Current advances in materials and fabrication of electronic devices continue to press towards the molecular level. The engineering of such devices is predicated upon a deep understanding of the fundamental and often quantum mechanical properties that give rise to a number of unique effects. Consequently, simultaneous strides have been made in the theoretical understanding and prediction of the properties of these new materials as well in the experimental detection and characterization of these properties. Furthermore, biomimetic strategies that actively use the connection to biological functional units like light-harvesting systems provide valuable guidelines and points of comparison. Thus, seemingly disjoint types of systems like organic semiconducting polymers and DNA are found to exhibit similarities in the elementary photophysical pathways that eventually determine their function. Likewise, the processes at single molecule junctions and polymer-polymer interfaces (heterojunctions) are a sensitive function of the intrinsic molecular properties of their molecular constituents.
By the fact that a quantum description of the electronic structure and electronic and nuclear dynamics is of key importance for the understanding of the elementary processes in these systems, the basic theoretical tools are closely related to techniques that have been developed in the context of atomic and molecular physics, theoretical chemistry, system-bath theory, quantum transport theory, scattering theory etc. However, the novelty and complexity of the systems under study requires re-thinking and adapting existing methods. The recent literature -- both experimental and theoretical -- reflects this process in several key areas:
(i) Excitation energy transfer (EET) and exciton migration, trapping and dissociation in nano-structured molecular arrays, beyond the conventional Foerster regime [1-4];
(ii) Ultrafast photoprocesses in a variety of functional systems ranging from DNA to semiconducting polymers, involving a crucial role of vibronic (electron-phonon) coupling [4,5];
(iii) Quantum transport in the specific context of electronic conduction through single-molecule junctions [6-9];
(iv) System-environment interactions adapted to molecular transport phenomena, involving bosonic and fermionic reservoirs and including particle exchange between system and reservoirs .
To tackle these systems, a considerable range of theoretical methods are applied, including explicit quantum dynamics , various mixed quantum-classical techniques, multi-dimensional vibronic coupling models , on-the-fly non-Born-Oppenheimer dynamics, density functional theory (DFT) and its time-dependent analog (TD-DFT) , non-equilibrium many-body Green's function techniques [6,7], path integral approaches , master equations , to name but a few approaches. To obtain quantitative insight, it is now recognized that all of these methods need to be adapted so as to include electron-nuclear couplings, as well as electronic, nuclear, and electronic-nuclear correlations. Furthermore, the theoretical treatment is often complicated by the fact that various dynamical effects compete and interfere (for example, exciton migration can typically interfere with phonon-assisted exciton dissociation yielding polaron pairs).
In a vast majority of cases, the relevant dynamical regimes are such that standard perturbative assumptions like weak coupling, adiabatic separability of nuclear vs. electronic motions, separation of time scales between system vs. bath degrees of freedom (Markovian limit), and rapid dephasing of quantum coherence, are not valid. This is highlighted by a number of recent time-resolved experiments which demonstrate the importance of vibronic coupling effects and the role of vibrational and electronic coherence which is often found to be surprisingly long-lived despite the presence of the environment . (This aspect is the main topic of the companion workshop on coherent many-body quantum dynamics mentioned above.)
While the overall scenario poses a formidable challenge to the existing theoretical techniques, the advances in ultrafast time-resolved and multi-dimensional experiments provide a unique opportunity to guide and test new theory developments. Against this background, the present workshop aims to give an overview of current state-of-the-art methodology and a discussion forum for various new approaches, while offering direct exchange with leading experimentalists.