This study synthesized a series of methoxystilbazolium analogues with π-conjugation extended by attaching a fused aromatic ring, i.e., 6-styrylisoquinolinium and 4-[2-(2-naphthyl)ethenyl]pyridinium derivatives with methoxy substituents. We evaluated their transition energies (EegS) and second-order hyperpolarizabilities (βs) experimentally using their absorption spectra and a hyper-Rayleigh scattering measurement, respectively. Subsequently, we calculated βs using the MOPAC PM3 method. These values were compared with those of 4-[4-(4-methoxyphenyl)-1,3-butadienyl]pyridinium derivative, which is a stilbazolium analogue with π-conjugation that is simply extended by an increase of the double-bond number between two aromatic rings. This study clarified that the fused-ring systems possess large βs, irrespective of their relatively short absorption wavelengths, compared to the double-bond elongation system.
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