Time-resolved fluorescence spectroscopy of individual molecular quantum systems

From steady-state absorption and emission for state structure to ultrafast nonlinear spectroscopy for energy transfer, practically all electronic excited-state properties of organic materials are known from optical spectroscopy. Prominent examples are excitation energy transfer in natural photosynthetic light-harvesting complexes and artificial organic solar materials, as well as excitation transfer between pairs of precisely placed labels reporting on protein structure and interaction at the sub-nanometre scale. However, basically, all the measurements and the resulting understanding are based on ensemble measurements that average over the vast number of the relevant systems. This is a fundamental limitation, since all the systems mentioned above are intrinsically highly disordered, fluctuating at several length and time scales. An approach that goes beyond the ensemble is so-called single-molecule fluorescence spectroscopy. True single molecules are, in a sense, boring, since the distribution of their properties is dictated by their immediate environment. In contrast, so-called single molecular quantum systems, i.e., supercomplexes that behave as individual emitting units, possess intrinsic complex dynamics with involved physics and direct functional implications. This PhD project is focused on the study of the static and dynamic disorder in such individual molecular quantum systems, with examples ranging from photosynthetic complexes to organic crystalline nano- and micro-domains. The core part of the thesis will be the construction of a sensitive, time-resolved ‘single-molecule’ confocal fluorescence detection setup, and its application to selected systems of interest. The time resolution is achieved in the emission using time-correlated single-photon counting, as well as in the excitation, using multi-pulse sequences as common in ultrafast spectroscopy. The project is realized in collaboration with experimental groups of Prof. Juergen Hauer,Technical University Munich, and prof. Jennifer Ogilvie, University of Michigan.

References:

Gruber et al., „From isolated light-harvesting complexes to the thylakoid membrane: a single-molecule perspective“, Nanophotonics 7, 81 (2018)

Malý et al., “Ultrafast energy relaxation in single light-harvesting complexes”, Proc. Natl. Acad. Sci., 113, 2934 (2016)

Thyrhaug et al., “Single-molecule excitation–emission spectroscopy”, Proc. Natl. Acad. Sci. 116, 4064 (2019)

Malý et al., “Fluorescence-detected Pump–Probe Spectroscopy”, Angew. Chem. Int. Ed. 60, 18867 (2021)

Fersch et al., “Single-Molecule Ultrafast Fluorescence-Detected Pump–Probe Microscopy”, J. Phys. Chem. Lett. 14, 4923 (2023)