Showing papers by "Gregory S. Engel published in 2013"

Engineering coherence among excited states in synthetic heterodimer systems.

Abstract: The design principles that support persistent electronic coherence in biological light-harvesting systems are obscured by the complexity of such systems. Some electronic coherences in these systems survive for hundreds of femtoseconds at physiological temperatures, suggesting that coherent dynamics may play a role in photosynthetic energy transfer. Coherent effects may increase energy transfer efficiency relative to strictly incoherent transfer mechanisms. Simple, tractable, manipulable model systems are required in order to probe the fundamental physics underlying these persistent electronic coherences, but to date, these quantum effects have not been observed in small molecules. We have engineered a series of rigid synthetic heterodimers that can serve as such a model system and observed quantum beating signals in their two-dimensional electronic spectra consistent with the presence of persistent electronic coherences.

Two-dimensional electronic spectroscopy of CdSe nanoparticles at very low pulse power.

Abstract: Nanoparticles have been proposed as a promising material for creating devices that harvest, transport, and manipulate energy and electrons. Ultrafast charge carrier dynamics represent a critical design aspect and are dependent on both size and shape of the nanoparticle. Spectroscopic investigation of the electronic structure and dynamics of these systems is complicated by sample inhomogeneity, which broadens peaks and leads to ambiguity in interpretation of both spectra and dynamics. Here, we use two-dimensional electronic spectroscopy to remove inhomogeneous broadening and to clarify interpretation of measured dynamics. We specifically investigate the effect of nanoparticle shape on the electronic structure and ultrafast electronic dynamics in the band-edge exciton states of CdSe quantum dots, nanorods, and nanoplatelets. Particle size was chosen to enable straightforward comparisons of the effects of particle shape on the spectra and dynamics without retuning the laser source. The spectra were measured with low pulse powers (generally <1 nJ/pulse), using short pulses (∼12 fs) to minimize interference from solvent contributions to the spectra, ambiguities in the dynamics due to pulse-overlap effects, and contributions to the dynamics from multi-exciton effects. The lowest two exciton states are clearly resolved in spectra of quantum dots but unresolved for nanorods and nanoplates, in agreement with previous spectroscopic and theoretical results. In all nanoparticles, ultrafast dynamics measurements show strong evidence of electronic relaxation into the lowest energy exciton state within ∼30 fs, a timescale not observable in previous dynamics measurements of similar systems. These dynamics are unambiguously assigned to hole relaxation, as the higher lying electronic excited states are not energetically accessible in these experiments. Clear evidence of coherent superpositions of the lowest two exciton states were not seen in any of the particles studied, in contrast to recent results from work on quantum dots.

Timescales of Coherent Dynamics in the Light Harvesting Complex 2 (LH2) of Rhodobacter sphaeroides.

Abstract: The initial dynamics of energy transfer in the light-harvesting complex 2 from Rhodobacter sphaeroides were investigated with polarization-controlled two-dimensional spectroscopy. This method allows only the coherent electronic motions to be observed, revealing the time scale of dephasing among the excited states. We observe persistent coherence among all states and assign ensemble dephasing rates for the various coherences. A simple model is utilized to connect the spectroscopic transitions to the molecular structure, allowing us to distinguish coherences between the two rings of chromophores and coherences within the rings. We also compare dephasing rates between excited states to dephasing rates between the ground and excited states, revealing that the coherences between excited states dephase on a slower time scale than coherences between the ground and excited states.

Energy Transfer Observed in Live Cells Using Two-Dimensional Electronic Spectroscopy

Abstract: Two-dimensional electronic spectroscopy (2DES) elucidates electronic structure and dynamics on a femtosecond time scale and has proven to be an incisive tool for probing congested linear spectra of biological systems. However, samples that scatter light intensely frustrate 2DES analysis, necessitating the use of isolated protein chromophore complexes when studying photosynthetic energy transfer processes. We present a method for conducting 2DES experiments that takes only seconds to acquire thousands of 2DES spectra and permits analysis of highly scattering samples, specifically whole cells of the purple bacterium Rhodobacter sphaeroides. These in vivo 2DES experiments reveal similar timescales for energy transfer within the antennae complex (light harvesting complex 2, LH2) both in the native photosynthetic membrane environment and in isolated detergent micelles.

Probing energy transfer events in the light harvesting complex 2 (LH2) of Rhodobacter sphaeroides with two-dimensional spectroscopy.

