Minding the Gap: Synthetic Strategies for Tuning the Energy Gap in Conjugated Molecules

While structure–property relationships are commonly developed in applications of physical organic chemistry to real-world problems at the graduate level, they have not been generally emphasized in the undergraduate chemistry curriculum. For instance, the ability to modify the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in π-conjugated small molecules and polymers is of particular interest in the fields of polymer chemistry and materials science, where small-gap systems can function as semiconductors in electronic devices. In order to demonstrate a general strategy for tuning the HOMO–LUMO energy gap for π-conjugated materials, a new experiment was designed for the undergraduate organic laboratory course that combines synthesis with theoretical calculations for π-conjugated biphenyl systems with differing electronic properties to generate a simple structure–property study of functional group substitution on the biphenyl core. The synthesis of electron-rich and electron-poor biphenyl compounds was accomplished by students with a modified procedure from the literature that uses green Suzuki conditions to yield 4′-(tert-butyl)-[1,1′-biphenyl]-4-ol (1) and methyl 4′-(tert-butyl)-[1,1′-biphenyl]-4-carboxylate (2) with average yields of ∼50% and ∼60%, respectively. The students characterized their products with TLC, NMR, IR, and UV–vis (from a representative sample) spectroscopies. A comparison of the UV–vis absorption spectra from density functional theory calculations and experiment showed how a π-conjugated system’s HOMO–LUMO gap was tunable with structural modifications to the core, which illustrated how physical organic principles could be used to understand the electronic properties of a π-conjugated system.