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Register a new userExact Simulation of Pigment-Protein Complexes: Vibronic Renormalisation of Electronic Parameters in Ultrafast Spectroscopy
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The primary steps of photosynthesis generate, transport and trap delocalised electronic excitations (excitons) in pigment-protein complexes (PPCs). Generically, PPCs possess highly structured vibrational spectra with a large number of discrete intra- and quasi-continuous inter-pigment modes while exhibiting electron-vibrational (vibronic) couplings that are comparable to electronic inter-pigment coupling. Consequently, establishing a quantitative connection between spectroscopic data and underlying microscopic models of PPC dynamics remains an outstanding challenge. We address this challenge with two numerically exact simulation methods that support an analytical theory of multimode vibronic effects. Vibronic coupling across the entire vibrational spectrum, including high-frequency modes, needs to be accounted for to ensure quantitatively correct description of optical spectra, where dynamic localization effects modulate the intensity of vibrational sidebands and multimode mixing shifts the absorption peaks. Furthermore, we show that high-frequency modes can support long-lived oscillations in multidimensional nonlinear spectra, which are not obtained in a coarse-grained description of the electron-vibrational coupling.
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We predict the enhanced light harvesting of a protein-pigment complex when assembled to a quantum dot (QD) antenna. Our prototypical nanoassembly setup is composed of a Fenna-Mattews-Olson system hosting 8 Bacteriochlorophyll (BChl) a dyes, and a near-infrared emitting CdSe$_x$Te$_{(1-x)}$/ZnS alloy-core/shell nanocrystal. BChl a has two wide windows of poor absorption in the green and orange-red bands, precisely where most of the sunlight energy lies. The selected QD is able to collect sunlight efficiently in a broader band and funnel its energy by a (non-radiative) Forster resonance energy transfer mechanism to the dyes embedded in the protein. By virtue of the coupling between the QD and the dyes, the nanoassembly absorption is dramatically improved in the poor absorption window of the BChl a.
We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using Ultrafast Transient Polarization Spectroscopy (UTPS), extending the well-known technique of Optical-Kerr Effect (OKE) spectroscopy to excited electronic states. The third-order non-linear response of the excited molecular ensemble is highly sensitive to details of excited state character and geometries and is measured using two femtosecond pulses following a third femtosecond pulse that populates the S1 excited state. By measuring this response as a function of time delays between the three pulses involved, we extract the dephasing time of the wave-packet on the excited state. The dephasing time measured as a function of time-delay after pump excitation shows oscillations indicating oscillatory wave-packet dynamics on the excited state. From the experimental measurements and supporting theoretical calculations, we deduce that the wave-packet completely leaves the S1 state surface after three traversals of the inter-system crossing between the singlet S1 and triplet T2 states.
We demonstrate that the coupling of excitonic and vibrational motion in biological complexes can provide mechanisms to explain the long-lived oscillations that have been obtained in non linear spectroscopic signals of different photosynthetic pigment protein complexes and we discuss the contributions of excitonic versus purely vibrational components to these oscillatory features. Considering a dimer model coupled to a structured spectral density we exemplify the fundamental aspects of the electron-phonon dynamics, and by analyzing separately the different contributions to the non linear signal, we show that for realistic parameter regimes purely electronic coherence is of the same order as purely vibrational coherence in the electronic ground state. Moreover, we demonstrate how the latter relies upon the excitonic interaction to manifest. These results link recently proposed microscopic, non-equilibrium mechanisms to support long lived coherence at ambient temperatures with actual experimental observations of oscillatory behaviour using 2D photon echo techniques to corroborate the fundamental importance of the interplay of electronic and vibrational degrees of freedom in the dynamics of light harvesting aggregates.
The possibility of using time-resolved vibronic spectroscopy for spectral analysis of mixtures of chemical compounds with similar optical properties, when traditional methods are inefficient, is demonstrated by using the method of computer simulation. The analysis is carried out by the example of molecules of a series of polyenes (butadiene, hexatraene, octatetraene, decapentaene, and decatetraene), their various cis- and trans-rotational isomers, and phenyl-substituted polyenes. Ranges of relative concentrations of molecules similar in their spectral properties, where reliable interpretation of time-resolved spectra of mixtures and both qualitative and quantitative analyses are possible, are determined. The use of computer simulation methods for oprimizing full-scale experiments in femtosecond spectroscopy is shown to hold much promise.
We study the generation of electronic ring currents in the presence of nonadiabatic coupling using circularly polarized light. For this, we introduce a solvable model consisting of an electron and a nucleus rotating around a common center and subject to their mutual Coulomb interaction. The simplicity of the model brings to the forefront the non-trivial properties of electronic ring currents in the presence of coupling to the nuclear coordinates and enables the characterization of various limiting situations transparently. Employing this model, we show that vibronic coupling effects play a crucial role even when a single $E$ degenerate eigenstate of the system supports the current. The maximum current of a degenerate eigenstate depends on the strength of the nonadiabatic interactions. In the limit of large nuclear to electronic masses, in which the Born-Oppenheimer approximation becomes exact, constant ring currents and time-averaged oscillatory currents necessarily vanish.
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