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.