Photochemical Stability of Pentacene and a Substituted Pentacene in Solution and in Thin Films

The organic semiconductor pentacene (1) has shown the highest field effect mobilities in thin films of any organic semiconductor, yet suffers from instability toward oxidation. 6,13-Bis(triisopropylsilylethynyl) pentacene (2) has been reported as an interesting functionalized pentacene which is soluble in common organic solvents and exhibits high carrier mobility (>0.1 cm2/Vs) in thin film transistor devices. In our investigations of 2, we were surprised by its remarkable stability in solution. Using UV-vis spectroscopy we observe that under ambient light conditions, 2 is approximately 50× more stable toward degradation in air-saturated tetrahydrofuran solution as compared to unsubstituted pentacene. Previous investigators have implicated oxygen in the mechanism of photodegradation of pentacene. In this study, quantum chemical calculations have been performed which demonstrate that alkynyl functionalization at the 6 and 13 positions reduces the rate of photooxidation in two ways. First, alkynyl substitution reduces the triplet energy of 2 considerably, thereby preventing singlet oxygen sensitization. Second, alkynyl substitution lowers the LUMO energy for 2 as compared to that of pentacene. We propose that the lower LUMO energy hinders photooxidation by reducing the rate of electron transfer from photoexcited 2 to oxygen. In thin films, pentacene is more stable to photooxidation than 2 when exposed to UV irradiation. The stabilization of pentacene in the solid state is discussed in the context of solid-state interactions.