Structural Explorations into Singlet Fission Chromophores for Solar Photochemistry with Femtosecond Stimulated Raman Spectroscopy

Abstract

Understanding physical, chemical or biological changes require monitoring the molecular vibrations aka nuclear coordinates on the timescale of their movements, which is hundreds of femtoseconds. Thus, ultrafast spectroscopic techniques capable of providing molecular conformational changes on the femtosecond timescale are desirable to provide in-depth knowledge of any photophysical process. This thesis discusses the strength of using a structurally sensitive ultrafast spectroscopic technique, femtosecond stimulated Raman spectroscopy (FSRS) to understand the photophysics in a chemical process called singlet fission. Thin films and single crystals which may undergo singlet fission are promising for solar energy conversion strategies as they can generate two charge carriers by the absorption of only one photon. A major bottleneck in the singlet fission field is the challenge of designing or discovering new molecules with enhanced photophysical properties, while not altering other parameters such as crystal packing or solubility which can hinder overall device performance. Chemical modifications to acenes may lead to singlet fission-based photovoltaics with improved solar energy conversion efficiencies, but identifying how molecular structural changes impact the rate and yield of fission is challenging to model and predict. Traditionally, spectroscopic measurements have not provided much insight for rational design of new chromophores, but rather have been used to understand why an existing system works well. To this end, I have used FSRS to examine the excited state structural dynamics in organic chromophores undergoing singlet fission and have provided a predictive model for molecular designing guidelines to obtain efficient singlet fission systems for their use in solar energy based devices. Specifically, I have used ultrafast FSRS to monitor the changes in the molecular structure in rubrene during singlet fission and observed that singlet fission in crystalline rubrene is associated with a loss of electron density in the tetracene backbone. Armed with this knowledge and a hypothesis to improve singlet fission rate by reducing electron density in the tetracene core, I screened rubrene derivatives to study their excited state dynamics using FSRS. From a series of derivatives, I screened two new rubrene derivatives with electron withdrawing substituents, FM-rubrene & F-rubrene, to prime the system for singlet fission, without impacting intermolecular interactions. Using these rationally designed rubrene derivatives I found that both the rate and yield of singlet fission are significantly improved, proving that spectroscopic insight is crucial to successfully designing new chromophores. This shows that the long-held promise of spectroscopy-informed small molecule design for organic optoelectronic materials can be realized in rubrene-based singlet fission materials. As the first graduate student to use FSRS in a microscope to look at crystals, I realized that distinguishing Raman from non-Raman features can be complicated in molecular crystals, where the narrowband transient vibronic couplings due to less heterogeneous broadening can overwhelm the excited state Raman bands. To overcome this, we redesigned the ultrafast FSRS experimental setup to help ease the data extraction and interpretation associated with this technique. By adding a pair of mirrors and a slit to the existing grating filter setup, we generated an additional Raman excitation pulse that identifies Raman peaks from a pool of non-Raman peaks. In this thesis, I present the successful implementation of FSRS to provide unprecedented structural dynamics in organic semiconducting materials with the aim to achieve better performing singlet fission based-photovoltaic devices and the experimental modifications made to support FSRS as a more user-friendly technique for its widespread adoption to understand chemical reactions.