Strong cooperative enhancement of two-photon absorption in dendrimers

We present, for the first time, unambiguous spectroscopic evidence of strong cooperative enhancement of two-photon absorption (TPA) in a series of dendritic macromolecules. The maximum TPA cross section increases in proportion to N-2, where N = 2, 4, 6 is the number of constituent identical chromophore units. We show that the enhancement is facilitated by the quasi-planar structure, which allows direct interbranch conjugation throughout the molecule.