Empty Level Structure of Boryl-Substituted Pentacyclic Heteroaromatics

Extended π-conjugation in molecular systems is being extensively exploited in a rapidly expanding range of electronic and photonic applications. The modification of such extended systems through the use of heteroatoms allows their tailoring to specific requirements. In particular, the use of the vacant boron pπ orbital can increase the electron affinity of the extended π system in an analogous fashion to the p-doping of crystalline silicon used in more classical microelectronic devices. Experimental data on such modifications of the empty level structures of boron-containing π-electron materials leading to an increase in their electron affinity are not available in the literature. Using Electron Transmission Spectroscopy, the energies of the vertical anion formation of 5-methyl-2-furanboronic acid pinacol ester (1) and 4-methyl-3-thiopheneboronic acid pinacol ester (2) are measured and compared with those of the reference unsubstituted heteroaromatics furan and thiophene. The results are interpreted with the support of density functional theory (DFT) calculations with the B3LYP functional. DFT calculations are also used to predict the effects of boryl substituents on the electronic and geometrical structures of trans and cis thienyl thiazole.