Probing intramolecular CH-π interactions in o-quinodimethane adducts of [60]fullerene using variable temperature NMR.

Several o-quinodimethane adducts of [60]fullerene were synthesized and their intramolecular aryl CH-fullerene π interactions were studied using variable temperature-NMR (VT-NMR). Evaluation of the rate constants associated with the first-order transition states for cyclohexene boat-to-boat inversions enables quantification of ΔG(‡) values for each inversion. A comparison between two constitutional isomers, only one of which is capable of intramolecular CH-π interactions, provides a lower limit of 0.95 kcal/mol for each aryl CH-fullerene π interaction.

Exploiting substituent effects for the synthesis of a photooxidatively resistant heptacene derivative.

Substituent effects have been exploited to produce an unusually persistent heptacene derivative. In total, four new heptacene derivatives with varying levels of photooxidative resistance (1 < 2 < 3 < 4) have been synthesized. A combination of p-(t-butyl)thiophenyl substituents at positions 7 and 16 (i.e., arylthio substituents attached to the most reactive ring) and o-dimethylphenyl substituents at positions 5, 9, 14, and 18 (i.e., steric resistance on neighboring rings) make heptacene derivative 4 especially resistant to photooxidation. It persists for weeks as a solid, for 1-2 days in solution if shielded from light, and for several hours in solution when directly exposed to both light and air. Heptacene derivative 4 has been fully characterized. It possesses a small HOMO-LUMO gap of 1.37 eV.

Substituent effects in pentacenes: gaining control over HOMO-LUMO gaps and photooxidative resistances.

A combined experimental and computational study of a series of substituted pentacenes including halogenated, phenylated, silylethynylated and thiolated derivatives is presented. Experimental studies include the synthesis and characterization of six new and six known pentacene derivatives and a kinetic study of each derivative under identical photooxidative conditions. Structures, HOMO-LUMO energies and associated gaps were calculated at the B3LYP/6-311+G**//PM3 level while optical and electrochemical HOMO-LUMO gaps were measured experimentally. The combined results provide for the first time a quantitative assessment of HOMO-LUMO gaps and photooxidative resistances for a large series of pentacene derivatives as a function of substituents. The persistence of each pentacene derivative is impacted by a combination of steric resistance and electronic effects as well as the positional location of each substituent. Silylethynyl-substituted pentacenes like TIPS-pentacene possess small HOMO-LUMO gaps but are not the longest lived species under photooxidative conditions, contrary to popular perception. A pentacene derivative with both chlorine substituents in the 2,3,9,10 positions and o-alkylphenyl substituents in the 6,13 positions is longer lived than TIPS-pentacene. Of all the derivatives studied, alkylthio- and arylthio-substituted pentacenes are most resistant to photooxidation, possess relatively small HOMO-LUMO gaps and are highly soluble in a variety of organic solvents. These results have broad implications for the field of organic molecular electronics where OFET, OLED, and other applications can benefit from highly persistent, solution processable pentacene derivatives.