Linear and nonlinear optical properties of photoresponsive [60]fullerene hybrid triads and tetrads with dual NIR two-photon absorption characteristics

Two C₆₀-(antenna)_x analogous compounds having branched hybrid triad C₆₀(>DPAF-C₁₈)(>CPAF-C _2M) and tetrad C₆₀(>DPAF-C₁₈)(>CPAF- C_2M)₂ nanostructures were synthesized and characterized. The structural design was intended to facilitate the ultrafast femtosecond intramolecular energy transfer from photoexcited C₆₀[> ¹(DPAF)*-C₁₈](>CPAF-C_2M) _1or2 or C₆₀(>DPAF-C₁₈) [>¹(CPAF)*-C_2M]_1or2 to the C ₆₀> cage moiety upon two-photon pumping at either 780 or 980 nm, respectively. The latter nanostructure showed approximately equal extinction coefficients of optical absorption over 400-550 nm that corresponds to near-IR two-photon-based excitation wavelengths at 780-1100 nm for broadband nonlinear optical (NLO) applications. Aside from their enhanced two-photon absorption (2PA) activity at 780 nm, we also demonstrated ultrafast photoresponses at 980 nm showing 2PA cross-section (σ₂) values of 995-1100 GM for the hybrid tetrad. These σ₂ values were correlated to the observed good efficiency in reducing femtosecond light-transmittance down to 35% at the light intensity of 110 GW/cm². Accordingly, 2PA characteristics of these nanostructures at multiple NIR wavelengths provided support for their suitability in uses as broadband NLO nanomaterials at 600-1100 nm that includes the 2PA ability of two antenna, DPAF (700-850 nm) and CPAF (850-1100 nm), and the fullerene cage at shorter wavelengths (600-700 nm).