3.

(a) Absorption spectra of neutral BF compounds 1–3 in THF solution show prominent π → π^* transitions in the 19–22 000 cm^–1 region along with a series of weaker transitions in the 22–33 000 cm^–1 region. (b) Raman spectra (excitation λ 532 nm) show a decrease in CC stretch corresponding with increased twist angle. (c) Rate constants for E/Z-isomerization of 4 was measured by EXSY NMR (50–70 °C) and by DNMR line shape analysis (120–150 °C) to estimate a barrier.