Solubility of N,N′-Di(1-naphthyl)-N,N′-diphenyl Benzidine (NPB) in Various Organic Solvents: Measurement and Correlation with the Hansen Solubility Parameter

Solubilities of organic semiconductor compounds are of great importance in controlling the coating, crystallization, and phase-separation processes in the fabrication of organic thin-film devices. Here we measured the solubility of N,N'-di(1-naphthyl)-N,N'-diphenyl benzidine (NPB), a typical hole-transport material, in 23 organic solvents by UV absorption spectroscopy. The solubility of NPB in mole fraction unit x(2) ranged widely from 2.4 x 10(-7) in methanol to 2.8 x 10(-3) in chloroform. The solubility data was correlated with the Hansen solubility parameter (HSP) as a function of the solute solvent distance R-a in the 3D HSP space. A least-squares regression to the equation -log x(2) = aR(a)(2) + b, based on the Scatchard - Hildebrand regular solution theory, was carried out to determine the coefficients (a = 0.0102 MPa-1 and b = 2.16) and the HSP of NPB (delta(D) = 22.6 MPa1/2, delta(P), = 6.8 MPa1/2, and delta(H) = 5.8 MPa1/2 for the dispersion, polarity, and hydrogen-bonding interactions, respectively). The correlation enabled us to describe the solvent dependence of the solubility of NPB within the root-mean-square deviation of 0.47 in log x(2) from the experimental data.