Indolonaphthyridine materials for singlet fission were systematically investigated. Firstly, cibalackrot was investigated as a singlet fission material. Single crystals of cibalackrot revealed insufficient electronic coupling was inhibiting singlet fission in the solid-state. A synthetically modified aza-cibalackrot was developed that had enhanced electronic coupling and exhibited singlet fission. Throughout the synthetic journey we developed a novel crystal engineering strategy that can be used to “turn on” singlet fission.

Following this, the mechanism of indolonaphthyridine intramolecular singlet fission was investigated. A series of indolonaphthyridine dimers were synthesised that were conjugated using different bridging groups. The rate of singlet fission was found to be highly dependent on the extent of electronic coupling. Charge-transfer states were also found to mediate singlet fission. We revealed the traditional design principles for singlet fission dimers cannot be applied to modern polarisable systems.

The relationship between molecular symmetry and charge-transfer character was then explored. A series of novel asymmetric indolonaphthyridines were synthesised that exhibited enhanced excited state charge-transfer character. Novel, high yielding bay-annulation chemistry was developed to synthesise the asymmetric indolonaphthyridines. The structureproperty relationships introduced in this chapter could potentially be used to develop high performance optoelectronic materials.

Lastly, the electronic structure and charge-transport properties of ultra-narrow band gap materials was investigated. Using novel polymerisation chemistry, indolonaphthyridine was co-polymerised with thiadiazole quinoxaline yielding a polymer with an exceptionally narrow band gap. Charge transport measurements revealed the polymer had ambipolar mobility. Interestingly, the polymer also showed open-shell character.