Two-dimensional electronic spectroscopy for mapping molecular photophysics

Abstract

We demonstrate the ability of two-dimensional electronic spectroscopy (2DES) to map ultrafast energy transfer and dynamics in two systems: the pigment-protein complex photosystem I (PSI) and aggregates of the conjugated polymer poly(3-hexylthiophene) (P3HT). A detailed description of our experimental set-up and data processing procedure is also given.First, we will give a brief introduction to 2DES and a description of our experimental set-up, followed by a detailed description of our data processing procedure. Finally, we demonstrate how 2DES can be used to elucidate ultrafast (0-400 fs timescale) photophysical events in two systems: poly(3-hexylthiophene) (P3HT) and photosystem I (PSI). TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPYA wealth of information on energy-transfer dynamics and electronic structure can be obtained by applying electronic spectroscopy to complex condensed-phase systems. Steady-state measurements such as linear absorption, fluorescence, excitation anisotropy, and circular dichroism can provide information on the electronic structure. Additional information can be obtained by conducting these measurements at cryogenic temperatures to reduce the effects of homogeneous line broadening; however, the distinction between homogeneous and inhomogeneous broadening is still ambiguous [12,13]. Time-resolved methods, such as transient absorption and pump-probe spectroscopies, can be used to gain further insight into electronic structure while also monitoring energy-transfer dynamics [14,15]. However, when using these techniques, one must choose between either high frequency or time resolution.2DES has emerged as an optical technique that can accomplish many of the objectives of conventional spectroscopies, while overcoming the limitations mentioned above. A 2D spectrum spreads information contained in a 1D pump-probe spectrum over two frequency axis resulting in a frequency-frequency correlation map, where each excitation frequency is correlated to each detection frequency [16,17]. To obtain a 2D spectrum, three femtosecond optical fields, E 1 , E 2 , and E 3 having wavevectors k 1 , k 2 , and k 3 interact with the sample leading to the emission of the signal, E s . In the most common implementation of 2D optical spectroscopy, the incoming fields are arranged in a box geometry so that the signal is emitted in the background-free direction, and the two relevant phase-matching conditions, referred to as "rephasing" (k signal = -k 1 + k 2 + k 3 ) and "nonrephasing" (k signal = +k 1k 2 + k 3 ), are collected independently. The beam geometry and pulse sequence are displayed in Fig. 1. The first pulse, E 1 , creates a coherent superposition between the ground state and a first excited state of the system effectively labeling the molecules. This superposition state evolves during the coherence time, t 1 . The arrival of the second pulse, E 2 , creates either a population or another coherence, marking the end of t 1 and the beginning of the waiting time, t 2 . The system is free to evolve during the waiting time, w...

Photosynthetic light harvesting: excitons and coherence

“…2DES has emerged as an optical technique capable of creating and probing coherences while still preserving information on both the excitation and emission energies. In addition to elucidating typical optical properties of molecular systems, 2DES can distinguish homogeneous and in homogeneous broadening and detect correlations between excitonic states [121][122][123][124][125][126][127]. 2DES is similar to transient absorption spectroscopy, however two excitation pulses are used cooperatively to excite the sample followed by the third 'probe-pulse' which interacts with the sample after a time delay and initiates the radiation of a signal depending on the optical response of the sample.…”

Crossing disciplines ‐ A view on two‐dimensional optical spectroscopy

“…Many aspects of the technique are related to nuclear magnetic resonance (NMR) methods developed decades earlier . 2D ES has now been used to study a variety of systems including atoms , molecules , molecular aggregates and related nanostructures , biological pigment‐protein complexes , and semiconductor nanostructures . Methods for measuring multidimensional spectra using entangled photons have been proposed .…”

Artificial quantum photosynthetic materials

“…Furthermore, BBPP data is often fundamental to the processing of 2DES data to ensure the phasing procedure is performed correctly. 19 2DES data collected from the LHC is provided as a video file in the ESI, and exhibit the same oscillations that are observed in the BBPP. Compared to 2DES data, BBPP offers a clear and straightforward method to present coherent oscillations, and thus is used as the focus of this discussion.…”