Dramatic enhancement of intrinsic two-photon absorption in a conjugated porphyrin dimer

We report strong cooperative enhancement of simultaneous two-photon absorption cross section in a porphyrin dimer. The two-photon cross section, sigma(2) = 6 x 10(-47) cm(4)s/photon, corresponds to a 400-fold increase over the value measured for the monomer, and is one of the largest ever obtained for organic chromophore of similar size.