| Achoimre: | Femtosecond two-photon absorption (2PA) spectra were measured of a series of butadiynelinked zinc-porphyrin oligomers assembled in single- and double-strand linear and cyclic structures containing 6, 12 or 24 porphyrin units, in the excitation wavelength range 900–1600 nm. We observe strong enhancement of the 2PA cross-section in the Soret region, with maximum values up to σ2PA ~10^5 GM depending on the geometry of the construct, which we quantitatively explain in terms of a three essential energy level model and cooperative enhancement of the transition dipole moments between the states. The Zn-coordinated templatebound ring of 6 porphyrin units, which resembles some natural light-harvesting systems, results in the highest 2PA cross-section per porphyrin unit (~10^4 GM). It is remarkable that this molecule gives such strong σ2PA ,when the S₀-S₁(0-0) transition is essentially forbidden.
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