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.

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

We report on ultrafast spectroscopy study of β-crustacyanin, the carotenoprotein responsible for the coloration of the lobster shell. β-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 β-crustacyanin.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.