Nonlinear Spectroscopic Theory of Displaced Harmonic Oscillators with Differing Curvatures: A Correlation Function Approach

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 insights into the dynamics of the system–bath interactions and reveal that multidimensional spectroscopies are sensitive to a difference in curvature between the ground- and excited-state adiabatic surfaces. This extension allows for the elucidation of short-time dynamics of dephasing that are accessible in nonlinear spectroscopic methods

Dissecting Hidden Couplings Using Fifth-Order Three-Dimensional Electronic Spectroscopy

We report the demonstration of single-quantum 3D electronic spectroscopy. Utilizing the recently introduced gradient assisted photon echo (GRAPE) methodology, the fifth-order nonlinear polarization of the solvatochromatic dye IR144 corresponding to evolution through three sequential single-quantum coherences is measured. GRAPE, which allows a 2D slice of data to be acquired in parallel, permits a practical implementation of 3D spectroscopy at optical frequencies in a matter of minutes instead of hours. By spreading frequencies into a third spectral dimension, we can resolve features in the spectra that are otherwise obscured. For IR144, a previously unresolved cross peak originating from high frequency vibronic modes is observed in the spectrum. Theoretical modeling based on the cumulant expansion truncated at second order reproduces the main features of the experimental results. This experimental approach will enable further high dimensional spectroscopic experiments

Time Scales of Coherent Dynamics in the Light-Harvesting Complex 2 (LH2) of <i>Rhodobacter sphaeroides</i>

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

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 time scales for energy transfer within the antennae complex (light harvesting complex 2, LH2) both in the native photosynthetic membrane environment and in isolated detergent micelles

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

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

Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets

There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm², lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials

Mutations to R. sphaeroides Reaction Center Perturb Energy Levels and Vibronic Coupling but Not Observed Energy Transfer Rates

The bacterial reaction center is capable of both efficiently collecting and quickly transferring energy within the complex; therefore, the reaction center serves as a convenient model for both energy transfer and charge separation. To spectroscopically probe the interactions between the electronic excited states on the chromophores and their intricate relationship with vibrational motions in their environment, we examine coherences between the excited states. Here, we investigate this question by introducing a series of point mutations within 12 Å of the special pair of bacteriochlorophylls in the Rhodobacter sphaeroides reaction center. Using two-dimensional spectroscopy, we find that the time scales of energy transfer dynamics remain unperturbed by these mutations. However, within these spectra, we detect changes in the mixed vibrational-electronic coherences in these reaction centers. Our results indicate that resonance between bacteriochlorophyll vibrational modes and excitonic energy gaps promote electronic coherences and support current vibronic models of photosynthetic energy transfer

Electronic Structure and Dynamics of Higher-Lying Excited States in Light Harvesting Complex 1 from Rhodobacter sphaeroides

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

Scalable Ligand-Mediated Transport Synthesis of Organic–Inorganic Hybrid Perovskite Nanocrystals with Resolved Electronic Structure and Ultrafast Dynamics

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 (<i>e</i>.<i>g</i>., 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

Redox Conditions Affect Ultrafast Exciton Transport in Photosynthetic Pigment–Protein Complexes

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