Design, synthesis and functionalization of organic π-conjugated donor-acceptor chromophores : from photophysics to optoelectronic application studies

Abstract

A series of two dimensional donor-acceptor enediyne-containing Pi-conjugated organic small molecules have been successfully synthesized and characterized. The structures of all the compounds have been characterized by 1H NMR, mass spectroscopy and elemental analysis. Different types of electron-donating and electron-accepting moieties with various electronic nature have been successfully introduced into the target molecules in order to modulate the photophysical and electrochemical properties of this class of compounds. The photophysical properties of the series of compounds have been studied by UV-vis absorption and emission spectroscopy. The electrochemical properties have been investigated by cyclic voltammetry and differential pulse voltammetry, while the thermal stability has been examined by thermogravimetric analysis. This class of compounds exhibited extended absorption window into the NIR region with high extinction coefficients in the order of 104 dm3 mol-1cm-1. Solvent-dependent UV-vis absorption studies showed that there were small hypsochromic shifts of the lowest-energy band maxima with increasing solvent polarity and linear relationship between the wavenumber maxima of this band and the Dimroth’s solvent parameter with rather gentle slopes, indicatingthe rather weak dipole moment associated with the ground state. All the compounds showed structureless emission bands in the range of 500 to 900 nm. The emission maxima of this class of compounds, on the other hand, showed large bathochromic shifts with increasing solvent polarity and linear relationships in the Lippert-Mataga plots with steep slopes of over 6000 cm-1 indicating a larger dipole moment in the excited state when compared to the ground state of this class of compounds. Together with large Stoke shifts of the compounds, the structureless bands of these compounds were believed to be of intramolecular charge transfer in nature. All target compounds exhibited high thermal stability with decomposition temperature of over 328 oC. The HOMO levels of this class of compounds were in the range of –5.69 to –5.16 eV while the LUMO levels were between –3.72 and –3.44 eV. The potential application of this class of compounds as active materials in solution-processable organic resistive memory devices has been explored by studying selected representative compounds. Binary logic was observed for devices based on these derivatives with switching threshold voltage of ca. 2.0 V. Current ratios of 1:106were exhibited between the “OFF” and “ON” conductivity states, which would be desirable for ensuring a low probability of signal misreading. The surface morphology of the thin films of the selected derivatives has also been investigated by AFM, showing smooth surfaces with small root-mean-square roughnesses.