Spectroscopic Characterisation of Di-tert-butylrubicene as an Efficient and Photostable Singlet Fission Chromophore

Sammanfattning

This thesis investigates the spectroscopic properties and singlet fission (SF) capability of 5,12-di-tert-butylrubicene (tBuRc), a soluble and photostable derivative of rubicene that has been proposed as a promising chromophore for next-generation photovoltaic and quantum information applications. The work focuses on the characterisation of tBuRc in both monomeric solution and aggregated forms, including thin films and colloidal nanoparticles, to evaluate its suitability as an efficient SF material. The monomeric form of tBuRc in solution exhibited a fluorescence quantum yield of approximately 22% and a fluorescence lifetime of approximately 5.5 ns. The experimentally determined singlet and triplet excited-state energies (2.2 and 1.1 eV, respectively) met the energetic requirement for singlet fission, further confirming tBuRc as a viable candidate for SF. Compared with tetracene derivatives, tBuRc also demonstrated significantly enhanced resistance to photodegradation. Thin films fabricated through spin-coating and drop-casting displayed altered photophysical behaviour due to intermolecular aggregation. Femtosecond transient absorption measurements revealed the formation of triplet-state signatures at early times after excitation, strongly suggesting intermolecular singlet fission in the aggregated films. In particular, drop-cast films exhibited substantial fluorescence quenching and transient spectral features consistent with triplet generation, in agreement with previous reports on vapour-deposited tBuRc films. Colloidal tBuRc nanoparticles were successfully prepared using varying compositions of polystyrene and surfactant. While the nanoparticles showed aggregation-dependent spectral shifts and partial fluorescence quenching, their transient absorption behaviour remained largely monomer-like, with no definitive evidence of efficient singlet fission. The results indicate that the aggregate morphology within the tBuRc nanoparticles is, although confined and resulting in short intermolecular distances, not the necessary morphology that enables SF. Overall, this work demonstrates that tBuRc combines excellent photostability with promising singlet fission activity in thin-film aggregates, highlighting its potential as a robust alternative to conventional tetracene-based SF chromophores.