Temperature and structural effects on singlet fission and intersystem crossing in organic semiconductor systems

Abstract

In this thesis, we investigate the fundamental mechanisms that contribute to triplet exciton generation in organic molecular systems, such as singlet fission, triplet-triplet annihilation, and intersystem crossing. We examine the effects of external conditions such as magnetic fields and temperature, as well as internal factors such as molecular structure, on exciton behaviours and triplet generation process. We initially studied the temperature and magnetic field dependence on photoluminescence of a diF-TES-ADT singlet fission system. We showed, through magnetic field-dependent photoluminescence spectroscopy and a range of different optical and magnetic resonance spectroscopic techniques, that singlet fission to form a weakly bound triplet pair state is highly temperature-dependent in this material. Then, we investigated the photophysical properties of different tetracene derivatives in solution, as well as illustrated the mechanism of triplet formation using excitation wavelength-dependent transient absorption spectroscopy. By providing a comprehensive analysis of the excited state dynamics, we showed excitation-dependent behaviour in a newly synthesized tetracene dimer and some monomers, displaying unique characteristics, along with the detection of ultrafast intersystem crossing triplet formation. Finally, we investigated the photophysical properties of a new synthesis macrocyclic parallel pentacene dimer. This dimer demonstrated an ultrafast intramolecular singlet fission process and selective generation of the quintet states. It also exhibits the longest room-temperature coherence time of a quintet state, to our knowledge at the time of publication, of 648ns.