Photochemical Stabilization of Pentacene and Substituted Pentacene in Solution and in Thin Films

The organic semiconductor pentacene has shown the highest thin film field effect mobilities of any organic semiconductor, yet it suffers from instability towards oxidation. 6,13-Bis(triisopropylsilylethynyl) pentacene (2) has been reported by Anthony et al. (J. Am. Chem. Soc. 2001, 123, 9482-9483) as a functionalized pentacene that is soluble in common organic solvents and exhibits high carrier mobility (>0.1) in thin film transistor devices. In our investigations of 2, we were surprised by its approximately 2 orders of magnitude more stable towards degradation in air saturated tetrahydrofuran solution than unsubstituted pentacene. Under ambient light conditions, oxygen has been implicated in the mechanism of photodegradation of pentacene. Alkynyl functionalization at the 6 and 13 positions is proposed to hinder photooxidation of 2 in the presence of oxygen, by reducing the rate of electron transfer from 2 to oxygen. In the solid state, unsubstituted pentacene is more stable to photooxidation than 2 in thin films exposed to UV radiation. The stabilization is proposed to be the result of interactions in the solid state that significantly reduce the excited state lifetime of pentacene.