The development of photoluminescent aromatic diimides (ADIs) remains an ongoing point of interest for research because of their potential to benefit our realisation of more advanced chemosensing, (bio)chemical diagnostics, functional nanomaterials and organic display technologies. With the exception of intrinsically emissive perylene diimides, smaller and more energy-dense ADIs such as benzene and naphthalene diimides are not naturally emissive and have traditionally required installation of electron-rich donor substituents on their aromatic cores to induce favourable charge distributions to “turn on” and “tune” photoluminescent properties. Other strategies to overcome this fundamental challenge have explored the embedding of electronegative (i.e., lone-pair possessing) heteroatoms to achieve light-emitting properties, as well as manipulating the degree of π-conjugation within ADI systems to tune their donor–acceptor charge-transfer and excited states.
This Thesis investigates similar π-donor conjugation approaches to induce and control the photoluminescence properties in two novel class of ADI-based redox-active molecules: (i) non-linear heteroaromatic diimides referred to as isoindole diimides (IDIs) and (ii) donor-functionalised [5]helical non-planar aromatic diimides (H). In Chapter 2, IDIs are established as a new class of inherently emissive aromatic diimides with PLQYs up to 0.93, that are further explored for their optical tunability via core functionalisation with substituents of different nature including aliphatic and aromatic moieties, polar groups to induce intramolecular interactions, and ultimately, development of dyads with electron-rich chromophores to discern their photoluminescence mechanism. Experimental and theoretical studies revealed IDIs to switch between locally-excited (LE) and charge transfer (CT) characteristic-S1 state depending on the core substituent with pathologically low T1 state for all, affording fluorescence via aggregation or intramolecular charge transfer (ICT). In the latter Chapters, ADI-based helicenes are functionalised with pyrene (Chapter 3) and pyridine (Chapter 4) to explore the influence of π-conjugation and heteroatom incorporation, respectively, towards “switching on” and in turn, accessing energy-efficient photoluminescence via intra/inter-molecular (excimeric) interactions and/or charge transfer in the excited states. Notably, upon pyridine substitution, not only are the helicenes found to be emissive, but also stimuli-responsive to produce white-tunable emission, which remains a current challenge for single-molecule organic light emitters. These advances in ADI chemistry enables access to a wide range of functional organic molecules in addition to offering a deeper understanding of the fundamental molecular interactions at play in affording energy-efficient photoluminescent properties that can be applied towards the development of advanced light-emitting technologies paving the way to a brighter, more sustainable future.
