Venkatakrishnan P

Merocyanine dyes that exhibit antithetic cyanine-like behaviour and giant first-order hyperpolarisability (β) values have been designed. These cyanine-type dyes open up an intriguing route towards molecular-based electro-optic materials as well as new second-harmonic generation dyes for imaging. Balanced properties: Strongly dipolar push-pull polymethine chromophores with huge dipole moments and excellent thermal stabilities exhibit suitable near-IR transparency and giant hyperpolarisability for applications in telecommunications (see figure).

New merocyanines dyes with tricyanopropylidene-based acceptor units connected to dihexylaminophenyl or dihexylaminothiophenyl donor moieties through polyenic bridges of different lengths have been designed. All derivatives exhibited a strong dipolar character and showed a typical intramolecular charge transfer (ICT) transition. NMR spectroscopy experiments combined with DFT calculations demonstrated that both the nature of the donor–acceptor pair and the length of the conjugated linker strongly impact the electronic structure of the dyes and induce alteration in the bond-length alternation (BLA) and marked shifts in the ICT absorption bands. Hyper-Rayleigh scattering experiments revealed an exponential increase in the second-harmonic generation response as the polyenic chain length was increased. Strikingly, the largest chromophores with the strongest donor–acceptor pair exhibited a very high first hyperpolarizabilty together with a cyanine-like electronic structure, which apparently contradicts the paradigm of optimal BLA predicted by the two-state model. Although it decreased as the polyenic chain length increased, all dyes also exhibited high thermal stability, which demonstrates their potential for applications in nonlinear optical devices.