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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - BORCOM (Borylated Conjugated Materials)

Teaser

Summary

Conjugated materials containing electron-rich donor (D) and electron-poor acceptor (A) units in a conjugated backbone have been widely used for organic electronic and optoelectronic devices, such as Organic Field Effect Transistors (OFETs), Organic Light Emitting Diodes (OLEDs) and Organic Photovoltaics (OPVs). The charge transfer between donor and acceptor units significantly reduces the band gap of such materials, leading to desirable properties such as high charge carrier mobilities in OFETs, near infrared emission in OLEDs and effective light harvesting in OPVs.

However, new strategies are essential to control the energies of the highest occupied or lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of such materials to effectively tune their electronic properties. Previous approaches have generally involved manipulation of the molecular framework of the donor and acceptor units using complex, multi-step synthetic procedures. Simpler, more efficient synthetic methods to reduce the band gap and tune the electronic properties are highly desirable. To this end transition metal free electrophilic borylation of aromatics and heteroaromatics has recently been developed at the University of Manchester. It has recently been extended to the fusion of D-A conjugated materials containing the acceptor moiety benzothiadiazole (BT) â€“ a group that is ubiquitous in organic electronics. The absorption and emission maxima of the borylated products are red shifted by over 100 nm from the starting materials; DFT calculations and electrochemistry show that this shift is primarily due to a lowering of the LUMO by more than 0.5 eV.

This simple two-step, one-pot procedure is applicable to a wide range of small molecules and polymers containing benzothiadiazole type acceptors. It leads to a significant increase in the electron affinity of the product and a considerable reduction in the band gap, consistent with near-IR emission in OLEDs, ambipolar OFET mobility, improved n-type stability and effective light harvesting in OPVs. The overall aim of BORCOM was to develop new molecules and polymers with higher electron affinities and reduced band gaps by functionalization of azole and azine containing acceptors in the conjugated backbone.

Work performed

In BORCOM we have extended the scope of the electrophilic borylation chemistry. We found that the reaction works well with flanking units to the BT group such as thiophene, simple arenes and substituted systems (e.g. alkylthiophenes, arenes) where the substituents are small or remote from the site of C-H activation. More sterically crowded systems such as those based on cyclopentadithiophenes do not work, as the directed C-H activation does not proceed. This work has lead to the synthesis of more than 10 new compounds and a better understanding of the benefits and the limitations of the electrophilic borylation approach. Extensive further work has successfully extended the scope of the reaction to conjugated polymers and also to new acceptor systems (e.g. benzoselenadiazole (BSe) and benzotriazole (BTz)). Precise control of the extent of borylation in conjugated polymers, from a few % to essentially quantitative functionalization, was achieved and more than 10 different polymers were prepared on an appropriate scale for device fabrication

The structure of all of the molecular and polymeric materials were characterised by multinuclear NMR and FT-IR spectroscopy, elemental analysis, mass spectrometry and GPC where appropriate. The optical properties of the materials were determined by absorption and photoluminescence spectroscopy including measurement of quantum efficiencies in solution. Cyclic voltammetry was used to determine the HOMO and LUMO levels and these values were correlated with DFT calculations of electronic structure. Thin films were prepared by spin coating to evaluate the use of the materials in devices. Fabrication of far red/near-IR OLED devices of the borylated BT materials by solution processing gave devices with encouraging performance using simple architectures, and further devices using more optimised structures and the new acceptors are currently in progress.

Final results

In the BORCOM project novel small molecules and polymers were prepared with higher electron affinities and reduced band gaps by a simple, high yielding chemical transformation conducted at room temperature. This borylation chemistry was able to reduce the band gap of a parent material by over 0.5 eV, resulting in materials that display very large Stokes shifts (>100 nm) and emission into the near-IR region that is of interest for biosensors and security applications. Measured solution and solid-state quantum efficiencies are very high for this region of the spectrum and initial OLED device performance is also promising. The results were presented at meetings across the globe and will form the basis of at least two publications in high-ranking international journals. The project results helped to facilitate a commercial license taken by the industrial collaborator CDT Ltd to intellectual property generated by the University of Manchester.