Vytautas Butkus

Vibronic coherence in oxygenic photosynthesis

Photosynthesis powers life on our planet. The basic photosynthetic architecture consists of antenna complexes that harvest solar energy and reaction centres that convert the energy into stable separated charge. In oxygenic photosynthesis, the initial charge separation occurs in the photosystem II reaction centre, the only known natural enzyme that uses solar energy to split water. Both energy transfer and charge separation in photosynthesis are rapid events with high quantum efficiencies. In recent nonlinear spectroscopic experiments, long-lived coherences have been observed in photosynthetic antenna complexes, and theoretical work suggests that they reflect underlying electronic-vibrational resonances, which may play a functional role in enhancing energy transfer. Here, we report the observation of coherent dynamics persisting on a picosecond timescale at 77 K in the photosystem II reaction centre using two-dimensional electronic spectroscopy. Supporting simulations suggest that the coherences are of a mixed electronic-vibrational (vibronic) nature and may enhance the rate of charge separation in oxygenic photosynthesis.

Vibrational vs. electronic coherences in 2D spectrum of molecular systems

Two-dimensional spectroscopy has recently revealed the oscillatory behavior of the excitation dynamics of molecular systems. However, in the majority of cases there is considerable debate over what is actually being observed: excitonic or vibrational wavepacket motion or evidence of quantum transport. In this letter we present a method for distinguishing between vibrational and excitonic wavepacket motion, based on the phase and amplitude relationships of oscillations of distinct peaks as revealed through a fundamental analysis of the two-dimensional spectra of two representative systems

Distinctive character of electronic and vibrational coherences in disordered molecular aggregates

Abstract Coherent dynamics of coupled molecules are effectively characterized by the two-dimensional electronic spectroscopy. Depending on the coupling between electronic and vibrational states, oscillating signals of purely electronic, purely vibrational or mixed character are observed with the help of oscillation maps, constructed from time-resolved spectra. Amplitude of beatings caused by electronic coherences is heavily affected by energetic disorder and consequently electronic coherences are quickly dephased. Beatings with vibrational character weakly depend on the disorder, ensuring their long-time survival. We show that detailed modeling of two-dimensional spectroscopy signals of molecular aggregates provides direct information on the origin of the coherent beatings.

Vibronic phenomena and exciton–vibrational interference in two-dimensional spectra of molecular aggregates

A general theory of electronic excitations in aggregates of molecules coupled to intramolecular vibrations and the harmonic environment is developed for simulation of the third-order nonlinear spectroscopy signals. It is applied in studies of the time-resolved two-dimensional coherent spectra of four characteristic model systems: weakly/strongly vibronically coupled molecular dimers interacting with high/low frequency intramolecular vibrations. The results allow us to (i) classify and define the typical spectroscopic features of vibronically coupled molecules, (ii) separate the cases, when the long-lived quantum coherences due to vibrational lifetime borrowing should be expected, (iii) define when the complete exciton-vibrational mixing occurs, and (iv) when separation of excitonic and vibrational coherences is possible.

Molecular vibrations-induced quantum beats in two-dimensional electronic spectroscopy

Quantum beats in nonlinear spectroscopy of molecular aggregates are often attributed to electronic phenomena of excitonic systems, while nuclear degrees of freedom are commonly included into models as overdamped oscillations of bath constituents responsible for dephasing. However, molecular systems are coupled to various high-frequency molecular vibrations, which can cause the spectral beats hardly distinguishable from those created by purely electronic coherences. Models containing damped, undamped, and overdamped vibrational modes coupled to an electronic molecular transition are discussed in this paper in context of linear absorption and two-dimensional electronic spectroscopy. Analysis of different types of bath models demonstrates how do vibrations map onto two-dimensional spectra and how the damping strength of the coherent vibrational modes can be resolved from spectroscopic signals.

Mapping energy transfer channels in fucoxanthin-chlorophyll protein complex.

