Design and synthesis of heterocyclic multi-branched dyes for two-photon absorption.

We report the first example of a heterocycle-based multi-branched two-photon absorption (TPA) dye. We show that the new pyridinium chromophore presents a very large TPA cross-section in the femtosecond regime. Comparison of the TPA cross-sections between the branched system and the corresponding \"monomeric unit\" shows a strong cooperative enhancement. A value of 22.6:1, corresponding to a 750% enhancement, was obsd.