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.

Organic/inorganic core shell nanoparticles have been synthesized using high K TiO(2) as the core nanoparticle, and polystyrene as the shell. This material is easy to process and forms transparent continuous thin films, which exhibit a dielectric constant enhancement of over 3 times that of bulk polystyrene. This new dielectric material has been incorporated into capacitors and thin film transistors (TFTs). Mobilities approaching 0.2 cm(2)/V.s have been measured for pentacene TFTs incorporating the new TiO(2) polystyrene nanostructured gate dielectric, indicating good surface properties for pentacene film growth. This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.

Ultraviolet absorbers such as Tinuvin P (2-(2-hydroxy-5-methylphenyl)benzotriazole), 1, achieve their exceptional photostabilities as a result of deactivation of excited singlet states through excited state intramolecular proton transfer (ESIPT). Adding a methyl group to the 6′ position of 2-arylbenzotriazoles reveals an additional excited singlet state deactivation mechanism in this class of molecules which does not require intramolecular hydrogen bonding. Steady state fluorescence and fluorescence lifetime measurements for a series of 6′-methyl-2-arylbenzotriazoles provides compelling evidence for a twisted intramolecular charge transfer (TICT) mechanism of excited singlet state deactivation. Due to the steric requirements of the 6′-methyl group, conformations are favored in which the phenyl and triazole rings are no longer coplanar. In the case of compound 11 (2-(6-methoxy-2,3-dimethylphenyl)-2H-benzotriazole), the presence of a 2′-methoxy group enhances nonplanarity and results in large deactivation rates. Compound 12 (2-(6-methoxy-2,3-dimethylphenyl)-5-(trifluoromethyl)-2H-benzotriazole), which possesses both twist and enhanced donor/acceptor properties, undergoes the most efficient fluorescence quenching for the methoxyarylbenzotriazoles. Compounds with both a 6′-methyl and a hydroxy group on the phenyl ring exhibit diffusion controlled quenching (k q ) 2 × 10 10 M -1 s -1) by DMSO. This quenching appears to result from either partial or complete excited state proton transfer to DMSO, which enhances TICT deactivation of the singlet excited state.

Computational and experimental studies have been performed to investigate the photostability of a series of 6,13-bis(arylalkynyl)-substituted pentacenes in the presence of oxygen. These studies indicate that photostabilization occurs through a selective LUMO orbital stabilization as has been seen previously for 6,13-bis(triisopropylsilylethynyl)pentacene. Marcus theory analysis suggests that the difference in vibrational reorganization energies across all compounds is small and that the thermodynamic driving force for forward electron transfer is primarily responsible for the observed photostabilization.