Design and synthesis of phosphole-containing donor-acceptor chromophores and their gold(I) complexes : from photophysics to organic electronics

Abstract

A library of dithienophosphole (DTP)-containing compounds with a donor−acceptor−donor structure has been synthesised and characterised. This class of compounds was found to exhibit broad and strong absorption in the visible light region and display intense yellow emission with high photoluminescence quantum yields of up to 0.68. As reflected by the photophysical and electrochemical studies, these DTP compounds were found to possess strong intramolecular charge transfer (ICT) character, which was further supported by the solvatochromic behaviours of the compounds. Together with the good thermal properties, vacuum-deposited organic photovoltaic (OPV) devices as well as organic light-emitting devices (OLEDs) have been realised with the selected phosphole compounds. By varying the overall planarity and the donor strength, satisfactory performances for OPV devices and OLEDs based on these compounds have been accomplished. These findings demonstrated the bifunctional properties of this class of dithienophosphole compounds and represented for the first time the use of phosphole-based small molecules as donor materials for the vacuum-deposited OPV devices.

Another class of solution-processable DTP-cored compounds with donor-acceptor-decorated π-bridges, multiple solubilising alkyl chains and strongly electron-deficient terminal units of octyl cyanoacrylate or 3-ethylrhodanin-5-ylidene has been prepared and characterised. These compounds were found to be intensely coloured and to possess good film-forming property. The photophysical, electrochemical and thermal properties of the compounds have been studied. In the thin-film absorption studies, the compounds displayed intense and broad absorptions spanning across the low-energy region of the visible light spectrum. The ICT characters of these π-extended DTP compounds were established from photophysical, solvatochromic and computational studies. Solution-processed resistive memory devices and OPV devices based on this class of compounds were fabricated by the spin-coating method. Interestingly, in the memory devices, the number of conductance states were found to be altered by the changes in molecular structures. With the correlation of the extent of ICT characters of the compounds, current−voltage characteristics of the devices and the fitting with theoretical conduction models, the tri-stable memory behaviour was believed to be contributed by the extra metastable field-induced charge-transfer state in the compounds with stronger ICT characters.

To explore the potential of phosphole-containing gold(I) complexes as functional electronic materials, a class of benzophosphole compounds and their related alkynylgold(I) complexes have been synthesised and characterised. The 2-aryl substituent of the phosphole unit and the aromatic ring of the alkynyl ligand have been varied with different donor or acceptor moieties. Interesting photophysical behaviours, such as temperature-responsive emission, have been observed and studied in some of the compounds. Selected compounds have been utilised in fabricating solution-processed resistive memory devices. The electronic behaviours of the two ligands of the gold(I) system were both found to be crucial for facilely tuning the donor-acceptor interactions and hence modulating the resultant memory performances, in which the lowest switching threshold value of 1.3 V has been realised in the device based on an alkynylgold(I) complex with donor and acceptor moieties located at the phosphole and alkynyl ligand respectively.