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1.
J Phys Chem Lett ; 11(10): 4163-4172, 2020 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-32391695

RESUMO

DNA scaffolds enable base-pair-specific positioning of fluorescent molecules, allowing for nanometer-scale precision in controlling multidye interactions. Expanding on this concept, DNA-based molecular photonic wires (MPWs) allow for light harvesting and directional propagation of photonic energy on the nanometer scale. The most common MPW examples exploit Förster resonance energy transfer (FRET), and FRET between the same dye species (HomoFRET) was recently shown to increase the distance and efficiency at which MPWs can function. Although increased proximity between adjacent fluorophores can be used to increase the energy transfer efficiency, FRET assumptions break down as the distance between the dye molecules becomes comparable to their size (∼2 nm). Here we compare dye conjugation with single versus dimer Cy5 dye repeats as HomoFRET MPW components on a double-crossover DNA scaffold. At room temperature (RT) under low-light conditions, end-labeled uncoupled dye molecules provide optimal transfer, while the Cy5 dimers show ultrafast (<100 ps) nonradiative decay that severely limits their functionality. Of particular interest is the observation that through increased excitation fluence as well as cryogenic temperatures, the dimeric MPW shows suppression of the ultrafast decay, demonstrating fluorescence lifetimes similar to the single Cy5 MPWs. This work points to the complex dynamic capabilities of dye-based nanophotonic networks, where dye positioning and interactions can become critical, and could be used to extend the lengths and complexities of such dye-DNA devices, enabling multiparameter nanophotonic circuitry.

2.
J Phys Chem Lett ; 11(7): 2658-2666, 2020 Apr 02.
Artigo em Inglês | MEDLINE | ID: mdl-32168454

RESUMO

Transition-metal dichalcogenides (TMDs) such as MoS2 display promising electrical and optical properties in the monolayer limit. Due to strong quantum confinement, TMDs provide an ideal environment for exploring excitonic physics using ultrafast spectroscopy. However, the interplay between collective excitation effects on single excitons such as band gap renormalization/exciton binding energy (BGR/EBE) change and multiexciton effects such biexciton formation remains poorly understood. Using two-dimensional electronic spectroscopy, we observe the dominance of single-exciton BGR/EBE signals over optically induced biexciton formation. We make this determination based on a lack of strong PIA features at T = 0 fs in the cryogenic spectra. By means of nodal line slope analysis, we determine that spectral diffusion occurs faster than BGR/EBE change, indicative of distinct processes. These results indicate that at higher sub-Mott limit fluences, collective effects on single excitons dominate biexciton formation.

3.
J Phys Chem Lett ; 10(2): 270-277, 2019 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-30599133

RESUMO

Light-harvesting complexes in photosynthetic organisms display fast and efficient energy transfer dynamics, which depend critically on the electronic structure of the coupled chromophores within the complexes and their interactions with their environment. We present ultrafast anisotropy dynamics, resolved in both time and frequency, of the transmembrane light-harvesting complex LH2 from Rhodobacter sphaeroides in its native membrane environment using polarization-controlled two-dimensional electronic spectroscopy. Time-dependent anisotropy obtained from both experiment and modified Redfield simulation reveals an orientational preference for excited state absorption and an ultrafast equilibration within the B850 band in LH2. This ultrafast equilibration is favorable for subsequent energy transfer toward the reaction center. Our results also show a dynamic difference in excited state absorption anisotropy between the directly excited B850 population and the population that is initially excited at 800 nm, suggesting absorption from B850 states to higher-lying excited states following energy transfer from B850*. These results give insight into the ultrafast dynamics of bacterial light harvesting and the excited state energy landscape of LH2 in the native membrane environment.

