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

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 (Φf) 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 Φf = 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.