Photophysical properties of coplanar and twisted 1,4-bis(9-ethynylanthracenyl)benzene. Rotational equilibration in the excited states of diaryalkynes

We report the synthesis and photophysical properties of 1,4-bis(9-ethynylanthracenyl)benzene. Results in dilute hexane solutions give a very high fluorescence quantum yield (Φ = 0.97) and a lifetime of τ = 1.8 ns. Studies at different temperatures, viscosities, and micromorphologies, as well as excited-state semiempirical calculations, indicate that conformations having aryl groups coplanar absorb and emit at wavelengths that are longer than those that have their aryl groups orthogonal or twisted. A simple MO picture is consistent with essentially frictionless aryl group rotation in the ground state and a relatively high rotational barrier in cummulene-like excited states.