4.
Proc Natl Acad Sci U S A ; 116(37): 18263-18268, 2019 09 10.
Artigo em Inglês | MEDLINE | ID: mdl-30093387

RESUMO

Recent work suggests that the long-lived coherences observed in both natural and artificial light-harvesting systems (such as the Fenna-Matthews-Olson complex) could be attributed to the mixing of the pigments' electronic and vibrational degrees of freedom. To investigate the underlying mechanism of these long coherence lifetimes, a sophisticated description of interactions between the molecular aggregates and the nonequilibrium fluctuations in the surrounding environment is necessary. This is done by implementing the hierarchical equations of motion approach on model homodimers, a method used in the intermediate coupling regime for many molecular aggregates wherein the nonequilibrium environment phonons play nontrivial roles in exciton dynamics. Here we report a character change in the vibronic states-reflective of property mixing between the electronic and vibrational states-induced by an interplay between system coupling parameters within the exciton-vibrational near-resonance regime. This mixing dictates vital aspects of coherence lifetime; by tracking the degree of mixing, we are able to elucidate the relationship between coherence lifetime and both the electronic energy fluctuation and the vibrational relaxation dephasing pathways.

5.
Phys Chem Chem Phys ; 20(47): 30032-30040, 2018 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-30480676

RESUMO

The efficiency of natural light harvesting systems is largely determined by their ability to transfer excitations from the antenna to the energy trapping center before recombination. The exciton diffusion length similarly limits organic photovoltaics and demands bulk heterojunction architectures. Dark state protection, achieved by coherent coupling between subunits within the antenna, can significantly reduce radiative recombination and enhance the efficiency of energy trapping. In this work we extend the dark state concept to the double-excitation manifold by studying the dynamical flow of excitations. We show that the lowest double-excitation state carries minimal oscillator strength, but relaxation to this state from higher lying double excitations can be relatively rapid such that the lowest double excitation state can act as a dynamical dark state protecting excitation from radiative recombination. This mechanism is sensitive to topology and operates differently for chain and ring structures, while becoming more pronounced in both geometries when the size of the antenna increases. When the exciton-exciton annihilation (EEA) mechanism is considered, the double-excitation population is quickly depleted and the dynamics change dramatically. However the efficiency and output power are still significantly different from those calculated using the single-excitation manifold alone, justifying the necessity of considering the double-excitation manifold. Remarkably, in certain scenarios, EEA can even increase the overall light harvesting efficiency by bringing population down from the double-excitation dark states to the single-excitation manifold.

6.
J Am Chem Soc ; 140(46): 15791-15803, 2018 11 21.
Artigo em Inglês | MEDLINE | ID: mdl-30285448

RESUMO

The ensemble emission spectra of colloidal InP quantum dots are broader than achievable spectra of cadmium- and lead-based quantum dots, despite similar single-particle line widths and significant efforts invested in the improvement of synthetic protocols. We seek to explain the origin of persistently broad ensemble emission spectra of colloidal InP quantum dots by investigating the nature of the electronic states responsible for luminescence. We identify a correlation between red-shifted emission spectra and anomalous broadening of the excitation spectra of luminescent InP colloids, suggesting a trap-associated emission pathway in highly emissive core-shell quantum dots. Time-resolved pump-probe experiments find that electrons are largely untrapped on photoluminescence relevant time scales pointing to emission from recombination of localized holes with free electrons. Two-dimensional electronic spectroscopy on InP quantum dots reveals multiple emissive states and increased electron-phonon coupling associated with hole localization. These localized hole states near the valence band edge are hypothesized to arise from incomplete surface passivation and structural disorder associated with lattice defects. We confirm the presence and effect of lattice disorder by X-ray absorption spectroscopy and Raman scattering measurements. Participation of localized electronic states that are associated with various classes of lattice defects gives rise to phonon-coupled defect related emission. These findings explain the origins of the persistently broad emission spectra of colloidal InP quantum dots and suggest future strategies to narrow ensemble emission lines comparable to what is observed for cadmium-based materials.

7.
J Chem Phys ; 148(20): 204307, 2018 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-29865841

RESUMO

Coupled excitonic structures are found in natural and artificial light harvesting systems where optical transitions link different excitation manifolds. In systems with symmetry, some optical transitions are allowed, while others are forbidden. Here we examine an excitonic ring structure and identify an accidental degeneracy between two categories of double-excitation eigenstates with distinct symmetries and optical transition properties. To understand the accidental degeneracy, a complete selection rule between two arbitrary excitation manifolds is derived with a physically motivated proof. Remarkably, symmetry analysis shows that the lack of certain symmetry elements in the Hamiltonian is responsible for this degeneracy, which is unique to rings with size N = 4l + 2 (l being an integer).

