Spatially-resolved fluorescence-detected two-dimensional electronic spectroscopy probes varying excitonic structure in photosynthetic bacteria

Abstract

Conventional implementations of two-dimensional electronic spectroscopy typically spatially average over ~1010 chromophores spread over ~104 micron square area, limiting their ability to characterize spatially heterogeneous samples. Here we present a variation of two-dimensional electronic spectroscopy that is capable of mapping spatially varying differences in excitonic structure, with sensitivity orders of magnitude better than conventional spatially-averaged electronic spectroscopies. The approach performs fluorescence-detection-based fully collinear two-dimensional electronic spectroscopy in a microscope, combining femtosecond time-resolution, sub-micron spatial resolution, and the sensitivity of fluorescence detection. We demonstrate the approach on a mixture of photosynthetic bacteria that are known to exhibit variations in electronic structure with growth conditions. Spatial variations in the constitution of mixed bacterial colonies manifests as spatially varying peak intensities in the measured two-dimensional contour maps, which exhibit distinct diagonal and cross-peaks that reflect differences in the excitonic structure of the bacterial proteins.

Before Förster. Initial excitation in photosynthetic light harvesting

“…Photo-electron detection has been used to achieve spatial resolution beyond the diffraction limit of the optical beams in 2D spectroscopy 19. Fluorescence detected 2D (FD2D) spectroscopy was used to investigate the conformation of molecular dimer complexes20 and spatial differences of the micro-distribution of the LH2 antenna composition due to growth condition variations 21. In all these approaches four collinear laser pulses bring the system to an excited state, which can generate the signals as photocurrent, photoelectrons or fluorescence.…”

Simulating Fluorescence-Detected Two-Dimensional Electronic Spectroscopy of Multichromophoric Systems

“…Recent F-2DES experiments 7,12,14 have reported early waiting time spectra of detergent-isolated LH2 exhibiting distinct positive cross-peaks, leading to the suggestion of unexpectedly large delocalization of the initial excitation. 12 Furthermore, the B850 band was reported to be round at early times in these experiments.…”

Characterization of thymine microcrystals by CARS and SHG microscopy

“…The Raman signal does not require macroscopic ordering, consequently, the generated signal from a crystalline sample will necessarily contain a background from noncrystalline phase. The same holds in fluorescence-detected two-dimensional electronic microscopy, which has been used in characterization of photosynthetic molecular complexes 40 . The situation is completely different in SHG where the signal generation requires macroscopic ordering.…”