Cyanine limit

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

Fig. 1.10 Left. Canonical resonance structures for the l-dimethylamino-l,3,5,7-octatetraen-8-al molecule, DAO. (Top) Neutral polyene limit (middle) cyanine limit (bottom) zwitterionic charge-separated limit. Right Corresponding evolution of bond length alternation BLA (in A) (x) and bond order alternation BOA ( ), as a function of the applied external electric field F.

The electric field dependence of M, . and l/ , are reported in the top of Fig. 1.11, where they are plotted versus BOA. The shape and magnitude of the curve for linear polarizability versus BOA are well reproduced by the simple model. The two-state model" thus constitutes a good approximation to describe the F dependence of the linear polarizability in such donor-acceptor polyenes. Through this model, the evolution of a can be easily understood. The first transition energy evolves with the applied electric field in such a way that, as a function of BOA, l/ , ., peaks at the cyanine limit (Fig. 1.1 la). Furthermore, the transition dipole moment A/, between the ground state and 5 evolves in a similar way as a function of BOA (note that M., . is maximized at the cyanine limit since in that situation the and 5i wave functions are most similar). The product l/ , times M, /, which constitutes the aulmodel], then evolves, as a function of BOA, in such a way as to be maximized at the cyanine limit, where BOA = 0. 

Scheme 11.22. Conjugated oligomers with terminal donor-acceptor substitution charged cyanines (symmetrical polymethine dyes) 51 below the so-called cyanine limit DAOPVs 52/DAOPEs 53 having electroneutral and zwitterionic resonance structures.

Studies by Meyers and co-workers have shown that the NLO response of many strong donor-acceptor molecules suffer from too much bond length alternation (on the order of 0.01 nm), which inhibits charge transfer in the excited state (132). jS is zero in the cyanine limit, however, when there is no bond length alternation. Interestingly, /S peaks with opposite sign when the BLA is either intermediate between the polyene/cyanine structure or intermediate between a cyanine/zwitterionic structure (BLA 0.004 nm) (133). One important result... 

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