Substituent effect on the ground- and excited-state torsional motions of pentiptycene-derived 1,4-bis(phenylethynyl)benzenes

The electronic absorption and emission spectra and fluorescence lifetimes of four pentiptycene-derived 1,4-bis(phenylethynyl)benzenes 2–5 and three segments of 2 (i.e., compounds 6–8) in MTHF at a variety of temperature between 80 and 300 K are reported. Compound 2 is an ethynylene-bridged pentiptycene trimer with octyloxy substituents at the two terminals. Compounds 3–5 differ from 2 only in the middle ring, which is a triptycene (3), a tetramethylphenylene (4), or the parent phenylene group (5). Despite the different ground-state conformer distribution in solutions, there is only a small difference in fluorescence properties for 2–5 at room temperature due to fast excited-state planarization reaction. However, in the solvent glass at low temperatures the planarization reaction is restricted with an extent depending on the middle-ring substituents. The well-resolved spectra and decay times for the planar and the twisted conformers of 2 allow one to deduce the excited-state planarization barrier (3.0 kcal/mol) and the overall phenylene torsional barrier (8.5 kcal/mol). These values are larger than those (0.47 and 3.54 kcal/mol, respectively) predicted by the AM1 and ZINDO/S calculations. The similarities in photophysical behavior between the twisted form of 2 and the model compounds 7 and 8 suggest that the former possesses large dihedral angles between the central and either one or both of the neighboring phenylene rings. Our results provide the first experimental approach toward the evaluation of the excited-state torsional barrier for the p-phenyleneethynylene systems.