Photophysical Characterization of a Benzo-Fused Analogue of Brooker’s Merocyanine: Solvent Polarity and pH Effects

The photophysical properties of 4-[2-(6-hydroxy-2-naphthalenyl)-ethenyl]-1-methyl-pyridinium (HNEP⁺) and its deprotonated form (NEP), a benzofused derivative of Brooker’s merocyanine (BM), were investigated through a combined spectroscopic and computational approach. Despite their structural similarities and similar pK_a values, HNEP⁺/NEP and BMH⁺/BM differ in the extent of charge delocalization in the ground and excited states. NEP exhibits the spectral characteristics of a charge transfer species in solvents in which BM exists in a charge-delocalized quinoid; however, quantum chemical calculations show that the CT absorption of NEP is not necessarily a consequence of the zwitterionic character. HNEP⁺ displays larger Stokes shifts than BMH⁺, and NEP demonstrates enhanced solvatochromism relative to BM as a consequence of benzofusion.