Studies of viologens, extended viologens, and triphenylmethyl radicals as isostructural dopants for molecular semiconductors

Almost all inorganic semiconductor devices are made with doped semiconductors. Semiconductors are doped by replacing a small amount of atoms with atoms with one more or one less valence electron. Organic semiconductors are doped by the interstitial addition of a strong oxidizing agent or a strong reducing agent. Interstitial dopants in organic semiconductors easily migrate and are a fundamental problem for molecular semiconductors.Isostructural doping is a potential method for doping molecular semiconductors. An isostructural dopant is a molecule that has the same skeletal structure as the molecular semiconductor, but has one or more atoms replaced by an atom of a different valence while maintaining charge neutrality of the molecule. For example, an n-dopant is created by replacing one or more carbon atoms within the molecular semiconductor with an atom that contains one more valence electron than carbon such as nitrogen. This dissertation presents the study of several isostructural n-dopants.First, the neutral form of phenyl viologen was isolated and characterized for the first time, and it was then investigated as an isostructural n-dopant for quaterphenyl. Although phenyl viologen could be substitutionally co-crystallized with quaterphenyl, the co-crystals exhibited no measurable electrical conductivity. Solution-phase cyclic voltammetry measurements showed that neutral phenyl viologen is not a strong enough reducing agent to dope quaterphenyl.Next, a neutral "extended viologen" isostructural with 4,4""-di- n-octyl-p-quaterphenyl was synthesized and studied as a potential isostructural dopant. Although the neutral extended viologen is the most reducing neutral organic molecule that has been isolated, it did not act as an n-dopant in 4,4""-di-n-octyl-p-quaterphenyl. Single-crystal X-ray diffraction indicated that the neutral extended viologen has a significant diradical contribution to its electronic structure.Finally, a pair of triphenylmethyl radical derivatives, tris(4-nitrophenyl)methyl and tris[(4-dimethylamino)phenyl]methyl, were synthesized because it was anticipated that they were close enough in structure to allow substitutional co-crystallization and that the tris(dimethylamino) derivative would spontaneously transfer an electron to the trinitro derivative. Samples of tris(4-nitrophenyl)methyl doped with tris[(4-dimethylamino)phenyl]methyl were electrically conductive, with conductivity increasing with doping fraction, and powder X-ray diffraction confirmed the substitutional co-crystallization.