The synthesis, structure and spectral properties of new long-wavelength benzodipyrroleninium-based bis-styryl dyes

Novel bis-styryl dyes based on a quaternized centrosymmetrical benzodipyrrolenine moiety were synthesized by reaction of a quaternized benzodipyrrolenine with two equivalents of 4-dimethylaminobenzaldehyde, 4-dimethylaminocinnamoic aldehyde or 4-(3,5-diphenyl-4,5-dihydro-1H-1-pyrazolyl)benzaldehyde. The molecular structures and spectral properties of these dyes were investigated using PM3 and PPP CI quantum chemical simulations and compared with biscyanines and parent “monomeric” styryl dyes. Conjugation of two uniform “monomeric” styryl chromophores to form a bis-chromophoric system was found to cause a pronounced red-shift (∼100 nm) of the absorption and emission maxima, an increase of the extinction coefficients but also a conformational rigidization of the molecules. This results in better vibrationally resolved absorption and emission bands and, in general, in increased quantum yields. Bis-styryl dyes absorb and emit in the red and near-IR spectral region (643–812 nm). They have high extinction coefficients (≤133,000 M−1 cm−1), large Stokes’ shifts (up to 55 nm in chloroform and 90 nm in methanol), and quantum yields in chloroform ≤10%, whereas polar solvents decrease quantum yields. Despite their symmetrical structures these bis-styryl dyes exhibit spectral properties in solvents of different polarity that are more resemble those of unsymmetrical “monomeric” styryls.

Research highlights
► We synthesized novel bis-styryl dyes based on a quaternized benzodipyrrolenine.
► Bis-styryls have ∼100-nm longer absorption and emission maxima vs. monomeric styryls.
► Bis-styryls absorb and emit between 643 and 812 nm.
► Bis-styryls have the increased extinction coefficients (80–133,000 M−1cm−1).
► Bis-styryl molecules are more “rigid” compared to monomeric styryls.