Large cross-section enhancement and intramolecular energy transfer upon multiphoton absorption of hindered diphenylaminofluorene-C ₆₀ dyads and triads

Sterically hindered fullerenyl chromophore dyad and triads, C ₆₀(>DPAF-C ₉) _x (x = 1 and 2, respectively), in an acceptor-donor (A-D) molecular linkage of C ₆₀-(keto-fluorene) _x were synthesized and fully characterized. Attachment of two 3,5,5-trimethylhexyl groups on C ₉ of the fluorene ring moiety greatly improves their solubility and makes direct intermolecular aromatic stacking contacts more difficult. They are the first series of fullerene derivatives showing high three-photon absorptivity (3PA). Accordingly, C ₆₀(> DPAF-C ₉) ₂ exhibits 2PA and 3PA cross sections in the values of 0.824 × 10 ^-48 cm ⁴ s (or 82.4 GM) and 6.30 × 10 ^-25 cm ⁶/GW ², respectively, in femtosecond region among the highest ones reported for many diphenylaminofluorene-derived AFX chromophores. Utilization of a keto linker located immediately between C ₆₀ cage and fluorene chromophore moieties facilitates molecular polarization of the DPAF ring toward the C ₆₀ cage. That may serve as the fundamental cause for correlation of enhanced A-D electron interactions to, ultimately, observed multiphoton absorption cross sections. By using nanosecond laser flash photolysis results taken at 355 nm as the reference, transient absorption data obtained from femtosecond pump-probe experiments at 800 nm unambiguously verified the occurrence of two-photon excitation processes of C ₆₀(>DPAF-C ₉) in air-saturated benzene and subsequent efficient energy transfer from the two-photon pumped DPAF-C ₉ moiety to the C ₆₀ cage moiety.