New organic molecules for non-linear optics

Organic chemists are continually looking for new molecules which have large non-linear optical response, particularly in the near-infrared, motivated for the need for such materials in telecommunication systems. Cyanine dyes (see above) are one candidate material which have attracted a lot of attention, particularly by Seth Marder and collaborators. A key feature is that the more delocalised the electrons in the ground and excited states the larger the non-linear response. This occurs when these quantum states are superpositions of two valence bond structures with distinctly different charge distributions. [For a more detailed discussion see this forthcoming J. Chem. Phys. paper by Seth Olsen and I].

A recent development has been the synthesis and characterisation of a family of porphyrin dimer carbocations (shown above), as described in this Angewandte Chemie paper. The large optical response is perhaps surprising because it involves triple bonds near the central carbon cation [these are used because they bond better to porphyrin rings "because they cannot twist out of conjugation"]. Chemical shifts in 13C nmr are used to monitor the charge distribution on the central carbon atoms in the ground state.

The authors claim that the electrons are delocalised over about 18 conjugated bonds. This estimate was based on comparing the dominant absorption frequency to the predictions of an old "particle in a box" model. I did not find this particularly convincing, because the energy of the first excited state is largely determined by the matrix element between the two relevant diabatic states. The family of Creutz-Taube ions show how this can be tuned without increasing the length scale of the charge delocalisation [see this Chemical Reviews].
I think a better measure of the amount of charge delocalisation would be the oscillator strength of the transition or (probably) bond lengths and vibrational frequencies in different parts of the molecule.

A quantum chemical study in JACS compared these "porphocyanine" dyes to a new class of cyanine dyes. They discuss how the triple bond structures are resonant with various valence bond structures with double bonds.