Periodic Reporting for period 1 - BORCOM (Borylated Conjugated Materials)
Reporting period: 2015-07-01 to 2017-06-30
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) – 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.
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.