Extremely large dipole moment in the excited singlet state of tris{[p-(N,N-dimethylamino)phenylethynyl]duryl}borane

Solvent effects on the spectroscopic and photophysical properties of tris{[p-(N,N-dimethylamino)phenylethynyl]duryl}borane (TMAB) and tris[(phenylethynyl)duryl]borane (TPhB) were studied in detail. Both TMAB and TPhB exhibited broad and structureless absorption and fluorescence bands ascribed to the charge transfer (CT) transition between the pi-orbital of the aryl group (pi(aryl)) and the vacant p-orbital on the boron atom (p(B)): pi(aryl)-p(B) CT. The fluorescence spectra of TMAB and TPhB were dependent on a solvent polarity, and the Stokes shifts of the compounds (Deltanu(S)) were shown to correlate linearly with a solvent polarity parameter: f(X) = (D(s) - 1)/(2D(s) + 1) - (n(2) - 1)/(2n(2) + 1) where D(s) and n are the dielectric constant and refractive index of a solvent, respectively. The slope value of the Deltanu(S) versus f(X) plot gave the excited singlet state dipole moment of TMAB or TPhB to be approximately 60 or approximately 30 D, respectively, demonstrating that the excited singlet state of TMAB possessed an extremely large dipole moment. The fluorescence quantum yields (Phi(f)) and lifetimes (tau(f)) of the compounds were also dependent on f(X). Detailed analysis of the present results demonstrated that the photophysical properties of TMAB were governed by the magnitude of the fluorescence rate constant (k(f)) and the k(f) value decreased almost one-tenth with an increase in f(X), which was the primary origin of the f(X) dependences of Phi(f) and tau(f). The results were discussed in terms of the characteristic pi(aryl)-p(B) CT excited states of the derivatives.