Coherence and Singlet Fission of TIPS-Pentacene Probed by Two-dimensional Electronic Spectroscopy

Abstract

Two-dimensional electronic spectroscopy (2DES) is a type of ultrafast pump-probe (PP) spectroscopy with high time resolution and broadband pump and probe pulses. The high time resolution makes 2DES ideal for tracking ultrafast processes, and the broadband pump and probe allows 2DES to detect correlation between species or states of the system, which manifests as cross peaks in the 2DES data. Additionally, the broadband pulses allow excitation to a quantum superposition of states, a phenomenon known as coherence. This manifests as oscillatory features in the 2DES data with frequency equivalent to the difference in energy between the two states in the superposition. Analysis of coherence signals can reveal details about the vibrational and electronic structure of the system under investigation. In this thesis, we begin with a study of the coherence and ultrafast relaxation of cresyl violet, a model two-level system with a single vibrational mode. We develop a flexible framework of data analysis techniques for the interpretation of 2DES data, incorporating Fourier transforms (FTs), time-frequency transforms (TFTs) and global analysis. With this framework, we find that cresyl violet exhibits vibrational coherence at 612 cm−1 with a dephasing lifetime of 600 fs. We find that the energy-dependence of this coherence is highly consistent with that predicted by a simple two-level model with the vibrational mode in both the ground and excited states. Additionally, we observe ultrafast relaxation associated with spectral diffusion and the dynamic Stokes shift with a rate constant of 1/k = 100 fs. This analysis demonstrates a novel algorithm for globally and simultaneously fitting the coherence and kinetics of 2DES data. We then use the analysis framework to investigate 6,13-bis(triisopropylsilylethynyl)- pentacene (TIPS-Pn) in dilute solutions and nanoparticles (NPs) of two morphologies. In concentrated solutions and solids, TIPS-Pn undergoes singlet fission (SF), an exciton multiplicative process that has the potential to increase the efficiency of photovoltaic devices by reducing energy loss through thermalisation. We use dilute solutions of TIPS-Pn to characterise the singlet states of TIPS-Pn when SF is absent. We observe coherence at 295 cm−1, 520 cm−1, 781 cm−1, 940 cm−1, 1159 cm−1 and 1341 cm−1 with lifetimes from 0.5 ps to 1.5 ps. The 940 cm−1 mode is attributed to non-resonant coherence of the solvent, tetrahydrofuran (THF), and displays surprisingly non-trivial energy-dependence, which is rationalised by a mechanism involving Raman scattering. The remainder of the modes are attributed to vibrational and vibronic coherence of the ground and first excited singlet state of TIPS-Pn. We then study amorphous TIPS-Pn NPs, in which we observe largely similar coherence to that observed in dilute solutions of TIPS-Pn, indicating that TIPS-Pn has similar vibronic structure in amorphous solids and dilute solutions. We are unable probe SF in amorphous TIPS-Pn NPs due to the SF process being relatively slow compared to the available time window of the instrument. Finally, we investigate crystalline TIPS-Pn NPs. In these NPs, ultrafast SF is observed with a time constant of approximately 75 fs. However, no significant coherence is found, which is likely due to any coherences being outside the detection limits of the instrument. These studies contribute to the current understanding of the coherence of TIPS-Pn as isolated and weakly coupled molecules, which is yet to be fully characterised in the literature.