Effects of boryl, phosphino, and phosphonio substituents on optical, electrochemical, and photophysical properties of 2,5-dithienylphospholes and 2-phenyl-5-thienylphospholes

Abstract

The optical, electrochemical, and photophysical properties of Mes₂B- (Mes = 2,4,6-trimethylphenyl), Ph₂P-, or Ph₂MeP⁺-substituted 2,5-dithienylphospholes and 2-phenyl-5-thienylphospholes are reported. The Mes₂B- and Ph₂P-substituted derivatives were prepared via regioselective lithiation and metathesis at the thiophene rings of the corresponding unsubstituted π-systems. The Ph₂MeP⁺-substituted derivatives (phosphonium salts) were obtained via methylation of the Ph₂P-substituted π-systems. The optical and electrochemical data and density functional theory calculations showed that the highest occupied molecular orbital and lowest unoccupied molecular orbital energies of these α,α′-linked thiophene–phosphole π-systems were sensitive to the intrinsic nature of the B and P substituents. The fluorescence quantum yields (Φ_f) of the Ph₂MeP⁺-substituted derivatives varied considerably depending on the counter anion, concentration, and solvent, but the π–π* transition energies were unchanged. The emitting ability of the Ph₂MeP⁺-substituted 2,5-dithienylphosphole π-system changed significantly from Φ_f = 0.07 to 0.87. These findings were investigated using time-resolved fluorescence spectroscopy and fluorescence titration measurements. The results show that phosphonium iodides were present in the equilibrium mixtures of three or two emitting ion pairs in CH₂Cl₂. The light-emitting abilities of the Ph₂MeP⁺-substituted derivatives rely on heavy atom effects derived from the counter halide anions.