8.
Chem Sci ; 9(15): 3694-3703, 2018 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-29780500

RESUMO

Förster Resonance Energy Transfer (FRET) is the incoherent transfer of an electronic excitation from a donor fluorophore to a nearby acceptor. FRET has been applied as a probe of local chromophore environments and distances on the nanoscale by extrapolating transfer efficiencies from standard experimental parameters, such as fluorescence intensities or lifetimes. Competition from nonradiative relaxation processes is often assumed to be constant in these extrapolations, but in actuality, this competition depends on the donor and acceptor environments and can, therefore, be affected by conformational changes. To study the effects of nonradiative relaxation on FRET dynamics, we perform two-dimensional electronic spectroscopy (2DES) on a pair of azaboraindacene (BODIPY) dyes, attached to opposite arms of a resorcin[4]arene cavitand. Temperature-induced switching between two equilibrium conformations, vase at 294 K to kite at 193 K, increases the donor-acceptor distance from 0.5 nm to 3 nm, affecting both FRET efficiency and nonradiative relaxation. By disentangling different dynamics based on lifetimes extracted from a series of 2D spectra, we independently observe nonradiative relaxation, FRET, and residual fluorescence from the donor in both vase to kite conformations. We observe changes in both FRET rate and nonradiative relaxation when the molecule switches from vase to kite, and measure a significantly greater difference in transfer efficiency between conformations than would be determined by standard lifetime-based measurements. These observations show that changes in competing nonradiative processes must be taken into account when highly accurate measurements of FRET efficiency are desired.

9.
J Chem Phys ; 148(6): 064304, 2018 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-29448771

RESUMO

Natural light harvesting systems exploit electronic coupling of identical chromophores to generate efficient and robust excitation transfer and conversion. Dark states created by strong coupling between chromophores in the antenna structure can significantly reduce radiative recombination and enhance energy conversion efficiency. Increasing the number of the chromophores increases the number of dark states and the associated enhanced energy conversion efficiency yet also delocalizes excitations away from the trapping center and reduces the energy conversion rate. Therefore, a competition between dark state protection and delocalization must be considered when designing the optimal size of a light harvesting system. In this study, we explore the two competing mechanisms in a chain-structured antenna and show that dark state protection is the dominant mechanism, with an intriguing dependence on the parity of the number of chromophores. This dependence is linked to the exciton distribution among eigenstates, which is strongly affected by the coupling strength between chromophores and the temperature. Combining these findings, we propose that increasing the coupling strength between the chromophores can significantly increase the power output of the light harvesting system.


Assuntos
Complexos de Proteínas Captadores de Luz/química , Pigmentos Biológicos/química , Transferência de Energia , Fotossíntese , Teoria Quântica
10.
Acta Crystallogr E Crystallogr Commun ; 74(Pt 1): 83-87, 2018 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-29416898

RESUMO

In the title compound, 4'-allyl-4,5,6,7,2',7'-hexa-chloro-fluorescein allyl ester {systematic name: prop-2-en-1-yl 2,3,4,5-tetra-chloro-6-[2,7-di-chloro-6-hy-droxy-3-oxo-4-(prop-2-en-1-yl)-3H-xanthen-9-yl]benzoate}, C26H14Cl6O5, accompanied by unknown solvate molecules, the dihedral angle between the xanthene ring system (r.m.s. deviation = 0.046 Å) and the penta-substituted benzene ring is 71.67 (9)°. Both allyl groups are disordered over two sets of sites in statistical ratios. The scattering contributions of the disordered solvent mol-ecules (both Ph2O and CHCl3, as identified by NMR) were removed with the PLATON SQUEEZE algorithm [Spek (2015 ▸). Acta Cryst. C71, 9-18]. In the crystal, tetra-meric supra-molecular aggregates linked by O-H⋯O hydrogen bonds occur; these further inter-act with neighboring aggregates through C-Cl⋯π inter-actions arising from the benzene rings, forming infinite two-dimensional sheets. Each C6Cl4 ring shifts in the direction perpendicular to the two-dimensional sheet, exhibiting a helical chain in which every C6Cl4 ring is utilized as both a donor and an acceptor of Cl⋯π contacts. Thus, these two-dimensional sheets pack in a helical fashion, constructing a three-dimensional network.

