Development of a Modular Synthesis of Rigid Polyacene Dimers for Mechanistic Exploration of Singlet Fission

Carey, Thomas James

Graduate Thesis Or Dissertation

Development of a Modular Synthesis of Rigid Polyacene Dimers for Mechanistic Exploration of Singlet Fission Público Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/v979v323p

Abstract

Singlet Fission (SF) is a non-radiative, spin-allowed, photophysical process wherein a singlet excited state converts into a pair of spin-coupled triplet states. This process has implications for improving solar cell efficiency, as SF offers a way to process high energy photons by proportionating their energy into pairs of electronic excitations rather than wasting excess energy as heat. Of particular interest has been the study of molecular dimers, as these are the simplest systems capable of SF and offer a platform for understanding mechanistic details for the development of efficient SF systems.

This dissertation describes the development of a modular synthesis of rigid polyacene dimers as well as a photophysical exploration of how structural features impact SF. Four polyacene dimers were synthesized; in all final products, the active chromophores are derived from tetracene or pentacene. The chromophores are connected via a norbonyl bridge with two points of attachment to the bridge per chromophore, providing a rigid geometry. The first dimer synthesized, an unsubstituted tetracene dimer (BT1), serves as a comparative baseline for related molecules. BT1 suffered from poor solubility and stability; these issues were addressed in the second dimer synthesized, a triisopropylsilylalkynyl-substituted (TIPS-alkynyl-substituted) tetracene dimer (TIPS-BT1). The route was then extended to the analogous TIPS-alkynyl-substituted pentacene dimer (TIPS-BP1) without success due to unreliable reactivity stemming from difficulties in handling the synthetic intermediates. This route was then modified in order to introduce solubility and stability earlier in the route, leading to the successful synthesis of constitutional isomers of TIPS-BT1 and TIPS-BP1, called TIPS-BT1’ and TIPS-BP1’.

The photophysics of this set of four dimers reveal interesting principles regarding the relative importance of energetics and coupling. All four dimers are expected to have poor coupling between the states relevant for SF due to their C_2v symmetry, and all of the tetracene dimers exhibit low SF yields as a result. By contrast, TIPS-BP1’ exhibits appreciable SF yield even in the absence of formal coupling due to its favorable energetics. These findings provide a synthetic as well as a photophysical foundation of mechanistic details for comparison to future dimers based on this molecular platform.

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