Large Cross-Section Enhancement and Intramolecular Energy Transfer upon Multiphoton Absorption of Hindered Diphenylaminofluorene-C60 Dyads and Triads

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Sterically hindered fullerenyl chromophore dyad and triads, C60(>DPAF-C9)x (x = 1 and 2, respectively), in an acceptor−donor (A−D) molecular linkage of C60-(keto-fluorene)x were synthesized and fully characterized. Attachment of two 3,5,5-trimethylhexyl groups on C9 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, C60(>DPAF-C9)2 exhibits 2PA and 3PA cross sections in the values of 0.824 × 10-48 cm4 s (or 82.4 GM) and 6.30 × 10-25 cm6/GW2, respectively, in femtosecond region among the highest ones reported for many diphenylaminofluorene-derived AFX chromophores. Utilization of a keto linker located immediately between C60 cage and fluorene chromophore moieties facilitates molecular polarization of the DPAF ring toward the C60 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 C60(>DPAF-C9) in air-saturated benzene and subsequent efficient energy transfer from the two-photon pumped DPAF-C9 moiety to the C60 cage moiety.