11.
J Phys Chem Lett ; 9(1): 89-95, 2018 Jan 04.
Artigo em Inglês | MEDLINE | ID: mdl-29236502

RESUMO

Pigment-protein complexes in photosynthetic antennae can suffer oxidative damage from reactive oxygen species generated during solar light harvesting. How the redox environment of a pigment-protein complex affects energy transport on the ultrafast light-harvesting time scale remains poorly understood. Using two-dimensional electronic spectroscopy, we observe differences in femtosecond energy-transfer processes in the Fenna-Matthews-Olson (FMO) antenna complex under different redox conditions. We attribute these differences in the ultrafast dynamics to changes to the system-bath coupling around specific chromophores, and we identify a highly conserved tyrosine/tryptophan chain near the chromophores showing the largest changes. We discuss how the mechanism of tyrosine/tryptophan chain oxidation may contribute to these differences in ultrafast dynamics that can moderate energy transfer to downstream complexes where reactive oxygen species are formed. These results highlight the importance of redox conditions on the ultrafast transport of energy in photosynthesis. Tailoring the redox environment may enable energy transport engineering in synthetic light-harvesting systems.


Assuntos
Complexos de Proteínas Captadores de Luz/química , Fotossíntese , Complexo de Proteínas do Centro de Reação Fotossintética/química , Transferência de Energia , Luz , Oxirredução , Análise Espectral
12.
Nat Commun ; 8(1): 988, 2017 10 17.
Artigo em Inglês | MEDLINE | ID: mdl-29042567

RESUMO

Photosynthesis transfers energy efficiently through a series of antenna complexes to the reaction center where charge separation occurs. Energy transfer in vivo is primarily monitored by measuring fluorescence signals from the small fraction of excitations that fail to result in charge separation. Here, we use two-dimensional electronic spectroscopy to follow the entire energy transfer process in a thriving culture of the purple bacteria, Rhodobacter sphaeroides. By removing contributions from scattered light, we extract the dynamics of energy transfer through the dense network of antenna complexes and into the reaction center. Simulations demonstrate that these dynamics constrain the membrane organization into small pools of core antenna complexes that rapidly trap energy absorbed by surrounding peripheral antenna complexes. The rapid trapping and limited back transfer of these excitations lead to transfer efficiencies of 83% and a small functional light-harvesting unit.During photosynthesis, energy is transferred from photosynthetic antenna to reaction centers via ultrafast energy transfer. Here the authors track energy transfer in photosynthetic bacteria using two-dimensional electronic spectroscopy and show that these transfer dynamics constrain antenna complex organization.


Assuntos
Transferência de Energia , Fotossíntese/fisiologia , Rhodobacter sphaeroides/metabolismo , Energia Solar , Proteínas de Bactérias/metabolismo , Fluorescência , Cinética , Luz , Complexo de Proteínas do Centro de Reação Fotossintética/metabolismo , Proteobactérias/citologia , Proteobactérias/metabolismo , Proteobactérias/efeitos da radiação , Rhodobacter sphaeroides/citologia , Rhodobacter sphaeroides/efeitos da radiação , Espectrofotometria/métodos
13.
J Chem Phys ; 147(13): 131101, 2017 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-28987085

