Rapid Scan White Light Two-Dimensional Electronic Spectroscopy on Artificial Light Harvesting Nanotubes

Abstract

Relaxation of electronically excited states across various systems, ranging from biological proteins to emerging energy materials, carries fundamental importance because it dictates the eventual fate of photoexcitations. Such relaxation proceeds through spectrally congested (overlapping) vibrational-electronic bands on femtosecond to picosecond timescales. Probing such systems thus requires time-resolved spectroscopic techniques that can provide fast time resolution along with spectral decongestion. Two-dimensional electronic spectroscopy (2DES) is a state-of-the-art technique that can resolve ultrafast phenomenon, routinely with sub-10 fs temporal resolution, as a 2D contour map of detection versus excitation frequency as a function of delay T between excitation (pumping) and probing. A broadband light source is naturally desirable in order to probe the entire energetic manifold of overlapping vibrational-electronic bands. The frontier of 2DES now lies at the development of high repetition rate 2DES approaches that work with a white light continuum (WLC) input. So far the only WLC-2DES approach that has been demonstrated is using highly complex and artifact prone acousto-optic pulse shaping (AOPS). This thesis outlines the development of a 2DES spectrometer that takes a WLC input, works with conventional optics, and generates 2D spectra with polarization-controlled input pulses that beat the state of the art in terms of throughput and sensitivity. We then demonstrate applications of this approach to reveal the nature of overlapping-vibrational-electronic bands in artificial light harvesting nanotubes. Our findings have broader implications for the mechanism of ultrafast internal conversion in naturally occurring photosynthetic aggregates. In the later chapters of this thesis, we utilize our spectrometer to study a series of self-assembled porphyrin aggregates. A critical factor often overlooked in designing artificial photosynthetic templates is the nature of the constituent molecule. Selecting a molecule that closely resembles the properties of those found in natural light harvesting is crucial for obtaining meaningful insights into the quantum dynamics that govern photosynthetic excitons. The key highlight in our approach is that self-assembled disordered aggregates closely mimic the structural hierarchy of aggregates found in photosynthetic proteins with similar electronic structure, nuclear displacements and overlapping vibrational-electronic bands. We use Pump-Probe spectroscopy (PP), 2DES and Polarization-controlled 2DES (P-2DES) to probe the exciton manifold of a hierarchy of self-assembled aggregates. Our experiments give conclusive evidence for strongly mixed QxQy type states in the Q band of the self-assembled nanotubular aggregates and the presence of low-lying dark states below the main Q band. Our observations also motivate the need to refine the existing theoretical models for the exciton bands of self-assembled nanotubes. Overall this work reports a novel approach to 2D spectroscopy and fresh insights into the nature of overlapping vibrational-electronic bands in disordered self-assembled aggregates where ultrafast population relaxation appears to proceed through vibronically mixed states. The presence of low-lying dark states is similar to the naturally occurring photosynthetic nanotubes (chlorosomes) and seems to be a general property of disordered aggregates. The general implications of our findings are that overlapping vibrational-electronic bands in large photosynthetic aggregates such as LH2 antenna proteins and chlorosomes, are likely strongly mixed due to vibronic couplings and that such effects do survive at room temperature in large aggregates.