High-performance n-channel carbonyl-functionalized quaterthiophene semiconductors: Thin-film transistor response and majority carrier type inversion via simple chemical protection/deprotection

The generalizable synthesis, comparative molecular physicochemical properties, film microstructures/morphologies, and field-effect transistor (FET) response characteristics of a series of six carbonylderivatized quaterthiophenes is described. These compounds are as follows: 5, 5‴-diheptanoyl-2,2′: 5′,2″:5″,2‴- quaterthiophene (1), spiro[4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4, 2′-[1,3] dioxolane], 2,6-bis-(5-hexyl carbonylthien-2-yl) (2), 2,7-[bis-(5-hexylcarbonylthien-2-yl)]-4H-cyclopenta[2,1-b:3,4-b′] -dithiophen-4-one (3), 5, 5‴-diperfluorohexylcarbonyl-2,2′: 5′,2″:5″,2‴-quaterthiophene (4), spiro[4H-cyclopenta[2, 1-b:3,4-b′]dithiophene-4,2′- [1,3]dioxolane], 2,6-bis-(5- perfluorohexylcarbonylthien-2-yl)(5), and 2,7-[bis-(5- perfluorohexylcarbonylthien-2-yl)]-4H-cyclopenta[2,1-b:3,4-b′] -dithiophen-4-one(6). Optical and electrochemical data demonstrate that terminal/central carbonyl-functionalization of the quaterthiophene core strongly lowers both HOMO and LUMO energies. However, the extent of LUMO lowering is far greater than HOMO lowering with the outcome that the carbonyl-containing quaterthiophenes exhibit lower energy gaps than the corresponding parent systems. This greater LUMO stabilization is confirmed by electrochemical data and fully explained by DFT computations. OTFT measurements show that all of the six semiconductors are FET-active, and very large n-type (up to 0.32 cm ²/Vs), p-type (up to 0.04 cm ²/Vs), and ambipolar (up to 0.12 cm ²/Vs for electrons, 0.008 cm ²/Vs for holes) mobilites are observed depending on the exact quaterthiophene backbone architecture. A simple Schottky injection barrier model in combination with molecular packing and thin-film molecular orientation/morphology characteristics of 1-6 explain the observed OFET performance trends. Finally, FET majority charge carrier inversion (p-type → n-type) via in situ chemical deprotection of the central carbonyl functionality (5 and 6) is demonstrated for the first time and is attractive for sensor functions as well as for patterning complementary circuits. The latter is demonstrated in a simple contact patterning process.