Fallon, Kealan J.; (2017) Synthesis of Naturally Inspired Conjugated Materials for Organic Electronics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The development of indolonaphthyridine as a building block for organic electronics was investigated. Firstly, indolonaphthyridine was polymerised with a series of simple co-monomers affording semiconductors with narrow optical band gaps absorbing in the near-IR. The materials exhibited high performance charge transport in organic field-effect transistors (OFETs) and high power conversion efficiencies in organic photovoltaics (OPVs). The observed electron transport were some of the highest achieved in the field to date. Theoretical analysis revealed that the high charge transport properties arose from the high degrees of backbone planarity as a result of low degrees of rotational freedom between the indolonaphthyridine and the neighbouring thiophene moieties. A series of analogous polymers but with branching-point-extended alkyl chains was then investigated. It was found that the charge transport properties were both positively and negatively affected in the series. X-ray studies confirmed that the extended alkyl chains did permit closer interfacial interactions due to the decrease in d-spacing in the (010) plane. The concept of cross-conjugation was then explored as a strategy to produce wider band gap indolonaphthyridine polymers. It was found that cross-conjugation was a successful method for widening the optical gap and theoretical analysis revealed a LUMO delocalised along the polymer backbone. Testing in OFETs revealed the materials facilitated n-type transport, the first examples of n-type cross-conjugated organic polymers in the field to date. Finally, the low triplet energies of indolonaphthyridine was investigated in small molecules for their potential to undergo singlet fission. Several family of compounds were screened computationally and the best candidates synthesised. Electronic paramagnetic resonance (EPR) was performed on the samples and showed strong evidence for singlet fission in indolonaphthyridine thiophenes compounds, which exhibited high degrees of tunability and chemical stability, in stark contrast to the linear acenes which dominate the field.

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