Methylenepyran based dipolar and quadrupolar dyes: synthesis, electrochemical and photochemical properties

This paper presents the synthesis of a series of push–pull and quadrupolar π-conjugated structures incorporating pro-aromatic methylenepyran electron-donor groups and various electron-attracting groups. Some of the methylenepyran derivatives were oxidized by I2 to give, after reduction by Na2S2O3, bismethylenepyran compounds via successive steps. The electrochemical redox properties of methylenepyrans 5–9 and extended bismethylenepyrans 10, 14, and 15 determined by cyclic voltammetry indicate the formation of redox bistable systems with high bi-stability. Oxidation of the dimers obtained from 5 to 9 was also described. All compounds are colored and slightly fluorescent (except some bismethylenepyran derivatives). Some compound second-order nonlinear optical properties were investigated, and large positive values of μβ were obtained. A positive dimer effect was also observed for bispyran derivatives.