Enhancement of Vibronic and Ground-State Vibrational Coherences in 2D Spectra of Photosynthetic Complexes

A vibronic-exciton model is applied to investigate the mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom recently proposed as the origin of the long-lived oscillations in 2D spectra of the FMO complex [Christensson et al. J. Phys. Chem. B 116 (2012) 7449]. We reduce the problem to a model BChl dimer to elucidate the role of resonance coupling, site energies, nuclear mode and energy disorder in the enhancement of vibronic-exciton and ground-state vibrational coherences, and to identify regimes where this enhancement is significant. For a heterodimer representing the two coupled BChls 3 and 4 of the FMO complex, the initial amplitude of the vibronic-exciton and vibrational coherences are enhanced by up to 15 and 5 times, respectively, compared to the vibrational coherences in the isolated monomer. This maximum initial amplitude enhancement occurs when there is a resonance between the electronic energy gap and the frequency of the vibrational mode. The bandwidth of this enhancement is about 100 cm-1 for both mechanisms. The excitonic mixing of electronic and vibrational DOF leads to additional dephasing relative to the vibrational coherences. We evaluate the dephasing dynamics by solving the quantum master equation in Markovian approximation and observe a strong dependence of the life-time enhancement on the mode frequency. Long-lived vibronic-exciton coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic resonance. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that both type have a similar magnitude at long time for the present model. The ability to distinguish between vibronic-exciton and ground-state vibrational coherences in the general case of molecular aggregate is discussed.Comment: 16 pages, 6 figure

Ultrafast Photo-Induced Charge Transfer Unveiled by Two-Dimensional Electronic Spectroscopy

The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.Comment: 14 pages, 11 figures, and Supporting informatio

Origin of Long Lived Coherences in Light-Harvesting Complexes

A vibronic exciton model is developed to investigate the origin of long lived coherences in light-harvesting complexes. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations in the nonlinear spectra of the Fenna-Matthews-Olson (FMO) complex with a dephasing time of 1.3 ps at 77 K. These oscillations correspond to the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. Purely electronic coherences are found to decay on a 200 fs timescale.Comment: 4 pages, 2 figure

Coherent Multidimensional Spectroscopy as a Probe of System-Bath Interactions

The interaction of molecules with their environment has a profound effect on dynamics in molecular systems. System-bath interactions, or the interaction of electronic and nuclear degrees of freedom, are important in order to understand chemical reactions and transport of energy and electrons in the condensed phase. In order to investigate system-bath interactions one needs to probe the underlying nuclear motion in real time. Ultrafast optical spectroscopy has this capability, and is a useful tool for such investigations. The work presented in this thesis utilizes coherent multidimensional spectroscopy to probe system-bath interactions in the time domain. We implement the three-pulse photon echo technique with various means of detecting the radiated signal field. One particular version of this experiment, the peak shift, directly gives the timescales of the system-bath interaction. To aid interpretation of the experimental results, analysis of the experiments is coupled to numerical simulations. In the first part of this thesis we explore how to obtain quantitative information about system-bath interactions. In this section we show that the chirp of the pulses needs to be considered in order to obtain quantitative information from experiments. We also show that the width of the three-pulse photon echo signal gives a direct and simple measure of the strength of the system-bath interaction. The second part of the thesis deals with coherent multidimensional spectroscopy of carotenoids. Here we show the presence of an excited state at roughly twice the S2 energy in many carotenoids. Another debated state in carotenoids is the so-called S* state. Our analysis of the experiments show that this state is an excited state, resolving a long-standing discussion about the position of this state. As the first to carry out coherent multidimensional spectroscopy on carotenoids, we have been able to address the system-bath interaction in these systems. Our results show that the structure of the carotenoid has a clear relation to the system-bath interaction, influencing it on multiple timescales

Determining Vibrational Huang-Rhys Factors by Photon Echo Spectroscopy

Electronic and vibrational dephasing dynamics of Rhodamine 800 has been studied with 3PEPS. With careful analysis, the S-factors of the vibrational modes can be accurately determined. The vibrational dephasing rate displays abnormal frequency dependence

Temperature dependent exciton-exciton annihilation in the LH2 antenna complex

Two-color pump-probe measurements of the peripheral light harvesting complex LH2 of Rb.sphaeroides reveal strong temperature dependence of the annihilation rate. The experimental results were modeled via multi-exciton density matrix theory. Based on available literature data we can set an upper limit for the feasible intramolecular internal conversion rate. We show that this also restricts the possible values of the still ill-determined energy of the doubly-excited molecular level of the bacteriochlorophyll, which is responsible for the annihilation process. (C) 2008 Elsevier B.V. All rights reserved

Solute specific polar solvation studied by photon echo spectroscopy

Polar solvation dynamics Of two Solute molecules in a series of 1-alcohols has been studied using the three-pulse photon echo peak shift technique. The inertial dynamics, on sub-200 fs timescale, is essentially independent of solvent in the 1-alcohol series. For a given solute, the solvent dependence is mainly manifested in the diffusive solvation dynamics. The solute dependence appears as a significantly stronger inertial component in one of the molecules. We ascribe this solute dependence to the differences in charge redistribution upon excitation. A detailed investigation of oscillations of the peak shift reveals a solvent dependent beating that can be connected to solvation of the vibrational mode in the excited state. The solvent dependence of the dephasing dynamics of the excited state mode can be explained by the electronic transition frequency correlation function and the beating pattern of the echo signal originates from interference between ground and excited state Feynman pathways. (C) 2008 Elsevier B.V. All rights reserved

Photon echo spectroscopy reveals structure-dynamics relationships in carotenoids

Based on simultaneous analysis of the frequency-resolved transient grating, peak shift, and echo width signals, we present a model for the third-order optical response of carotenoids including population dynamics and system-bath interactions. Our frequency-resolved photon echo experiments show that the model needs to incorporate the excited-state absorption from both the S-2 and the S-1 states. We apply our model to analyze the experimental results on astaxanthin and lycopene, aiming to elucidate the relation between structure and system-bath interactions. Our analysis allows us to relate structural motifs to changes in the energy-gap correlation functions. We find that the terminal rings of astaxanthin lead to increased coupling between slow molecular motions and the electronic transition. We also find evidence for stronger coupling to higher frequency overdamped modes in astaxanthin, pointing to the importance of the functional groups in providing coupling to fluctuations influencing the dynamics in the passage through the conical intersection governing the S-2-S-1 relaxation