Charge Transporting Supramolecular pi-Donor/Acceptor Arrays Based on Redox-Active Metallacycles, Metal-Organic Cages, and Daisy-Chain Coordination Polymers

With the support from the Macromolecular, Supramolecular, and Nanochemistry (MSN) program in the Division of Chemistry, Sourav Saha of Clemson University will be developing charge-conducting and light-harvesting supramolecular assemblies consisting of precisely organized electron donor and acceptor units confined to supramolecular containers. Electron donor and acceptor aromatic molecules contain conjugated pi-systems that allow electron delocalization through bonds via resonance as well as through space when they are cofacially pi-stacked like a deck of cards.

In addition, electroactive aromatic molecules often absorb and emit light, thereby displaying fascinating optical properties. When electron rich (pi-donors) and deficient (pi-acceptor) pi-systems are organized in an alternating fashion, their ability to conduct electrical current is further accentuated due to facile through-space electron delocalization from the donor to the acceptor units via charge-transfer and even partial electron transfer interactions.

In this research conducted by the Saha group at Clemson, repeating electron donor and acceptor units will be stacked by encapsulating them within specifically designed supramolecular containers. The containers will be constructed using platinum metal ions and organic linkers to precisely control their dimensions and electronic and optical properties.

As such, they will accommodate pi-donor and acceptor units of different sizes to regulate their three-dimensional ordering and orientation. The results generated by this work can potentially to lead to new class of semiconductors, transistors, light-harvesting systems, and electrochromic and thermochromic materials. Dr.

Saha will work to train and mentor diverse groups of undergraduate and graduate students, provide research experience to high school seniors through Clemson’s EUREKA! program, and offer career guidance and motivation to largely minority students from Georgia Southern University to pursue graduate education through a ‘Be a Grad Student for a Day’ initiative. In addition, local outreach programs at Beck Academy Middle School and Roper Mountain Science Museum will be conducted to engage K-12 students and encourage them to consider pursuing higher education in STEM (science, technology, engineering and mathematics) fields.

This research will focus on the synthesis, assembly, and structure–function relationship studies of robust supramolecular pi-donor/acceptors arrays using redox-active multicomponent metallacycles, metal-organic cages, and daisy chain coordination polymers as supramolecular containers that can encapsulate redox-complementary guest pi-systems inside their cavities. Tricomponent rectangular metallacycles and tetragonal prismatic metallacages consisting of two parallel redox-active faces and orthogonal linkers with variable lengths connected by heteroligated platinum corners will be employed as supramolecular containers to encapsulate redox-complementary guests.

The containers and their guest-inclusion complexes will be coordinatively linked to assemble daisy-chain polymers. Emphasis will be placed on controlling the formation of discrete pi-donor/acceptor stacks through systematic variations of the cavity size of the hosts and the redox properties of the hosts and guests. The structures and compositions of inclusion complexes will be determined using a suite of crystallographic, spectroscopic, and electron microscopic techniques.

Systematic studies will be conducted to examine how the host/guest and guest/guest charge-transfer and electron-transfer interactions will influence the color, emission, bandgaps, and charge transport capability of the resulting inclusion complexes. This work, if successful, will develop a novel bottom-up strategy to assemble robust supramolecular pi-donor/acceptor stacks using redox-active supramolecular containers and pave way to their applications as supramolecular wires, semiconductors, and light-harvesting materials in future electronic and photovoltaic devices.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.