Fucoxanthin-chlorophyll protein (FCP) is the key molecular complex performing the light-harvesting function in diatoms, which, being a major group of algae, are responsible for up to one quarter of the total primary production on Earth. These photosynthetic organisms contain an unusually large amount of the carotenoid fucoxanthin, which absorbs the light in the blue-green spectral region and transfers the captured excitation energy to the FCP-bound chlorophylls. Due to the large number of fucoxanthins, the excitation energy transfer cascades in these complexes are particularly tangled. In this work we present the two-color two-dimensional electronic spectroscopy experiments on FCP. Analysis of the data using the modified decay associated spectra permits a detailed mapping of the excitation frequency dependent energy transfer flow with a femtosecond time resolution.

Coherence and population dynamics of chlorophyll excitations in FCP complex: Two-dimensional spectroscopy study

Energy transfer processes and coherent phenomena in the fucoxanthin-chlorophyll protein complex, which is responsible for the light harvesting function in marine algae diatoms, were investigated at 77 K by using two-dimensional electronic spectroscopy. Experiments performed on femtosecond and picosecond timescales led to separation of spectral dynamics, witnessing evolutions of coherence and population states of the system in the spectral region of Qy transitions of chlorophylls a and c. Analysis of the coherence dynamics allowed us to identify chlorophyll (Chl) a and fucoxanthin intramolecular vibrations dominating over the first few picoseconds. Closer inspection of the spectral region of the Qy transition of Chl c revealed previously not identified, mutually non-interacting chlorophyll c states participating in femtosecond or picosecond energy transfer to the Chl a molecules. Consideration of separated coherent and incoherent dynamics allowed us to hypothesize the vibrations-assisted coherent energy transfer between Chl c and Chl a and the overall spatial arrangement of chlorophyll molecules.

Discrimination of Diverse Coherences Allows Identification of Electronic Transitions of a Molecular Nanoring

The role of quantum coherence in photochemical functions of molecular systems such as photosynthetic complexes is a broadly debated topic. Coexistence and intermixing of electronic and vibrational coherences has been proposed to be responsible for the observed long-lived coherences and high energy transfer efficiency. However, clear experimental evidence of coherences with different origins operating at the same time has been elusive. In this work, multidimensional spectra obtained from a six-porphyrin nanoring system are analyzed in detail with support from theoretical modeling. We uncover a great diversity of separable electronic, vibrational, and mixed coherences and show their cooperation in shaping the spectroscopic response. The results permit direct assignment of electronic and vibronic states and characterization of the excitation dynamics. The clear disentanglement of coherences in molecules with extended π-conjugation opens up new avenues for exploring coherent phenomena and understanding their importance for the function of complex systems.

Disorder-Induced Quantum Beats in Two-Dimensional Spectra of Excitonically Coupled Molecules

Quantum superposition of molecular electronic states is very fragile because of thermal energy fluctuations and the static conformational disorder induced by the intimate surrounding of constituent molecules of the system. However, the nature of the long-lived quantum beats, observed in time-resolved spectra of molecular aggregates at physiological conditions, is still being debated. We present our study of the conditions when long-lived electronic quantum coherences originating from recently proposed inhomogeneous broadening mechanism are enhanced and reflected in the two-dimensional electronic spectra of the excitonically coupled molecular dimer. We show that depending on the amount of inhomogeneous broadening, the excitonically coupled molecular system can establish long-lived electronic coherences, caused by a disordered subensemble, for which the dephasing due to static energy disorder becomes significantly reduced. On the basis of these considerations, we present explanations for why the electronic or vibrational coherences were or were not observed in a range of recent experiments.

On origin of coherence dynamics in biological complexes

Dynamic quantum effects in biological systems is a hotly debated question, which since recently has been experiencing renewed wave of broad attention [1]. This discussion was initiated by the application of the multidimensional spectroscopy techniques for investigating dynamics in biological complexes. Is nature really optimized to use non-trivial quantum effects, like dynamically evolving coherent superpositions to optimize biological functions, or we observe the beatings of quantum coherences, because we use lasers that are coherent light sources, which are quite different from incoherent sunlight utilized by biological organisms? These are difficult questions that require consideration of the essence of quantum mechanics. An important step towards improved understanding in this area is investigation of the origin of coherent beatings observed in the measurements. Only the beatings with electronic character can in principle indicate the importance of dynamical quantum mechanical effects.