homo- (Ni) and heterotrinuclear (Ni-Cu-Ni) coordination motifs. The solid-state structure of the NiPor-CuCor-PorNi hybrid was revealed by single-crystal X-ray diffraction studies. The Ni(II) porphyrins are significantly ruffled and tilted by 83° from the plane of corrole. The robustness of the synthesized hybrids was reflected in the electrochemical investigations and the redox behaviour of the hybrids show that the oxidation processes are mostly corrole-centred. In particular it is worth noting that the Por-Cor-Por hybrid can further be manipulated due to the presence of substituent-free meso-positions on both the terminals.

Spacer-Modulated Differentiation Between Self-Assembly and Folding Pathways for Bichromophoric Merocyanine Dyes.

We have synthesized a large series of bis(merocyanine) dyes with varying spacer unit and investigated in detail their self-organization behavior by concentration- as well as solvent-dependent UV/Vis spectroscopy. Our in-depth studies have shown that the self-organization of the present bis(merocyanine) dyes is subtly influenced by the nature of the spacer unit. The utilization of rigid spacers results in the formation of self-associated bimolecular complexes with high binding strength, while flexible spacers drive the respective bichromophoric dyes to intramolecular folding. Our thorough investigations on the impact of alkyl spacer chain length on the folding tendency of the present series of bis(merocyanine) dyes revealed a biphasic behavior, that is, a steep increase of the folding tendency for the dyes containing C4 to C7 chains and then a gentle decrease for dyes with longer alkyl spacer chains as evidenced by free energy (ΔG) values for the folding of these dyes. Furthermore, analyses of aggregates' optical properties based on exciton theory as well as quantum chemical calculations suggest a bimolecular aggregate structure for the dye possessing a rigid spacer and a rotationally twisted pleated structure for the bis(merocyanine) dyes having spacer units with less than seven carbon atoms, while the application of longer alkyl chain linkers (≥C7) provides enough flexibility to orient the chromophores in electrostatically most favored antiparallel fashion.

Relationship between molecular stacking and optical properties of 9,10-bis((4-N,N-dialkylamino)styryl) anthracene crystals: the cooperation of excitonic and dipolar coupling.

Five 9,10-bis((4-N,N-dialkylamino)styryl) anthracene derivatives (DSA-C1-DSA-C7) with different length alkyl chains were synthesized. They showed the same color in dilute solutions but different colors in crystals. The absorption, photoluminescence, and fluorescence decay indicate that there exist both excitonic and dipolar coupling in crystals of DSA-C1-DSA-C7. X-ray crystallographic analysis revealed that all the crystals belong to the triclinic space group P1 with one molecule per unit cell and that the molecules in every crystal have the identical orientation. This offers ideal samples to investigate the impact of the molecular stacking on the optical properties of the crystals. For the first time, the cooperation of excitonic and dipolar coupling has been comprehensively studied, and the contribution to the spectral shift from the excitonic and dipolar couplings quantitatively obtained. The experiments of amplified spontaneous emission (ASE) together with measurements of the quantum efficiency further confirmed this interpretation. The results suggest that the excitonic and dipolar couplings between the adjacent molecules are both important and jointly induce the spectral shifts of the crystals.

Mixed quantum-classical description of excitation energy transfer in supramolecular complexes: screening of the excitonic coupling.

A mixed quantum-classical description of excitation energy transfer (EET) in large chromophore complexes with significant conformational flexibility is improved by considering screening and local-field effects. To account for the environmentally induced modification of the excitonic coupling J(mn)(ε>1) between chromophore m and n, the Poisson-transition-charges-from-electrostatic-potential (Poisson-TrEsp) method is utilized. A parameterization scheme for the such derived screening/local field factors is introduced, which allows for their incorporation to a mixed quantum-classical description. The method is applied to the supramolecular complex P16 formed by sixteen pheophorbide-a molecules covalently linked to a butanediamine dendrimer and dissolved in ethanol. Data calculated using the novel parameterized screening method are compared to those obtained by alternative screening approaches. Averaging the screening factors in different ways may reproduce ensemble experiments on EET well, while the description of single molecule properties requires the consideration of individual screening factors.

Slow Hot-Exciton Cooling and Enhanced Interparticle Excitonic Coupling in HgTe Quantum Dots.

Rapid hot-carrier/exciton cooling constitutes a major loss channel for photovoltaic efficiency. How to decelerate the hot-carrier/exciton relaxation remains a crux for achieving high-performance photovoltaic devices. Here, we demonstrate slow hot-exciton cooling that can be extended to hundreds of picoseconds in colloidal HgTe quantum dots (QDs). The energy loss rate is 1 order of magnitude smaller than bulk inorganic semiconductors, mediated by phonon bottleneck and interband biexciton Auger recombination (BAR) effects, which are both augmented at reduced QD sizes. The two effects are competitive with the emergence of multiple exciton generation. Intriguingly, BAR dominates even under low excitation fluences with a decrease in interparticle distance. Both experimental evidence and numerical evidence reveal that such efficient BAR derives from the tunneling-mediated interparticle excitonic coupling induced by wave function overlap between neighboring HgTe QDs in films. Thus, our study unveils the potential for realizing efficient hot-carrier/exciton solar cells based on HgTe QDs. Fundamentally, we reveal that the delocalized nature of quantum-confined wave function intensifies BAR. The interparticle excitonic coupling may cast light on the development of next-generation photoelectronic materials, which can retain the size-tunable confinement of colloidal semiconductor QDs while simultaneously maintaining high mobilities and conductivities typical for bulk semiconductor materials.

Circular dichroism in two-dimensional BC6N and B3C2N3in absence of intervalley excitonic coupling.

Two-dimensional (2D) noncentrosymmetric systems offer potential opportunities for exploiting the valley degrees of freedom for advanced information processing, owing to non-zero Berry curvature. However, such valley polarization in 2D materials is crucially governed by the intervalley excitonic scattering in momentum space due to reduced electronic degrees of freedom and consequent enhanced electronic correlation. Here, we study the valley excitonic properties of two 2D noncentrosymmetric complementary structures, namely, BC6N and B3C2N3using first principles-based GW calculations combined with the Bethe-Salpeter equation, that brings the many-body interactions among the quasiparticles. Thek-resolved oscillator strength of their first bright exciton indicates their ability to exhibit valley polarization under the irradiation of circularly polarized light of different chiralities. Both the systems show significant singlet excitonic binding energies of 0.74 eV and 1.31 eV, respectively. Higher stability of dark triplet excitons as compared to the singlet one can lead to higher quantum efficiency in both the systems. The combination of large excitonic binding energies and the valley polarization ability with minimal intervalley scattering make them promising candidates for applications in advanced optical devices and information storage technologies.