Kinetically stabilized 1,3-diarylisobenzofurans and the possibility of preparing large, persistent isoacenofurans with unusually small HOMO-LUMO gaps

We show that isoacenofurans of any size possess smaller HOMO-LUMO gaps than the corresponding acene bearing an isoelectronic p-system (i.e., the same total number of rings). Large isoacenofurans show limited stability due in part to the highly reactive 1,3-carbons of the furan ring. Placing sterically congesting phenyl substituents bearing ortho alkyl groups at the 1,3-positions of 1,3-diarylisobenzofurans force these molecules to adopt non-planar conformations with the ortho alkyl groups above and below the most reactive 1,3-carbons of the furan ring. These bulky substituents provide a strong measure of kinetic stabilization. Thus, 1,3-dimesitylisobenzofuran and 1,3-di(2’,4’,6’-triethylphenyl)isobenzofuran are at least 500 times less reactive than 1,3-diphenylisobenzofuran in their reactions with the strong dienophiles DMAD and acrylonitrile. The insights gained here suggest that the synthesis of large, persistent, kinetically stabilized isoacenofurans with unusually small HOMO-LUMO gaps is within reach. As such, these molecules deserve increased attention as potential p-type organic semiconductors.