Synthesis and Photochemical Characterisation of Photoactive Compounds for Molecular Electronics

Abstract: In this modern age of technology, communication is highly reliant on computing devices such as mobile phones and computers. The advent of artificial intelligence (AI) and internet of things (IoT) as well as the widespread use of user-generated contents (UGC) are demanding more computing power and large data storage. However, traditional silicon based semiconductor technology is struggling to fulfill the required infrastructure due to fundamental physics and fabrication challenges. To overcome these challenges, molecular electronics has been proposed as potentially viable solution. With judicious design, organic molecules can be synthesized with tailored properties that can emulate the functions of conventional electronics devices such as diodes, switches and transistors. To this end, photochromic molecules attract significant attention since they can be switched with light/voltage between a less conducting and highly conducting forms. This property can be exploited to perform logic operations similar to transistors. This thesis explore the potential of norbornadiene-quadricyclane (NBD-QC)-based photochromic system, to serve as a switch for molecular electronics application. Four NBD derivatives, terminated with a thiol and thiophene groups, to enable tethering between gold electrodes, were synthesized. The compounds photochemical and photophysical properties were investigated using absorption and fluorescence spectroscopy. The results showed the compounds ability to switch between the NBD form and QC form upon photoirradiation. Moreover, the compounds were found to exhibit intrinsic emission. In particular, the long conjugated NBD form were found to be highly emissive, FF= 49%. Moreover, it was discussed that the emission can be tuned by the use of light, this makes them a potential candidate for optical memory device application. To test the robustness of the switching, more than 100 switching cycles were performed in solution and little or no degradation was observed, particularly under inert atmosphere. Additionally, the charge transport through the molecules were studied as well, using Scanning Tunneling Microscope-Break Junction (STM-BJ) technique. The results showed higher conductance values for the NBD forms and lower conductance values for the QC forms. Furthermore, we tested the potential of 2-nitrobenzyl-based photocleavable protection group (PPG) to release terminal alkynes on plasmonic surfaces by selective light activation. The terminal alkynes may then react, for example, with azido groups embedded on nanoparticles to create a dimer linked by a single molecule. By using the tools of template self-assembly the dimers can be aligned and placed on electrodes made by lithography. Initial findings showed promising result moving us closer to create single molecule devices based on parallel fabrication.