RESUMO

In photosynthetic organisms, the pigment-protein complexes that comprise the light-harvesting antenna exhibit complex electronic structures and ultrafast dynamics due to the coupling among the chromophores. Here, we present absorptive two-dimensional (2D) electronic spectra from living cultures of the purple bacterium, Rhodobacter sphaeroides, acquired using gradient assisted photon echo spectroscopy. Diagonal slices through the 2D lineshape of the LH1 stimulated emission/ground state bleach feature reveal a resolvable higher energy population within the B875 manifold. The waiting time evolution of diagonal, horizontal, and vertical slices through the 2D lineshape shows a sub-100 fs intra-complex relaxation as this higher energy population red shifts. The absorption (855 nm) of this higher lying sub-population of B875 before it has red shifted optimizes spectral overlap between the LH1 B875 band and the B850 band of LH2. Access to an energetically broad distribution of excitonic states within B875 offers a mechanism for efficient energy transfer from LH2 to LH1 during photosynthesis while limiting back transfer. Two-dimensional lineshapes reveal a rapid decay in the ground-state bleach/stimulated emission of B875. This signal, identified as a decrease in the dipole strength of a strong transition in LH1 on the red side of the B875 band, is assigned to the rapid localization of an initially delocalized exciton state, a dephasing process that frustrates back transfer from LH1 to LH2.


Assuntos
Proteínas de Bactérias/química , Proteínas de Bactérias/metabolismo , Complexos de Proteínas Captadores de Luz/química , Complexos de Proteínas Captadores de Luz/metabolismo , Transferência de Energia , Rhodobacter sphaeroides
14.
Nature ; 543(7647): 647-656, 2017 03 29.
Artigo em Inglês | MEDLINE | ID: mdl-28358065

RESUMO

Coherence phenomena arise from interference, or the addition, of wave-like amplitudes with fixed phase differences. Although coherence has been shown to yield transformative ways for improving function, advances have been confined to pristine matter and coherence was considered fragile. However, recent evidence of coherence in chemical and biological systems suggests that the phenomena are robust and can survive in the face of disorder and noise. Here we survey the state of recent discoveries, present viewpoints that suggest that coherence can be used in complex chemical systems, and discuss the role of coherence as a design element in realizing function.


Assuntos
Biofísica , Modelos Biológicos , Modelos Químicos , Elétrons , Transferência de Energia , Metais/química , Modelos Moleculares , Movimento (Física) , Teoria Quântica , Análise Espectral , Fatores de Tempo , Vibração
15.
ACS Nano ; 11(3): 2689-2696, 2017 03 28.
Artigo em Inglês | MEDLINE | ID: mdl-28195690

RESUMO

Colloidal perovskite nanocrystals support bright, narrow PL tunable over the visible spectrum. However, bandgap tuning of these materials remains limited to laboratory-scale syntheses. In this work, we present a polar-solvent-free ligand-mediated transport synthesis of high-quality organic-inorganic perovskite nanocrystals under ambient conditions with photoluminescence quantum yields up to 97%. Our synthesis employs a ligand-mediated transport mechanism that circumvents the need for exquisite external control (e.g., temperature control, inert-gas protection, dropwise addition of reagents) required by other methods due to extremely fast reaction kinetics. In the ligand-mediated transport mechanism, multiple equilibria cooperatively dictate reaction rates and enable precise control over NC size. These small nanocrystals exhibit high photoluminescence quantum yields due to quantum confinement. Nanosecond transient absorption spectroscopy experiments reveal a fluence-independent PL decay originating from exciton recombination. Two-dimensional electronic spectroscopy resolves multiple spectral features reflecting the electronic structure of the nanocrystals. The resolved features exhibit size-dependent spectral positions, further indicating the synthesized nanocrystals are quantum-confined.

