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.

Vibronic and vibrational coherences in two-dimensional electronic spectra of supramolecular J-aggregates.

In J-aggregates of cyanine dyes, closely packed molecules form mesoscopic tubes with nanometer-diameter and micrometer-length. Their efficient energy transfer pathways make them suitable candidates for artificial light harvesting systems. This great potential calls for an in-depth spectroscopic analysis of the underlying energy deactivation network and coherence dynamics. We use two-dimensional electronic spectroscopy with sub-10 fs laser pulses in combination with two-dimensional decay-associated spectra analysis to describe the population flow within the aggregate. Based on the analysis of Fourier-transform amplitude maps, we distinguish between vibrational or vibronic coherence dynamics as the origin of pronounced oscillations in our two-dimensional electronic spectra.

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.

Weak exciton scattering in molecular nanotubes revealed by double-quantum two-dimensional electronic spectroscopy.

The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry.

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.

Excitons and disorder in molecular nanotubes: a 2D electronic spectroscopy study and first comparison to a microscopic model.

The efficiency of natural light-harvesting complexes relies on delocalization and directed transfer of excitation energy on spatially well-defined arrangements of molecular absorbers. Coherent excitation delocalization and long-range molecular order are also central prerequisites for engineering energy flows in bioinspired devices. Double-wall cylindrical aggregates have emerged as excellent candidates that meet these criteria. So far, the experimental signatures of exciton relaxation in these tubular supramolecules could not be linked to models encompassing their entire spatial structure. On the basis of the power of two-dimensional electronic spectroscopy, we characterize the motion of excitons in the three-fold band structure of the bitubular aggregate C8S3 through temporal, energetic, and spatial attributes. Accounting for intra- as well as interwall electronic interactions in the framework of a Frenkel exciton basis, we employ numerical computations using inhomogeneous and homogeneous microscopic models. The calculations on large but finite structures identify disorder-induced effects, which become increasingly relevant for higher energy states and give insight into the topology of the excited state manifold. Calculations in the infinite homogeneous limit capture the phenomena evidenced in the experimental two-dimensional patterns. Our results provide a basis for understanding recently reported correlated fluctuations of excitonic absorption bands and interband coherences in tubular aggregates.

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.

Compact phase-stable design for single- and double-quantum two-dimensional electronic spectroscopy.

We report a compact, easy to align, and passively phase-stabilized setup for recording two-dimensional (2D) electronic spectra in three different phase-matching directions in the boxcar geometry. Passive phase stabilization is achieved by a diffractive optical element, the use of refractive optics for introducing pulse delays, and the use of common optics for all pulses. Representative 2D spectra correlating single- and double-quantum coherences in a molecular aggregate are presented.

Two-dimensional electronic spectroscopy of molecular excitons.

Understanding of the nuclear and electronic structure and dynamics of molecular systems has advanced considerably through probing the nonlinear response of molecules to sequences of pulsed electromagnetic fields. The ability to control various degrees of freedom of the excitation pulses-such as duration, sequence, frequency, polarization, and shape-has led to a variety of time-resolved spectroscopic methods. The various techniques that researchers use are commonly classified by their dimensionality, which refers to the number of independently variable time delays between the pulsed fields that induce the signal. Though pico- and femtosecond time-resolved spectroscopies of electronic transitions have come of age, only recently have researchers been able to perform two-dimensional electronic spectroscopy (2D-ES) in the visible frequency regime and correlate transition frequencies that evolve in different time intervals. The two-dimensional correlation plots and their temporal evolution allow one to access spectral information that is not exposed directly in other one-dimensional nonlinear methods. In this Account, we summarize our studies of a series of increasingly complex molecular chromophores. We examine noninteracting dye molecules, a monomer-dimer equilibrium of a prototypical dye molecule, and finally a supramolecular assembly of electronically coupled absorbers. By tracing vibronic signal modulations, differentiating line-broadening mechanisms, analyzing distinctly different relaxation dynamics, determining electronic coupling strengths, and directly following excitation energy transfer pathways, we illustrate how two-dimensional electronic spectroscopy can image physical phenomena that underlie the optical response of a particular system. Although 2D-ES is far from being a "turn-key" method, we expect that experimental progress and potential commercialization of instrumentation will make 2D-ES accessible to a much broader scientific audience, analogous to the development of multidimensional NMR and 2D-IR.

Two-dimensional electronic spectra of an aggregating dye: simultaneous measurement of monomeric and dimeric line-shapes.

We present a sequence of two-dimensional electronic spectra of a prototypical cyanine dye, whose spectral properties in aqueous solution are determined by the formation of a monomer-dimer equilibrium. Quantum-chemical methods are utilized to calculate the structure and absorption properties of the two species involved. Our spectroscopic results simultaneously characterize the spectral line-shapes of the two species in terms of underlying dynamic and static disorder, and demonstrate how the two-dimensional technique allows the exploitation of high spectral and temporal resolution in one and the same experiment. The distinctly different spectral relaxation dynamics are quantified in a two-dimensional line-shape analysis, by extracting the time dependent ratios of the diagonal and anti-diagonal peak-widths. Our findings are in line with theoretical considerations, that predict the fluctuational dynamics of an excitonic dimer state to be exchange-narrowed by excitation delocalization.