Final Report Summary - SUPRAL_SAS (Supramolecular active layer, self-assembly on surface)
The aim of this project was to find general methods for facile access to supramolecular n-p heterojunction (silicon-heterojunction (SHJ)) photosystems with a view to the future improvement of the architecture of photovoltaic (PV) devices. The desired SHJ structure had to efficiently direct the photogenerated charges (electrons and holes) in opposite directions along transporting channels that are separated and aligned on the molecular level. In order to build molecular pathways for electron and hole, the Matile group has designed the self-organising surface-initiated polymerisation (SOSIP). Here, naphthalenediimide (NDI) stacks (and only recently also perylenediimide (PDI) and oligothiophene stacks) were grown from a solid surface and covalently fixed by disulfide bond (S-S) formation. The resulting molecular assembly (electron-acceptor, n-type channel) possessed hydrazide functionality for dynamic covalent capture of electron-donating units to introduce the second charge transport channel (p-type channel). To this purpose, electron-rich NDIs, arylmines and oligothiophenes have been successfully incorporated in SOSIP NDI stacks, however a common component for molecular electronics, such as phthalocyanine (PC), was not previously explored in the host group.
While pristine PCs are widely used in organic electronics, the synthesis of structurally modified PCs is still an open challenge. Their poor solubility is problematic for the purification of asymmetric multi-functional PC derivatives. Nevertheless, PCs are particularly interesting in this context because their intense absorption in the visible region usually centred around 620 - 700 nm (Q band), their capacity to self-organise into highly ordered supramolecular structures through pi-pi stacking interactions, and their ability to undergo charge transfer when photo-excited (electron-donor, p-type material).
Preparation of PC-based SHJ architectures required, amongst the other things, the familiarisation with self-organising surface initiated polymerisation method developed in the host group, and the design, synthesis and characterisation of PC-based units for the stack exchange approach. In order to obtain the desired SHJ structures, PCs were arranged in conductive assemblies by means of NDI-based scaffoldings. The latter, were prepared starting from initiator 1 and propagator 2. Both units contained structurally supporting 'peptide' side chains, polymerisable disulfides, and hydrazides for dynamic covalent capture of electron-donating units to introduce the second charge transport channel (p-type). The initiator further contained two lateral NDI spacers and four diphosphonate functionaliies to bind to an indium tin oxide surface. Treatment of the initiator 1 with dithiothreitol (DTT) reduced the disulfate functionalities to thiolates. These thiolates then were reacted with the strained disulfides in the propagator to undergo ring-opening disulfide polymerisation. The resulting NDI assemblies showed poor photoactivity as charge-transport channels (n-type). To introduce a lateral stack for the transport of holes, the benzaldehydes were removed with hydroxylamine, and reacted with aldehydes of PCs and porphyrins (PC analogue) of free choice.
In order to minimise the recombination between the charges in the conductive pathways, two approaches were engineered and investigated: oriented multicoloured antiparallel redox gradient (OMARG) and lateral multiple channel. The first method, OMARG, makes use of an organised molecular assembly to generate orthogonal redox gradient channels, while the second method made use of electron donating and accepting dyads with variable distance between n- and p-lateral transport channels. The resulting findings were published in Chem. Sci., in a compressive review in Org. Biomol. Chem., and in J. Am. Chem. Soc. (manuscript submitted).
The outcomes of this project are expected to increase the prospects of using PC and supramolecular n-p heterojunction in more practical applications. It is plausible to think that these results could benefit not only the scientific community at the academic level, but also companies and industries for the design and fabrication of optoelectronic materials. Moreover, the molecular assemblies on surface investigated in this project, may even be of interest for advanced organic-based transistors, which commonly find application in displays, sensors and electronic barcodes.
While pristine PCs are widely used in organic electronics, the synthesis of structurally modified PCs is still an open challenge. Their poor solubility is problematic for the purification of asymmetric multi-functional PC derivatives. Nevertheless, PCs are particularly interesting in this context because their intense absorption in the visible region usually centred around 620 - 700 nm (Q band), their capacity to self-organise into highly ordered supramolecular structures through pi-pi stacking interactions, and their ability to undergo charge transfer when photo-excited (electron-donor, p-type material).
Preparation of PC-based SHJ architectures required, amongst the other things, the familiarisation with self-organising surface initiated polymerisation method developed in the host group, and the design, synthesis and characterisation of PC-based units for the stack exchange approach. In order to obtain the desired SHJ structures, PCs were arranged in conductive assemblies by means of NDI-based scaffoldings. The latter, were prepared starting from initiator 1 and propagator 2. Both units contained structurally supporting 'peptide' side chains, polymerisable disulfides, and hydrazides for dynamic covalent capture of electron-donating units to introduce the second charge transport channel (p-type). The initiator further contained two lateral NDI spacers and four diphosphonate functionaliies to bind to an indium tin oxide surface. Treatment of the initiator 1 with dithiothreitol (DTT) reduced the disulfate functionalities to thiolates. These thiolates then were reacted with the strained disulfides in the propagator to undergo ring-opening disulfide polymerisation. The resulting NDI assemblies showed poor photoactivity as charge-transport channels (n-type). To introduce a lateral stack for the transport of holes, the benzaldehydes were removed with hydroxylamine, and reacted with aldehydes of PCs and porphyrins (PC analogue) of free choice.
In order to minimise the recombination between the charges in the conductive pathways, two approaches were engineered and investigated: oriented multicoloured antiparallel redox gradient (OMARG) and lateral multiple channel. The first method, OMARG, makes use of an organised molecular assembly to generate orthogonal redox gradient channels, while the second method made use of electron donating and accepting dyads with variable distance between n- and p-lateral transport channels. The resulting findings were published in Chem. Sci., in a compressive review in Org. Biomol. Chem., and in J. Am. Chem. Soc. (manuscript submitted).
The outcomes of this project are expected to increase the prospects of using PC and supramolecular n-p heterojunction in more practical applications. It is plausible to think that these results could benefit not only the scientific community at the academic level, but also companies and industries for the design and fabrication of optoelectronic materials. Moreover, the molecular assemblies on surface investigated in this project, may even be of interest for advanced organic-based transistors, which commonly find application in displays, sensors and electronic barcodes.