Enhancement of vibronic and ground-state vibrational coherences in 2D spectra of photosynthetic complexes.

A vibronic-exciton model is applied to investigate the recently proposed mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom. We study a dimer system to elucidate the role of resonance coupling, site energies, vibrational frequency 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 two coupled bachteriochloropylls of the FMO complex, long-lived vibronic coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic energy difference. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that, due to the dephasing of the former, both type of coherences have a similar magnitude at longer population time.

Origin of the bathochromic shift of astaxanthin in lobster protein: 2D electronic spectroscopy investigation of ß-crustacyanin.

We report on ultrafast spectroscopy study of ß-crustacyanin, the carotenoprotein responsible for the coloration of the lobster shell. ß-Crustacyanin is formed by two closely positioned astaxanthin molecules encapsulated in protein. The 2D electronic spectroscopy together with two-color pump-probe was applied to investigate the electronic structure, the excited-state dynamics, and the influence of the excitonic interaction between the two carotenoids in ß-crustacyanin. By using the ∼20 fs laser pulses tuned to absorption bands of the S0-S2 and S1-Sn transitions of carotenoids, we were able to trace full excitation relaxation dynamics, starting with S2-S1 relaxation on the ∼30 fs time scale and finishing with the ground-state recovery of 3.2 ps. Superimposed on the relaxation dynamics in the 2D spectra, we observed long-lived beating signals at the characteristic frequencies of astaxanthin vibrational modes. We assign these oscillations to the ground-state vibrational wavepacket dynamics. All major features of the 2D spectra, including amplitude and phase maps of the long-lived oscillations, were reproduced by employing the exciton-vibronic model. Consistent modeling of all optical properties of ß-crustacyanin (including absorption and circular dichroism spectra) points to the relatively weak coupling between the two astaxanthin molecules (∼250 cm(-1)). This implies that the excitonic coupling provides insignificant contribution to the bathochromic shift in ß-crustacyanin. We discuss the origin of the shift and propose that it is caused by two major effects: conformational changes of astaxanthin molecules (increase in effective conjugation length) together with increased charge-transfer character of the S2 state. We put the bathochromic shift in the broad perspective of other "blue" carotenoids properties.

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.

Origin of long-lived coherences in light-harvesting complexes.

A vibronic exciton model is applied to explain the long-lived oscillatory features in the two-dimensional (2D) electronic spectra of the Fenna-Matthews-Olson (FMO) complex. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations with dephasing times of 1.3 ps at 77 K, which is in a good agreement with the experimental results. These long-lived oscillations originate from the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. The oscillations obtain a large amplitude due to excitonic intensity borrowing, which gives transitions with strong vibronic character a significant intensity despite the small Huang-Rhys factor. Purely electronic coherences are found to decay on a 200 fs time scale.

System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra.

In this work, we examine vibrational coherence in a molecular monomer, where time evolution of a nuclear wavepacket gives rise to oscillating diagonal- and off-diagonal peaks in two-dimensional electronic spectra. We find that the peaks oscillate out-of-phase, resulting in a cancellation in the corresponding pump-probe spectra. Our results confirm the unique disposition of two-dimensional electronic spectroscopy (2D-ES) for the study of coherences. The oscillation pattern is in excellent agreement with the diagrammatic analysis of the third-order nonlinear response. We show how 2D-ES can be used to distinguish between ground- and excited-state wavepackets. On the basis of our results, we discuss coherences in coupled molecular aggregates involving both electronic and nuclear degrees of freedom. We conclude that a general distinguishing criterion based on the experimental data alone cannot be devised.

Double-quantum two-dimensional electronic spectroscopy of a three-level system: Experiments and simulations.

Double-quantum coherence two-dimensional (2Q2D) electronic spectroscopy is utilized to probe the dynamic fluctuations of electronic states in a solvated molecule at approximately twice the energy of the ground state bleach transition. The 2Q2D spectrum gives insight into the energetic position and spectral fluctuations (system-bath interaction) of the probed excited states. Combining it with single-quantum two-dimensional (1Q2D) electronic spectroscopy enables one to determine the strength of the excited state absorption transition and the relative detuning of electronic states, as well as the dynamics of the single-quantum coherence. To investigate the correlation of spectral fluctuations in different electronically excited states, we have carried out experiments on a solvated dye (Rhodamine 6G) with 23 fs pulses centered at the maximum of the linear absorption spectrum. The 2Q2D spectrum reveals three peaks of alternating signs with the major negative peak located at higher frequencies along the emission axis compared to the single positive peak. The 1Q2D spectrum, on the other hand, shows a negative peak stemming from excited state absorption at lower frequencies along the emission axis. Analysis of the signal in the homogeneous limit fails to account for this observation as well as the number of peaks in the 2Q2D spectrum. Employing a three-level model in which all time correlations of the third-order response function are accounted for via second-order cumulant expansion gives good agreement with both the 1Q2D and 2Q2D data. Furthermore, the analysis shows that the fluctuations of the probed electronic states are highly correlated, reflecting the modulation by a common nuclear bath and similarities in the nature of the electronic transitions.

Weakly chirped pulses in frequency resolved coherent spectroscopy.