16.
Nat Chem ; 9(3): 219-225, 2017 02 21.
Artigo em Inglês | MEDLINE | ID: mdl-28221350

RESUMO

In multidimensional spectroscopy, dynamics of coherences between excited states report on the interactions between electronic states and their environment. The prolonged coherence lifetimes revealed through beating signals in the spectra of some systems may result from vibronic coupling between nearly degenerate excited states, and recent observations confirm the existence of such coupling in both model systems and photosynthetic complexes. Understanding the origin of beating signals in the spectra of photosynthetic complexes has been given considerable attention; however, strategies to generate them in artificial systems that would allow us to test the hypotheses in detail are still lacking. Here we demonstrate control over the presence of quantum-beating signals by packing structurally flexible synthetic heterodimers on single-walled carbon nanotubes, and thereby restrict the motions of chromophores. Using two-dimensional electronic spectroscopy, we find that both limiting the relative rotation of chromophores and tuning the energy difference between the two electronic transitions in the dimer to match a vibrational mode of the lower-energy monomer are necessary to enhance the observed quantum-beating signals.


Assuntos
Nanotubos de Carbono/química , Teoria Quântica , Dimerização , Elétrons , Fluoresceína/química , Conformação Molecular , Análise Espectral Raman
17.
Phys Chem Chem Phys ; 18(46): 31845-31849, 2016 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-27841390

RESUMO

The high efficiency of the photon-to-charge conversion process found in photosynthetic complexes has inspired researchers to explore a new route for designing artificial photovoltaic materials. Quantum coherence can provide a mean to surpass the Shockley-Quiesser device concept limit by reducing the radiative recombination. Taking inspiration from these new discoveries, we consider a linearly-aligned system as a light-harvesting antennae composed of two-level optical emitters coupled with each other by dipole-dipole interactions. Our simulations show that the certain dark states can enhance the power with the aid of intra-band phononic dissipation. Due to cooperative effects, the output power will be improved when incorporating more emitters in the linear system.

18.
Biophys J ; 111(10): 2125-2134, 2016 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-27851937

RESUMO

Phytochromes are red/far-red photoreceptors that are widely distributed in plants and prokaryotes. Ultrafast photoisomerization of a double bond in a biliverdin cofactor or other linear tetrapyrrole drives their photoactivity, but their photodynamics are only partially understood. Multiexponential dynamics were observed in previous ultrafast spectroscopic studies and were attributed to heterogeneous populations of the pigment-protein complex. In this work, two-dimensional photon echo spectroscopy was applied to study dynamics of the bacteriophytochromes RpBphP2 and PaBphP. Two-dimensional photon echo spectroscopy can simultaneously resolve inhomogeneity in ensembles and fast dynamics by correlating pump wavelength with the emitted signal wavelength. The distribution of absorption and emission energies within the same state indicates an ensemble of heterogeneous protein environments that are spectroscopically distinct. However, the lifetimes of the dynamics are uniform across the ensemble, suggesting a homogeneous model involving sequential intermediates for the initial photodynamics of isomerization.


Assuntos
Processos Fotoquímicos , Fitocromo/química , Isomerismo
20.
J Phys Chem A ; 120(24): 4124-30, 2016 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-27232937

RESUMO

Light harvesting in photosynthetic organisms involves efficient transfer of energy from peripheral antenna complexes to core antenna complexes, and ultimately to the reaction center where charge separation drives downstream photosynthetic processes. Antenna complexes contain many strongly coupled chromophores, which complicates analysis of their electronic structure. Two-dimensional electronic spectroscopy (2DES) provides information on energetic coupling and ultrafast energy transfer dynamics, making the technique well suited for the study of photosynthetic antennae. Here, we present 2DES results on excited state properties and dynamics of a core antenna complex, light harvesting complex 1 (LH1), embedded in the photosynthetic membrane of Rhodobacter sphaeroides. The experiment reveals weakly allowed higher-lying excited states in LH1 at 770 nm, which transfer energy to the strongly allowed states at 875 nm with a lifetime of 40 fs. The presence of higher-lying excited states is in agreement with effective Hamiltonians constructed using parameters from crystal structures and atomic force microscopy (AFM) studies. The energy transfer dynamics between the higher- and lower-lying excited states agree with Redfield theory calculations.


Assuntos
Elétrons , Complexos de Proteínas Captadores de Luz/química , Complexos de Proteínas Captadores de Luz/metabolismo , Rhodobacter sphaeroides/metabolismo , Membrana Celular/metabolismo , Fotossíntese , Rhodobacter sphaeroides/citologia
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