Syntheses, X-ray crystal structures, and optical, fluorescence, and nonlinear optical characterizations of diphenylphosphino-substituted bithiophenes

A series of bithiophene derivatives that are either symmetrically disubstituted with two Ph(2)(X)P groups (X = O, S, Se) or monosubstituted with one Ph(2)(X)P group (X = O, S, Se) and an organic functional group (H, CHO, CH(2)OH, CO(2)Me) have been synthesized. The X-ray crystal structures of Ph(2)(Se)P(C(4)H(2)S)(2)P(Se)Ph(2), Ph(2)(O)P(C(4)H(2)S)(2)H, Ph(2)(S)P(C(4)H(2)S)(2)H, and Ph(2)(O)P(C(4)H(2)S)(2)CH(2)OH exhibit very different solid-state structures depending on the type of intermolecular π-π interactions that occur. The compounds have been characterized by electronic absorption and fluorescence studies. Of particular interest is that the quantum yields of Ph(2)(O)P(C(4)H(2)S)(2)H, Ph(2)(O)P(C(4)H(2)S)(2)P(O)Ph(2), Ph(2)(O)P(C(4)H(2)S)(2)CO(2)Me, and Ph(2)(O)P(C(4)H(2)S)(2)CH(2)OH are significantly larger than that of bithiophene (factors of 13, 14, 14, and 22, respectively). This behavior is quite different from that of analogously substituted terthiophenes in which substitution results in only modest increases in the quantum yields over that of terthiophene (factors of 0.94, 2.7, 1.3, and 1.5, respectively). DFT studies of the emission process suggest that modifying the Ph(2)(X)P group affects both the fluorescence and nonradiative rate constants while modifications of the organic substituents primarily affect the nonradiative rate constants. The higher quantum yields of the substituted bithiophenes make them promising for application in organic light-emitting devices (OLED). The optical power limiting (OPL) performances of these Ph(2)(X)P-substituted bithiophenes were evaluated by nonlinear transmission measurements in the violet-blue spectral region (430-480 nm) with picosecond laser pulses. The OPL performances are enhanced by heavier X groups and when by higher solubilities. Saturated chloroform solutions of Ph(2)(O)P(C(4)H(2)S)(2)H and Ph(2)(S)P(C(4)H(2)S)(2)H exhibit significantly stronger nonlinear absorption than any previously reported compounds and are promising candidates for use in broadband optical power limiters.