Donating Strength of Azulene in Various Azulen-1-yl-Substituted Cationic Dyes: Application in Nonlinear Optics

Three new azulene-based substituted pyridinium cations, azulene-1-azo( N -methyl-4‘-pyridinium) ( 1 + ), azulene-1-[( E )-2-( N -methyl-4‘-pyridinium)ethenyl] ( 2 + ), and azulene-1-azo[( N -methyl-5‘-quinolinium)] ( 3 + ) were synthesized, and their crystal structures solved by X-ray diffraction. A set of crystallographic, spectroscopic, and computational investigations reveals that, while the pyridinium is a very strong acceptor unit, the azulene counterpart acts as an extremely efficient electron donor in these chromophores. The static quadratic hyperpolarizabilities (β 0 ), measured by the hyper Rayleigh scattering method, are equal to 67 × 10 - 30, 108 × 10 - 30, and 80 × 10 - 30 cm 5 esu - 1, for 1 +, 2 +, and 3 +, respectively. Interestingly, these values are slightly reduced versus those recorded on the related stilbazolium derivatives built up from the less efficient (dimethylamino)phenyl donor substituent. This observation suggests that the donor strength of azulene is beyond the point which maximizes β in the “push−pull” (arylethenyl)pyridinium series. An important charge delocalization in the ground state is suggested to account for the reduced β value in these chromophores, in agreement with the calculated charge-transfer properties, Mulliken charges, and experimental solvatochromism. These investigations suggest that azulene could attract an additional interest in the design of chromophores with cubic optical nonlinearities.