The role of weakly chirped pulses (time bandwidth product, DeltanuDeltatau<0.61) on three-pulse photon echo signals has been systematically studied. Pulses with varying chirp were characterized with frequency resolved optical gating (FROG) and used to measure spectrally resolved three-pulse photon echoes of a dye in solution. The weakly chirped pulses give rise to markedly different echo signals for population times below approximately 100 fs. The chirped pulses can decrease or enhance spectral signatures of an excited state absorption transition in the echo signal. Furthermore, the observed dephasing dynamics depend on the phase of the electric fields. Simulations based on a three-level model and the electric fields retrieved from the FROG traces give a good agreement for photon echo experiments with both transform limited and chirped pulses. The simulations also allow for a numerical investigation of effects of chirp in two-dimensional spectroscopy. For a two-level system, the chirped pulses result in nonelliptical two-dimensional spectra that can erroneously be interpreted as spectral heterogeneity with frequency dependent dephasing dynamics. Furthermore, chirped pulses can give rise to "false" cross peaks when strong vibrational modes are involved in the system-bath interaction.

Electronic Double-Quantum Coherences and Their Impact on Ultrafast Spectroscopy: The Example of ß-Carotene.

The energy level structure and dynamics of biomolecules are important for understanding their photoinduced function. In particular, the role of carotenoids in light-harvesting is heavily studied, yet not fully understood. The conventional approach to investigate these processes involves analysis of the third-order optical polarization in one spectral dimension. Here, we record two-dimensional correlation spectra for different time-orderings to characterize all components of the transient molecular polarization and the optical signal. Single- and double-quantum two-dimensional experiments provide insight into the energy level structure as well as the ultrafast dynamics of solvated ß-carotene. By analysis of the lineshapes, we obtain the transition energy and characterize the potential energy surfaces of the involved states. We obtain direct experimental proof for an excited state absorption transition in the visible (S2→Sn2). The signatures of this transition in pump-probe transients are shown to lead to strongly damped oscillations with characteristic pump and probe frequency dependence.

Two-dimensional electronic spectroscopy of beta-carotene.

Two-dimensional electronic spectroscopy (2D) has been applied to beta-carotene in solution to shine new light on the ultrafast energy dissipation network in carotenoids. The ability of 2D to relieve spectral congestion provides new experimental grounds for resolving the rise of the excited state absorption signal between 18,000 and 19,000 cm(-1). In this spectral region, the pump-probe signals from ground state bleach and stimulated emission overlap strongly. Combined modeling of the time-evolution of 2D spectra as well as comparison to published pump-probe data allow us to draw conclusions on both the electronic structure of beta-carotene as well as the spectral densities giving rise to the observed optical lineshapes. To account for the experimental observations on all time scales, we need to include a transition in the visible spectral range from the first optically allowed excited state (S(2)-->S(n2)). We present data from frequency resolved transient grating and pump-probe experiments confirming the importance of this transition. Furthermore, we investigate the role and nature of the S* state, controversially debated in numerous previous studies. On the basis of the analysis of Feynman diagrams, we show that the properties of S*-related signals in chi(3) techniques like pump-probe and 2D can only be accounted for if S* is an excited electronic state. Against this background, we discuss a new interpretation of pump-deplete-probe and intensity-dependent pump-probe experiments.

Ultrafast excited-state isomerization dynamics of 1,1'-diethyl-2,2'-cyanine studied by four-wave mixing spectroscopy.

Excited-state dynamics and solvent-solute interactions of 1,1'-diethyl-2,2'-cyanine iodine (1122C) in alcoholic solutions are investigated using time-integrated three-pulse photon-echo spectroscopy. 1122C serves as a model compound for ultrafast photoinduced isomerization-a key process in the light reception of plants, bacteria, and human vision. The photoreaction in 1122C is interrogated in dependence on solvent and excitation wavelength. The wavelength-dependent three-pulse photon-echo peak shift indicates strong alterations of the reaction pathways and points to the existence of a direct internal conversion channel in close proximity to the Franck-Condon point of absorption. The solvent-dependent S1-S0 internal conversion time does not follow conventional sheared viscosity dependence, suggesting that the solvent local friction has to be considered to account for the observed isomerization kinetics. The concerted discussion of transient grating and three-pulse photon-echo peak-shift data allows us to derive a complete picture of the solvent-solute interaction-controlled photoreaction. The results obtained are related to other work on reactive systems and are discussed in the framework of multilevel response functions.

Appearance of intramolecular high-frequency vibrations in two-dimensional, time-integrated three-pulse photon echo data.

An alternative experimental outline to measure homodyne detected three-pulse photon-echo data is presented. The novel experimental approach allowing for online monitoring and correction of experimental timing and stability is discussed in detail using the paradigm system of Nile blue in alcohol solution. It is shown that excellent signal-to-noise ratios together with high reproducibility of the data can be routinely achieved. We report in detail on the appearance of high-frequency intramolecular vibrations in the two-dimensional three-pulse photon-echo data and suggest that besides the conventionally discussed three-pulse photon-echo peak-shift the width of the integrated echo signal as a function of population time contains identical and easily accessible information on high-frequency intramolecular vibrations. A comparison of experimental data with theoretical modeling is performed showing that the observed echo-width oscillations are in line with predictions of the Brownian oscillator model.

New fluorophores based on trifluorenylamine with very large intrinsic three-photon absorption cross sections.

[structure: see text] A new fluorophore, tri(9,9-diethyl-9H-fluorenyl)amine, was synthesized by the Buchwald-Hartwig reaction of 2-aminofluorene, and based on this molecule three more fluorophores were prepared that exhibit a very large intrinsic three-photon absorption in the near-IR region, which scales as a third power of the bridge length.

Quantum interference in organic solid.

We demonstrate high contrast quantum interference between one-photon and three-photon absorption pathways in an organic solid at room temperature. Illumination of a thin polymer film activated with fluorescing dendrimer chromophores of large three photon absorption cross section with two simultaneous femtosecond pulses at near-IR frequency omega and its third harmonic UV frequency 3omega results in a spatial interference fringe pattern observable by eye.