Abstract: Excitation energy transfer events in the photosynthetic light harvesting complex 2 (LH2) of Rhodobacter sphaeroides are investigated with polarization controlled two-dimensional electronic spectroscopy. A spectrally broadened pulse allows simultaneous measurement of the energy transfer within and between the two absorption bands at 800 nm and 850 nm. The phased all-parallel polarization two-dimensional spectra resolve the initial events of energy transfer by separating the intra-band and inter-band relaxation processes across the two-dimensional map. The internal dynamics of the 800 nm region of the spectra are resolved as a cross peak that grows in on an ultrafast time scale, reflecting energy transfer between higher lying excitations of the B850 chromophores into the B800 states. We utilize a polarization sequence designed to highlight the initial excited state dynamics which uncovers an ultrafast transfer component between the two bands that was not observed in the all-parallel polarization data. We attribute the ultrafast transfer component to energy transfer from higher energy exciton states to lower energy states of the strongly coupled B850 chromophores. Connecting the spectroscopic signature to the molecular structure, we reveal multiple relaxation pathways including a cyclic transfer of energy between the two rings of the complex.

Nonlinear spectroscopic theory of displaced harmonic oscillators with differing curvatures: a correlation function approach.

Abstract: We present a theory for a bath model in which we approximate the adiabatic nuclear potential surfaces on the ground and excited electronic states by displaced harmonic oscillators that differ in curvature. Calculations of the linear and third-order optical response functions employ an effective short-time approximation coupled with the cumulant expansion. In general, all orders of correlation contribute to the optical response, indicating that the solvation process cannot be described as Gaussian within the model. Calculations of the linear absorption and fluorescence spectra resulting from the theory reveal a stronger temperature dependence of the Stokes shift along with a general asymmetry between absorption and fluorescence line shapes, resulting purely from the difference in the phonon side band. We discuss strategies for controlling spectral tuning and energy-transfer dynamics through the manipulation of the excited-state and ground-state curvature. Calculations of the nonlinear response also provide...

Independent phasing of rephasing and non-rephasing 2D electronic spectra.

Abstract: Assigning absolute phase to two-dimensional (2D) third-order nonlinear optical signals generally requires acquiring both the rephasing and the non-rephasing signals and comparing the sum of the two to spectrally resolved pump-probe spectra. To date, however, Gradient Assisted Photon Echo Spectroscopy (GRAPES) has only been able to acquire rephasing spectra. Such a constraint requires a new phasing protocol. Here, we analytically prove that the rephasing and non-rephasing spectra can be phased independently using pump-probe signal. We verify this result holds even for finite duration pulses by simulation. This relationship holds for all 2D spectroscopies, not only GRAPES. In addition, we present improvements to GRAPES that enable acquisition of rephasing and non-rephasing signals in different phase-matched directions. We employ our phasing protocol to phase the data for laser dye IR-144, leading to reconstruction of purely absorptive 2D spectrum.

The dependence of exciton transport efficiency on spatial patterns of correlation within the spectral bath

Abstract: Spatial correlations in spectral bath motions have been proposed to explain long-lived coherence in exciton transport. Systems of interest, ranging from photosynthetic complexes to organic photovoltaics, contain inhomogeneous environments. We consider the possibility that the degree of spatial correlation varies throughout an exciton transport system. We model exciton transport in the Fenna–Matthews–Olson complex (FMO), a photosynthetic light-harvesting complex. Although it remains unclear whether significant spatial correlations exist in FMO, its very high exciton transport efficiency makes it an interesting case for studies of exciton transport. We also simulate a highly symmetric ten-site model system. We use an extension of the environment-assisted quantum transport model to simulate transport, allowing the spatial correlation function to vary throughout the system. We demonstrate both via analysis and via simulation that exciton transport efficiency is most sensitive to changes in correlation between the site coupled to the trap and its neighboring sites. This asymmetry in sensitivity is highly robust and appears irrespective of changes in parameters such as transition dipole orientations and initial conditions. Our results suggest that in the design of exciton transport systems, efforts to increase efficiency by controlling spatial correlation should be focused on the region near the site of exciton trapping.

Correction to “Energy Transfer Observed in Live Cells Using Two-Dimensional Electronic Spectroscopy”

Abstract: In a recent publication, we made an error while producing final figures for publication.1 The coherence frequency axis, ωτ, and the rephrasing frequency axis, ωt, were exchanged. This error appears in figure 3 and supporting figures S3, S4, and S5; the error does not affect any conclusions drawn in the paper. Corrected figures are included in supporting information to this correction. We apologize for any inconvenience these errors may have caused our readers.