Periodic Reporting for period 4 - DYNAMO (Energy and charge transfer nonadiabatic dynamics in light-harvesting molecules and nanostructures)
Reporting period: 2019-12-01 to 2020-05-31
There is growing experimental evidence that nonadiabatic dynamics triggered upon light absorption plays a fundamental role in determining the efficiency of energy and charge transfer in functional materials.
In addition to the intramolecular nonradiative transitions through conical intersections, well known from photochemistry, the coupling between the chromophores in multichromophoric assemblies gives rise to novel intermolecular nonadiabatic relaxation channels through funnels between the delocalized excitonic and/or charge transfer states. In order to simulate coupled electron-nuclear dynamics in multichromophoric nanostructures we will develop and implement light-induced surface hopping methods and combine them with efficient electronic structure methods.
We apply our methodology to investigate energy and charge transport in nanostructures of self-assembled organic molecules in low band-gap organic polymers (e.g. squaraines) and in hybrid plasmon-exciton architecturess.
(1) The development and implementation of the multichromophoric field-induced surface hopping method (McFISH) for the simulation of exciton dynamics in molecular aggregates .
(2) The development and implementation of the long-range corrected tight-binding density functional theory (LC-TDDFTB)  and its combination with the surface hopping non-adiabatic dynamics.
This led to the publication of a generally applicable code DFTBaby that has been made publicly available .
(3) The development of the coupled coherent state dynamics  that generalizes the surface hopping simulation technique and allows including quantum effects (such as e. g. Berry phase or tunnelling) in the nuclear dynamics.
(4) The extension of the field-induced surface hopping method (FISH) in order to simulate the coupled electron-nuclear dynamics in chiral systems driven by laser-fields with arbitrary polarization .
(5) The development of a fully atomistic and fully quantum-mechanical approach for the simulation of exciton dynamics and spatio-temporal plasmonic field distributions in hybrid plasmon-exciton systems by combining fully quantum simulations of the exciton dynamics with computational electrodynamics [6|
(6) The development of the mulistate metadynamics for automatic sampling of conical intersections and its implementation into the program package MetaFALCON .
The developed methods have been applied in collaboration with experimental partners to study the light-induced dynamics processes in a number of systems. This led to the following achievements:
(1) The simulation of the optical properties of squaraine dye aggregates that enabled the identification of different foldamers present in polar and non polar solvents and led to the interpretation of their spectroscopic properties and exciton transfer dynamics .
(2) The discovery of the origin for strong solvent dependence of the fluorescence quantum yield in merocyanine dyes. The latter arises due to the presence of solvent-dependent activation barrier leading to a conical intersection that mediates ultrafast nonradiative relaxation .
(3)The first systematic microscopic study on different regimes of exciton transfer in molecular aggregates shedding new light on the possible mechanisms of energy transport in organic molecular excitonic materials .
(4) Identification of the mechanism for the excimer formation in organic materials involving large amplitude vibrational dynamics and intermolecular conical intersections .
(5) Identification of Benzyne as an intermediate in the photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions .
(6) Identification of the low-energy excitonic states in DNA-silver cluster hybrids, which enable ultrafast energy transport between metal clusters, and give insight into the origin of the fluorescence signal in coupled DNA-stabilized silver clusters, which have been recently experimentally detected .
(7) The combination of the coherent two-dimensional microspectroscopy with theoretical modelling of excitonic spectra allows us to obtain a map of the local optical coherence length within a hierarchically structured molecular film. The nanoscale coherence length was
found to correlate with microscale topography, suggesting a perspective for controlling structural coherence on molecular length scales by appropriate microscopic growth conditions .
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has for the first time enabled us to perform fully atomistic simulations of the energy and charge transport dynamics in multichromophoric aggregates. In these simulations, both electronic structure as well as the coupled electron-nuclear dynamics are treated from first principles in the real time.
The applications to several classes of multichromophoric assemblies has given us a full dynamical picture of the exciton transport and has revealed the time-scales of the processes such as excimer formation
which fundamentally limit exciton transport. Furthermore, we have identified conical intersections between the excitonic states which are responsible for the efficient
nonradiative relaxation and have developed efficient methods for their systematic exploration. These achievements emphasise the role of the dynamical effects
in the functionality of light-harvesting materials.
Altogether, our methodological developments have allowed us to push the limit of the excited state non-adiabatic dynamics simulations towards complex functional materials. The improvement of the function
of such materials should lead to the development of new efficient and environment-friendly functional materials.