2,3-Diphenylphenanthro[9,10-b]furan Derivatives as New Blue Fluorophores

A series of 2,3-diphenylphenanthro[9,10-b]furans were prepared by the reactions of phenanthrene-9,10-dione with two equivalents of benzylidenetriphenylphosphoranes as a key step. The bis(4-bromophenyl) derivative can be readily lithiated by the action of n-butyllithium in diethyl ether. The corresponding 4-trimethylsilyl and 4-methylthio derivatives were obtained by quenching the dilithio derivative with trimethylsilyl chloride and dimethyl disulfide, respectively. The Buchwald-Hartwig amination of the bromide with diphenylamine afforded the diphenylamino derivative in a good yield. Crystallographic analysis of the parent diphenyl derivative reveals that the phenanthrofuran moiety has a slightly twisted helicene-like structure, and the compound forms a columnar stacking in the crystal. The phenanthrofurans display intense blue fluorescence both in CH2Cl2 and in the solid state. Their electrochemical properties obviously indicate high HOMO energy levels of the furans, as predicted by density functional theory calculations.