Linear and Nonlinear Optical Properties of Tricyanopropylidene-Based Merocyanine Dyes: Synergistic Experimental and Theoretical Investigations

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.