Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads

We synthesized four C(60)-(light-harvesting antenna) dyads C(60) (>CPAF-C(n)) (n = 4, 9, 12, or 18) 1-C(n) for the investigation of their broadband nonlinear absorption effect. Since we have previously demonstrated their high function as two-photon absorption (2PA) materials at 1000 nm, a different 2PA wavelength of 780 nm was applied in the study. The combined data taken at two different wavelength ranges substantiated the broadband characteristics of 1-C(n). We proposed that the observed broadband absorptions may be attributed by a partial π-conjugation between the C(60) > cage and CPAF-C(n) moieties, via endinitrile tautomeric resonance, giving a resonance state with enhanced molecular conjugation. This transient state could increase its 2PA and excited-state absorption at 800 nm. In addition, a trend of concentration-dependent 2PA cross-section (σ(2) ) and excited-state absorption magnitude was detected showing a higher σ value at a lower concentration that was correlated to increasing molecular separation with less aggregation for dyads C(60)(>CPAF-C(18)) and C(60)(>CPAF-C(9)), as better 2PA and excited-state absorbers.