The aim of this project is to explore new concepts for the control of nanoscale energy transport in organic donor-acceptor materials. Using two-dimensional electronic spectroscopy (2DES) we could recently observe for the first time that intermolecular conical intersections (CoIns) govern the light-induced, sub-50 fs energy relaxation dynamics in organic molecular aggregates used as photoactive materials in organic electronics (1). CoIns are ubiquitous in molecules but were not yet observed in solids. Their discovery opens up interesting new prospects for the control of ultrafast energy and charge transport dynamics in organic functional materials, for example by coupling the materials to external light modes. Through controlled light-induced modifications of the potential energy surface, this approach opens up new opportunities for the control of ultrafast energy relaxation and transport dynamics in organic nanostructures with potential implications for energy technologies.

To this end, we combine the complementary expertise of the PIs in 2DES (1-4), supercontinuum polarization shaping (5) and photoelectron tomography (6-8). Using polarization-tailored femtosecond excitation pulses, we aim at manipulating the direction and speed with which the optically launched wave packet approaches the CoIn. While optical 2DES probes the dynamics on optically bright potential energy surfaces, photoelectrons provide access to the ensuing dynamics on optically dark states. To validate the photoelectron tomography method, we will perform reference experiments on model molecules in the gas phase and investigate the signatures of a nuclear wave packet traversing a CoIn in the three-dimensional photoelectron momentum distribution (6,7). The combination of all-optical and photoelectron detection methods with polarization shaping holds great promise for a comprehensive understanding of the ultrafast nonadiabatic dynamics through CoIns and for their control. Our studies will contribute to the development of future organic optoelectronic devices with tailored nanoscale energy transport.

(1) A. De Sio et al, Nature Nanotechnology 16, 63-68 (2021)

(2) X.T. Nguyen et al, The Journal of Physical Chemistry Letters 10, 5414-5421 (2019)

(3) A. De Sio, C. Lienau, Physical Chemistry Chemical Physics 19, 18813-18830 (2017)

(4) A. De Sio et al., Nature Communications 7, 13742 (2016)

(5) S. Kerbstadt et al, Optics Express 25, 12518 (2017)

(6) D. Pengel et al., Physical Review Letters 118, 053003 (2017)

(7) S. Kerbstadt et al, Nature Communications 10, 658 (2019)

(8) K. Eickhoff et al., Physical Review A 104, 052805 (2021) (Editor’s Suggestion)

Photo-catalysis is expected to evolve to a main technology for sustainable production of commodity chemicals and green energy in the future. Despite a large number of experiments, the fundamental principles of photoactive processes are not yet understood at the atomic scale. This concerns, in particular, the influence of local structural and stoichiometric inhomogeneities, such as line defects, interfaces and grain boundaries, on the diffusion, live time and recombination probability of the photo-induced carriers. Typically, the necessary experiments are hindered by the large structural and electronic complexity of photo-active materials that cannot be accessed by common microscopic and spectroscopic techniques.

Aim of this project is to overcome this ‘structure gap’ by a combination of high-resolution physical and chemical methods. For this purpose, two model systems will be prepared, (i) crystalline V-doped cuprous oxide films grown by physical vapor deposition and (ii) copper-vanadate powder samples produced by hydrothermal and sol-gel synthesis. The atomic structure of the resulting mixed oxides will be probed either by electron diffraction and scanning tunneling microscopy for the thin-film samples, or by X-ray diffraction and electron microscopy for the powder materials. The chemical and optical character of both samples will be accessed by photoelectron and NMR spectroscopy, as well as UV-Vis, absorption and luminescence spectroscopy, respectively. Whereas the macroscopic activity of the powder samples, e.g. in photocatalytic water splitting, is probed with a batch reactor, the reactivity of the crystalline films is determined locally by monitoring photo-driven charge transfer into suitable adsorbates bound to the surface. For this purpose, molecules with high electron (W₃O₉) and hole affinity (methyl viologen) are placed and optically activated at the surface. Changes in the molecular conformation due to photo-driven charge transfer can now be probed at the single-molecule level directly by STM imaging. In addition, the morphology and local optical activity of the samples can be correlated by means of STM luminescence spectroscopy, opening a pathway to determine the impact of grain boundaries and exciton traps on the photo-induced charge generation. By applying physical and chemical methods onto similar model systems, a better understanding of photocatalytic reactions and a substantial decrease of the ‘structure gap’ are expected from this project.