Two-dimensional structural motif in thienoacene semiconductors: Synthesis, structure, and properties of tetrathienoanthracene isomers

Stille coupling of 2- and 3-(tributylstannyl)thiophene derivatives with tetrabromobenzene followed by oxidative cyclization provides a simple, two-step synthetic route to two isomeric tetrathienoanthracene structures (13 and 14). The materials are characterized by a remarkable thermal stability, both in air (T-dec approximate to 400 degrees C) and under nitrogen (sublimed with no decomposition at 450-500 degrees C). Optical studies of the parent and alkylated compounds showed sky-blue photoluminescence with quantum yields ranging between 0.17 and 0.40 and Stokes shifts of 0.03-0.16 eV. Both the optical properties and electrochemical behavior depend strongly on the position of the heteroatoms. Structural studies with X-ray crystallography (for 3D single crystals) and scanning tunneling microscopy (for 2D monolayers) indicate a high level of order, with similar intermolecular interactions for both isomers. The alkylated materials 13b and 14b have been used to fabricate thin-film transistors by both vacuum evaporation and solution processing. The preliminary studies show (unoptimized) hole mobilities of up to 7.4 x 10(-2) cm(2) V-1 s(-1) and very high on/off ratios of up to 1 x 10(8) for vacuum-deposited films. The isomer 13, which showed higher thermodynamic stability of the radical cation and stronger intermolecular S center dot center dot center dot S contacts, also revealed better device performance.