Thesis Colloquium- Development of White-light Two-dimensional Electronic Spectroscopy and Its Applications on Light Harvesting Aggregates

 We first demonstrate an alternative approach^3–5 to AOPS that combines the simplicity of broadband WLC generation, with conventional birefringent optical elements⁶ for pulse pair generation and mechanical delay lines for introducing time delays. By combining high repetition rate shot-to-shot detection with rapid scanning of the mechanical delays, we demonstrate measurement of a final averaged 2DES absorptive spectrum with as fast as 1.2 seconds of sample exposure per 2D spectrum, with signal-to-noise ratio (SNR) of 6.81 for sample OD down to 0.05 with 11.6 seconds averaging at 100 kHz repetition rate. Crucially we demonstrate that a combination of rapid scanning of mechanical delays and conventional optical elements can provide 1.6x higher sensitivity along with 5x faster throughput compared to the AOPS approach² which is fundamentally limited by the acoustic velocity in the crystal.

In the later chapters of this thesis, we utilize our spectrometer to study a series of self-assembled porphyrin aggregates. Research on designing artificial photosynthetic templates to mimic the near unity energy and charge transfer efficiency of natural photosynthesis has largely focused on strategies such as designing donor-acceptor systems⁷, DNA-templated aggregation⁸, and pi-stacked aggregation on carbon nanotubes⁹. A critical factor often overlooked in many of these approaches 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 excitons¹⁰. The key difference 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.

While probing smaller systems such as the monomer and the H-dimer has proven challenging due to limitations in pulse energies, our results on rings, nanotube and bundle aggregates have some surprising findings. First we show that all previous theoretical models¹¹ for the exciton bands of porphyrin nanotubes need to be revisited because their starting point incorrectly assumes the ring aggregate as a monomer. Our measurements conclusively show that this species is a small J aggregate. Second, we show that the weak vibrational progression like shoulders in the Q bands of nanotubes and bundles are in fact strongly mixed Q_x-Q_y type states that directly show up as a cross-peak in the 2DES spectra at the earliest pump-probe waiting time T. These states likely mix strongly due to a combination of energetic disorder and linear vibronic coupling. The 2D spectra also report excitation wavelength dependent kinetics within the Q bands. Additionally, we observe a rapid loss of frequency-frequency correlation, occurring nearly 10x faster than typical spectral diffusion timescales for molecules in solution, indicating rapid exciton relaxation within these bands.

These intra-band Q_x-Q_y couplings in the nanotubes are then further investigated through polarization-controlled 2DES and PP measurements. We first characterize the polarization-controlled WLC 2DES (P-2DES) pulse sequences which can selectively probe the mixed Q_x-Q_y states. When applied on nanotubes and bundles, we find concomitant decay and rise of population within the main Q band of the porphyrin nanotubes and bundles thus providing direct evidence of fast (240 fs in tubes and 440 fs in bundles) intraband population relaxation within the broad overlapping vibrational-electronic bands. Slower decay on a 1-2 ps timescale is attributed to the presence of low-lying dark states below the main Q band. Our results also suggest that inter-tube interactions are weak enough so as not to strongly perturb the fast intra-tube dynamics.

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 proteins¹² and chlorosomes¹³, similar to those found in a vibronically resonant photosynthetic dimer¹⁰, are likely strongly mixed due to vibronic couplings and that such effects do survive at room temperature in large aggregates.