Tuning the Optical Properties of 1,1′‐Biphospholes by Chemical Alterations of the P–P Bridge

2,5‐Di(1‐naphthyl)‐1‐phenylphosphole ( 2 ) was prepared from phenylphosphine and dinaphthylbutadiyne in 70 % yield, and oxidized to its oxide 3 . Treatment of 2 with a solution of potassium naphthalide results in selective formation of phospholide anion 4 , which can be oxidized with iodine to afford biphosphole 5 . The crystal structure of 5 exhibits an unusual s‐ trans conformation across the P–P bridge that is most stabilized by π–π stacking of the naphthyl substituents. Biphosphole 5 can be oxidized to its mono‐oxidized 6 and dioxide 7 . Generally, all phosphole oxidations lead to decreased pyramidalization of the P centres and, consequently, to significantly redshifted lowest‐energy absorption maxima. Mono‐oxidized biphosphole 6 , which consists of an ordinary phosphole and an electron‐deficient oxidized phosphole, features a unique low‐energy optical transition (shoulder at 430 nm), which is tentatively assigned to a charge‐transfer transition. The UV/Vis spectrum of dioxide 7 lacks this transition. At the same time, the emission maximum of 7 is bathochromically shifted by 80 nm relative to that of 5 .