In our group we discuss, build and apply theoretical methods to study molecular dynamics in both natural and artificial systems, addressing fundamental questions such as the role of coherence in energy transport, the wavepacket dynamics, the exciton delocalization and others.
As an example, the observation of coherent oscillations in two-dimensional electronic spectroscopy (2DES) with attributes partially due to electronic coherence has been startling because of the unanticipated long dephasing times of the coherences. At first it was thought that these signals were incisive probes of electronic coherences, but subsequent work uncovered a more complicated picture. Consequently, several groups are seeking alternative explanations for the coherences, and typically pose the question of whether the beats are electronic or vibrational or some kind of mixed electronic-vibrational state. We consider different directions to help elucidate the nature and properties of coherent oscillations in femtosecond spectroscopy. We foresee the importance of elucidating from experiment data that can be analyzed to retrieve information on delocalization, in which case it is important to choose the measures carefully.
How to quantify excitonic delocalization in complex systems?
At the foundation of the most interesting coherence effects in light harvesting systems are molecular excitons which can be seen as are electronic excited states where the wavefunction is shared by two or more molecules. The electron density remains localized on the molecules individually, but the transition density for the exciton is a well-defined superposition of molecule-localized transition densities. In the kinds of exciton coherence this delocalization evolves in time.
Exciton delocalization, if strong, can be indicated by spectral features that are perturbed, shifted, or split. Similarly, changes in the radiative rate (“superradiance”) are quite sensitive to delocalization. Coherent superpositions of states that contribute oscillating cross-peaks to 2DES data have recently been proposed to reveal delocalization and coherences— and particularly how they dephase. From a physical point of view, reasons behind changes in delocalization include exciton localization—the interplay of the Stokes shift in the site basis and exciton delocalization with a lesser nuclear reorganization—or an explicit dependence of energy resonance on various nuclear coordinates.
GD Scholes, C Smyth, “Perspective: Detecting and measuring exciton delocalization in photosynthetic light harvestin” J. Chem. Phys. 140,
F Fassioli, R Dinshaw, PC Arpin, GD Scholes ”Photosynthetic light harvesting: excitons and coherence” Journal of The Royal Society Interface 11 (92), 20130901 (2014)
Can sun-light drive coherence?
Recent 2DES experiments have reported evidence of coherent dynamics of electronic excitations in several light-harvesting antennae. However, 2DES uses ultrafast coherent laser pulses as an excitation source; therefore, there is a current debate on whether coherent excitation dynamics is present under natural sunlight − incoherent − illumination conditions. We showed that even if incoherent light excites an electronic state with no initial quantum superpositions among excitonic states, energy transfer can proceed quantum coherently if non equilibrium dynamics of the phonon environment takes place. Such non equilibrium behavior manifests itself in non-Markovian evolution of electronic excitations and is typical of many photosynthetic systems. We therefore argue that light-harvesting antennae have mechanisms that could support coherent evolution under incoherent illumination.
F. Fassioli, A. Olaya-Castro, G. D. Scholes, “Coherent Energy Transfer under Incoherent Light Conditions” The Journal of Physical Chemistry Letters 3, 3136–3142 (2012)
DB Turner, PC Arpin, SD McClure, DJ Ulness, GD Scholes ”Coherent multidimensional optical spectra measured using incoherent light” Nature communications 4 (2013)
Design of molecular circuits that use coherence
We are interested in the question of whether it is feasible to realize photo-initiated quantum dynamical effects in condensed phase supramolecular systems. If so, what would we gain from such effects? Conversely, if photo-induced dynamics can be influenced by non-trivial quantum effects, how will that inspire the design of supramolecular systems?
F Fassioli, DG Oblinsky, GD Scholes, “Designs for molecular circuits that use electronic coherence” Faraday discussions 163, 341-351 (2014)
Exciton Modeling in Energy transfer processes
We have determined the exciton model and energy transfer scheme of the protein LH complex. We combined MD-MMPol methodology (recently developed) with a quantitative simultaneous fit of the 2D spectra and transient absorption kinetics using the modified Redfield approach. The MMPol method allows for a consistent atomic detail description of electrostatic and polarization pigment−protein interactions in the estimation of site energies, transition dipole strengths, and electronic couplings. In addition, conformational flexibility of the system is accounted for through classical molecular dynamics simulations. (collaboration with Benedetta Mennucci and Carles Curutchet).