Large concentration-dependent nonlinear optical responses of starburst diphenylaminofluorenocarbonyl methano[60]fullerene pentads

Abstract

We demonstrate an approach toward the design of starburst C60-keto-DPAF assembly by applying a starburst macromolecular configuration with C60 as the core center, which is encapsulated by multiple bulky groups leading to the increase of intermolecular separation and aggregation barrier. Molecular compositions of the resulting C60(>DPAF-C9)2 triad and C60(>DPAF-C9)4 pentads were clearly confirmed by MALDI-MS (positive ion) detection of protonated molecular mass ions. Both C60(>DPAF-C9)22 and C60(>DPAF-C9)4 (structural isomers, 3a and 3b) exhibited nonlinear optical transmittance reduction responses in the femtosecond (fs) region with a lower transmittance value for the latter at the high laser power above 80 GW cm−2. This was attributed to the larger fs 2PA cross-section values of 3a and 3b than that of 2 at the same concentration and, apparently, correlated to a higher number of DPAF-C9 subunits in the structure of 3. As the concentration was decreased to 10−4 M, a clear monotonous increase of the σ2 value change (Δσ2) from 13.9, 33.2, to 48.1 and 68.2 × 10−48 cm4 s photon−1 molecule−1 (or 6820 GM for the latter) for the structural variation from the monoadduct 1, bisadduct 2, to tetraadducts 3b and 3a, respectively, was observed. We interpreted the concentration-dependent phenomenon as being due to the high tendency of fullerene-DPAF chromophores to form nanoscale aggregates at concentrations above 10−3 M. We also proposed that starburst structures, as exemplified by C60(>DPAF-C9)4, in a multipolar arrangement resembling encapsulation of C60 by DPAF-C9 pendants, provide a useful means to increase the degree of molecular dispersion and maintain high nonlinear optical efficiency.