Hemibiquinones: synthesis and computational validation of an asymmetric d—a biphenyl system
In order to help determine the governing characteristics behind asymmetric current flow in molecules (molecular rectification), it is important to establish size limitations for the electron transfer processes which make rectification possible. Here I present the rational design and synthesis of a molecular template which has been tested for its rectification properties, alongside important derivatives. The molecule template in question, hemibiquinone (HBQ), is an asymmetric biphenyl derivative composed of a dimethoxybenzene ring covalently bonded to a benzoquinone (2,5-cyclohexadiene-1,4-dione) ring. It contains a 2-position group allowing the molecule to self-assemble. Early recognition that the molecule already possessed electroactive donor (dimethoxybenzene) and acceptor (benzoquinone) sections led the author to hypothesize that the torsion angle between the rings of biphenyl is orbital isolating enough to be the requisite tunneling barrier necessary for unimolecular rectification. Spectroscopic and electrochemical data are presented to validate predictions made by Density Functional Theory. A monolayer of the molecule sandwiched between gold is found to rectify with a forward/reverse current ratio approaching 200. This result demonstrates that larger, saturated carbon bridges are not necessary components in the design of a molecular diode. Any way of breaking conjugation in the system will suffice. Beyond establishing a lower limit for D-σ-A rectifiers, this work also lays the foundation to experimentally test how properties such as polarity, torsion angle, end group effects and HOMO-LUMO gap energy affect the rectification efficiency for a given structure. Future work will focus on the systematic change of functionality to the hemibiquinone motif, and relating the measured conduction and rectification ratio to one another.