Benzodipyrrolenine-based biscyanine dyes: Synthesis, molecular structure and spectroscopic characterization

A novel method of synthesizing long-wavelength absorbing and emitting bis-trimethine dyes that consists of condensing benzodipyrrolenine dialdehyde with quaternized heterocyclic CH-acidic compounds was used to prepare a series of biscyanines. The new method is more convenient than its conventional counterepart, which relies upon condensation of a quaternized benzodipyrrolenine with Fisher's aldehyde, for the synthesis of a symmetrically substituted dyes that contain various heterocyclic end-groups. Investigations of the spectral and luminescent properties of the dyes in solution revealed that the absorption (596-717 nm) and emission (629-773 nm) maxima of the biscyanines were red-shifted by ∼100 nm compared to the parent "monomeric" cyanine that contained only one chromophoric polymethine system. The prepared dyes have high extinction coefficients (≤251,000 M^-1 cm^-1) and quantum yield (≤28%). Substitution of both terminal benzoxazole moieties with indolenine, benzothiazole, 2- and 4-quinoline imparted a red-shift in the absorption and emission maxima but lowered quantum yield.