Spectroscopic analysis demonstrates cooperative enhancement of two-photon absorption in zinc-porphyrin nanostructures, highlighting molecular architectures for efficient light harvesting.
Femtosecond two-photon absorption (2PA) spectra were measured of a series of butadiyne-linked 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 σ 2PA ∼ 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 template-bound 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 σ 2PA, when the S 0 –S 1 (0–0) transition is essentially forbidden for one photon transition.
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Mikhaylov et al. (2016) studied this question.
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