Benzo[b]phosphole-Containing π-Electron Systems: Synthesis Based on an Intramolecular trans-Halophosphanylation and Some Insights into Their Properties

The intramolecular trans-halophosphanylation of 2-(aminophosphanyl)phenylacetylenes mediated by PBr3 followed by the oxidation with H2O2 produces 3-bromobenzo[b]phosphole oxide derivatives. This cyclization is also used for the synthesis of a 3-iodo derivative by conducting the reaction in the presence of LiI. Based on this synthetic method, various benzophosphole-containing pi-conjugated compounds, including a phosphoryl and methylene-bridged stilbene 10, 2,3,6,7-tetraphenylbenzo[1,2-b:4,5-b']diphosphole-P,P'-dioxides 11, and their phosphine sulfide derivatives 12, are synthesized. The study of the structure-property relationships in a series of the bridged stilbenes, including a bis(methylene)-bridged stilbene 10, and a bis-(phosphoryl)-bridged stilbene, reveals that as the contribution of the phosphoryl groups increased, the absorption and emission maxima substantially shift to longer wavelengths. The intrinsic substituent effects of the phosphoryl group in this skeleton are to decrease the oscillator strength of the electronic transition and thus decrease the radiative decay rate constants from the singlet excited state. Nevertheless, these compounds maintain high fluorescence quantum yields (Phi(F) > 0.8) owing to the significantly retarded non-radiative decay process. In the study of the benzodiphosphole derivatives 11 and 12, their cyclic voltammetry revealed that both of the phosphoryl and phosphine sulfide derivatives have low reduction potentials (-1.7 to -1.8 V vs ferrocene/ferrocenium couple) with the high reversibility of the redox waves. These compounds also showed high thermal stabilities with the high glass transition temperatures of 147-159 degrees C, indicative of their potential utilities as amorphous materials.